Cliff's Quadrajet Parts and Rebuild Kits
General Category => Quadrajet Carb Talk and Tips => Topic started by: jberenyi on July 29, 2021, 03:59:40 AM
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Just fired up a newly rebuilt 1974 350 Corvette engine and the idle is rough and slightly unstable. Above 850+ RPM and driving the car its great. But in drive at idle its a low rough idle and it wanders from 650 to 850 rpm. The idle mixture screws are about 3 turns out. There is no hesitation either its just an unstable rough idle. BTW, the engine is bone stock. What could the problem be? Do I have to wait for the engine to break in due to a lower compression till the rings seat?
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Have you tried adjusting the mixture screws?
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A 1974 350 Corvette engine should idle like a kitten.
What camshaft is used? Part-#?
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Please elaborate on newly rebuilt.
Cam part number
Heads
Compression
Piston part number if available
Head gasket
Etc.
Could be in the boat i am. Not quite enough compression to get a solid idle but just enough to run fine everywhere else.
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I looked in my QuickBooks program and it looks like that job was done back in 2017. No details on the engine build and I don't remember the job. It's lost in the shuffle as I've done close to 15,000 of them since I started keeping records. We'll just have to wait for details.......
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The newly rebuilt engine is a matching number 350 bone stock except for .030 over on bores. Clifford built this carb from scratch because I did not have a carb for him to build. This Vette uses 7044206. When I got it back it ran just fine with the tired engine but now its a completely different animal. I also had Cliff rebuild the original distributor and he re-curved it to be in at 36 deg at 2800 rpm which I have confirmed it is. Initial timing is set to about 12 deg at 700 rpm. Vacuum is about 15Hg at 1000 rpm and it will get better as she breaks in. I confirmed there are zero vacuum leaks. When I last spoke to you all I noticed that with 5 turns out on the mixture screws and cupping the airhorn with my hand the rpm would rise signifying a lean condition. I turned the idle screws out to about 8 turns and and it got rid of the lean condition about 90%. If she idles at 750 rpm and then cup with the hand it will go up to 800 rpm but that's it. The carb was provided with 75 jets and a 44 primary which is concerning because the specs say it should a 46. But me being at 4500 ft. above sea level maybe its okay. Anyhow, at 8 turns out she's running better but not perfect with respect to idle only in drive or park. All other things like cruising, acceleration, etc. she's perfect. I really think we have an IFR that needs attention on sizing. Thought's anyone?
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In 1974 Corvette uses a std performance Qjet or a high performance Qjet.
The std 7044206/7 units uses 46B pri rods #75 jets .034" idle tubes .100" idle bypass air and CH secondary rods.
The HP 7044210/1units uses 43B pri rods .037" idle tubes .120" idle bypass air and DA secondary rods.
In any case i would open the idle needle holes to .095" and idle tubes .002" to start with.
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In 1974 Corvette uses a std performance Qjet or a high performance Qjet.
The std 7044206/7 units uses 46B pri rods #75 jets .034" idle tubes .100" idle bypass air and CH secondary rods.
The HP 7044210/1units uses 43B pri rods .037" idle tubes .120" idle bypass air and DA secondary rods.
In any case i would open the idle needle holes to .095" and idle tubes .002" to start with.
Good info. One thing to note, I know this carb's casting number is not the one I listed above. Also, my baseplate is missing a port for the vent hose on my charcoal cannister! After paying about $800 for this carb I'm disappointed its not working 100% on a spec build bone stock 1974 350.
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What are the engine specs?
You've mentioned "bone stock build", but no details.
What cam was used in the engine (specs).
Overbore, dished pistons, flat tops, and what head gaskets?.....
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Sorry to the original poster/thread starter. I know you pm'd me, but i dont call numbers off of the internet man. Post up your build info here and we can help. I know you said bone stock rebuild, but many parts are not to factory spec
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Also, being at 4500 ft you may need to throw a few more degrees of timing into it at idle.
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99 if not 100 percent of the "stock" engines I see are not actually built to stock specs.
This is why I ask for details. The statement was made below which makes me think changes were made or the engine is not actually built and blueprinted back to the original "stock" specifications:
"When I got it back it ran just fine with the tired engine but now its a completely different animal."
Right to start with it is difficult to obtain a real factory "stock" camshaft for the early 350 engines. Those engines with the stock cam made close to 20" vacuum at idle speed without much timing in them.
Another problem is that no one will use the correct .020" steel shim head gaskets nor will they verify that the pistons used are only .015-.018" in the holes at TDC.
Instead they purchase "rebuilder" pistons that can end up .030" or ever further in the holes at TDC and then throw a thick "builder" head gasket on the engine. So instead of having .035-.038" quench they end up with .060"-.070" or even more in some cases.
This LOWERS the static compression ratio WAY below where it was on the original engine and idle vacuum and quality will suffer. So basically the owner thinks he's duplicating the stock engine and compression ratio is 8 to 1 or so when it can actually be much lower. Not uncommon to see a SBC build with 882 heads (stock in 1974 on those engines and complete pieces of bovine excrement....I would NEVER use them on one of my engines even if I was hired to build it back to "stock" specifications......FWIW), thick head gaskets, pistons WAY below the deck at TDC, and an "RV" cam substituted for the stock one. With the compression ratio now in the 7 to 1 range and camshaft being used having 10 degrees more duration than a stock one the engine will not make nearly as much vacuum at idle speed as it should be. This may require MORE fuel from the carburetors idle system than a stock carburetor can deliver, or you may find yourself backing out the idle mixture screws till they are nearly falling out of the carburetor to get decent idle quality.
I have no idea if that's what's going on here as we aren't getting any details.
I'll ask once more, provide the "details" of your new engine build and we'll get to the bottom of the issues and see if we can help get a good end result here.......Cliff
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Regarding the engine specs here are the details. When I bought the car (which I sold 2 years ago to a friend) it was somewhat rebuilt at .030 over. The heads were original and untouched and so was everything else other than pistons and rings of course. This was all I could find out about the engine when I bought it. Fast forward, one month ago my friend let the oil get low and spun a bearing so we had the block cylinders honed and replaced the rings, crank, crank bearings and one connecting rod. While at it we ground the valves and put new seals in which were original still and in sad shape. The cam is the same one when I bought it. No other changes were made to the engine and this is all I know. Other than these few changes I don't understand why the carb has a rough idle in park and drive when before it was fine. The only thing I can think of is maybe wait for the rings to seal better and break in the engine more. She only has about 15 miles on her. The lean condition this engine has makes no sense to me considering the changes noted above.
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The correct .020" steel shim head gaskets would NOT have been in your gasket set.
So most likely you installed a "rebuilder" set of head gaskets instead. This would LOWER the compression ratio close to half a point.
Lowering compression would make for a lower vacuum reading at idle speed.
You may have to advance the timing at idle to compensate. Even so I doubt is lowering the compression with the thick head gaskets would make for "rough" idle quality.
Another possibility is that there is a vacuum leak someplace.......
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"Do I have to wait for the engine to break in due to a lower compression till the rings seat?"
No. Should be no difference
"one month ago my friend let the oil get low and spun a bearing "
Why would the engine oil go low on a good engine?
"While at it we ground the valves and put new seals in"
When you say we I hope you mean a Professional
"The cam is the same one when I bought it."
A used cam?
"I don't understand why the carb has a rough idle"
Have you ever thought it might not be the carb. Why was the carb fine before the motor was taken apart
"Vacuum is about 15Hg at 1000 rpm"
Maybe you should be checking your engine repairs / valve job.
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Another possibility is that there is a vacuum leak someplace.......
As noted earlier we have a lean condition. Cupping the airhorn raises the rpm. After changing the mixture screws from 5 turns out to 8 turns out the roughness got better. Probably 90% there. I plugged all vacuum sources and sprayed all gasket areas with starter fluid and no change. We don't have a vacuum leak.
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"Do I have to wait for the engine to break in due to a lower compression till the rings seat?"
No. Should be no difference
So your saying an engine with 10 miles on it has the same compression as when it has 1000 miles on it? Really?
"one month ago my friend let the oil get low and spun a bearing "
Why would the engine oil go low on a good engine?
I can't account for someone's lack of checking their oil.
"While at it we ground the valves and put new seals in"
When you say we I hope you mean a Professional
Yes.
"The cam is the same one when I bought it."
A used cam?
Original used cam as noted in my thread above.
"I don't understand why the carb has a rough idle"
Have you ever thought it might not be the carb. Why was the carb fine before the motor was taken apart?
That's the $50,000 question
"Vacuum is about 15Hg at 1000 rpm"
Maybe you should be checking your engine repairs / valve job.
I don't question the valve job. I do question how long it will take the rings to seal though. BTW when the idle was at about 650 to 700 it was 10Hg if I remember correctly.
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Bump your timing to 16 degrees btdc and see gow it reponds.
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Bump your timing to 16 degrees btdc and see gow it reponds.
At 14 deg she's doing fine. At 2800 rpm she pulls in at 36 deg with vacuum cannister blocked. I don't think I want to go any higher than that. When Clifford did my disty he also made sure the cannister was correct. Lately I've done a fair amount of study how the idle metering circuit works and IMHO I think I don't have enough manifold vacuum to pull fuel up the idle tube on through to the down restrictor. With having the cylinder bores professionally honed and new rings maybe we have to wait for full break in to happen so we can get the increased vacuum like it had before. Looking at all the variables this is the only thing I can come up with other than the valves being jacked up which I know is not the case.
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How much manifold vacuum did the engine make before the "rebuild"?
There is not going to be any improvement in engine vacuum based on ring seal. If it was done correctly the rings were sealed as good as they will ever be a few minutes after you fired up the engine.
Did you change the timing set?
If so what did you use?
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10 in/hg at 650-700 rpm´s is about half of what´s expected on a std 1974 350.
Check valve settings, adjust with engine running. 1 turn down from just stopped clatter is fine.
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"BTW when the idle was at about 650 to 700 it was 10Hg if I remember correctly."
If this is correct then there is the answer. You have a mechanical problem.
Do as Kenth said. Compression test and cylinder leak down test also. You could have an intake leak on the valley side. You need to chase down down the reason for low vacuum.
Last year I had a fellow show up saying he had a carburetor problems on his 383 mopar. I looked at the vacuum gauge, wandering from 8 to 10 inches and and told him it wasn't the carburetor. Never seen the car on the road since.
Four years ago I built a new 454 motor for my Monte Carlo. Bored and finished with a boring plate, new pistons and Total Seal rings. I tour with this car and have never added a drop of oil since firing it up. I change the oil once a year.
A few years ago someone asked me to assemble a 455 Pontiac engine that just came from the machine shop. They bored the cylinders , machined the crank, did a valve job and valve guides, supplied all parts for assembly. This was an original numbers matching 1970 GTO. The new pistons were "rebuilder pistons" with the wrong compression height. Sent them back. I checked the valves for leakage and sent the heads back. The machinist said the valves would seat in when you start the motor. Not good enough for me. We also sent the Camshaft back.
A couple months later I was rebuilding an AMC motor and doing a valve job ( I have my own equipment) . The owner of the 455 pontiac walked in and watched me do a leak test on the valves. He asked , any leaks. No there wasn't.
A few weeks ago a mechanic friend told me he was going to "freshen up" a motor for a numbers matching 1968 Cougar. He asked if I thought it would need boring. My answer was. I've never seen one that didn't need boring.
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+2
I've never seen a used block that didn't need bored either. Even if it was decent, the pistons will ALWAYS have wear in the ring lands.
The blocks will also have tapered wear, less at the bottom and more a the top. This is why you find a "ridge" in the bores and even if it is very minimal you'll see a dark area about 1/4" below the deck surface when you hone them.
Between taper in the bores and wear in the piston ring lands your freshly "rebuilt" engine is doomed from day one. The piston rings, traveling up and down the bores many times per second will have the rings moving all over the place. They will loose their tension quickly and it only gets worse the more time you put on it.
This is why ring seal and ring life is never good on an engine that just gets honed, new rings and put back in service. The bores absolutely need to be round and the rings need to fit tight in the pistons.
Putting a used cam back in an engine is another recipe for disaster. I've seen a few folks try that over the years and knock lobes right off of them which puts millions of particles of ground up iron thru the assembly.
Anyhow, with a "stock" cam in one of those engines, even if the builder wasn't smart enough to check piston deck height and use the OEM .020" thick head gaskets to keep quench tight and compression high it still should be making up near 17-20" vacuum with normal timing in it. I've seen folks try to run manifold vacuum advance on some of those engines (most were ported) and they idled so smooth with so much vacuum they weren't able to get the idle speed slow enough even with the throttle plates completely closed.
I'm talking about the L-48 or base engine. The L-82 will not make nearly as much vacuum and pretty much require higher compression and a LOT of timing at idle to make decent vacuum.
The L-82 camshaft is actually too much cam for one of those engines using the 882 castings. They never did run all that great right off the showroom floor. I was heavy into the muscle car thing at that time and can't say "turd" loud enough here. From 1974 on the 350's topped with 882 heads were gas guzzling gutless pigs.
GM didn't figure out that low compression, poor flowing heads with huge combustion chambers and restrictive exhaust ports isn't the correct approach to reducing emissions. About all it does is kill fuel efficiency and power for the end user just to reduce emissions a tad to get by EPA mandates at that time........FWIW.......
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I appreciate everyone's input thus far. I have learned much from you all during this effort. I instructed the owner of the Vette to come back in 500 miles when the break-in oil needs changing. At that time I will perform a compression and vacuum check on the engine and report back my findings. I'll be curious to see the results and how the engine is running at idle.
BTW, I know that much more could have been done to the engine like boring instead of honing and replacing the camshaft, etc.; but the purpose of this build was to get it back like before and keep cost down for the current owner. A local performance engine building shop with 40+ years experience did the tear down analysis of the complete engine and said the bores were very nice commenting all it needed was a hone. They also said the camshaft was fine. We did have to get another crankshaft off the shelf from them though. I don't think much mileage was on the engine since its last rebuild when the cylinder bores were bored .030 over. With respect to the 882 heads; I agree their not the best but it is what it is. And just to confirm with you all, after the valves came back ground we installed them with new seals and adjusted the preload per these instructions:
How To Set Valve Lash (SBC)
To get started, make sure the engine is cold. You can use timing tape on the balancer or very carefully measure and mark the balancer every 90 degrees to double-check the engine's rotation.
Be sure to apply a generous amount of high-quality assembly lube like Isky Rev Lube or Red Line oil on all of the contact surfaces before starting.
With the valve covers off and spark plugs removed, rotate your engine to the top dead center compression stroke on the number one cylinder. Do this by watching the movement of the lifters or rocker arms – the exhaust will open first, then the intake. As soon as the intake returns to its fully closed position you’ve reached the compression stroke.
Now you can set the valve lash on both the intake and exhaust valve. Most hydraulic lift camshafts will require zero lash for the proper adjustment. The process will be the same whether you have stamped steel rockers or roller tip rockers.
To achieve zero lash, tighten down the rocker as you spin the pushrod in your fingers, do this until you notice slight resistance as you spin the pushrod (this is zero lash). You should also no longer have any lateral movement in the pushrod.
When you know you’ve hit zero lash, tighten the rocker another 1/2 turn to set the optimum preload on the lifter, pushrod, and rocker arm.
Move to the number two cylinder in the firing order and the intake valve should already be open. Then rotate the engine over another 90 degrees or 1/4 turn clockwise while watching the lifters to ensure you're on the base circle of the camshaft lobe and repeat the adjustment procedure.
As you move through the firing order the intake valve should be open on each cylinder. Simply follow the procedure on each cylinder and be consistent as you tighten each rocker.
Again, thank you all for assisting me in this troubleshooting effort. It has been frustrating to say the least.
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Did you change the timing set?
If so what did you use?
There are some flaws in that method of adjusting pre-load on the lifters.
The goal is to make sure that you are making adjustments with the lifter well onto the base-circle of the camshaft.
Following the firing order will work with a marked balancer, you can also set half of the lifters at TDC on the #1 firing position then again on #6. Anyone wanting to use that method can Google up which valves to adjust at each position.
Following the firing order it is easy to get confused as you also have to know the firing order of the engine and make sure that you are on the correct cylinder when adjustments are made. The balancer is also marked in four places at 90 degrees, most will not be so you have to make sure that happens correctly.
One must also take lifter travel into consideration. Some lifters have more travel than others but typically 1/2 turn down from zero lash will be fine. Some "high performance" short travel lifters are designed to be ran with less preload.
In any and all cases here I will compute lifter plunger travel first, especially with new lifters and I've seen variations from about .060" of travel to .150". This is happening these days because there are a lot of manufacturers that have stepped into the lifter business and quite a bit of what we see now is coming from off-shore. There are also a lot of different types of lifters out there, so very little if any "standards" in that industry these days.
The off-shore lifters are horrible and to be avoided. Problem is that there is also a good bit of "re-boxing" going on, but that's a story for another thread on another day. In any and all cases IF you are using a flat tappet camshaft make sure it's getting top quality USA made lifters on it or you will have issues.
Excessive noise is one of them, and lifter/lobe failure is also very common and not a day goes buy you don't read about someone having an issue on one of the Forums. The cam lobe failure thing is directly related to using bad lifters, although there are others that are blaming break in procedures, not removing inner springs during break-in, break in lube, lack of trace metal particles in modern oil (zinc), and about half a dozen other things which have nothing to do with the issues we are seeing.
These cheap imported lifters are NOT made of good quality high nickel iron, nor are they finished and crowned correctly so they spin in operation.
The info above doesn't necessarily relate directly to this thread, but put up there to educate folks as to what's going on with these things.......Cliff
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Did you change the timing set?
If so what did you use?
The original timing chain and gear was replaced. There was no slack at all in the original setup.
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OP, you need to give more details and answer the questions being asked. I just rebuilt a fresh 350 and had none of the issues you are having (low vacuum,poor running,etc). The carb has needed some minor work as it was a factory calibration minus the primary jets and rods supplied from Cliff. it sounds like not enough attention to detailed was paid to engine build. i hate to say this, but this is not a cheap hobby and when trying to go cheap you get what you pay for. been there done that. I've also found a lot of engine builders (even the "performance" shops) will put there nose up to some of the things you want. i had to be firm what I wanted and had to measured and check the machine shops work. i kept the quench tight even though machine shop warned me I was over 10:1 w/ iron heads. i didn't go crazy on camshaft and degreed it. so far it seems to run just fine on 89 octane, but I still have distributor tuning to do. i also am a firm believer in putting the engine on the dyno before putting it in your car/truck/boat. it is worth the few hundred dollars IMO to not have to worry about things once in vehicle. not trying to rain on your parade here....
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With that low of vac. on a stock type build, could possibly have retarded cam timing.
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The original timing chain and gear was put back in. There was no slack at all in the original setup.
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it has been asked twice if timing set was replaced and if so what TYPE. single roller, double roller, link belt (oem), non-adjustable, adjustable, etc. probably would be a good idea to include if the cam was degreed/checked.
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Are you saying you reused to old timimg chain and gears?
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One reply says they replaced it, another reply says they used it over again.
I'm confused here for sure. We're getting "spoon fed" information a little at a time and it's conflicting.
The most important information given is that the engine only makes 10" vacuum at 650-700rpm's, and that would be with "normal" timing in it. It should be nearly double to that reading.
The timing is advanced some and it's got low vacuum, something is fundamentally wrong someplace for sure.
Has anyone done a cranking compression test?.......
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it has been asked twice if timing set was replaced and if so what TYPE. single roller, double roller, link belt (oem), non-adjustable, adjustable, etc. probably would be a good idea to include if the cam was degreed/checked.
I keep saying everything is bone stock except for bored cylinders .030 over and then honed. Everything else is OEM stock stock stock!
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I keep saying everything is bone stock except for bored cylinders .030 over and then honed. Everything else is OEM stock stock stock!
Ok i realize you are getting frustrated, but to the guys that build engines this doesnt make sense man.
You had heads machined correct? This is a change from the original engine, so there could be a problem here.
You reused the old cam... did you keep track of which lifters went in which holes? If not you will wipe the cam lobes fairly soon stop running the engine and buy a new cam and lifters. This could also be your problem.
Reused the old timing set... looks slack or not this is not a good plan. Was it a single keyway sprocket or multiple keyway?
Did you use the thick felpro style head gaskets or the very thin sheet metal gaskets? Thick ones will change compression which will dictate different base timing needed. .. and can lead to similar issues.
You stated that the carb ran fine before the engine was rebuilt. This leads me to believe it is not the carb but an engine issue
Some guys here are probably just unsure of where to go with all the old parts reusing. Ive done it myself out of several buckets of worn parts.... BUT never expected the engine to idle perfect, just run enough to move the vehicle around.
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Fast forward, one month ago my friend let the oil get low and spun a bearing so we had the block cylinders honed and replaced the rings, crank, crank bearings and one connecting rod.
Replaced the crank and one rod? Guys that are in the know are there more than one keyway? Pretty sure one is 2 degrees right of tdc and one is 10 degrees right. And the 86 and newer cranks are like 30 degrees. If this is the case and the new crank keyway doesnt match your balancer and timing cover your timing will LOOK right but be wrong.
Also if it is the original balancer the outer ring may have slipped in the process of removal and installation. So get a piston stop and find tdc with that.
Were the rod ends resized? If the crank was damaged enough to warrant replacement, one would think the large end of the rods would be damaged too.
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Chevy 350 crankshafts has the keyways at the same spot.
What differs are the dampers and timing scale that needs matching.
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"I keep saying everything is bone stock except for bored cylinders .030 over and then honed. Everything else is OEM stock stock stock!
That's BS right there and I'm calling it."
I asked about head gaskets, got no info.
Did you even take the time to measure how thick they were? I went into detail about it earlier on this thread.
"Stock" head gaskets were .020" thick. Even with .020" thick head gaskets the compression ratio with dished pistons and 882 heads is a dismal 8 to 1. Doing the actual "math" on some of those mid-1970's engines I've actually calculated less than 8 to 1 BEFORE you throw on a Detroit .060" thick head gasket!
IF you installed a "builder" gasket it will be .040-.060" thick so you knocked at least 1/2 a point of compression right out of this build from where "stock" would have been.
It's still unclear what you did for a timing set but likely a cheap POS double roller set was installed since this build appears to have been done with the absolute MINIMUM amount of investment.
No one ever puts a used cam back into one of these engines, that's a recipe for disaster and we never got an answer as to whether you put all the old lifters back on it or bought new ones?
It also appears that you sold this vehicle at some point and are having tuning issues with the engine. Not sure what dog you have in the new owners fight, but coming on here bellyaching about how my you paid for the work back in 2017 then having issues on this low-end half-ass attempt at a rebuild isn't working to well with me.
I pride myself in doing things right and don't get all caught up in those sort of things. If someone wants me to re-install a used cam and pick the lifters I pushed out onto the dirty shop floor back into their engine I would very quickly hook the cherry picker to it and drop it in the back of their truck and send them to a local "guru" who deals with that sort of thing.
Engines are EXPENSIVE, time consuming and require great attention to detail and the best parts you can find for them. Honing one with a rock and stuffing it back together with a bunch of used parts is a poor plan, and likely you'll end up on a Forum bellyaching about how it's not working well.
That's where this thread came from and has gone to. I'm renaming the thread and will let it run a little longer. I should have canned it from the start, but I don't get my panties all wadded up when someone starts busting my balls and trying to insinuate that something I did isn't making the grade.
My goals are to help and get a good end result with these things. If this thread goes any other direction I'm going to can it.......Cliff
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I can't believe how far off base this thread has gone and it's all my fault. I never meant to offend anyone. I was only looking for help. I offered what I knew and looks like it wasn't enough or done correctly. I should have let my friend who did the assembly after it left the shop do all the typing and info sharing since he could have answered all your questions accurately and in a timely fashion. I was only involved in the timing and carb adjustment after it was completed and installed. I honestly didn't know there were so many variables in an engine build that can cause issues later. So sorry for everything everyone. I promise to do better next time. Clifford, if you want, just delete the thread and again I want apologize to you and everyone else involved :( Stay healthy and be safe everyone. :(
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I can't believe how far off base this thread has gone and it's all my fault. I never meant to offend anyone. I was only looking for help. I offered what I knew and looks like it wasn't enough or done correctly. I should have let my friend who did the assembly after it left the shop do all the typing and info sharing since he could have answered all your questions accurately and in a timely fashion. I was only involved in the timing and carb adjustment after it was completed and installed. I honestly didn't know there were so many variables in an engine build that can cause issues later. So sorry for everything everyone. I promise to do better next time. Clifford, if you want, just delete the thread and again I want apologize to you and everyone else involved :( Stay healthy and be safe everyone. :(
Hey, it happens man.
I see where this got all wonky now.
So you didnt do the assembly, you just got called at the end of the deal to be "the tune guy".
This is like when i show up for night shift at work after day shift had been fiddlefudging with a machine all day and have to take over with half the info on what was done.
Yeah, if you sisnt personally assemble the engine and ( IF YOU ARE CONFIDENT IN YOUR TUNING CAPABILITIES) you can not get the tune right.... assume someone assembled something wrong. Especially, if it idled fine before... something mechanical changed to make the old settjngs of the carb inadequate. Simple disassembly and reassembly with " the same stock parts" wouldnt change that. So if you didnt assemble it, just maybe something isnt as "stock" as you were told.
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In only delete threads and ban users when we don't follow the rules for posting.
There are a few things you need to be aware of with engine "rebuilds".
The SBC engine is a perfect example.
99 percent of the "builder" pistons supplied for them in oversizes have a REDUCED pin height. This is some lame attempt to put the pistons lower in the holes at TDC and not raise the compression proportionally with the overbore.
Few if anyone out there knows this, not even machine shops, engine builders and local "guru's".
Most SBC builds also get a "double roller" timing chain, recommended by every "guru" on every Forum out there as some sort of "upgrade". 90 percent of them are JUNK, and even the top end varieties aren't as strong as a factory Morse type chain nor to they provide constant tooth contact and "smooth" drive for the camshaft, distributor and oil pump. Speaking oil pumps it will also get a "high volume/high pressure pump" that does nothing more than rob 10hp or so from the assembly and put a LOT of additional load on the distributor gear and oil pump drive. I don't even use the HP pumps on full race stuff as the SBC has an EXCELLENT oiling system and if you know how to clearance things its fine with stock pressure and stock pump volume.
Continued.....
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Moving on here, the factory had the pistons pretty close to the top of the block at TDC and used .020" thick head gaskets. This was done to keep quench pretty tight for higher compression, improved combustion efficiency, cooler running temps etc.
ALL replacement gaskets showing up in "builder" kits will be much thicker, typically in the .040-.060" range.
So between your pistons being .030 or deeper in the holes at TDC and your thick head gaskets quench is no longer in the ideal .030-.39" range and up in the .055-.095" range someplace. SBC engines with that much quench run hotter, overheat, don't make chit for power, take more timing and more fuel, etc, etc. To add insult to injury folks doing these things think their compression ratio is nearly a point higher than it actually is.
Then most poorly informed enthusiasts bolt on the common 882 castings which don't flow for chit and have a very restrictive exhaust port to help provide some natural EGR and make the EPA happy. Doesn't matter at this point what else you do, fancy cam, intake, headers, exhaust, etc, you just built a "turd" and you're stuck with it.
It's easy to spot one of these engines, they don't sound "clean", "deep/heavy" exhaust note at idle (low vacuum), poor throttle response, LOUD exhaust when they aren't pulling a load, lackluster power everyplace, REQUIRE a lot of timing to be happy, and they consume huge amounts of fuel for "normal" driving.
Sadly many of the owners don't know all this because they haven't had a professionally built SBC with all the right things done to it and correct parts used in it to compare it to.
As all of this applies to this thread there are many unknowns as to exactly what was used and the parameters, but what we do know is that the engine is only make about half the vacuum it should at idle speed.
Could be as simple as the lifters adjusted too deep, some worn cam lobes, retarded cam timing, LOW compression, retarded ignition timing, etc. I can tell anyone reading this for certain, IF you have a correctly rebuilt stone stock 1974 350 engine it will make great vacuum at idle without a lot of timing in it. You will not need to back the idle mixture screws way out on the stock carb either, and the throttle plates will be nearly closed at idle speed.........Cliff
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Based on your knowledge of engines, this guy must be desperate if he needs you to "tune" it.