Cliff's Quadrajet Parts and Rebuild Kits
Quadrajet Problem Solving => Dialing in your rebuilt Quadrajet carburetor => Topic started by: 68rs/ss on April 09, 2022, 05:51:25 PM
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Looking for possible solution for new engine/carb. This is fresh 396 BBC and reconditioned using cliff's parts and recommendations.
Engine is 9.5 CR using closed chamber heads, with mild bowel cleanup and good flowing valves. Cam is Crower with 224intake, 232 exhaust, .542 lift intake, .514 exhaust. Lobes are at 112 degrees.
using Q-jet RPM and headers.
This is fresh engine and is on test stand for break-in and carb adjustments before putting in car.
After initial break-in, can not get it to idle down. Primary throttle blades are open too much causing idle drip. Does not run smooth with blades turned down to stop drip. Mixture screws have little effect. Pretty sure no vacuum leaks as all the ports are plugged. Initial timing is roughly 8 degrees.
Carb is as follows.
Jets:79
primary rods: #46 tapered from cliff
Upper Idle air bleed:.070
Lower idle air bleed: .059
Idle tube: .038
Idle down Channel: .053
Mixture screws: .090
Idle by-pass air: .110
Float was set a little high so lowered it to 1/4"
Needle and seat vacuum tested and held for hours.
Looking through cliffs book, seems like the idle down, mixture passages and lower idle air bleeds are all a little small compared to some of the recipes.
Where should I look to cure this?
Thanks.
Phil
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Your carburetor "recipe" should be adequate for what you are doing. Before making any carburetor changes get the idle speed as low as you can and gently "tip-in" the choke flap or place your hand over the choke housing area and see if the engine speeds up and smooths out.
I would also try more initial timing. Your compression ratio is pretty "low" for the CID and camshaft specs so it's going to enjoy more timing at idle speed.
Do both of those things before adding any more idle fuel......Cliff
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OK, I will put it back together for another try. It did smooth out with more timing and it did run better when I put my hand over the carb.
It's a little discouraging hearing the cam is not the best choice for the application. I sent my engine and car details to "cam guy" at company and that's what was recommended.
Do you have any recommendations? Hate to do a cam change, but I do want this to run without any issues. Better to do it now before it goes in car. You've been very helpful in the past.
Phil
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You simply don't have enough compression IF you are needing to add a lot of timing and fuel at idle speed for the cam being used.
Doesn't always make it a bad choice, but from what I've seen with these things if you find your engine wanting a lot of timing at idle, and you aren't making a lot of vacuum without adding a bunch of timing, the cam choice isn't optimal.
Sadly I see a LOT of this sort of thing these days. Started back when someone (who needed to go very quickly back to their day job and stay the hell off the Forums) came up with some proverbial "brick wall" of 9.5 to 1 compression for these engines if you want to burn pump gas in them.
That statement alone is probably the most inaccurate statement ever made related to these engines......FWIW and IMHO.
Anyhow, next time it's up and running do a vacuum reading at 700-750 RPM with 8 degrees timing. Increase it to 10, 12, 14 and keep going till it's pretty happy, and take a vacuum reading at each point.
Once you reach your "happy spot" shut the engine down, let it sit 3-5 minutes and see if it "bucks" the starter on a hot restart........post the results.......Cliff
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You might try the 1965 Impala High Performance vacuum advance (STANDARD MOTOR PRODUCTS VC181) and use full manifold vacuum at idle.
Starts @ 5-7 in/hg and have 16° @ 11-13 in/hg.
This will make a stronger "pull" thru the idle ciruit due to bigger difference between atmospheric pressure and vacuum beneath the trottle blades.
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Your cam and compression are alright . Correct timing for the situation is the number one starting point .
Kenth is on the right track if you are using a points distributor and you need more timing with vacuum advance at idle. Like Cliff said post results after stepping up timing with vacuum readings.
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In addition to the above make sure your secondary throttle blades are absolutely closed all the way. Be sure there's a little "slop" in the linkage rod that connects the primary and secondary blades. Hold the secondary air valves (flaps) open by hand and hold the carb up to a light to see if the sec blades are shut tight.
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Good advice Kenth but we're a bit early in the game here to start running 25-30 (or more) degrees initial timing. Before that happens we need to know vacuum readings and how much timing it's going to take to get good idle quality.
The carb already has PLENTY of idle fuel for this application, but we're also assuming no fundamental issues either.
The cam may be "alright" for the application, but that remains to be seen. It's only around 400CID and dubbed 9.5 to 1 compression. If it's actually 9.5 to 1 and doesn't make at least 12" vacuum with that cam with only 12-14 degrees initial timing then it's seeing it as s pretty "hefty" camshaft for the combination of parts, or the compression ratio isn't as high as we think it is.
Doesn't mean it woln't work OK, but personally I consider having to run the timing clear off the scale at idle and open up the carbs idle system quite a bit more of a "crutch" fix than a good method of tuning.....FWIW.....
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68RS, you never gave the main air bleed sizes. Unless I overlooked this.
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Took Sunday off.
Put carb back together today after checking everything carefully and really didn't see any problems. Although, I've only done a few of these and may have missed something. I wonder if the new idle tubes are plugged or something? I did run this on a 350 for a test fire last year and it did work ok then. So not sure. So, Put it back on engine and tried to do the test that Cliff had suggested. Really could not get it to run smooth enough to get any reliable reading at any initial advance. Vacuum was fluxuating between 10 and 15 in/va. Started at 8 degrees and moved up to 12 or so. Eventually, I did advance it to around 30 degrees and it seemed to work a little better. vacuum there was around 12, but not steady. Was really hard to get accurate reading as engine was "hunting" so bad at idle. I could get it down to 750-800 rpm . Hot restarts are not happening. I do not have vacuum advance hooked up.
It did smooth out when i cupped my hand over the choke to restrict the air flow. Seemed to like this. although, didn't cure it completely.
I've never had an engine this hard to set. I will be trying a different carb and different distributor in the next couple of days, just to see.
I am starting to wonder if I have a intake gasket leaking. It is a new Q-jet RPM and it did need to have some of the intake bolt bosses files down so it would fit.
This engine is a true 9.5. compression. Heads were cc'ed, piston/deck checked at .017 ands a .020 steel shim gasket to get a good quench. There were basically only two forged piston options to choose from, so it was a thought out build. I didn't want to deck block to loose right engine code for my car. So pretty happy to get what I did with the few options available.
If it turns out that this is what the engine personality will be like, I certainly have no issues with a cam change and using my cast intake. I want a strong, fun, reliable street engine. Something that can be driven on a day trip. Of coarse, I will run her through the gears once in a while too.. ;D
Main air bleeds are aprox. .113"
So tomorrow, i will get back to basics. Check plugs and may remove intake. Its easy sitting on test stand.
Will keep you posted with any new developments. Thanks for all the help. Definitely learning here.
Phil
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If it smoothed out by choking a bit, needs more idle fuel or it has a vac. leak.
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"Really could not get it to run smooth enough to get any reliable reading at any initial advance. Vacuum was fluxuating between 10 and 15 in/va. Started at 8 degrees and moved up to 12 or so. Eventually, I did advance it to around 30 degrees and it seemed to work a little better. vacuum there was around 12, but not steady. Was really hard to get accurate reading as engine was "hunting" so bad at idle."
If you couldn't get it to idle smooth and decent vacuum there is a fundamental issue someplace probably unrelated to the timing and idle fuel delivery from the carburetor.
Could be as simple as how I cured one today. Was getting my ars handed to me testing a carb done by a good friend. It was correctly rebuilt, used my parts, and I went over every detail on the carb and couldn't find any issues anyplace. My friend told me that it ran horrible on his test engine no matter what size idle tube he tried, main jet or APT setting....Hum?
Testing on my engine confirmed his findings. The carb resulted in HORRIBLE idle quality so I had him take it apart and make several changes to it and with each test no improvement whatsoever. I was doing other things in the shop each time he made small changes to it but did look it over closely and couldn't find anything obviously wrong.
Right before the last time it went back together for another test I noticed that the carb had been sealed over the bottom plugs with JB Weld at some point. I hadn't noticed this earlier, but did ask the builder and he said he just left it on because it passed a leak test so I never gave it another thought. Just to make sure it wasn't causing an issue I did a closer inspection. I found that on one side there was a very slight witness mark on the glue and low and behold it was sticking up just high enough it was holding the baseplate away from the main casting when the screws were tightened up.
I ground it down flush, put the carb back together and ALL the problems at idle disappeared. No more lean idle, surging, hunting, no response with the mixture screws, etc. Yes, one little detail was causing ALL the problems we were seeing.
For what you are seeing it sounds like a vacuum leak someplace, and probably a pretty decent one as you can't get the engine to be happy no matter where the timing is at. That isn't enough cam in your build to need 30 degrees initial timing to start to get happy......IMHO........Cliff
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What type of ignition system? Does it have correct voltage/current to the coil?
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"Engine is 9.5 CR using closed chamber heads, with mild bowel cleanup and good flowing valves. Cam is Crower with 224intake, 232 exhaust, .542 lift intake, .514 exhaust. Lobes are at 112 degrees. "
The static CR must be based not only on fuel octane but also intake valve closure point.
If the compression ratio isn't in line with the cam duration , (the closure point of the intake valve) , the dynamic CR, and cranking compression , will consequently be low. This not only kills torque everywhere in the rev range, but also significantly reduces the engines low end output.
A good test to establish that an engine has a high enough CR is to do a cranking test. Assuming that the rings and valves seal perfectly, then even a modest street motor should show at least 180 psi if cam and compression are roughly right. For a regular use performance street motor expect to see 200 psi.
Assuming no pressure is lost to leakage, anything below these levels indicates too low a CR.
For these reasons it is important to follow the CR requirement given for each cam. If for some reason the required compression ratio for a cam can't be obtained choose a shorter duration cam; you will like the results better.
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Thanks for all the replies. I did manage to get it to idle with no nizzle drip. RPM hovered between 790 and 840. Vacuum was between 12-13. Initial timing was 12-13 degree's. I will see if i can get it any better tomorrow. Hot restarts are not happening. I went through carb again and changed both top and bottom gaskets. I did find the new base gasket was split and separating. Replaced with old one. This seemed to help.
The past few days, I have been doing alot of reading on cam selection and CR. Thanks "Old Cars" for the reply. So what is a desirable Dynamic CR for a good street engine. My cam card says intake closes at 40 degree's ABDC. used a calculator on line and it came up with 8.78: for my combination (if I did it right).
Phil
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Why are hot starts "not happening"? Is it "bucking" the starter or flooded?
That tiny gap in the carb to intake gasket has nothing to do with your issues as it would compress the second the bolts were tightened down holding the carb to the intake.
12-13" vacuum is pretty "low" for the combination of parts. I would have expected closer to 14-16" and relatively smooth idle quality with maybe at most a "deep/heavy" sound in the exhaust.
Might want to go back thru the valves and make sure you don't have one that's too tight holding a valve open just a bit.
When it comes to cams Crower grinds will act bigger when compared to others as they use longer seat timing providing more effective duration. I use and prefer them as they have gentle ramp profiles and don't require a butt-ton of spring pressure like other companies that use "fast ramp" technology.
I'd also add that on the dyno Crower grinds really deliver and we've made more power with them pound for pound.
For example, in a Pontiac 400 build with unported #16 heads at 10 to 1 compression the Crower 60916 (221/229/112) cam made 419hp/453tq. With the next cam in their line-up the 60243 (228/234/112) the same engine made 424/465tq.
Idle vacuum for the 60916 cam was 14" at 750rpm's, the 60243 was down to 12" at 750rpms. This is with 10 degrees initial timing. Both cams were relatively smooth for idle quality with just a hint of "lope" with the larger cam.
I've done more dyno testing than most who will read this, and back to back cam testing on my nickel to see what works and what doesn't.
The Comp XE lobes are just pure CHIT despite the excellent advertising for them. They also have issues at high RPM's loosing control of the valve train as the seating velocity is just too quick and even using their recommended springs (which have too much pressure for street use IMHO) we still saw a quick and abrupt end to power production on the dyno right around 4900-5200rpm's every time we tried to test one.
Sorry to wonder off track but I use these opportunities to educate others.
I'm not seeing why you are having so much trouble here and still think that you have a fundamental issue someplace.
Did you degree the cam when it was installed?
What timing set was used?........
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The calculator you used may have been for seat to seat intake closing . Your cam card may be intake closing at .050" spec. Try this one
https://www.gofastmath.com/Compression-Ratio-Calculators/Static-And-Dynamic-Compression-Ratio-Calculator
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Cliff, Thanks for the reply. After taking a couple days off to get some other stuff done, found cause for problems I was having. I suspected leaking intake on bottom side and was confirmed today after I took it off. 7 of the 8 ports were sucking air/oil from the bottom. Checked intake machined flange and found one side to have high spot where heat riser port is. Will be sending this new intake back for refund. It never fit properly from the beginning, with filing bolt boss's and holes to fit. One would think, with everything being new there would be no issues like this. I did use a brand name gasket and torqued bolts to proper torque and order in progressive higher settings till proper torque value was reached.
Will be using cast Q-jet intake in its place. Modified plenum to make two sides and smoothed out corners. Only disadvantage is the thing weighs a ton. Hopefully this will work OK.
I kind of enjoy Cliff sharing his knowledge here. Most of us learn by either reading or with someone's help.
When this is running again, will post update.
Thanks for the guidance and suggestions. learned some stuff.
Phil
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The factory cast iron intakes are excellent, especially the early high rise variety.
Don't sell the later "flat" big block intakes short either. I've helped out quite a few folks in FAST and Pure Stock using them and they make great power.
Not that long ago I helped a customer from Canada with his 1970 Chevelle 454 being built for FAST Class racing. He must use the stock flat iron intake. The engine was stroked to 496cid, well prepared heads, high compression and I provided the carb and helped with the cam selection.
It cranked out over 580hp and nearly 600ft lbs torque with the "flat" intake and no spacer under the carb! I know a lot of folks who ditch those intakes for an aluminum "high rise" intake on engines making a LOT less power thinking they are helping the engine out someplace. Then they loose even more power "cobbling" things up to get the air cleaner to fit under the hood or their Cowl induction to fit, etc.......
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I still remember the last Edelbrock intake I installed on a 454 Chevy engine. It didn't fit for chit and required taking an angle grinder to it so it would sit down and seal the gaskets. I'm not all that fond of them right to start with and even less when you have to modify them to make them fit......FWIW......
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“Modified plenum to make two sides and smoothed out corners. “
What does modified plenum mean? Can you post pictures?
“Checked intake machined flange and found one side to have high spot where heat riser port is.”
Similar to checking block deck and cylinder head surface this is done with a machinists straight edge/bar. How many thousands was it out?
“It never fit properly from the beginning, with filing bolt boss's and holes to fit.”
Have your heads been planed and/or engine deck?
Machining deck/heads can cause misalignment for the intake manifold including end end gasket squish / thickness required on chevy engines. Also BBC intake manifold gaskets come in different thickness and quality. Such as Felpro 1212 Printoseal .060”
thickness or Edelbrock 7203. End gaskets are not recommended. Use RTV Silicone 1/4” inch bead aprox.
This applies to both aftermarket and OE intake manifolds.
Cliff
If your example did not have rules to follow for his class that 750 Quadrajet on a dual plane aftermarket intake manifold would have been too small on cfm by a couple hundred . This is where the aftermarket manifolds shine / The cork in the system becomes
the carburetor, especially on a true dual plane intake.
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Two photos of cast intake. Original and after. Used RPM as template to remove casting which pretty much went to floor of plenum. Just smoothed out transition to runners. Was pretty much at right angles to runners.
Block is not decked. Heads did have a few thousands planed off to make true. Edelbrock intake has four bolt boss's that interfered with ridge along top of mating surface, just below valve cover. Couple of holes needed to be enlarged to fir bolts.
One side has .010 high spot at heat riser. Yes, checked with straight edge. Used Fel-Pro .060 gaskets with silicon bead on end rails.
I did not check angles of machined surface on intake, when I get my other intake back from soaking, I will compare.
So, in my mind, for the amount of money spent for this product, I feel quality was pretty low quality for issues found. Will it may make more HP? Maybe....? Only way to tell is with dyno or strip time. Don't have either.
Phil
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What was the Edelbrock number?
Cutting the divider between the two plenums (moderate to ? ) , you allow cylinders to draw from all four barrels instead of just two. It converts a dual plane intake into a single plane intake that has longer but more tortuously routed runners. A true dual plane intake , where the left and right hand plenums are separated , each cylinder sees only half the carb's CFM capability. This is why a true dual plane can require a bigger carb than normally thought of.
Think of the Z28 1969 302 and 1970 350 which had a true dual plane fully divided intake and a 780 holley.
But you can always rely on the plenum cutout when the carb cfm falls short. Big blocks in the 454 cubic inch and up, are already hurting for flow.
If a certain power output is targeted , in practice , an oval port single plane intake with a short cam
( this is what I did ) makes as much or more low end output as a dual plane with a big divider cutout and a longer cam.
I think this could be a very good test of your cast iron intake.
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"Cliff
If your example did not have rules to follow for his class that 750 Quadrajet on a dual plane aftermarket intake manifold would have been too small on cfm by a couple hundred . This is where the aftermarket manifolds shine / The cork in the system becomes
the carburetor, especially on a true dual plane intake."
Some race Classes require the factory intake to be used. Nothing more, nothing less. My example simply shows that a flat iron big block Chevy intake is fully capable of making big power. It's not meant to compare it to anything else.
You also know nothing about the carburetor used, it was done here and specifically built for FAST Class racing. To maintain the privacy for my customers I will not reveal any details on those particular units. I have other Q-jets out there in FAST Class applications, several run into the high 9's and those cars do it on 8" bias ply tires, so the level of performance we are getting is most likely WAY above the vehicles owned by most who are reading this..
I'd also add here that on more than one occasion I've back to back tested intake manifolds with no other changes. I've also tested them in conjunction with 4 different 1" spacers.
On one outing (private track rental) I tested my "modified" cast iron Pontiac intake against the new (at that time) Tomahawk intake. I also added a 1" spacer to the Tomahawk as well for comparisons. The Tomahawk intake is basically a well modified Holley Street Dominator single plane intake professionally ported by Dave at SD Performance then used to supply the scan for the final product originally marketed/sold by Ace at Pacific Performance Racing.
Without a spacer it ran SLOWER everyplace, 60', 1/8th and 1/4 mile, worse in ET and less MPH.
With a 1" spacer and eventually swapping over to my very well prepared 4781-2 850 Holley DP carb it ran almost 2mph faster, but slightly slower in ET by .02 seconds to my factory cast iron intake with no spacer at all. I repeated the test and same thing buy .03" seconds slower but still almost 2 MPH faster.
Where it really gave up power was 60' and short times. Even with a stronger top end charge (it obviously made more upper mid-range and top end power) it was not enough to offset the loss of low RPM and average power.
So it was NOT hitting the converter as hard, the tires as hard, or pulling as hard off the line thru the 60', but pulling harder at the top end of the track.
I know testing like this is difficult for some folks to understand or accept. Edelbrock (and many other companies selling "high performance" parts) have to tell us how much better they or they'd have trouble selling them in the first place. So if you don't test anything you'll simply be like everyone out their and think that all of these products are superior to factory parts at every level.......FWIW
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"Cutting the divider between the two plenums (moderate to ? ) , you allow cylinders to draw from all four barrels instead of just two."
NEVER cut down or remove the divider in a dual plane intake. I've had to put dividers back in quite a few over the years.
It can help to remove a small amount of material between the secondaries, about have of a 50 cent piece, but removing part or all of it KILLS off more power and it can/will induce stumble/hesitation/bog going quickly to full throttle that is difficult and often impossible to tune out.
Yes, I've done a LOT of testing in that area as well and dyno and drag strip tested my Pontiac cast iron intake many times against aftermarket intakes. The final result will outrun Edelbrocks RPM intake on the dyno and at the track to engines making at least 500hp. Plus it fits under the hood just fine and doesn't require "cobbling" to use Shaker or factory Ram Air set-ups, or a drop base air cleaner (they can move the lid too close to the carb and rob power) for hood clearance.......
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Just to clarify, I did not touch the divider. The two cross overs cut out between the two sides are factory.
So, I guess I would be wise to use a four hole gasket with stainless shield that I had got from u when I got my kit.
This thing is not going to cause me any problems I hope.
Phil
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You must use the factory gasket arrangement to effectively seal off the "hot slot" in the front of the intake. If the holes are left open on each end of the "channel" then there will also be hot exhaust gasses traveling across under the carb to heat things up for improved thermal efficiency (allows a leaner mixture to be burned as it's heated a bit).
Good idea in theory but it can be problematic in actual use so many engine builders and owners of vehicles with engines using those intake block them off.
You'd be AMAZED at how many early Chevy carburetors are sent here with a carbon track across the bottom of them clearing showing that the wrong gaskets were being used and there was a HUGE vacuum/EGR leak into the engine.
Anyhow, the gasket goes down against the intake and the stainless steel shim gasket directly against the carburetor. No other gaskets are used or needed with that set-up.......
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"50 cent piece" This is like a history lesson . Have not seen a 50 cent coin here in Canada for years. Don't even remember the size of it. Anyways I googled 50 cent US coin (1.2" diameter ) so half of that is .6 inch ( moderate ) . I should have specified a dimension.
Looking at the picture of his intake wouldn't it be better to drill/tap/plug the channel , unless you live in a cool/cold climate driving.
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There are still thousands of early GM engines in use out there that use additional passages in the intake to heat the bottom of the carburetor. Once in a while we'll get a complaint or tech call to the shop from someone having fuel percolation issues that they believe is being caused directly by too much heat to the carb from that system.
Many times we actually find out that they have other issues instead, but we still do get some legitimate complaints from folks who have those set-ups. It is ALWAYS in the hottest summer months, never in cool/cold weather.
I don't leave it open here on most builds we've done simply because of the way these cars are used these days. Most only see use in summer months and it's usually pretty hot out as well. I've never seen any negatives anyplace from blocking off the holes as the intake eventually gets pretty hot anyhow and transfers a good bit of heat to the carburetor unless the later type thicker gaskets or spacers of some sort are used........
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Just a update on my carb and new engine build. Engine dyno today. Engine fired right up and performed very well. Carb was very close with modification recommendations by Cliff. A/F ratio was good on primary side and secondary side needed a little richer rod. I had turned down some extra DR rods by by .010 and .020 in case. Used the .010 one in the end. Timing was 34 total with 10 initial. Idle speed at 750. Did find a couple small issues. After full throttle pull, idle went rich. After some investigations, we found secondary side was hanging open a little. Throttle shafts worn, so will be bushing this side as well. I didn't think the secondary side was a issue as for as worn shafts/bushing. Most play was up and down and not front to back. Will watch for this in future.
Top cover leaking, so will need to check float and gasket.
I expect some of my initial issues when I had first posted were poor ignition wires and old distributor. Half dead battery didn't help much either.
440HP and 451tq in my little 396. Happy with this. I expect I may need to do some fine tuning when it is the car.
Will be calling shop soon to get required parts.
Thanks Cliff for your assistance in all this.
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Good numbers!
The secondary side hanging up after going to WOT is not in need of "bushings". Most of the time when that happens the secondary throttle plates are not well indexed and centered.
I've also seen too much epoxy applied to the bottom plugs cause that issue as it binds things up when the baseplate is tightened to the main body.
Third cause is insufficient clearance between the lever on the shaft and link that opens them.
The factory left some clearance on the secondary side for a reason and if you get it too tight back there the huge secondary throttle plates will not be able to effective close and seat in the bores.......