Cliff's Quadrajet Parts and Rebuild Kits
Quadrajet Problem Solving => Diagnose a Quadrajet carburetor problem => Topic started by: RyanAK on July 27, 2020, 09:22:17 AM
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Hey, gang. I've searched through just about every thread here and on 67-72 Chevy Trucks trying to find someone that's had a similar issue but came up short. So gotta post. Sorry if this is long winded. This is my daily driver and my alternate vehicle isn't available for a while, so I'm wound pretty tight about this. This has been going on since June. Appeared out of nowhere after driving all winter and spring... about 8,000 miles.
So... the truck: 1978 K10 Suburban pretty bone stock with a GM crate installed by the previous owner. I believe the 190hp version. About 18,000 miles on that motor now. Carter 4MV Quadrajet 1705821? ('78 GM Truck), TH350, NP203 (stock), 3.73 gears. The only ‘upgrade’ to the drivetrain would be 16” wagon wheels and 275/75R16 tires.
At highway speeds of 60-75mph (2500-3000rpm), at load (uphill or long periods of acceleration), the truck feels like it gets starved for fuel. It bogs and goes in quick time, chugging, and feels like it’s sucking more gas than what’s available.
I added a “diagnostic tool” in the way of a vacuum gauge to help figure this out. I’m good as long as I keep the load to where there’s 10” or more of vac. (Idles in P at 20" @750rpm). Unfortunately this can drop my speed to 50 mph or less on a grade... with traffic zinging by at 75. If I spend any time with my foot in it and vac is below 10”, eventually it will start stumbling. When it bogs, the vacuum drops. So if I’m climbing a hill and engine vacuum is at something like 7”, when it starts lugging the gauge will bounce in time with the lugging between 7” (power) and 4” (stumble). If I take my foot out of it it smooths out and we pull along. Just slowly. Right now I'm limited to about 1/4 throttle or less. I just got back from a 1,200 mile trip between Pennsylvania and Maine, so I've had a LOT of time and a LOT of hills to think about this issue.
This will happen on the level or even going down hill if I put my foot into it, too.
I also put the back glass down. When it stumbles the exhaust smells rich. Hard to tell if the rich condition CAUSES the stumble, or it smells rich from when it catches itself.
Engine temps remain steady. Ditto oil pressure. Idles fine with 20” at 750 rpm. It’s been hot and muggy. This truck came from Montana at 5,000ft and now it's on the East Coast at between sea level and 2,000ft.
TBC...
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Stuff I did on this past trip to try to improve things:
Air cleaner lid flip (to bypass the valve in the air cleaner)
Wired the choke open (thought the choke could be closing and choking the engine)
Insulated the hard fuel line between the fuel pump and carb where it sneaks between the block and alternator with rubber fuel line (hot fuel?)
Fiddled with the choke pull off - seems as though it won't pull in on its own when engine is first fired. I have to 'nudge' it to retract.
TBC...
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Not sure what's happening that my posts keep getting clipped...
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Stuff I've done in the last few months chasing this:
Complete ignition tuneup (plugs, wires, cap, coil, rotor, module)
Timing dialed in at 10* initial, 32* all in @2800rpm, 15* vac on manifold vacuum
Idle air/fuel mixture - screws responsive and set to max vac
New mechanical fuel pump
New fuel filter with check valve
New soft lines at pump - old were soft and kinking. looked like an artery pumping!
Confirmed clear fuel lines from tank by blowing air back to tank
New air cleaner
Moved vac advance from ported to manifold
Double shot of Sea Foam
Cleaned carb with carb cleaner without disassembly
New PCV valve (suspected sucking blow by and causing lean condition...)
New divorced choke spring
I have video of this chugging happening.
I feel pretty confident this is carb-related at this point. And I'm determined to fix it, educate myself, and have it reliable. I have faith. But I haven't rebuilt a carb since I was 10 and helping out my Pap. So any help with diagnostics and guidance to a fix is very welcome.
I wish I had a known good carb to put on just to keep my rig mobile so I could pull this and really dig into it...
Sincerely, Thank you for any help that comes my way.
Ryan
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You are running lean not rich, most if not all hesitation is due to too lean A/F mixture
I would open the idle down channels from .049" to at least .052" and replace the std #63 jets with .066".
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Thanks, Keith. Appreciate the response. Two quick things...
Truck ran strong and without incident all winter and spring. Seems to have become an issue with the onset of hot, humid weather. Is this something to consider, or just a red herring?
And...
When it does this, it
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Couple of things I would do.
First order one of my SR rebuild kits and go completely through the carb.
My kit will have the larger solid (not windowed) .135" fuel inlet set in it. I'll bet at some point the carb was kitted with over the counter parts and it has the tiny N/S assembly. Even with the little 350 stock crate engine I've seen them suck the bowl low or dry with the smaller fuel inlet seats.
During the rebuild install my filter/spring w/o the check valve in it. The check valve is not needed when you use the solid fuel inlet seat and it is very restrictive.
Check the power piston and polish it up with some 600 grit auto body sandpaper and brush the PP bore with the appropriate rifle brush.
I'll supply a new correct PP spring with the kit.
Set the float to factory specs and check the power piston hanger arms to make sure they are level and exactly even.
Once the carb has the correct parts in it and correct settings it will take it out of the equation when troubleshooting.
If your fuel delivery condition still exists make sure all the rubber hoses on the supply side of the pump get replaced and clamps are tight.
If all this fails to correct the issue I'd drop the tank and take a look at the pick-up inside the tank to see if it's cracked and sucking air or the "sock" is plugged up with debris.
It wouldn't hurt to move up a jet size with that carb number but if it was working fine then things went South it's going to be something else......Cliff
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This sounds exactly like what my '73 Monte Carlo did in the mid 70's. The culprit was a clogged fuel filter "sock" (nylon mesh) on the fuel pickup in the tank. Going up hills or on level with heavy throttle it would bog then die if I pushed it. I could pull off the road, turn it off (if not already off), and wait a few minutes. Then it would start and run fine for a while. Rust and/or sediment would evidently get sucked onto the sock and clog it, then after sitting turned off for a while the crud would evidently fall off the sock and it would run fine til the cycle would start over. The fix was removing the sock from from the pickup (tank must be removed to get to it), cleaning out the tank, and installing an in-line fuel filter between the tank and fuel pump. No problems after that even when I was towing a Corvette on a trailer at times.
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Great stuff. Thank you.
Ya know... When this first happened and I dove into it, I changed the filter to one without a check valve because that's the only one the parts place could get. The issue seemed to (seemed to) have gone away. Later I sourced one with a check valve and the issue returned. Could it be as simple as that?
Cliff - I'll call you to place an order for book, kit, jets, etc.
Dropping the tank will be my last move. It's a 40 gal tank and I'd need to cut the big tube steel hitch off the frame to remove the tank. Next time I'm low on gas I do intend to stick a borescope down the filler neck to try to get a look at the pick up and filter sock.
Much appreciated fellas. Now to find my spare no-check-valve filter...
R
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If a slight restriction at the filter is causing fuel delivery issues I'm betting you have bigger issues elsewhere. In any case I'd still do two things. Get rid of the check valve in the filter and rebuild the carb with the larger N/S. I'm betting your troubles will be gone just doing those two things.......
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New developments...
Cliff, I got jammed up at work so didn
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Trouble is with looking at the in-tank sock with a bore scope, is that when the engine isn't running under load the rust and sediment don't get sucked onto it and what was there has fallen off. Maybe you can see any stuff on the bottom of the tank though and general tank condition.
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My posts keep getting clipped for some reason... here's the cliff notes version of what I had typed above. (pun!)
It's super hot here. And humid. Truck still acting the same. Yesterday on the way home from work I stopped at a gas station for a soda and a cigar. Came out, started up, didn't get past the gas pumps before the truck shut off. Wouldn't restart.
Pulled the fuel filter. Clean and dry. And the check valve seemed stuck to the rubber gasket. Did a 'gatorade bottle test' to see if I was getting fuel. Took about 10 seconds of cranking, but eventually a good flow of fuel came from the pump. Buttoned up without the filter and got on my way.
Same starvation issues as before even without a filter. If I keep my foot in it too long it chugs like it's out of gas.
Stopped at the parts store for an alternator. Came out, started truck, same thing. Didn't get out of the parking lot before it shut off. Another 'gatorade bottle test'. 10 seconds of cranking before fuel flow but then it was strong. Fuel in the bottle looks very clean. Buttoned it up, cranked for a bit, got it started and headed for home.
What the heck!? Is the delay in fuel from the pump to the carb normal? Is underhood heat an issue?
This truck drove so well all winter and spring. 15,000 trouble-free miles. Then all of a sudden this started in June. Heat? Summer fuel? Hmmm...
I'm definitely going to rebuild the carb. Not sure that's all that's going on though.
Cliff, I got tied up at work yesterday but I'll be calling you shortly for an order. Book, kit, jets, accelerator pump, whatever else I'll need to get this sorted and tuned.
I appreciate everyone's input. Von, that's a good point about the sock and the truck not being under load.
Here's a few photos....
(http://67-72chevytrucks.com/vboard/picture.php?albumid=2607&pictureid=25843)
(http://67-72chevytrucks.com/vboard/picture.php?albumid=2607&pictureid=25987)
(http://67-72chevytrucks.com/vboard/picture.php?albumid=2607&pictureid=25988)
(http://67-72chevytrucks.com/vboard/picture.php?albumid=2607&pictureid=25990)
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Check all the lines and clamps before the mechanical fuel pump. Also check the routing of those lines to see if any are too close to the exhaust system.
I've seen these issues plenty of times in really hot weather and it is often caused by a restriction in the system or a leak in the system prior to the fuel pump. When the pump puts the fuel under suction and it's really hot outside and under the vehicle these problems car rear their ugly head.
I just went thru a LOT of "drama" with a customer that I built a carburetor for with similar issues. His issues were even worse and his entire fuel line was draining all the way back to the tank so he had to crank it for long periods of time before fuel returned to the carb after long periods of being shut down.
He was convinced the carb was draining down and wanted to send it back. I told him to check ALL the connections and replace all the rubber hoses before the fuel pump. He insured me that all of it was new when the vehicle was being restored over the last couple of years and that it got a new fuel tank.
So we go back and forth over a week and finally he goes under the vehicle and low and behold he finds a loose clamp on a metal line where it goes into the fuel pump on the engine. It was allowing the fuel pump to suck air but not loose enough to show a fuel leak after shut down as the air coming in was letting all the fuel return to the tank and empty the line.
That loose clamp caused him a LOT of issues including the carb going empty on a hard run, empty fuel line after sitting and LOTS of cranking to re-fill the carb, etc.
I've seen defected or old rubber hoses do the same thing, suck air but not leak fuel.......
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Hi, Cliff. Thanks for taking the time to respond. I went through the fuel lines yesterday. Sucking air and breaking the vacuum created by the pump makes perfect sense as to why it would drain back or suck air while pumping (sucking). More sense than the new pump is wonky.
Supply and return soft lines at the pump are new and clamps are tight.
Hard line from pump to carb is clear and I put rubber line around it to insulate it where it was near the block. Temp on that line (meat thermometer taped to line) just before the carb is 140* after running for a few hours.
Factory heat shield is in place where the exhaust maneuvers around the transfer case and gets near the frame where the fuel lines are.
Soft lines mid-frame are in good shape and clamps are tight. I'll replace anyway.
I can't get to the soft lines at the tank... dropping this tank will be a huge PITA. 40 gallons and there's a massive hitch welded in around it. I'm gonna see what I can do to maneuver myself into a position to replace that line.
I had a thought to try to check all of these with a stethoscope with the engine running to see if I can hear a leak where it might be sucking air.
Are the Quadrajets susceptible to heat soak/fuel boiling issues like some of the Eddy carbs are?
And I'll still rebuild the carb. Left you a message yesterday.
And... I'm trying to not be one of those 'drama' guys. Ha!
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the line that is hardest to get to is probably the problem, murphy's law. haha
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Posts keep getting clipped...
Rigged up a soft line from the pump to carb with a clear plastic inline filter so I can see what
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AAAARRRRGGGGHHHH!!!...
Rigged up a soft line from the pump to the carb with a clear plastic inline filter so I can see what’s happening. Seem to get good fuel on first start on a cool engine. When I shut it down, it immediately drains back. After a 10 min wait on a hot engine, the carb will suck the filter dry before the pump can push fuel to the filter. Maybe 10 seconds from when the engine fires until fuel starts arriving in the filter.
I have a return line on the pump. Are these mechanical pumps supposed to allow fuel to drain back? Could something in the carb not be closing/sealing to allow the fuel to drain?
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I am trying how to understand how the carby can "suck" the filter dry
I see a suck as a high pressure trying to get to a low pressure. so would need a pressurized line with a low in the carb to move fuel that way. Once pump has stopped and no pressure in line fuel shouldnt be going into carb unless you have heat causing a high pressure in the line. Is it possible your tank is vacuum locking and you are getting a huge low in the tank as the level drops causing movement of fuel back to the tank once pump is stopped?? This would also explain why it would need time to reprime. If you dont have an intank pump then you are rellying on gravity to get the fuel from tank pickup to first pump.
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Thanks for chiming in, Hiy_Po. I
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Thanks for chiming in, Hiy_Po. I’ll try to get video. The tank isn’t under vacuum. The vent is clear. It will do this with the gas cap removed... I checked. Pump is mechanical on the block. Not sure if it is supposed to have some sort of check in it to keep fuel from draining back... either through the supply line or the return line. But it drains as soon as the engine is cut. As for the fuel remaining in the filter after drain-back... after a few seconds after startup, it gets pulled from the filter to the carb. I’d guess the low vacuum in the intake manifold (20”) might be enough to pull the fuel from the filter to the carb. A few seconds after that the pump is pushing and refills the filter. If the engine were under load right after starting, the carb would get sucked dry before the pump could get fresh fuel to it and the engine would stall.
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I tried calling yesterday, then saw this morning where you called again.
Was out of the office most of the afternoon running parts and working on my shop and driveway.
I'll try again in a couple of hours, orders don't ship out till around noon......
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Posts keep getting clipped...
Rigged up a soft line from the pump to carb with a clear plastic inline filter so I can see what
trying pressing "Preview" button first then "Post"
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lol I think I may have read it wrong. I read it as the carb was emptying the fuel filter after shut down. With no manifold vacum or pressure in fuel line I couldnt see how that was happenning. I get it that the carb is running dry before the filter is primimg on start up.
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I'm thinking that hot fuel / vapor lock is contributing here. I'll check the fuel return line is functioning from the fuel pump and explore ways to keep the pump and lines cool, along with the carb rebuild. Anyone have thoughts?
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Fuel comes out the return nipple on the pump at idle. Stops when revving the engine. I'll need to check back at the soft line sections at the between the pump and tank to make sure fuel is getting all the way back and not turning this into a deadhead system. Lines were blown all the way back to the tank when I first had this issue to make sure they were clear, so I'm thinking it's not a return line issue.
Rigged up a fuel pressure gauge after the temporary clear filter between pump and carb. 6.5psi at idle. 6.5psi at 2500rpms. No load, sitting in the yard. Engine up to temp.
Next to rig the gauge so I can see it while driving. (short drives... this is all hillbilly rigged together for diagnostics only!!)
The soft supply line going into the pump from the hard line on the frame was resting on a heater hose. Moved that and put a split piece of fuel line on it in that area as an insulator.
Fuel pump gets HOT. Now I'm thinking the heat on the pump may be the culprit. How do you keep a block mounted fuel pump cool?
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Well that was enlightening! Took the truck for a drive. 82* here today. Truck up to temp. Fuel pressure at idle and lazy acceleration stayed at 6.5psi. When I pulled onto the highway and accelerated the fuel pressure slowly and steadily dropped to under 2.0psi. Then the stumble and chug routine. Let off the throttle, pressure stayed low for quite some time until it began to rise again.
When the engine is turned off, pressure bleeds off immediately. I would have thought with the check valves in the pump that some pressure would hold in the line from the pump to the carb...
Pump is HOT.
So... Heat? Restriction? Wonky pump?
Hmmmm....
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is it starving from fuel or the new fuel pump defective? I think it's time to drop the tank, replace rubber hoses, inspect hardlines on sending unit and install a new pickup/sock.
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Alright, fellas. For any of you still reading this saga, thanks for sticking around!
Rebuilt the carb with Cliff's kit - my first solo rebuild. Went very well. This thing was pretty damn clean inside. Just a few issues that I ran into...
As a reminder - 4MV 17058213 on a '78 Suburban, 350 GM 190hp crate, TH350, NP203, 3.73 gears, stock, stock, stock.
This 4MV doesn't have an "Idle Vent Valve" as shown on the instruction sheet. Not sure if this is an issue...
Also... the "Idle Compensator" on the back of the carb is missing.
I didn't get the APT screw exposed because I wanted to get this buttoned up... but I will eventually do that.
Also didn't replace the Secondary Air Valve Cam because I wasn't sure how to remove the staked screws without buggering something up.
As I said, this was pretty clean and everything seemed to be correct. The accelerator pump didn't have the skirt spring in it, and the Secondary Air Valve tension spring was a bit loos, but everything seemed "right" otherwise.
Some specs that are a mystery to me and I could use some input on:
Primary Rods - 40B
Jets - 64C
Power Piston - arms level and even, Cliff's orange spring
Secondary Rods - DP
Hanger - C
N/S - Cliff's .135" non-window, no clip on needle
Accelerator Pump - Cliff's
Float - was A67, now Cliff's A61 set to stock 15/32"
All linkage adjustments in spec.
Need to re-tune idle rpms, idle f/a mixture, etc. Now that the carb isn't an issue, I can go back through my timing. I had it slightly retarded to 8* initial to limit the pinging that apparently was my carb going lean from fuel starvation and not a detonation issue.
It was fun! My only worry for future rebuilds now is that Cliff will retire and the good parts won't be around!
All that said... the carb rebuild didn't fix my issue. Same starvation/chug when under load.
What DID fix my issue after everything I went through... was replacing the 2-month-old NAPA fuel pump! Ordered in a Delphi unit from NAPA and after some struggles getting the fuel line to seat and not leak, I have fuel pressure and flow to the carb. The NAPA pump fell apart in my hands when I pulled it off the block. Spring fell out, lever fell out, and there was a black rubber seal of some sort chewed up and jammed near the pump piston. UGH!!
New pump... pressure seems a bit high, but no flooding yet and no starved condition. At hard acceleration I was getting <2psi out of the pump. Now at WOT it holds a steady at 6psi with the new pump. Only thing that makes me nervous is at idle and cruise the pressure can be 10psi or a smidge higher. Am I looking at trouble with that pressure and this particular pump? As I said, zero issues so far. And no starvation/stumble/chug.
Appreciate any further comments on the pump pressure or the way the carb is set up. I learned a lot and had fun doing it!
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That carb can handle higher pressure w/o issues with the correct float setting.
The "staked" portion of the screws for the flaps need to be removed before attempting to remove them to replace the plastic cam. You can change the spring w/o removing the shaft, but I recommend taking the extra time to replace the cam as well. They get worn down and loose on the shaft and cause issues with the secondaries that are difficult to troubleshoot.
A working APT system is a nice feature and worth the extra time to remove the factory screw and install the custom APT screw I make here.
Not surprised at all about the failed pump, I see this ALL the time with newly purchased mechanical fuel pumps. I believe Airtex (probably made in Mexico, Korea or China) supplies a lot of mechanical fuel pumps these days. It is difficult to obtain on OEM pump, the prices for them on Ebay are really getting up there on them........Cliff
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PS: HIC was not used on that carburetor number.
The idle vent valves weren't used either and neither one is needed for what you are doing......
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Awesome. Thank you to everyone. I
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Awesome. Thank you to everyone. I'll get to the cam and APT screw soon. Good to know that I'm good to go. Jets, rods, hanger look good?
I'm supremely impressed with the quality of knowledge here. Both carbs and general engine. I think I'll stick around! :)
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Cool! 8)
Lots of good knowledge on this Forum for sure.
I encourage others respond and help out as well, some folks who are members here have extensive experience with these things and a lot to share. One of my main goals here is to make sure folks get a good end result with these things.
It's also nice when we sell a few parts along the way, helps keep my bills paid and the wife happy!......
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Totally agree. This place is great. I've been reading other threads and the knowledge is broad. I can't imagine running anything other than a Quadrajet now. Well... unless I eventually do that 4-banger Model A traditional hot rod I've been thinking about...
I may order ahead so that I have good parts on hand in the future. Rebuild kits, rods, jets, etc. You know... just in case!
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As way of an update...truck is running well down the highway. No fuel starvation issues. Can't believe it was a bonked out new fuel pump!
New issue though... when the truck is at operating temp and idling - whether in Park or in gear - the engine will occasionally stumble hard and then recover. Idle in P is ~900RPM and it'll stumble down to under 500 and then come back. In gear were around 700 and it'll stumble to almost stalling out before smoothing back out.
I popped the air cleaner so I could watch. I can see the fuel level increase in the bowl by looking down the secondary rods. Up it comes...stumble... down it goes... smooths back out. Does it just once occasionally, or it'll start cycling like this every 20 seconds.
I have a video but can't seem to post.
Float is set at 15/32. New pump is pushing ~8psi. Non-check-valve filter. Zero issues while driving.
Thoughts?
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Video!
https://youtu.be/Iz53f_OCZ6g
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Flooding.
Haven't ran into one doing what yours is but for some reason it's having fuel control issues.
It will be float, N/S, fuel pump/pressure related....