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73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: Smithaj1981 on December 01, 2017, 05:49:00 PM
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My 1974 GMC C2500 with automatic transmission runs great when the the engine is cold but after it warms up it stalls and idles rough at red lights, in parking lots or whenever I'm not giving it gas. It seems like it's flooding because the exhaust turns dark and is really strong smelling. I put it in neutral and give it some gas to keep it from stalling and after o rev it up it runs okay for 5-10 seconds and then starts idle rough again. I recently replaced the fuel sending unit, valve cover gaskets and replaced a split hose that runs from the vapor canister back to the tank. I adjusted the idle mixture screws to try to fix the problem and also checked the pcv valve and it rattles and has suction. Other than those few minor repairs that is all I've done. FYI the carb is a Rochester quadrajet. Any help with this would be awesome.
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Is the choke open when it's warm? Which red lake does it happen at?
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The choke does open when it's warm. One thing that I noticed is there is a port on the passenger side of the carb near the choke vacuum brake diaphragm that when I spray carb cleaner near the idle speed increases. Is that supposed to be plugged?
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Yes or something hooked to it.
Do you have vacuum advance hooked up?
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Hi Smithaj:
Yeah, if you can send a photo of the passenger side of the carb that would help to verify linkages and hoses are correct. On the surface it sounds as if everything is OK when the engine is cold and when it heats up something gets stuck in the carb...like the float and maybe the power valves which may get stuck in one position allowing too much fuel to flow. If it turns out that everything is OK on the outside of the carb, then it is probably time to take the top of the carb off and inspect the float chamber and power valves for gunk.
Regards,
Henry
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Correction. It bogs down when I spray carb cleaner near that port that I circled.
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Open manifold vacuum leak should have an extension on it and capped.
Could be used for straight manifold vacuum for the distributor or anything else that requires manifold vacuum.
Should not be open, causes mixture to be lean.
(https://uploads.tapatalk-cdn.com/20171203/58f3b753e59eaa8b9e4bc02e720860e9.jpg)
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Cool. I'll cap it somehow. Any suggestions?
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Hi AJ:
Yes, that area you have circled and sprayed into is where the THERMAC vacuum source would normally connect to. As the photo Blazer74 sent, there should be a 2" long metal pipe pressed into the carb body where a vacuum line would slip over. Your photo indicates that little metal pipe is not there...? If it was there it would be a simple measure of slipping a rubber vacuum cap over it. If it is just a hole in the carb, you either need to get that little steel pipe to press into the hole first or find a rubber stopper to press into it. It needs to be connected or plugged one way or the other to get your carb to work right. The THERMAC system was part of the oem air cleaner assembly to help the engine warm up faster when it is cold. That vacuum port in question operated a door in the air cleaner snorkel to allow varying amounts of hot air from the pass side exh manifold to be pulled into the carb. If you dont have this oem air cleaner anymore it is no big deal but after you plug that port you are probably going to need to retune your carb to see if problem was solved. Do you have any other unplugged ports on your carb?...My 76 C-20 with basically same carb has two more ports on front of carb plugged with rubber cap (one on pass side above fuel filter and one on drivers side close to mixture screw). Another thing to check is the real short little vacuum line from the choke dashpot to make sure it is not split or dried out and leaking. Your photo of pass side carb was too fuzzy to make out details but it looks like all your linkages are there.
Regards,
Henry
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Plugged it and no change. I had to put it in neutral and feather the gas like crazy to get it back to the house during the test drive. When I got back and removed the breather I notice gas was pooled on the manifold.
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Hi AJ:
Yes, that area you have circled and sprayed into is where the THERMAC vacuum source would normally connect to. As the photo Blazer74 sent, there should be a 2" long metal pipe pressed into the carb body where a vacuum line would slip over. Your photo indicates that little metal pipe is not there...? If it was there it would be a simple measure of slipping a rubber vacuum cap over it. If it is just a hole in the carb, you either need to get that little steel pipe to press into the hole first or find a rubber stopper to press into it. It needs to be connected or plugged one way or the other to get your carb to work right. The THERMAC system was part of the oem air cleaner assembly to help the engine warm up faster when it is cold. That vacuum port in question operated a door in the air cleaner snorkel to allow varying amounts of hot air from the pass side exh manifold to be pulled into the carb. If you dont have this oem air cleaner anymore it is no big deal but after you plug that port you are probably going to need to retune your carb to see if problem was solved. Do you have any other unplugged ports on your carb?...My 76 C-20 with basically same carb has two more ports on front of carb plugged with rubber cap (one on pass side above fuel filter and one on drivers side close to mixture screw). Another thing to check is the real short little vacuum line from the choke dashpot to make sure it is not split or dried out and leaking. Your photo of pass side carb was too fuzzy to make out details but it looks like all your linkages are there.
Regards,
Henry
The vacuum line attached to the air cleaner was attached to the vacuum hole on the front drivers side of the carb. There are 2 ports there.
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Hi AJ:
Was that gas that was pooled on the intake only at that one spot you photographed? Has it done this before? Is there evidence on where it came from on the side of the carb? Just plugging a vac line should not have caused this. Typically this pooling occurs when the choke plate (top casting of carb) is leaking at the gasket interface with the float bowl (center casting of carb).
If you took if for a test drive and had to feather the throttle to keep it from dying that is an indication that the accelerator pump is not working: the acc pump sends an extra shot of fuel under higher engine demand to keep the air/fuel mixture correct...when the shot doesnt happen when the throttle opens the engine leans out and wants to die until you release the throttle. This symptom sounds different from the symptom you described earlier. I think the inside of your carb is gunked up and causing the float and power valves to stick. It also sounds like the seal on your acc pump is leaking or going out slowly.
When was the last time you rebuilt the carb? Another thing that happens on Q-jets when they get real old is the welch plugs on the bottom of the carb start to leak and this gives all kinds of hard to diagnose drivability problems: occasional stalling, rough running, and refusal to start all after the engine is warm and been running fine for a while. It will run fine when cold and initially when warm. When this is a problem you can look down the throttle bores and see a puddle of gas on the floor of the intake manifold after the engine has stalled.
Regards,
Henry
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I just bought the truck 3 months ago so I don't know when the carb was last rebuilt. I've never rebuilt a carb before but it seems like the next step.
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Hi AJ:
Rebuilding a Q-jet is a medium size job for an experienced shade tree mechanic and a BIG job and learning experience for a shade tree mechanic who has not done it before. If you have the will, the tools, the shop space, and time you can do it yourself. If not, you will have to find a old school mechanic who can do it. There are also companies on the internet who will sell you a rebuilt one in exchange for yours. If you think you can do it yourself, there will be some downtime on your truck....maybe a week to get it all done and re-installed on your truck. An experienced Q-jet professional mechanic can do it in a day or two. Buying a rebuilt is fastest way but also the most expensive. Either way you go there are plenty of people on this forum to help out and answer questions.
Regards,
Henry
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Hi AJ:
Rebuilding a Q-jet is a medium size job for an experienced shade tree mechanic and a BIG job and learning experience for a shade tree mechanic who has not done it before. If you have the will, the tools, the shop space, and time you can do it yourself. If not, you will have to find a old school mechanic who can do it. There are also companies on the internet who will sell you a rebuilt one in exchange for yours. If you think you can do it yourself, there will be some downtime on your truck....maybe a week to get it all done and re-installed on your truck. An experienced Q-jet professional mechanic can do it in a day or two. Buying a rebuilt is fastest way but also the most expensive. Either way you go there are plenty of people on this forum to help out and answer questions.
Regards,
Henry
Thanks. I found a carb from guaranteed carburetors for $178. I think that's my best bet.
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Check the warranty!!!!
Make sure it can be returned/exchanged if u have any issues.
Remanufactured can be hit or miss.
What have others used and experiences on this site.
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I would also recommend that if the carb u have is original I would keep it.
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Thanks everyone for your help. I rebuilt the carb and it runs like a champ now. Now on the these gas gauges!
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