Author Topic: 1974 GMC C2500 Runs rough and stalls at red lake gets and in parking lots.  (Read 5262 times)

Offline Smithaj1981

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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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Offline VileZambonie

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Is the choke open when it's warm? Which red lake does it happen at?
,                           ___ 
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              ⌠ŻŻŻŻŻ'   [☼===☼]
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74 GMC, 75 K5, 84 GMC, 85 K20, 86 k20, 79 K10

Offline Smithaj1981

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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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Offline blazer74

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Yes or something hooked to it.
Do you have vacuum advance hooked up?

Offline Henry

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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

Offline Smithaj1981

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Offline Smithaj1981

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Offline Smithaj1981

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Offline Smithaj1981

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Correction. It bogs down when I spray carb cleaner near that port that I circled.


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Offline blazer74

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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.


Offline Smithaj1981

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Cool. I'll cap it somehow. Any suggestions?


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Offline Henry

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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

Offline Smithaj1981

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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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Offline Smithaj1981

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Offline Smithaj1981

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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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