As I was driving home last night I noticed that my truck ('75 GMC 290 hp/350 crate, 222/222 cam, Edelbrock manifold and carb) was running rough at stoplights, and it got worse as I drove home. The engine ran smoothly in the morning, and all this started while I was driving.
It felt like it might be a vacuum leak, so I did some troubleshooting and it seemed to be a problem with the check valve in the brake booster. I got it to idle better by richening the mixture, and I had engine vacuum back up to about 17" (normal is about 16-17" for this engine) and I thought the problem was fixed enough that I could get a new check valve and hose today. I also found out from the GM manual that sometimes the valves just stick, and this might be the source of the problem.
This morning the truck started and idled a little rough as it warmed up. However, when I put it in gear the idle dropped so much that it won't stay running. I have it fully warmed up, and I pulled off and capped each vacuum connection in turn to try to find a leak (brake vacuum line, manifold vacuum to TH350/HVAC, and the vacuum and PCV ports on the front of the carb). Nothing seemed to resolve the problem and no lines had an obvious leak. I also sprayed WD-40 around the carb base and had no change in idle.
I had also taken the top off the Edelbrock carb this weekend to check the float levels. I did notice a little sediment in the bowls, which I wiped out with a clean rag. I took it off again last night and checked everything carefully as I put it back on. I I did not see any problems and everything went together smoothly.
Any ideas about what might be happening to drop the idle this much? Could a malfunction in the TH350 cause that much idle drop? The truck will run normally at road speed, so it seems more like a vacuum or carb idle circuit problem. But I can't find what to fix.
Thanks,
Bruce
It felt like it might be a vacuum leak, so I did some troubleshooting and it seemed to be a problem with the check valve in the brake booster. I got it to idle better by richening the mixture, and I had engine vacuum back up to about 17" (normal is about 16-17" for this engine) and I thought the problem was fixed enough that I could get a new check valve and hose today. I also found out from the GM manual that sometimes the valves just stick, and this might be the source of the problem.
This morning the truck started and idled a little rough as it warmed up. However, when I put it in gear the idle dropped so much that it won't stay running. I have it fully warmed up, and I pulled off and capped each vacuum connection in turn to try to find a leak (brake vacuum line, manifold vacuum to TH350/HVAC, and the vacuum and PCV ports on the front of the carb). Nothing seemed to resolve the problem and no lines had an obvious leak. I also sprayed WD-40 around the carb base and had no change in idle.
I had also taken the top off the Edelbrock carb this weekend to check the float levels. I did notice a little sediment in the bowls, which I wiped out with a clean rag. I took it off again last night and checked everything carefully as I put it back on. I I did not see any problems and everything went together smoothly.
Any ideas about what might be happening to drop the idle this much? Could a malfunction in the TH350 cause that much idle drop? The truck will run normally at road speed, so it seems more like a vacuum or carb idle circuit problem. But I can't find what to fix.
Thanks,
Bruce
