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Old 02-03-2009, 07:54 AM
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SBC 350 running very rough at idle

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

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Old 02-03-2009, 08:21 AM
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Quote:
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,
Ayuh,..

My WAGuess is,...
There's $h!t in the idle circuit,+ it's Plugged up....
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Old 02-03-2009, 10:20 AM
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Sounds like a good guess, so I pulled off the carb top and pulled out the float and needles - no sediment or anything holding them open, and they do close off flow as the float drops.

While checking for vacuum leaks I did find another strange symptom.
- If I pull the line off the manifold fitting - idle drops due to vacuum leak (makes sense)
-- If I pull the manifold vacuum line off the front of the carb - idle drops due to the leak, and because the vacuum advance doesn't have vacuum (makes sense)
-- If I pull the vacuum line off that connects to the metal line out of the back of the carb (runs to brake booster) - idle rises if the metal line is open to atmosphere, and drops slightly when its capped - Does that make sense? Shouldn't the idle drop?

I feel like I'm chasing a mystery problem. The truck ran yesterday morning, and literally started running bad as I was driving home. Something had to have come apart, fallen off, gotten plugged, or whatever, but I can't even figure out what it is. I'm assuming its either a carb problem or a vacuum leak, but I've found no obvious problem in either area.

Bruce
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Old 02-03-2009, 05:38 PM
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I finally took the carb apart further, and found debris plugging the primary venturi booster on one side. It looks like a sliver of silicone sealer from when I changed my thermostat. I scraped the sealer off the manifold & thermostat housing, and later on started up the engine to check for leaks. That little piece of silicone (about the diameter of a sewing needle) must have been sucked into the carb.

Now I have a slightly different problem - a light throttle bog when accelerating slowly. The mid throttle bog seems to be a common complaint for an Edelbrock, but I've never had it before. I now have a good idle, and full throttle seems good, but light throttle seems like its fuel starved.

I've had the carb top removed several times during troubleshooting, and somewhere along the way it looks like I've scratched the surface of one metering rod in one section. I'm not sure if this is making it stick and make the engine bog, or if I have some other factor to consider.


Bruce
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