[Solved] 351c Idle Worsens Until it Dies

I have a 351c with extensive work done (heads, cam, bored, etc) and it had been running great all summer until it needed a carb tune, but then we were off to the races again. Just recently it had slowly been deteriorating in the ease of starting the car (went from clean start up and idle, to holding the throttle around 1k rpm until it warmed up, to now where she fluctuates RPM then dies.) On top of this idle issue, a high pitched sound which I believed to be the fan or one of the belts has appeared (around 10 sec mark of video). The weather has dropped around 8 degrees celsius, could this impact the car this much? Lastly, while waiting for a new part to come, I have driven with the valve cover to carb PCV valve removed but that presented no issues for two weeks of driving. I apologize for the flood of information, but I am at a loss so figured I'd include everything.

Here is a video of the sound, and just today it backfired through the carb..

http://sendvid.com/cq7htj9w

Any help would be great! Feel free to ask as many questions as you'd like I appreciate the effort!

Thanks,

Adam

Check the ignition timing.. the backfire thru the carburetor on startup indicates ignition while an intake valve is open. I can't determine exactly what the noise is and my thoughts are on a vacuum leak at the base of the carb.. A very lean mixture will also cause a backfire thru the carb or tail pipe..

Thru a series of elimination, you should be able to narrow down the problem.

+1 on the vacuum leak. I'm surprised you didn't have problems with the PCV valve removed, that should have caused a major vacuum problem. Maybe the hose was plugged and broke loose.

Check the ignition timing.. the backfire thru the carburetor on startup indicates ignition while an intake valve is open. I can't determine exactly what the noise is and my thoughts are on a vacuum leak at the base of the carb.. A very lean mixture will also cause a backfire thru the carb or tail pipe..

Thru a series of elimination, you should be able to narrow down the problem.

Click to expand...

Thanks for the help, will check for vacuum leaks and timing for sure. Just praying it's not a bent valve... Could it also be carbon build up on the valves?

+1 on the vacuum leak. I'm surprised you didn't have problems with the PCV valve removed, that should have caused a major vacuum problem. Maybe the hose was plugged and broke loose.

Click to expand...

Yea was running great for a couple weeks without it, so will seek to get that replaced for sure. Thanks!

Sounds like a vacuum leak to me as well. Spray carb cleaner around the base of the carb, vacuum hoses, intake gasket etc. If the idle changes in any way, you've got a leak.

Carbon build up at the base of the valve stem would cause a constant backfire, or popping thru either the carburetor or the tail pipe, depending on which valve was dirty..

I had a brand new Olds Cutlass Supreme, right off the truck with a skipping 350 engine.. Removed the right cylinder head and disassembled the defective valves... After taking it apart, I found a burr on the valve stem at the base which prevented the valve from closing.. A little emery cloth on the burr did the trick.. Engine ran as it should have from the factory..

You just never know what's going on inside these engines and a good diagnosis can save you a lot of time and expense..

Hey guys thanks for all the help! So messed around with the idle mixture and put a cap on the carb where the pcv valve goes and that solved all my problems. It seems blatantly obvious now that that was the source of a huge vacuum leak, but I dismissed that as a problem since my car ran without a problem for two weeks. Sounds like Don C was right and it was plugged and broke loose or something... Anyways, problem solved thanks for all the help!