I have rebuilt my 350 1980 vet and am still having a problem with idling. At first everybody including my brother the mechanic thought a bigger cam was installed due to the fact that it idles rough and erratic. However when accelerating or cruising off idle it runs fine. However after pulling the cam it was stock. i replaced the cam anyway with a comp cam with the same stock cam specs. Had the heads cleaned up and put back together. Had the intake valvle checked for cracks and was told it had cracks. Replaced it and idle still was rough. Did not check that one because i got it cheap and still idled rough. Replaced that one with a new edlebrock performer and still idle was poor. pretty much everything on the car was either checked or replaced. When checking the idle I plugged all the vacum ports off except for the distributor to see if it was vacum line problems but no improvement. I had the carb rebuilt twice over the years before deciding to tear the motor apart and it still had a rough erratic idle. My question is could the carb be the culprit for the rough idling. Off idle whether accelerating or cruising the car runs fine no missing or hesitation, only at idle does the tach fluctuate around 50 -75 rpms and almost want to die. The ony other parti haven't addressed is the EGR valve which looks original. Could that be causing vacum leak. Any ideas would be greatly appreciated

 

A little hard to follow your post, but as I understand it;
you rebuilt the engine
new stock cam
new intake manifold
not sure about the intake valve, but there are 8 of those.
Sounds like you did a lot of motor work.
Have you ever had the carb checked by a competent tuner?
Sounds like you have an idle issue requiring idle circuit tuning. The EGR does not operate at idle, so unless it has failed (possible) thats not the problem. You can verify it by removing and cleaning the EGR, test it with a vac pump, and block off the egr port. Stick a vacuume guage on the engine, plug off all the extra vac ports and see what the vac does, if it is "Hunting" there is a vac leak. Get the carb properly tuned.

 

 

Replace the EGR valve. Their cheap. If it is original it should be replaced just because. If it is leaking at idle it will cause the problem you describe. If this doesn't fix it then you have some trash in an idle circuit somewhere. This is the original carburetor for this engine isn't it?

BigBlockk

Later.....

 

Yes, it is the original had it rebuilt twice with no improvement, thinking maybe they did,nt replace or inspect something. so i am down to what i think are two things that could be the problem, which is the egr valve or the gasket plates or something along that line, which is original, and the carb because i have plugged all the vacum hook ups and still get vacum fluctuation at idle but steady vacum as i increase throttle

 

how do you check the idle circuit

 

Have you checked for vacuum leaks particularly around the carb base?. Spray some carb cleaner around the base of the carb, if idle speed increases when you hit a certain spot, you have a leak there. This is a very common problem, even with brand new base gaskets. God bless, Sensei

 

as long as everything else is correct with the engine and you recurved the distributor for the new cam (barry grant has a good reference for initial timing by cam duration at .050 valve lift) the carb could be lean at idle. if the car runs good at idle wh en you choke the car by hand at idle or add propane to the carb air it may be lean. you can enlarge the ICR (idle channel restriction) in the carb to .055" and some times we have to enlarge the idle tube (idle jet) by a few thousands of a inch. any decenct carb shop should have the reams or replacement idle tubes if you need them. the best way to check the air/fuel mixture is a exhaust gas analyzer but they are expensive tools i use one everyday and could not work without one good luck

 

If the carburetor is original the idle restriction should be ok unless there have been modifications to the motor. Vacuum fluctuations could be a burnt valve or broken ring in one cylinder. Have you done a compression test on this engine? I would suggest this before you go any ferther. A compression test will show you more about the internal health of an engine than any other single test. I think you need to do one before you do anything else.

As far as the EGR valve goes, the easiest way to test it is to simply remove it and see if the engine idle improves. Just take it off and seal all of the vacuum and exhaust openings. If the idle improves the valve should be replaced.

BigBlockk

Later.....

 

I did a compression test before taking engine apart. Test showed good compression throughout if i remember correctly it was roughly 130 throughout. I turned up the idle screws from 4 turns to 8 and the engine idles much better fluctuation decreased dramatically and does not feel like it is going to staul with trans in drive at a stop. idling at 750 in drive yet i do not have to hold the brake firmly like i did before with engine stumbling and shaking. Still has a little miss but it doesn't seem to be carb related. just can't figure out what else to check.