I'm hoping somebody out there will be able to help me. My '72 302 is running like crap. It was running great until I did 2 things; I installed a set of headers and I replaced the automatic choke with a manual conversion (the kind that fits into the autochoke housing). Trouble is I did both at the same time so I don't know which is causing the engine to run rough.

It starts OK but idles rough and will not accelerate smoothly from idle. It hesitates and then acts like its running on 6 or 7 cylinders. Even when I'm driving along slowly at a steady speed it misses sometimes.

I checked all the plugs and they are all light brown and dry. I put the timing light on all the plug leads and get a steady pulse on each one so I think the ignition is OK. I thought it might be the cold air entering the carb. because, with the headers, I don't have the original warm air shrowd, so I built a duct that picks up air near the headers.

I've tried adjusting the idle mixture but that doesn't help. Is there something I'm missing with regard to the choke...it opens and closes. Isn't that all there is to it?

Any ideas?

 

Does it have the problem after the engine is hot? Do not need any choke then.
I cant see what the headers would do to make ir run rough....... Unless......did you get the sparkplug wires back on right ....if you did remove them to put the headers on?

Sparky

 

More thoughts The warm air is only used during engine warmup as is the choke, so if it still runs rough after warmup it isnt a choke or lack of warm air into the carb problem.
So how about a loose vaccuum line. Maybe hit one when you were installing the headers. Or was there a vaccuum line on the auto choke you removed that maybe left an open vac port?

Sparky

 

When you took the automatic choke off did you take it completely off of the carburetor and bolt on one of those plastic cans with a cable attached to it? Or did you just take off the can and all the linkage to run a straight cable to the butterfly?

If you took the can off and didn't replace it with something you might have missed the stupid little air passage that connects to the little black thermostat housing. If you look there is a connecting passage that uses a small gasket to seal it off. It looks like a bolt hole, but it doesn't have threads. If you didn't plug this passage you will have a huge vacuum leak in internal of carb causing problems you have.

Did you do away with the pipe that picked up hot air from the intake and sent it too the auto-choke? If you did and you didn't seal the pipe back up you might have a vacuum leak there. It's been awhile, but I can't remember if the EGR passage (If there is one on yours) had any vacuum to it or not. I know that chevy's are plagued with this when you take the EGR valve off of them, there is a vacuum passage that goes under the carb. ( That one had me looking for days.)

If all else fails, try putting the carb back together and start from there. I've never heard of the problems you have being caused by headers alone. I've also had a vacuum advance go bad on me causing some of the problems you have. But that only happens at higher rpm than idle.

Keep looking it's probably something so simple you will want to kick yourself when you find it.

 

Sam, I think you're on to something. The choke kit was the type the fits over the existing choke housing...but I didn't do anything to plug the vaccum port. Maybe if I take the choke cover off and plug that port it'll be OK. I'll keep you posted.

 

 

OK, I took the cover off the choke housing and with the engine running I did have a vaccum leak. So I plugged it up and put everything back together and took it for a test run, sure that I had fixed the problem. SAME THING...when it gets completely warmed up it starts missing.

One thing...when I put that choke kit on I used one of the intake manifold bolts to fasten the cable bracket. I torgued it down again but do you think I could have a vaccum leak from the manifold now? How would I test for this...would a vaccum gauge show this up? Could I spray something on the manifold to head joint?

Any other ideas?

Thanks for you help

 

I use carb cleaner to spray for vacuum leaks - just watch for the RPM to change.

Reggie

 

Hey everybody, I guess we all knew it would be something really stupid. Turns out I had a sparkplug with a crack in the insulator. Spotted it when the engine was idling at night with the hood up in the dark. There was a little light show going on around one of the plugs. Put a new set of plugs in and she runs fine now.

Thanks for everybody's help.:-X11

 

Its the little problems like those that drive you CRAZY! huh?