Rough Idle, 77 Nova

[uadmlj1]

I have a 77 Nova, small block, 4bbl, no smog. As the motor gets warmer, 10-20 minutes, the idle gets progressively worse.

[DanD]

There are so many things that it could be that we’re going to need you to tell us some service history on this car. As in when was it last tuned up, the carb adjusted, compression test results, is the auto choke functioning properly and the list could go on for quit awhile.
Not trying to give you the bums rust here or anything but we need more info.
Once we get to know what has and hasn’t been tried to remedy the problem I think we’ll be of more help.
Vacuum leak comes to mind.
Dan.

[uadmlj1]

Recent tune. Just passed smog with great numbers. Probably 50K miles on re-built motor. I was also thinking vacuum leak. Any advice on what to check and how? Thanks for the response.

[DanD]

Well I guess one thing we can start with is to see if it is running lean or not.
The next thing I’m going to say is BECARFULL while trying what I’m going to suggest; not that you’re going to hurt yourself, but maybe the engine; if you let something get sucked in.
Remove the air breather plug any of the vacuum lines you may have had to remove while doing so. Start the engine and allow it to warm up to where it starts to run rough.
Allow the engine to run at idle; then slowly start covering the top of the carb’s air horn with a large rag or towel. If the engine starts to smooth out as you again slowly restrict airflow into the carb; then the engine is running to lean.
Notice how I said SLOWLY cover the carb if you go to fast you may go right past the lean point and flood the engine and it won’t tell us anything. If no matter how slow you cover the carb it starts to run worse then it’s not lean.
Tell us what you find and then we’ll go to round two.
I an’t da goodest at splainen tings somertimes so if’n I’a loose ya just ask what I’a is talken aboot. LOL
Keep it fun, we all here want to help and still remember what a carb is all about.
Dan.
PS: Sorry for my small attempt at humour, it's not directed at you; just my ablity to explian what I'm thinking.

[re-tired]

Love that canadian humor . Myself and the rest of the peanut gallery are watching the master at work. Can't wait for episode 2. ;D ;D

[DanD]

LOL

[way2old]

Hokd on foniks werkd fer hm!!!!! ;D

[re-tired]

thats FONE ITS you hillbellie

[uadmlj1]

Tried it. Not lean.

[DanD]

Is the choke opening fully as in the choke air valve being vertical after the engine is warm?
Next would be to pull the spark plugs and have a look how they are running as in not all black from excessive fuel and whether they are all running the same or not. A plug running normally should have a light brownish tinge to them.
While there out do a compression test on the engine that will confirm the engines mechanical is where it should be.
Ding Ding round 2.
Dan.

[uadmlj1]

Will check.

[uadmlj1]

Thanks for hanging in there on this one with me. Butterfly is full open and stays there when hot so that's not the problem. Plugs 3 months old. Half of insulator is brand new white, the other half is black. No brown. But no build up on electrode. If it were running rich enough to make it idle rough, how did the recent smog test come out so good? I'm still leaning towards a vacuum leak. Cheers.

[re-tired]

uadmlj1 said:

Tried it. Not lean.

If you had a vac leak it would be running lean and when you did the test that dan mentioned the idle would have picked up. It may be just one or two cyl. Do a cyl drop test. Pull one plug wire at a time and replace. See if all cyl drop the same rpm.

[DanD]

What was the compression like on all cylinders?

[uadmlj1]

Compression test and drop test will have to wait for the weekend. I drive it daily. Thanks.