Rough idle at start.

[brown7373]

I continue to have a ridiculous idle on a cold start. I live in south Florida, so I don't mean freezing temps, just sitting over night. It is a crate 350 engine 350/290 4 bbl Q-jet. It seems like only running on 1/2 the cylinders. But If I put it in gear it idles perfectly. Once it gets a little heat in the engine it runs fine. Carb has been rebuilt and it and the choke function as designed. Oh, and it is lucky to get 8 mpg. The original engine got 11-12 mpg. I have heard I need a different torque convertor, but that would have nothing to do with the idle. And once started, and warmed just a tad, it will continue to run fine all day long.

[HO455]

Can you post a photo off the carburetor with the air cleaner removed just after starting it cold? That will help us determine what the issue may be. If you can frame the photo like this one it would be great.

[brown7373]

This is the picture after starting. Ther engine is running and kicked off fast idle.

[weq92f]

.

Flip fa ya...

[HO455]

It appears that the fast idle cam is in the midway position. Which is good. Meaning that you should be able to turn the fast idle screw in and raise the idle to help smooth things out.
First photo see your fast idle cam position.
Second photo see the cam completely down.
Fast idle adjustment screw is circled in last photo.
Try turning the fast idle up before changing other settings. The RPM should be in the 1000 to 1200 range.

[brown7373]

I'll give that a try.

[LS short box]

Maybe a bit too much choke pull off at cold start?

[leftybass209]

I agree that starting with the carb fast idle is most logical. Most quadrajets just need the proper love and setup. However, there's a whole other list of things just to consider after your rough idle is sorted...

These crate motors are really low compression. They seem to like a lot of timing... in the 14-16+ range i've found after messing with 3 or 4. The engine break in instructions call for weirdly low static timing, and no vacuum advance. Great in theory for racing, and punishing for fuel mileage.

The cam in that 290hp motor is nearly too much for the low compression, and i'd argue the lobe centerline is all wrong as well for the compression.

If the throttle shafts weren't rebushed on that quad, then there may be a vacuum leak around the throttle shaft. Small vacuum leaks may seem like they iron out after a motor warms up, but they can wreak havoc on a cold motor

[Steeveedee]

Originally Posted by leftybass209

I agree that starting with the carb fast idle is most logical. Most quadrajets just need the proper love and setup. However, there's a whole other list of things just to consider after your rough idle is sorted...

These crate motors are really low compression. They seem to like a lot of timing... in the 14-16+ range i've found after messing with 3 or 4. The engine break in instructions call for weirdly low static timing, and no vacuum advance. Great in theory for racing, and punishing for fuel mileage.

The cam in that 290hp motor is nearly too much for the low compression, and i'd argue the lobe centerline is all wrong as well for the compression.

If the throttle shafts weren't rebushed on that quad, then there may be a vacuum leak around the throttle shaft. Small vacuum leaks may seem like they iron out after a motor warms up, but they can wreak havoc on a cold motor

Truth, about those little vacuum leaks. I bought a '69 Skylark years ago for the engine and trans for my '68 Skylark. The guy had done all kinds of stuff to it- HEI, rebuilt the carb, etc. I drove it home, and when troubleshooting it later I discovered the vacuum line for the AC/heater control not connected to the nipple on the back of the carb. Couldn't hear that leak, but man it sure smoothed out the idle when I connected it.