Help bad idle

[adrianMCmoreno]

Ok so I have a 1980 monte with a 350 tbi swap. Carburated. Rebuilt top end and has been running for about 2 months and all of a sudden it has developed a really bad mis. I've changed plugs, wires, swapped to a different dizzy and have checked for vacuum leaks with carb fluid and nothing.😳 I've changed carbs from a quadrajet to a holley and still no change. I've checked compression on all cylinders and get 155 across. My question is what else could I be missing

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• adrianMCmoreno
• Sep 16, 2019
• 1

 

[adrianMCmoreno]

I did notice on the quadrajet any adjustment I made to the air mixture screws did not change idle

 

[Clone TIE Pilot]

Vacuum leak, try testing the intake manifold gaskets and the manifold itself for leaks. Also check for HVAC, EVAP, and brake booster leaks.

 

[adrianMCmoreno]

Vacuum leak, try testing the intake manifold gaskets and the manifold itself for leaks. Also check for HVAC, EVAP, and brake booster leaks.

The booster is new and brakes have never been as good as they are now. How would I check the vacuum on the booster

 

[fleming442]

Squirt some Brakleen on the vacuum hoses, around the carb base, and the intake to head sealing surface. If there's a vacuum leak, the idle will rise. Don't hose the whole thing down at once; just try to isolate areas.

 

[pontiacgp]

have you taken the valve covers off to check the valve train for any issues

 

[Clone TIE Pilot]

The booster is new and brakes have never been as good as they are now. How would I check the vacuum on the booster

Use a Mityvac vacuum pump to test it.

 

[adrianMCmoreno]

have you taken the valve covers off to check the valve train for any issues

No issues under valve covers

 

[pontiacgp]

I would remove one spark plug wire at a time with the car running to find out what cylinder is misfiring

 

[57 Handyman]

A bad miss is usually related to ignition if the miss occurs consistently. In addition to removing plug wires to isolate issue to a particular cylinder, plug wire, or spark plug, you should also closely examine the cap. It could be a cracked cap.