Idle RPM Increases When Engine Hot

[Dave Darke]

Hello. I am driving a 74 Barracuda with a 340 auto. Holley 650 double pumper carb. My idle increases when the engine warms up ? I was told this may be a vacuum leak ?

Carb adjustment ?


Thanks ! Happy New Year !

 

[moper]

When was the Holley last rebuilt and properly tuned? Does it have a choke? Electric or manual?

 

[Dave Darke]

When was the Holley last rebuilt and properly tuned? Does it have a choke? Electric or manual?

Hello. Rebuilt last year. Never been professionally tuned. Manual choke. Thanks !!

 

[moparleo]

If you want to check for vacuum leaks SAFELY, first is after it is warmed up slowly block off the airhorn with your hand. If it wants to idle faster, you have a leak. Second, take a spray bottle filled with WATER, not any flammable liquid. The whole point with this is to get a change in rpm. Start at the base plate and slowly work your way around the base and then move to the throttle blade/linkage area. If you have a leak, the rpm will drop. Also check the PCV by blocking the open end with your finger. If you have power brakes, check the valve/hose at the Booster. Both open and blocked off. This should keep you busy. Is it a problem idling in gear ? Does it diesel when you shut it off after running a while ? Do you know the proper way to adjust the idle air/fuel screws ?
how to adjust the idle on a holley carburetor - Bing video

 

[Dave Darke]

If you want to check for vacuum leaks SAFELY, first is after it is warmed up slowly block off the airhorn with your hand. If it wants to idle faster, you have a leak. Second, take a spray bottle filled with WATER, not any flammable liquid. The whole point with this is to get a change in rpm. Start at the base plate and slowly work your way around the base and then move to the throttle blade/linkage area. If you have a leak, the rpm will drop. Also check the PCV by blocking the open end with your finger. If you have power brakes, check the valve/hose at the Booster. Both open and blocked off. This should keep you busy. Is it a problem idling in gear ? Does it diesel when you shut it off after running a while ? Do you know the proper way to adjust the idle air/fuel screws ?
how to adjust the idle on a holley carburetor - Bing video

Thanks Moparleo. Yes it does diesel when I shut it off. I will do these checks.

Much appreciated !!

 

[moparleo]

Dieseling is normally a sign that the throttle blades are not closing fully. Try to back off the idle screw on the linkage and recheck the timing at idle with vacuum blocked off.
Tuning Tips

 

[Dave Darke]

Dieseling is normally a sign that the throttle blades are not closing fully. Try to back off the idle screw on the linkage and recheck the timing at idle with vacuum blocked off.
Tuning Tips

Thank you.

 

[dadeo1852]

Also seen where distributor advance weights were not returning completely when hot (weak spring) and snapping the throttle would be enough to bring advance back down therefore idle. (just in case carb checks out)

 

[moper]

Sounds like the tune up isn't where it should be.