High Idle Speed

[Randyi00]

I started up my car today for the first time from winter hibernation and instead of it's typical 700-800 RPM, it wouldn't come down below 1300 RPM. For reference the carb is a Holley 750 DP with choke removed. I checked the fast idle screw and it's backed off all the way already. The only think I did over the winter that could have possibly affected this is I removed the carb to check something on my spacer plate. Vacuum lines and everything went back on just as it was before so I'm a little confused what's going on here.

Any advice is appreciated.

 

[Don C]

Did you check the idle adjustment on the secondaries? Double check the linkage and return springs, make sure it's not hanging up on something, including carburetor linkage contacting the intake manifold.

 

[cv_72mustang]

Vacuum leak ? Cracked hose?

 

[Randyi00]

Did you check the idle adjustment on the secondaries? Double check the linkage and return springs, make sure it's not hanging up on something, including carburetor linkage contacting the intake manifold.

I didn't check the corner idle screws since they were dialed in fine before and I'm sure they haven't changed. I've looked at everything else and it's all fine to.

 

[Randyi00]

Vacuum leak ? Cracked hose?

I took a look at that already too and didn't see anything, but I'll likely have a closer look for any vacuum leaks.

 

[Zach]

With the carb running place your hand lightly over the choke opening (not hard enough to cause suction) if rpm raises you most likely have a leak. Spray some starting fluid or map gas safely around the base to check for cracks. I’m used to quadrajets, they will physically stop the primaries from fully shutting and lowering idle, pretty impossible to remove the choke from that system. Throw the carb on your bench and flip it over you should see the transfer slot in the primaries, screw the idle screw in till you barely see the transfer slot (square it). Make sure nothing stops it from being that shut. That’s where your idle screw should stay.

Aside from those tips not sure, I chased a lot of fast idles and rough idles in my TA but there are more lines and a lot more complicated, good luck.

 

[Graham Man]

I would take it out and drive it long enough to get to full temperature, then see how it runs. Many times, weird stuff happens after sitting. A good heat cycle will cure lots of ailments.

 

[Cleveland Crush]

I started up my car today for the first time from winter hibernation and instead of it's typical 700-800 RPM, it wouldn't come down below 1300 RPM. For reference the carb is a Holley 750 DP with choke removed. I checked the fast idle screw and it's backed off all the way already. The only think I did over the winter that could have possibly affected this is I removed the carb to check something on my spacer plate. Vacuum lines and everything went back on just as it was before so I'm a little confused what's going on here.

Any advice is appreciated.

Is the 1300rpm idle with an automatic in neutral or drive? Or do you have a manual transmission? Also what was the issue with your spacer plate?

 

[Randyi00]

Is the 1300rpm idle with an automatic in neutral or drive? Or do you have a manual transmission? Also what was the issue with your spacer plate?

High idle was in park with automatic transmission. There was no issue with the spacer plate, I just couldn’t remember if it was open style or four hole and I just wanted to check.

 

[Randyi00]

So the problem ended up being the gasket between the spacer and the manifold. I guess when I took it off I tore it a little, and it must have been pulling some vacuum in from there. I put a new gasket on and problem solved.

 

[c9zx]

Thanks for reporting what the problem was. It may help someone in the future. Chuck

 

[mrgmhale]

I am glad you conquered it. Thanks for sharing the cause.