Engine idle surging

[TBP]

I have finally gotten settled into my new house enough that I can start working on the Mustang again.

The good news is it starts right up, pretty easily in fact.

The bad news is the idle is all over the place. It surges, then settles back to 1000-1600, then will jump up to 3000 RPM. I have followed the basic carb tuning steps, but it doesn't seem to make it better. Possibly a vacuum leak?

 

[TOM B]

Yep sounds like a vacuum leak, check the carb base and all hoses including the vacuum advance pod on the distributor also check your power brake chamber if you have one. Spray carb cleaner around the manifold and carb base to check for leaks. If you have an automatic dont forget to check the hose to the vacuum diaphram on the trany.

 

[TBP]

I checked the carb base and the intake manifold, and that doesn't seem to be it. It's a 4-speed, so that's not it, and I have manual brakes. Any other ideas?

I startedit up this morning, and it's "idling" at around 2200 RPM, if it drops much below that, it starts to sputter and even backfires (very mildly more like a spit) out the carb.

 

[TOM B]

OK so if you found no vacuum leaks then pull the coil wire out and crank the engine then quickly look down the carb bowls to check if gas is running down the bowl but it sound like a vacuum leak to me. if you have a 4bl check the secondary plates are closed.

 

[D.Hearne]

I have finally gotten settled into my new house enough that I can start working on the Mustang again.

The good news is it starts right up, pretty easily in fact.

The bad news is the idle is all over the place. It surges, then settles back to 1000-1600, then will jump up to 3000 RPM. I have followed the basic carb tuning steps, but it doesn't seem to make it better. Possibly a vacuum leak?

Are you running a factory carb spacer on that Performer intake? If so, there's your vacuum leak. The hollow factory PCV spacer doesn't mate with a Performer's carb base pattern. All that seals it there is the gasket, and it'll get sucked up into the spacer's hollow.

 

[TBP]

Are you running a factory carb spacer on that Performer intake? If so, there's your vacuum leak. The hollow factory PCV spacer doesn't mate with a Performer's carb base pattern. All that seals it there is the gasket, and it'll get sucked up into the spacer's hollow.

No, I am not running a spacer at all. Do you recommend that I do?

On a good note. I pulled the carb and intake, replaced the intake gaskets, and retorqued everything back down. As I was about to put the carb back on, I decided to double check all of the vacuum plugs. The power booster port plug was in really bad shape. It had several cracks in it. I replaced it with a new one, buttoned everything back up and she cranked right up and idled at around 800 RPM. It still needs a little tuning, it seems to be running a little rich.