Won't idle

[Sopp]

I'm new to propane and IMPCO products ( also this forum ). IMPCO 425 on a slightly built up Ford 302. Starts great but jumps up to about 3000rpms immediately. All 4 butterflies are closed properly. Has not been started in about 2 years, stored indoors, and ran great last time I started it. Searched this forum for answers to this problem but did not find anything directly addressing this issue. I'm betting it's something real easy that I'm just not seeing.
Any and all help will be greatly appreciated.

Sopp

[Sopp]

Come on guys ( and gals ), 33 views and no responses. Someone out there must have an idea of what my problem is. If I push the button on the top of the mixer, it will slow and then kill the engine. Can't figure where the air is coming from to make the engine rev like that. All 4 venturis are closed, pcv valve is working properly, and I'm baffled.

Is propane different than gas as far as the amount of air needed to make it rev up like that? I started it several times and it did the same thing each time. Is there other info that you need to help solve this?

Sopp

[Tom68]

Brake booster line and booster diaphragm OK ?

[Sopp]

Thanks for the suggestion, Tom. I disconnected the line and plugged it and started it again-----------------right up to 3000rpms. Checked all the other ports on the carb and all are capped off.
I will recheck everything again tomorrow for any leaks.

Sopp

[storm]

If it not something mechanical like the accelerator cable holding the rpm high or the advance weights in the distributor stuck it will probably be vacuum related. Something like a vacuum leak or vacuum pulling the vacuum advance in the distributor.

Is the centre hole in the Holley baseplate blocked off? If not it will bleed air and totally stuff your ability to idle. If it isn't block it off with a grub screw and a small amount of sealant. Is the baseplate gasket in good condition. Spray some soapy water around all potential vacuum leak locations and see what happens.

[C3H8]

Sorry you didn't get immediate responses. An increased idle speed is almost always a major vacuum leak somewhere. 3000 rpm usually is an intake manifold gasket sucked in at the front or rear of the manifold. The end gaskets have a tendency to fall off the small ledge when placing the intake in position. Other culprits can include the EGR gasket or an EGR stuck open if equipped with one. Lastly the gaskets under the throttle body plate or openings that Storm mentioned. To conclude it is almost certainly a vacuum leak.