• Welcome to ClassicBroncos! - You are currently viewing the forums as a GUEST. To take advantage of all the site features, please take a moment to register. It's fast, simple and absolutely free. So please join our community today!
    If you have problems registering or can't log into your account, please contact Admin.

Jekyll and Hyde Carburetor


Sr. Member
Jun 28, 2008
I have a '75 Stock 302 with the Motorcraft 2150 Carb. It's freshly rebuilt, but I'm having trouble with it.

Parked with transmission in P the engine sounds great at idle. As soon as I put the trans in D the engine idle drops down really low. So low, it almost sounds like it's stumbling. There is also light stumbling and hesitation when accelerating from a stop

It runs like a champ in Park. No stumbling at idle or when I move the throttle. What adjustment am I missing?

I do have an electric choke, but that is working great. Truck is starting up fine when cold, high rpms until it warms up, then rpms kick back down to normal. The idle speed in gear is much lower than what I'm calling "normal".


Total hack
Jun 4, 2002
Vacuum leak.
Also check the timing. May need to retard the base timing a little bit so it isn't so sensitive to loads.
And while at it, check the vacuum advance is connected correctly to ported vacuum and not manifold vacuum.


Bronco Guru
May 22, 2003
Also keep in mind that idle speed should be set with the tranny in drive with a automatic. it really doesnt matter what the idle speed is in park. Although it really should only be a little higher maybe 8-900 rpm. Over time things do need slight adjustments.