runability issue

[74ranger]

Hey guys, have a 74 with a 351w, c4, 2 bbl holly 2300, and Hydroboost. bronco ran great last summer but I cannot get it to stay running this year! Runs fine when "cold" but after driving for 20 min or so it loses rpm(4-450 in gear, 6-650 in neutral). Idle is set at 6-650 in gear, carb rebuilt with no change, timing reset/checked, pulled vacuum on the advance and no leaks, new coil, new duraspark, 15in vacuum. Has to be something iam overlooking?

Anyone have any ideas? Was trouble free last year with no mods throughout the winter

 

[spap]

Could the choke be sticking. Maybe a little dark grey exhaust

 

[74ranger]

Could the choke be sticking. Maybe a little dark grey exhaust

choke is free, pulled air cleaner when running poorly and choke is wide open

 

[BruceAH]

Is the fuel supply to the carb adequate? Clogged fuel filter or line?

 

[74ranger]

Is the fuel supply to the carb adequate? Clogged fuel filter or line?

Tank cleaned, New fuel pump(mechanical) and new fuel lines run year before last, new fuel filter beginning of summer

 

[blubuckaroo]

Is it also running rough?

 

[74ranger]

Is it also running rough?

Yes, runs smooth until it warms up, has to be driven, sitting in the driveway idling for half an hour won't produce the issue, drive for 20 min or so and it pops up

 

[jckkys]

It sounds like your idle circuit is running lean enough to need choke enrichment to run well. The automatic choke seems to be working properly by opening the choke plate and getting the throttle off the fast idle cam as the engine warms up. This provides a leaner A/F mixture the engine needs when at operating temp. If readjusting idle mixture doesn't help, you may have a blockage in the IFR or idle feed restrictions. What 2300 Holley model do you have? The List# is on the front of the choke horn. Many 2300s made for '70s Fords as replacements were emissions models that have reverse idle circuits. Fuel supply issues are manifested when an engine is accelerating or climbing a grade at speed.

 

[B RON CO]

Hi, my stab in the dark question is is there an electric idle dashpot that is no longer working or way out of adjustment? I do like also like trouble in the idle circuit. How does adjusting the idle screw affect the engine? Most engines will run OK at two full turns out, and start to stumble when the screw is slowly turned in
Good luck

 

[74ranger]

Tracked down to a intake leak on the back of the manifold, didn't get any response when I sprayed ether back there but spiked around 200rpm with carb cleaner. Wouldn't think it would make a difference what is being sprayed but I guess it does?

 

[Justafordguy]

Vacuum leaks can be tricky. Glad you found the problem.