1984 351 4bbl

Having some problems with my Bronco's idle. Doesnt stumble or want to die, but the idle is always different everytime I get into the truck. Sometimes when I get in, it will idle high at 1400rpm in park, and 900-1000 in drive. Sometimes not. Today when I went to the gas station for a coffe and a bite it idled in park at 900rpm, then idled at a red light in drive at 550-600. Other than the fact that I think the carb (stock) is of crappy quality anyways, it works well. There are no vacuum leaks that I know of besides the brake booster which hisses. I think that even when this is replaced that I will still have these strange idling ranges.

If there is anyone knowledgabe about general check up, and tuning procedures for these Motorcraft 4bbl carburators that came on the HO 351's or any links that I would be able to go to to see for myself it would be much appreciated. i have always been spoiled with EFI, and my knowledge of carburators is limited at best.

-Matt

 

Matt,
I had a similar problem on an 1983 HO 4 barrel. It turned out to be the Choke Pull-Off coil on the side of the carb. It's apparently supposed to increase RPM to warm motor, and then as heat builds, so does resistence and the coil backs the RPM's off. MY buddy jumped it out with a paper clip and it straightend out the idle until I could get a replacement.
Dennis

 

I'm with denuch, if the choke pull-off is thermal and not electric, the spring steel gets old and doesn't perform as well as it did when new. Chased this problem in a 69 Cadi for two months before I figured out the problem.

 

Just to let you know that the problem had corrected itself when the brake booster was replaced. But I do believe that the choke is not operating as Greystreak said. When starting cold, the truck will have a high idle but will fail to lower the rpm's when it warms up until I give the gas a little shot.

-Matt