I've searched and not been able to synthesize quite enough wisdom...so here's my situation.

Resurrected the 1986 F-150 351W / AT / Holley 4180. The carb was purchased from a rebuilder a little over a year ago...but the vehicle sat all that time with the carb installed.

Anywho...the truck will not idle at low rpm...for example...sitting at a stop light...nor will it idle at low rpm in park. The shop that replaced the timing chain ***'y said that it coughed and backfired a few times while they were working on it... The truck is running okay at 40 mph or better...even gets 10mpg!

So...the question is this: is my idle problem most likely a vac leak or a power valve? If it is a power valve...what size? (I know...get a vac guage...) The only sizes available locally are between 4.5 and 6.5.

It's a little chilly to spend all day troubleshooting...so...just looking for a little wisdom and guidance.

Michael

 

Does it act like it's flooding at an idle? Black smoke? If so, it could be the power valve or a sticking needle and seat. If not, then possibly a vacuum leak.

 

Jimmy...

No black smoke at all. As the idle deteriorates it acts like its not getting enough fuel. I have to ride the accelerator at stop signs...and feather it creeping in traffic.

So, based on your post, something may be amiss with the carb...but the power valve probably isn't it. I'll go back to square 1 and look at vacuum. Well...after I replace the thermostat...BRRRRR!! It's 24 degrees outside!!!

Thanks,
Michael

 

There's 4 idle mixture screws at the base of that carb. If they were changed, the rebuilder had to break off the anti-tampering covers to get to them. Check to see. Either way, you need an allen wrench to adjust them. The two front ones for each primary are fairly easy to get to. The rear ones will be impossible to get to if you re-installed the factory heat shield between the carb and the intake.

I found this out after buying and installing a remanned 4180 from the parts store fo my '84 F150 with 351W HO. I had to take the carb back off, screw the mixture screws all the way in, then back them off 1 and a half turns. I reinstalled the carb without the shield so I could get to all the screws if need be. I don't know if you have the vacuum assisted idle system, but I took that off. Then I just followed the procedure on the sticker near the radiator to set the idle, adjusting the idle screw at the the throttle lever. I can't remember but I may have fooled around with the primary mixture screws a little also to get it to sound right.

This was done a few years ago and I've had no problems.
-Tom