A Friend Of Mine Has An 86 F250 W/ A 460 In It, And When He Goes To Take Off It Back Fires Through The Carb. But When You Are Sitting Still In The Drive Way It Idles Great With No Rough Idle. It Has New Plugs,wires, Cap Rotor, Distributor, Carb Is Rebuilt, Timing Is Right On The Money And I Can't Find Any Vacuum Leaks.
What Am I Over Looking? This Truck Has Been Sitting For About A Year Before He Bought It A Week Ago.
Can Anyone Give Me Any Ideas?

79 Bronco Nut

 

Sounds to me like a carb issue, something in the settings for the power valve. Just because it was rebuilt, doesn't mean it can't take a little tweaking. Was it purchased rebuilt or did someone do the work?

Welcome aboard, --Mike

 

Thanks For The Info, Someone Rebuilt The Carb, I Was Also Informed That The Vacuum Line Going To The Automatic Tranny To The Modulator Could Be Leaking Or The Valve It Self Could Be Bad. Did You Ever Here That Problem?

 

Could be. Was the truck running when it was brought home? How is the gas in the tank/engine. Was it drained and filled new? A rebuilt carb will have good innards but still require set up. Most do not run perfect, even out of the box... Can you take the whole vehicle to the individual who rebuilt the carb and get them to fine tune to your motor?

--Mike

 

I have had a couple with a flat cam act that way but smell the exhaust,you can tell if its lean by the smell.

 

Well I was just informed that with it idling, you could take off the No. 1 plug wire and the engine doesn't make no differance. It does this on a couple of cylinders. I'm starting to think he has a cam or valve problem.

 

You checked ignition timing but did you check the timing chain set for wear? This could be another possibility that can be quickly fixed.

 

thanks for all the info guys, he found the problem, it was a stuck EGR valve. He said its running like a rapped ape know.

 

a stuck EGR valve....who'da thunk it? I would still be looking at the carb....wow....Where did he get that idea?

 

The EGR valve is designed to only open once the engine is off idle speed. Before that and the intake mixture is flooded with exhaust gases. Hence a bad idle.