1986 F250 w/460 7.5 most of the smog had been removed b4 we got it. the backfire was really really bad like blowing up the pipes. we r in mexico and the techs down here are not what they r up north so we were having problems getting the work done we wanted. we tell them to fix backfire and check timing.. it comes back with a new (used) trany..not really but close... so we did all we could do.. even tryed to put in new dizzy and it still backfired so took it back (yes they will take back here. we put in new coil, ecm and so on..spark plugs and still huge backfire... today the guy says "let me try one more thing my friend" i pick it up "knowing that it is still going to do the same thing" but i took it up to 85 and let off the gas i dropped ito first and trew it into second and put it through tests (everyone I could think of) and not one backfire not even a pop or fart. also b4 I could smell gas everytime I shut the engine off I mean smell it bad from the engine compartment. now just normal smells.. oh ya forgot to say what he did... HE TOOK THE BELT OFF THE SMOG PUMP.. WHY WOULD THAT MAKE A DIFFERANCE... VACUUME?

 

Well, that sounds about right. The smog pump injects fresh air into the exhaust (I'm not sure exactly where the fresh air is injected, but I'd guess the backs of the cylinder heads), and sometimes the fresh air will mix with the unburnt fuel/air mixture in the exhaust pipe and BOOM! The mixture finishes burning in the tailpipe, causing the "backfire".

Now, unless I'm way off, you might want to tune your carburetor, because I think you might have an excessively rich mixture. Pull some spark plugs and get back to us.

Later...
Pat

 

Update: When We Did The Other Things Coil, And Ecm And So On The Tech Also Rebuilt The Carb?????????? Remember We Are In Mx.

I Could See New Gasket At Intake And Some Oil Type Substance Around Fittings But In No Way And I Mean No Way Was The Carb Rebuilt As You All Would Do It If We Were North... But For The Carb "rebuild" And Adjust. And Vacuume Line Fix And The Smog Pump Issue It Was $20 And A 6 Pack... It Runs Strong Now And Hell No Backfire You Get What You Pasy For You May Have To Take It Back To The Guy 37 Times... But He Got It

 

The only way I can think of the smog pump making the engine backfire is if the pump was rotating backwards. It would explain the gas smell when you shut the engine off. When you kill the ignition the gas in the cylinders is not burned as the engine is still rotating for a second or so. Air is normally injected into each exhaust port through a small hole drilled to a central passage that runs the length of the head.

Them changing the intake manifold gaskets makes sense if the heat crossover passage seal was leaking to a intake port. The oil substance, was it dark brown? If it is aviation gasket maker that I use on all my fittings to seal the threads from leaking air or coolant. The good thing about that stuff is it doesn't dry fully and stays tacky.

 

WHAT??????? That makes less sense than the ECM change. Does an '86 460 have an ECM? If the pump was running backwards it couldn't pump air into the exhaust to cause the backfire.

 

So confusing a ECM with a Dura-a-spark brain box is a really far stretch of the imagination as well. But then again I don't think like a normal person.

 

mayby I am calling something an ECM that is really somthing diff.. the box on the drivers side fender wall that has the wires that go to dizzy? was changed.. have no clue about manifold gasket..i never said we did anything with that... also dont know if pump was pumping one way or another...all i know is that after we did everything else it stopped backfiring when they took the belt off the smog pump..

 

Ah! That's the Dura-spark ignition box. ECM usually stands for Electronic Control Modules or Engine Control Module. The Dura-spark just produces voltage to the coil to generate the spark and is controlled by the distributor. I have an '84 which is the same set up.

 

i have a problem of calling everything in a metal box on a fender wall an ECM...lol
this so called "carb rebuild" I think was the real problem.. I found out today that while the guy had the truck he took it to another mech in town to ask for help...i think he may have showed him that the carb was not adj. right? not sure dont car just happy that it does not go boom.. i am also happy to know that you only need three vacuume lines to run the truck right trans, dizzy and carb? right? well this truck is never going back to ca reg. so i think we will take everything else off and keep out on the ranch...what a mess..this $1500.00 truck has now cost way over that in my time..lol

 

Trans, yes or they shift real weird. The dist. has one line to it. The carb has one line to it which originally went to a temp switch at the thermostat housing. The was a second hose to the switch from the intake manifold and the third line went to the dist. If the temp got too high the switch changes from the carb ported vacuum to the manifold vacuum to increase the idle speed and help cool the engine down. I disconnected the switch and used carb ported vacuum straight to the carb. Since then I had to heating problems while towing and in summer in town traffic and have gone back to the temp switch system.