Backfire

Reply Subscribe

Thread Tools

Search this Thread

Jul 26, 2010 | 10:23 AM

epidemic73

Thread Starter

New User

Joined: Jul 2010

Posts: 4

Likes: 0

Backfire

Need some help here fellas, I have a 85 F250 w/ a 460. I have 2 problems that might be related. First I have a miss at idle that won't go away, second if I snap the throttle I get a backfire through the carb. I put in new plugs, wires, cap, rotor, changed the coil, put on brand new Summit 600 cfm carb, torn down the distributor to make sure everything is moving and cleaned, found vacume advance to be bad,so changed it. If I come up slow on the throttle everything is is fine, miss goes away and no backfire. I check the timing and it is dead on 6 degrees and holds, bring the rpms up and it advances just like it should. I have also checked for vacume leaks, none. What am I missing? Any help would be great!

Jul 26, 2010 | 12:59 PM

Franklin2

Moderator

Joined: Jan 2001

Posts: 56,992

Likes: 2,741

From: Virginia

Have you played with the timing at all? Loosen it and turn it while the engine is running till the engine speeds up a little bit, and see if it helps.

Jul 26, 2010 | 09:16 PM

fordmanjoe1987

Junior User

Joined: Dec 2007

Posts: 89

Likes: 1

You said that you had changed the carbuerator... you need to verify that it is properly adjusted and not running rich. you can use a vacuum gauge and try to find the highest vacuum. if you changed the vacuum advance, then there is a problem also, they dont come set for your vehicle and most are adjustable units, they take like a 3/32 allen wrench inside where the vacuum tube plugs on, if i remember correct counter clockwise retards and clockwise advances. Try adjusting that, i just go one way, if it gets worse, reverse and go until it just stops backfiring then drive and make sure it doesnt do it under partial throttle as vacuum drops off on full throttle....or should

Jul 28, 2010 | 09:10 AM

epidemic73

Thread Starter

New User

Joined: Jul 2010

Posts: 4

Likes: 0

Thanks for the advice, last night I richened the carb and advanced the timing 4 degrees and the back fire went away, seems to be running smoother also. Vacuum is now at 18", as where before it was around 14". I was wondering though, my book says 6 degrees btdc, I'm now at 10, wonder if I have a streched timing chain, the motor is only suposed to have around 30,000 miles on it. any ideas?

Jul 28, 2010 | 09:18 AM

Franklin2

Moderator

Joined: Jan 2001

Posts: 56,992

Likes: 2,741

From: Virginia

Are you sure this engine is all original? Possibly someone has the wrong harmonic balancer on it. Also, the harmonic balancer outer ring can slip throwing the timing marks off, but with a very low mileage engine that seems unlikely. Is this engine rebuilt with 30,000 miles on it, or it's all original factory with 30,000 miles on it.

Sorry to say, you also can't really trust what people tell you if you just bought this engine.

Jul 28, 2010 | 09:25 AM

epidemic73

Thread Starter

New User

Joined: Jul 2010

Posts: 4

Likes: 0

The man I bought it from builds race engines. He told me that the 460 was a Jasper rebuild that he had bought for a customer. He was going to tear it down and just use the short block but the customer backed out. So i bought the whole truck off of him.

Jul 28, 2010 | 09:37 AM

Franklin2

Moderator

Joined: Jan 2001

Posts: 56,992

Likes: 2,741

From: Virginia

The only way to know for sure(and he can probably help you with this) is take a sparkplug and modify it to make a piston stop out of it(he might have one of these). Screw it into the #1 cylinder and then slowly turn the engine by hand till it stops. Make a mark on the damper across from the pointer at this point.

Then turn the engine by hand the opposite direction till it hits the stop again. Make another mark across from the pointer.

You will now have two marks on the balancer. Carefully measure the distance between the two marks and put a mark exactly in the middle between the two marks. That's top dead center for your engine. You can see how it compares to the marks already on the balancer.