Hello All, new to the forum and have read alot of good posts. I have a 66 Ford F100 with a 390 V8 engine from a '68. I have a new edelbrock carburetor 650 scfm and a custom exhaust.

When I drive it around at normal speeds the exhaust has some backfire. I am assuming I might have and exhaust leak somewhere. but more than likely the exhaust and intake backfiring are related.

When I really step on it, I get backfire through the carburetor. I had an engine compression check and the results were good. Nothing wrong with the valves. my vacuum is a little lower than normal I run about 15". I have changed the vacuum lines but it also has not improved. I have been told my truck does run lean so maybe I need to adjust the secondaries on the carburetor to get more fuel in the engine when I really accelerate. Any comments or suggestions?

I am fairly new to these trucks and I would consider myself a novice mechanic. But I can get things done when I have a plan. Just want to get some comments before I start messing with it. She runs okay now but If I could fix the backfiring she would run like a champ. Thank you.

 

timing? Sounds like its retarded to me. Did you check that?

 

timing is what i was going to say

 

Back fireing threw the carb, ussually means timeing is to far advanced... Now that being said, it could be that your 1 tooth off,when you put in the dist... did you time it?

Second... back fire threw the exhaust, is most common caus from a rich condition... When the hot rich exhaust hits cold air/ oxygen,from an exhaust leak, it explodes, hence the back fire... unless you have a burned valve, but you would have a weak cylinder, which you said you didnt so...

Vacuum lines really wont help vacuum, unless one is broken. 15 isnt bad either... A distributer, 1 tooth off could cause poor vacuum, since the engine wont be operating efficiently, (simply put)...

What is you Compression? did they tell you??

 

Sound like taking another look at the timing is my next thing to do. I originally had it checked by a mechanic when I first purchased the truck a few weeks ago. I can check it out myself this time. Would bad timing be the cause of backfire through the Carb during acceleration?

The backfire through the exhaust is nothing very loud or unbearable. it is more a a constant popcorn popping sound.

The engine compression was in the 150# range on all cylinders. Thank you for the quick replies. I'll update with what I find on the timing check.

 

backfire, when accelerating, could be caused from a lean condition, but I would rule out the timeing first...

Has the vehicle sat for a long time, If so, also rule out bad gas.( No octane)

You have an exhaust leak causing the poping, propbably mostly on deceleration...

 

Rey, the secondaries do not come into play unless you are romping the throttle. You are backfiring through the carb under moderate throttle because of an ignition timing problem, lean primary jets, a bad accelerator pump, air suction in the intake path or a hanging intake valve.
Check the distributor indexing and do all the things that the pros above have suggested. Tighten carb and manifold bolts and nuts. Be suspicious of anything connected to the intake manifold, especially rubber. Then toss a few ounces of Marvel Oil into a full tank of gas. It is a light penetrating oil and will work its way into the valve guides as you drive. At worst you are out a few bucks but it may free up a sticky intake. You may have a backfire because of an intermittently hanging intake valve.
Assuming your carb is proper for the motor you should not have a backfire problem from lean mixture.
Do all of the above and report back. We will get you going.
Semper Fi

 

Sounds most like the timing is too low (retarded).

If the timing were advanced too far, your problem would be spark knock, as the plugs would fire prior to the piston reaching top dead center.

 

Found out the problem.... One bad spark plug. the insulation had broke off and was covering the gap.

 

I would still check your timing. Ive never heard of that being a cause for backfire through a carb. Mine was doing the same thing and i was to far advanced and to lean on the primaries.

 

I have a 1985 Ford F250 with a 460 engine. Went on a trip towing a 5th-wheel camper was fine for the first 150 miles as I started to accelerate up a hill, it backfired through the carb one time. Its never done this before been driving for 5 yrs. After getting home & driving it with no load it does it at high RPMs. Could the distributor be the problem??? Any kind of advice will be greatly appreciated.

 

Rey, it makes sense that a failed plug will cause backfire through the intake system. When a plug is supposed to fire, the collapsing magnetic field lines in the coil cause an electrical disaster in the secondary circuit- tens of thousands of volts. The rotor is pointed at the lug of the intended cylinder and the path of least resistance is through the wire to the .035 gap on the plug where the arc happens. If you disrupt this path, like breaking a plug, the arc will happen at the next path of least resistance which will likely be an adjacent wire or lug on the distributor cap. If that cylinder is on the intake stroke, pop! back up through the carb.

Carb backfire is associated with ignition advance, not retard. If the spark is retarded, the fire happens late in the power cycle and unburnt fuel is shoved out into the red-hot manifolds on the exhaust stroke. The fuel accumulates in the manifold or muffler until a spark of carbon comes by and pop! out the exhaust pipe.

Listen to the graybeards: Check timing, tighten down everything around the carb and intake, check all the rubber that attaches to the intake and maybe invest in a new set of plug wires.

Semper Fi

 

I changed the distributor & it solved the problem.....