I have an 86 F-150 with a rebuilt 302 that backfires through the carb under a load and during hard acceleration,Why? I had the timing set to spec so i tried to tune by ear, I don't think it is the timing.

engine-1972 302 block
edelbrock F4B manifold
Comp cam, Adv Dur 268 degr, Lift 456
Holley 600 cfm carb
dynomax headers

Rear gears 3.08
Trans 4spd manual with creeper

 

Sounds like it could be an accelerator pump issue. Have you verified that the vacuum advance is working and is hooked to the port on the side of the front metering block on your Holley?

 

advance is working.
what would you suggest i do to the accelerator pump

 

First verify that it is squirting into the carb while opening the throttle.
The carb linkage doesn;t have a cheater screw in the slot on the drivers side rear barrel linkage does it? This will cause a lean condition by opening the rear barrels too fast.

 

Fuel does squirt into the carb when moving the throttel, as for a cheater screw there is none. my first thought was that the secondaires were opening to fast so I installed a heavier secondary spring.

 

It sounds like you need to go to a larger squirter. You probably have a #25. I would try a #28.

 

thanks for the info, i will try a larger squirter

 

Why would the accelerator pump cause a backfire???
The most likely cause would be timing or burnt intake valves\sloppy guides allowing the valve to not seat. Under load timing should increase and the backfire become not as bad unless the timing advance mechanism is not functioning properly. On the newer vehicles that is set by the ECM. It can detect that base timing is off a b it and reset engine operating timing for proper spark. Base timing really effects the start up of the engine. Does the engine start harder then it used to??? Does your vehicle have an ECM, if so look to see if the spout connection is disconnected which will not allow the timing to advance thru the ECM. Do a timing check with the spout disconected and with it connected. Timing should advance with it connected. If is doesn't the problem could be in the ignition module.
Backfire occurs when the timing is retarded to the point when the plugs light off a little too soon. Backfire of this nature can also be caused by a cylinder that is on its intake stroke lighting off when its not supposed to. With intake valve open the light off fires back into the intake creating the backfire. This can be caused by plug wires laying too close to each other, an inductive spark is generated in the the opposite cylinder. Ford engines are noted for this problem. If I remember correctly #5 & #7 cylinders wires on most 8 cylinder engines are the the two most effected. They must cross over each other at right angles and be so much distance from each other to stop this from occuring. There are some articles out on the internet about this, do a search. It could be part of your problem. If the cause is timing adding a larger accelerator pump or setting the existing arm to add more fuel is going to cause a larger backfire.

If someone has some information on the added fuel causing the backfire please send me that data. The root cause is always something else. A crossed ignition wire, etc. An engine can get to a point where it leaks by so bad the valves do not seal well enough under high load conditions to stop backfire. Valve springs get weak etc, this will eventually lead to burnt valves.
If the engine has solid lifters and the intake valves are set a little to high when the engine heats up the valves will set half a thousandth os so off their seats and cause backfire and low power, low compression. As the engine cools off the problem drops off from expansion and contraction of the metals.
All of these issues should be looked at to find the root cause, if the problem just started happening its probably not engine internals. as these will begin slowly and creep up to a larger problem. If you worked on the vehicle and the problem began occuring look for what you worked on and the areas you were working in. As an example, the disconnected spout wire may have happened while you were changing valve covers

 

Because if you are not squirting enough fuel, then you get a lean condition which will cause a backfire thru the intake/carb.

 

Thanks for the information.
I understand the principle you are referring to. I have always found something else causing this condition. Maybe I have been wrong on this in all the years I have worked on vehicles. I guess until you are actually have a particular situation the you can prove that this really is the problem you have nothing else to go on.
The thought of adding the extra fuel creates an air fuel ratio that won't burn until the heat of compression is added on the compression stroke.
I am thinking it can mask another problem.

 

Nightstalker; If you want a demonstration, get a carbed vehicle when it's cold, take the aircleaner off, and tie the choke open to disable it. Then try to start it. You will see lots of spitting back up through the carb.

 

i instaled a larger squirter and it solved the problem, it's amazing how such a small adjustment can make such a big difference!

 

Glad it worked out for ya

 

I guess this might be one of those things that until you have been there you wouldn't think to investigate. Thanks guys, I learned something new on this one.