Back Firing - What Is Causing It --

My 80 MGB ( 32/36 Weber - mechanical choke ) has suddenly started back firing if that is the right word. Hubby has checked choke and it is working fine so that is not the problem.

Each morning I turn on the key, listen for the pump, leave it out of gear, pull the choke all the way out, and it starts on the first turn of the key. I then warm it up for about 30-45 seconds, take off, and slowly push the choke all the way in. When I really kick the throttle down after the choke is all the way in it starts to backfire and then runs fine afterwards. During the day if I change gears from 2nd to 3rd or 3rd to 4th before I really should it backfires again for just a few seconds. Oh, also noticed that if I leave overdrive on at a dead stop and then take off and am really going to slow for 4th but yet put it in it will backfire when the overdrive first kicks in. Runs great otherwise and most of my roads are either uphill or downhill with a few straight stretches.

Help -- Again --Please!


Your turn :)

If you've had wet weather for a while try popping the distributor cap off and letting it dry out in there. Otherwise it could be the accelerator pump isn't working right.

Accel pump was my first though.

Isnt backfiring a symptom of leaness?

It just became much worse and I had to turn around and bring it back home. Thanks JDW for the reply and what else should the hubby check ??

Could be a timing issue also.

Na:

Do you still have the air pump, injectors and gulp valve hooked up? The reason I ask is that when I got rid of all that stuff, my backfiring problems all went away.
I've read that a bad gulp valve will cause backfiring.

tfisher7621 Wrote:

Accelerator pump was my first reaction, but I didn't know if Webers even have one. Apparently from the other replies they do. The fix is to replace the diaphragm or piston or whatever the pump device is in a Weber. The pump is apparently leaking when you accelerate.. This is probably your problem:

http://www.pegasusautoracing.com/productdetails.asp?RecID=4133

When you say backfire, do you mean you hear a popping sound out of the exhaust when you let off the gas, or you hear it popping in the carbs and feel the car jerk as a result?

Those are two different things and are caused by opposite reasons. If it is backfire out the exhaust, you could be running to rich. It the carbs are backfiring, you could be running too lean.

Fred, not to put down your wisdom but I thought it was the other way around.

Backfiring out exhaust(afterfire) was too lean?