backfiring in carb

[jgparker]

While I was adjusting the Webers on my new 2.8 this weekend, I noticed that cylinder #4 was occasionally backfiring into the carb at idle. It did every 20 seconds or so, and appeared unrelated to idle mixture (did it rich and lean). I could see the flash under throttle plate, so I know it was always #4 and in the carb.

My first guess is that the intake valve is not seating properly, and I ether have debris on the intake valve seat, or the valve has somehow gotten to an improper valve adjustment. My other thought is that maybe I'm wrong about the mixture, and the idle circuit is too lean. Any thoughts? Should I panic?

Thanks,

[john walker's workshop]

too rich or too lean will cause that. compare the airflow thru each barrel with a syncrometer and adjust the bypass screw to even up the flow in all holes before adjusting the mixture.

[snowman]

THe usual culprit of backfire in the carbs is to lean a mixture. There are many possibilities for the lean condiiton. One common one is dirt. Dirt in one of the jets. If it is due to this condidion you can solve it by taking out every jet and blowing out the circuits as well as the jet itself. This is often a result of the engine sitting for a long period without any use. The "dirt" may be due to corrosion due to moisture in the fuel.

In any case this "backfire" is called a lean condition backfire. If you have electric fuel pumps, turn them off and run the engine for a while. At the time the fuel starts to run out the engine will start to backfire.

Other causes of this condition are float problems, ie set to low or sticking, bad fuel pump, clogged fuel filter.

[rw7810]

Jack - You don't mention worn throttle body shafts which can also allow air to be sucked in causing an overly lean condition I think. I suffer from this on the old zeniths.

[Wayne 962]

Jets, clogs, etc. will indeed cause this, as would problems with your valves. Check the valve clearances too...

Try everything else first, and then if you still have problems, swap the left and right carbs. That will tell you if the problem is with the long block or the carb.

-Wayne

[snowman]

Originally posted by rw7810
Jack - You don't mention worn throttle body shafts which can also allow air to be sucked in causing an overly lean condition I think. I suffer from this on the old zeniths.

Bob,
Thank you. You are absolutely correct, throttle shaft wear is another possibility. Leaks in the intake manifold or carb gaskets are another source. One way to identify these is to use a propane tank and spray raw (no fire) propane at the suspect areas. IF the engine smooths out as you add propane, you have found the leak.

Wayne has also suggested another possibility. The valves can cause this condition. Just hope it isn't so, as a valve job would be in order to fix it.

There may be other sources of a lean condition but the basic point is: Backfiring is typicaly a LEAN condition. ANYTHING that can cause a LEAN mixture may be the cause. I mentioned shutting off the electric fuel pumps to hear this condition. Its good to learn exactly what this sounds like as it will be very helpful in solving many a problem.

[Wayne 962]

The float meter should be able to detect whether or not your throttle valves are leaking air...

-Wayne