Backfiring G1

[Spicyunicorn]

Only way huh? Visual inspection,? spray carb cleaner while it's running and listen for raise in rpm? These little seals will blow out if you could manage to susesfully seal off exhaust and intake to pressurize the motor

[austindebalski]

Originally Posted by juicedstang999

I just put a brand new carb on my 85 G1 for the very same reason of backfiring and it turned out to be the crank shaft seals were leaking creating a lean misfire... You might want to look into a leak down test.

How do you go about that?

[Lochlin]

A leak down test is actually a series of tests to see if an engine holds pressure or vacuum when it isn't running. If you google "leak down" test you'll find a lot of information on it. Personally, I have never done it. To be honest, on a 1 cylinder, 2 stoke engine (that has no valves) I am not sure such detailed testing is necessary.

The vast majority of backfires are caused by throttles that don't close all the way when the accelerator is at rest or coming to rest. The cart's ignition is stopped but there is still fuel going into the cylinder by way of the throttle butterfly not being fully closed. Hit the accelerator again and "BANG" because there is a lot of excess gas in the cylinder.

Now, I am still a bit new to the G1s so hopefully someone with a bit more G1 knowledge can chime in on this next point:

In a lot of carts, backfires can be caused by the ignition circuit starting / stopping at the wrong time when the accelerator is depressed. In other words, the accelerator is depressed allowed gas to enter the cylinder but spark is delayed due to improper accelerator adjustments. Once the delayed spark starts BANG due to too much fuel in the cylinder. Now, I know on a lot of carts there are ways to adjust when spark starts via a microswitch / accelerator switch that the accelerator pedal activates. I am not sure on the G1 though. Hopefully, a real G1 guru will chime in on it. Bottom line is that you always want ignition to start before fuel enters the cylinder and and stop after fuel stops entering the cylinder.

Make sure your throttle cable is good and lubed. If there is friction there it will slow the throttle butterfly from closing.

Lastly, make sure the choke butterfly is fully closed when at rest as well. There is a chance it isn't adjusted right causing too much gas to get into the engine.

The "lean misfire" described by juicedstang is new to me (by no means am I saying it's wrong though). My experience backfiring is usually too much gas / mistimed spark.

[nota]

there is also wrong time sparking or extra sparks caused by ?
that will sound like a back fire

the G-1 runs forward and backward so can't be too far off top dead center
aka advanced timing

that said my old G-1 will spark at odd times way before top dead center
that can stop or really slow the motor
I suspect bad wiring causing that
but have yet to find the wire/short that is the direct cause