backfire

[camojfk]

having trouble with my 82 2 stroke backfiring. have the spring on the acc. cable. it only does it after full throttle and is deaccelerating almost coming to a stop

[Ol Jake]

Check to see that the butterfly in the carb is closing all the way when you let off the throttle.....

[SamCarroll]

mine back fires when i flutter the gas trying to control a take off or going slow around a corner. I hate it and it scares me every time. I guess i have to adjust the screw some?

[Dave Box]

Guys, think of the mechanics of what is happening here. A backfire is an explosion in the exhaust system. The only way this can happen is if you have fuel build up in the exhaust AND an ignition source. So you need to find out why you have too much fuel first. As Ol Jake says you need to ensure that the throttle butterfly is fully closed when you release the accelerator. Unfortunately the only visual way to be certain is to remove the carburetor, but you can be fairly certain by observing the throttle arm in the top of the carburetor.
As far as ignition goes there are two possible sources. One, of course, is the spark plug itself. The ignition is designed to turn off when the throttle is released and the spring is there to ensure that the throttle closes BEFORE the spark is turned off thus exploding any residual fuel in the system. The first issue then is to ensure that the spring is set correctly, and it may well be worth experimenting with that. Another source of ignition is carbon build up on the piston. During normal use this will glow red hot and cause, what is known on a 4 cycle as, run on. Now modern fuels have largely eliminated carbon build up but 2 cycle engines are running a nice oily mix and carbon build up is a pointer to running your engine at the correct ratio i.e. 128:1 or one ounce of oil to one gallon of gas.
Camojfk are you saying that you release the throttle and then you never press it again and as the cart comes to a halt it backfires? If that is the case your ignition must be coming from carbon in the engine.
Sam your symptoms sound slightly different as clearly your throttle is still depressed when you experience the backfire. Do you have the spring on the throttle cable. I can tell you that having driven with and without a spring, you do need the spring.
Of course there could be other explanations and it is difficult (at least for me) to diagnose problems without touching and feeling. All I can say is that this is what I would do.

1. Check the operation of the microswitch. You do not want it operating half way down the pedal travel as this would mean that as you backed off you could end up with the ignition off but the throttle partially open. The engine will continue to rotate and suck in fuel that doesn't get burnt but rather collects in the exhaust pipe. When you press the pedal again the gas in the cylinder explodes and sets fire to the gas in the exhaust pipe.

2. Check the throttle butterfly and the arm/spring operation. You want the throttle to be closed with say half and inch of pedal travel left before the pedal is fully up (and the microswitch still on).

3. I think that I might be tempted to BBQ the muffler. It kind of a long shot but it is an easy task and it will certainly not hurt anything.

4. If all else fails I would remove the cylinder head and see if it is full of coke. If you do need to de-coke it then use a blunt tool as the piston and head are soft materials. Any gouges will heat up red hot and you will be in the same position as if you had not de-coked it.

Dave

[racin161]

Dave just fired mine up tonight and now i'm going to apply the tuning you've posted.Great stuff for a new guy thats only raced cars

[camojfk]

i pulled the engine today and cleaned the piston it was alittle drity but not bad put it all back togother and backfire. WTF ajusted the mircoswitch and no help. i just ordered new exhaust gaskets see if that helps. the butterfly is closing all the way.

[GoTeeMan]

Ah backfiring, I knew you well. Here's the deal, the spring on the accelerator cable should be between the cable stop and the throttle lever and should be FULLY compressed when your foot is OFF the pedal.
Leave the key off, take the seat off, slowly push on the pedal and make sure that for the 1st inch or so of travel of the pedal that the throttle lever doesn't move (stays closed). If the spring is not compressed then the throttle butterfly is not fully closed, air and fuel are still getting in and BANG!!
Are you able to run at VERY low speed (practically crawling)?, if not the spring is not adjusted correctly.

Before you change anything take a picture and post it here for us to see, then we can help more.

Good luck!

[camojfk]

here s the pics.could it have something to do with the float? to much fuel

[GoTeeMan]

Jack up the rear of the cart, put it on jackstands, take the seat off, start it up and spray carb cleaner around the crankshaft where it goes into the case of the engine to check for vacuum leaks at the crank seals(if found the rpm will increase). If nothing there run the cart and spray around the carb and listen for rpm changes......if the rpm changes then you can pinpoint the source by slowly spraying different spots until you find the culprit.........the throttle looks correct

[GoTeeMan]

I just noticed the choke cable bracket is bent, which would be "choking" the engine even if the choke is all the way in which would cause a rich condition and lead to backfiring.....just a thought