engine backfires! Please help

taddthewadd · #1 03-01-2009, 11:13 PM

I just helped my friend put together his bike and it ran perfectly right from the first start. Then the next day he called me and said it was not running right and is sputtering and backfiring. We tried a new spark plug and that didn't help. We also tried adjusting the needle valve on the carb a little. We tried slightly richer then slightly leaner and nothing seemed to help. Also a very small amount of fuel is leaking from the carb. Not enough to drip though. Do you guys have any ideas? Thank you in advance.

Clotho · #2 03-02-2009, 03:09 AM

Backfiring is usually caused by improper ignition timing or hot carbon in the combustion chamber. What sort of motor is it? 2 stroke or 4? Who is the manufacturer?

taddthewadd · #3 03-02-2009, 10:57 AM

It is a 80cc 2 stroke grubee engine kit. (I think its actually 66cc)

**Bikeguy Joe** · #4 03-02-2009, 02:35 PM

Look to see if the mag side key is sheared off.

**Bikeguy Joe** · #5 03-02-2009, 02:36 PM

Also, check to see if the float is sticking or full of fuel.

MB-Monkey · #6 03-02-2009, 05:41 PM

backfiring could also be a sign of an air leak between the carb and the motor

taddthewadd · #7 03-02-2009, 08:27 PM

Thank you for the ideas but what is a mag side key? I asume a part of the magneto. Where is it and how do I know if it is sheared off? I will check for air leaks inbetween the carb and engine and I will check the float. Is there a good way to know where to adjust the float bowl? Maybe a good starting point? Thank you.

toytime · #8 03-02-2009, 08:51 PM

The mag side is under the cover on the left side of engine. The key fits into a slot on the crankshaft. When you go to slide the mag into place, it also has a slot that fits over the key. This is to make sure that there is only one way for the parts to fit, making it impossible to make a mistake. The key is smaller than your little fingernail.
I would leave this alone for now and work on the carb. Because you mentioned the gas at your carb, I'm thinking that you are getting that extra gas going into the intake when it's running. The float is like in your toilet, when the water level goes low, the ball drops and allows more water to enter. The carb is just the same but a toilet is built much better. (I just had to get a dig in at these kits)
With carb apart, you will notice a little brass lever that moves up and down. The little plastic float pushes up on the brass lever as the bowl fills up with gas. The lever can be bent and a general rule is that it should be level when it is raised up all the way.
If this is hard for you to understand , go over to your toilet, take the tank cover off and push down on the float or ball until it's below the surface of the water. You will hear the water entering the tank until you let go .

taddthewadd · #9 03-02-2009, 11:16 PM

ok I get it. I will give that a try, I understand how all of that works but I just wanted to make sure I wasn't mis diagnosing and tweaking the float when that might not be the problem. I will try bending the arm a little and checking the torque on all of the engine hardware and specifically the hardware on the intake, (manifold to head and carb to manifold). Thank you.

**Norman** · #10 03-03-2009, 07:10 PM

You might need to look at your wiring make sure its all tight(no loose connections) and not shorting out. The white wire will need to be covered with tape or heat shrink tubing. If the end of the white wire touches any metal it can shut off the ignition mater of fact if you have any wire with cut or abraded insulation or just one little bare wire touch the frame it can shut off the ignition and make it back fire. Some people have had trouble with the kill switch acting up make sure it is working correctly you can test it with an ohm meter.
Norman