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Wayne S.
Wayne S., Motorcycle Mechanic
Category: Motorcycle
Satisfied Customers: 1255
Experience:  Experienced and Licensed Motorcycle Mechanic - all sizes and brands welcome.
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I'm working on a 2003 Yamaha yfm350 warrior. It backfires

Customer Question

I'm working on a 2003 Yamaha yfm350 warrior. It backfires out the exhaust real bad out of the exhaust when you increase the gas. It will also die out when increase the gas. It seemed like it was getting to much gas so I changed main jets, jet nozzle, and
needle jet. But no luck. It seems to me it is not advancing the timing like it should. I checked the coil, the pickup coil, and the direction detection coil (both coils are located at the stator). The only one that is not in limits is the direction detection
coil which I measured at 0.15 ohms with the limits of 0.083.0101 ohms. It kind of points to the CDI unit. But I'm just not sure. Can you help/
Submitted: 2 years ago.
Category: Motorcycle
Expert:  Wayne S. replied 2 years ago.
Backfiring is usually related to lean fuel mixtures like leaking intake rubber or head gasket. Could also be a sticky valve or valve clearance is incorrect. I would check the carb slide for wear as it may be leakingvacuum. See if adding partial choke helps or not. Pipe leaks near the head can also cause backfires. Last resort is try or buy a pulse coil and finally CDI box. Keep battery up and check for corrosion on connectors. Plug color can help in diagnosing the problem.WS--
Customer: replied 2 years ago.
What do you mean when you stated "Plug color can help in diagnosing the problem."?
Expert:  Wayne S. replied 2 years ago.
Go to this link and use it to determine your engine running conditions:
Did you check for intake air leaks?
Does partial choke make the problem better or worse?
Battery is up?
Customer: replied 2 years ago.
I preformed the following
Leak Down Test - 8% leakage
Compression – 120 psi Limit of 121 psi. This would indicate head gasket is good.
Exhaust Valve Clarence – measured 0.008 (limit – 0.0063-0.00787)
Replaced carburetor to manifold gasket, inspected carburetor manifold to cylinder head o-ring. All ok. Removed and inspected exhaust gasket and location of exhaust pipe to head. All ok. Measured and changed float level from 14mm to 12mm. Installed original main jet (145) and changed needle jet clip to setting when it came in (third space below top of jet). Changed out needle jet and jet nozzle. With choke out it dies out. Spark Plug color is clean but wet.
But it still backfires out the exhaust when gas is increased but runs ok at idle.
Any thoughts?
Expert:  Wayne S. replied 2 years ago.
Try this:
Under the headlights open the panel. On the side with the parking brake there are 2 connectors unplug them and plug the male into the female connector on the harness side this grounds out the rev limiter, see if that helps or not.
Customer: replied 2 years ago.
According to the wire diagram, what you're describing would be the clutch switch, correct? According to the diagram the black/yellow wire comes from the CDI unit to the switch and when the clutch lever is pulled in then voltage will go to ground, correct? I presume that the rev limiter is in the CDI unit, correct? Checking continuity in this switch I get 5.0 ohms. When connecting the black/yellow connector to the black connector and start the ATV, it still backfires out of the exhaust. It does sound like a rev limiter is engaging. Does this sound correct? If so, this would mean the CDI unit is not operating correctly?
Expert:  Wayne S. replied 2 years ago.
I was thinking there is a switch on the parking brake which might
activate a limiter. That is the one that might be the problem
if you have it on your bike.
Maybe the brake is released but the switch is still limiting the
bike from gaining any RPMs.
I will have to check the wiring and get back tonight.
Customer: replied 2 years ago.
Found the problem, it is the parking brake switch. When I disconnect it at the connector the ATV run like it should. Thanks
Expert:  Wayne S. replied 2 years ago.
Good job!