I have a 2000 Honda Foreman 400 TRX FW. When I run at full throttle in 5 th gear, then let off the gas completely I get a loud single backfire. What could be causing this? Other info...my exhaust is getting pretty loud, and I just had it into the local Honda dealer to have the valve clearence adjusted, and a new spark plug/and air filter. They dealer said that the Carb was opperating properly.

Thanks.

 

This is from the mikuni website






4: Backfires in Exhaust

Note:

It is normal for many high performance exhaust systems to moderately backfire or pop when the throttle is closed from mid-to-high rpm. In fact, one should expect a well-tuned high performance engine to "pop" and "crackle" when the throttle is closed at high rpm.

The popping is a result of the air/fuel mixture becoming very lean when the throttle is closed and the engine is rotating well above idle speed. It is also necessary that the exhaust system have rather open mufflers.

Why This (normally) Happens:

1) When the throttle valve is in the idle position, fuel does not flow out of the main system (needle, needle jet, main jet). Fuel is only delivered to the engine by the pilot (idle) system.
2) The combined effect of the closed throttle and elevated engine rpm is to create a fairly strong vacuum in the intake manifold. This vacuum, in turn, causes a high air flow rate through the small gap formed by the throttle valve and carburetor throat.
3) Under these conditions the pilot (idle) system cannot deliver enough fuel to create a normal, combustible air/fuel ratio. The mixture becomes too lean to burn reliably in the combustion chamber. It gets sent into the exhaust system unburned and collects there.
4) When the odd firing of the lean mixture does occur, it is sent, still burning, into the exhaust system where it sometimes ignites the raw mixture that has collected ---- the exhaust then pops or backfires.

If the exhaust is getting loud, then it means it getting more free flowing, and that will enable more backfiring. Try going 1/4 to 1/2 turn more rich on the mixture screw. That should stop the backfiring. If this doesnt stop it, check for intake leaks.

 

Thanks for the help!!! You probably just saved me $300 at the local Honda Dealer.

 

ha guess that my old 200s has a high performance(rusted out) exhaust!

 

turn the fuel screw on the bottom of the carb out a 1/2 turn or so. remember, most mikuni carbs have an air screw. most keihins have a fuel screw. to richen things up with a fuel screw, turn it out. the opposite does the same thing for an air screw.

you will notice air screws are placed near the side of the carb and controling air. the fuel screws are place near the bottom of the carb or on the bottom actually. this controls fuel flow. turning the fuel screw OUT enables more fuel to go in. turning an air screw IN lets less air in meaning your now having more gas than before.

 

I am having the same backfiring at all deceleration in most gears on my /96 foreman 400 . It has gradually got worse . The muffler now sounds like it has lost a baffle (rattling). I was told by the dealer the same about it being acceptable as long as it is only on deceleration . But this doesn't seem right to me . I am loosing an expensive mufler and my nervs are shot from the constant gun shots . I could not see any jet adjusting screws so I looked at changing the main needle on the air flow piston of the carburator . I couldn't find any easy way to do this . I feared that I would break a perfectly good component , so I put it back unchanged . I thought it might be running too rich a fuel mixture as my plugs are black and not brown . Question - Can anyone advise me how to change my fuel /air mixture ? Any input will be appreciated .

 

A header pipe that isn't tight will definetly cause backfiring under deceleration. Try checking for any leaks that you may have around the header area of your exhaust.

Good luck!

Brian C.