When going down a hill under compression it backfires. It's an 02 Foreman 450. The seller said he had just adjusted the valves and said when it starts to backfire it's time to adjust them again. I've had it a couple weeks now and it has backfired since I bought it. Is this a valve issue or should I check timing? I'm new to working on wheelers but I have a service manual and would rather not take it to my local Honda dealer if I can do it myself. Any help from those who have experience in this area is much appreciated.

Thanks,

Dan

 

Deceleration backfire is a classic sign of lean jetting....

 

Yea I would have to agree recon...Take out your spark plug and check to see what color is on the electrode...If it is white you are lean, tan=right on, and black=rich...Does it have any exhaust or airfilter on it that is aftermarket, baffles removed, or anything else done to it? Also does it only do it when going down a hill or when you are decelerating on flat ground also.

 

<div class="FTQUOTE"><begin quote>Originally posted by: 4-man luver

When going down a hill under compression it backfires. It's an 02 Foreman 450. The seller said he had just adjusted the valves and said when it starts to backfire it's time to adjust them again. I've had it a couple weeks now and it has backfired since I bought it. Is this a valve issue or should I check timing? I'm new to working on wheelers but I have a service manual and would rather not take it to my local Honda dealer if I can do it myself. Any help from those who have experience in this area is much appreciated.



Thanks,



Dan</end quote></div>

If it pops or backfires during deceleration it means that your pilot system is lean (not the main jet). There should be an adjustment on your carb for the pilot air/fuel mix - generally turning it out will richen the pilot system. You may have to readjust the idle screw afterwards. (Pilot system is frequently referred to as the idle system).
When you get off the gas the carb goes off the main jet directly to the pilot jet (a lot less gas) this causes a vacuum and a big rush or air to be sucked into the combustion chamber (because of high rpm's) this air/fuel mix is way too lean to burn and is send out to the exhaust unburned, where it collects. Then on odd firing the gas (still burning) is sent to the exhaust and ignites the collected unburned gas - thus a backfire or pop.
You need only to richen the Pilot jet screw. Generally it is located under the carb.
The procedure for setting it: Turn the screw all the way in then back it out 2 turns - this is your reference point and the engine should idle fine. Turn it in until it stops to idle smoothly and make a note. Then turn it out going past the 2 turns and keep turning it out until it stops to idle smoothly and make a second note. Split the difference of your two noted points and you should be dead on - I generally will tweak 1/16 to 1/8 turn for the richer (buffer).

 

I don't know what aftermarket stuff it has. It doesn't seem to do it on flat ground when slowing down. I'll check the plug to see what shape it's in and put in a new one while I'm at it.

Thanks,

Dan

 

Exactly as Buckaroo says. But I will add that make sure the engine is warmed up real good before adjusting the idle mixture.

 

buckaroo is right on again. very good advise. Probably the best explanation to the pilot system i've ever heard. You wont notice anything on the plug since it is on the bottom end. I just cant believe some of these dealers either dont know what they are doing or they are blatently trying to rip you off. i dont trust any of them anymore