Backfiring problem

Hi I just recently bought a 2006 yz450f with about 30 hrs on the rebuild it wasn't running a when I bought it so I ended up having to retime it and rebuilt the carb but now when I'm riding and I let off from high rpms the bike starts to backfire but this only happens when the choke is off can someone give me some ideas?

How does the bike start?? Idle??

Hard to start would be valves to tight or jets....starter jet and pilot

Sounds like it's probably lean as well...what's ur jetting look like??

It could even be as silly as the o-ring, washer and spring missing off the idle fuel screw as I found though if the carb has been rebuilt that seems unlikely. Fitting an after market, extended, knurled numbered knob, idle mixture screw has transformed my 04. It may be that it just needed tweaking but in any case that's so much easier now.

Well I'll try adjusting the fuel screw first I thought it was running too rich because of all the unburnt fuel was causing it to backfire it idles and starts at ease

Also when I kick the motor over to start it I give it gas and it backfires

When you say "backfire" do you mean actually firing back through the carb or popping/banging in the exhaust system? As grayracer said you need to check for air leaks in the exhaust system.

It backfires out of the exhaust system

I will play around with the idle fuel screw and check exhaust for leaks tomorrow and get back to you guys

Backfiring while cranking is unrelated to the backfiring that may occur on deceleration.  On starting, it usually indicates a wet plug or too rich condition (flooded), in which fuel pumped through the engine without igniting ends up in the exhaust and is ignited by the "extra" spark that happens during the valve overlap on the "wrong TDC".

I had that on my tuned Gilera Nordwest when it was reluctant to start in the winter due to a weak spark. Turning over with the odd attempt to fire and then an almighty bang as the unburned mixture in the exhaust system ignited. A bit like the infamous Kill Switch Backfire trick. Getting the CDI system in to tip top condition (particularly the rotor/sensor clearance correct) helped, but when it was really sulky and it was very cold I would swap in a grade softer plug which cured it. That had twin 33mm Keihin CR smoothbores fitted in place of the standard Teikei XT600 type carb and the starting enrichment was a rather crude. Too much 'choke' and it would put the spark out I think. It was sometimes better to leave the cold start lever alone and catch it on the throttle as it was cranked.

So you think I could have the wrong timing?

I just got mine and have the same problem, backfiring because it's too lean, changing to a 45

I also would recommend cleaning the carb thoroughly so nothing gets stuck

I was told to never split the carb past the safety screws..? Anyone ever heard that?

I was told to never split the carb past the safety screws..? Anyone ever heard that?

NEVER SPLIT THE CARB

 

Do the easy things first.

Clean Pilot or change it, check fuel screw, check air leaks (Particularly the rubber between carb and cylinder), check hot start isn't stuck open, check needle model and clip isn't some weird experiment by previous owner.

Okay so some news I started the bike today and when I shut it off it backfired right after and the &%$#@!ing thing shot a flame out of the exhaust my guess has got be I timed it off a tooth

You can split the carb, and on some of the older 400's, it's the only way to fix them.  I believe it's JD Jetting that carries the formed "O"-ring gasket for them.  Get one in hand prior to opening the carb at that point.  Your old gasket will not be reusable.

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