Hey guys im new to the forum just looking to see if I can get a few answers to a problem we are having... when we go to start the bike doesnt matter cold or warm it backfires... Its backfired so hard that it has actually broken the kickstarter. so we rebuilt the bike no problem it had low compression and what not so now its fine then all of a sudden it started backfiring againchanged the plugs from the one out of the 2008 250f we have its seemd to cure the backfiring when starting for 10 starts then its started again we made sure there was no exhaust leaks the timing is right.. we are going to put a new plug in and new packing in the pipe.. any ideas on what it could be ? we are gonna clean the carb now
2009 YZ450f Backfiring problem
Started by jcartier314, Feb 05 2012 11:16 AM
10 replies to this topic
Posted 05 February 2012 - 11:16 AM
Hey guys im new to the forum just looking to see if I can get a few answers to a problem we are having... when we go to start the bike doesnt matter cold or warm it backfires... Its backfired so hard that it has actually broken the kickstarter. so we rebuilt the bike no problem it had low compression and what not so now its fine then all of a sudden it started backfiring againchanged the plugs from the one out of the 2008 250f we have its seemd to cure the backfiring when starting for 10 starts then its started again we made sure there was no exhaust leaks the timing is right.. we are going to put a new plug in and new packing in the pipe.. any ideas on what it could be ? we are gonna clean the carb now
Posted 05 February 2012 - 02:19 PM
jcartier314, on 05 February 2012 - 11:16 AM, said:
Hey guys im new to the forum just looking to see if I can get a few answers to a problem we are having... when we go to start the bike doesnt matter cold or warm it backfires... Its backfired so hard that it has actually broken the kickstarter. so we rebuilt the bike no problem it had low compression and what not so now its fine then all of a sudden it started backfiring againchanged the plugs from the one out of the 2008 250f we have its seemd to cure the backfiring when starting for 10 starts then its started again we made sure there was no exhaust leaks the timing is right.. we are going to put a new plug in and new packing in the pipe.. any ideas on what it could be ? we are gonna clean the carb now
Your choke plunger is probably stuck part way open. When you rebuild the carb, be sure to rebuild the choke plunger, the hot start plunger, the apump diaphrahm, and clear the apump nozzle
Posted 05 February 2012 - 02:48 PM
Doesn't matter where the choke is it will still backfire... And the carb was done when the top end was done and it backfired before that anyways so I don't think it's the carb..
Posted 05 February 2012 - 04:56 PM
When you say backfire, do you mean kickback? A backfire is a popping sound you hear in the exhaust caused by unburnt fuel igniting (and is normally heard when running, it wouldnt be possible to break a kickstarter with a backfire), a kickback is the mixture in the cylinder firing before the piston's over top dead causing the engine to cycle backwards and kick the kickstarter up into your foot as youre trying to kick it down (it hurts, and can sometimes launch you off the bike..)
I always get a kickback on myWR if i'm holding the throttle even slightly open when i try kicking it.. The starting conditions for the engine must be that the throttle slide is as closed as possible, otherwise too much air is admitted to the cylinder, you can't compress it with your leg effort unless you kick really really hard and you might find that kickstarter coming back at you.. Check that nothing is stopping the throttle slide closing fully, like an excessively wound tickover, stuck throttle cable etc
I always get a kickback on myWR if i'm holding the throttle even slightly open when i try kicking it.. The starting conditions for the engine must be that the throttle slide is as closed as possible, otherwise too much air is admitted to the cylinder, you can't compress it with your leg effort unless you kick really really hard and you might find that kickstarter coming back at you.. Check that nothing is stopping the throttle slide closing fully, like an excessively wound tickover, stuck throttle cable etc
Posted 05 February 2012 - 04:59 PM
cjard, on 05 February 2012 - 04:56 PM, said:
When you say backfire, do you mean kickback? A backfire is a popping sound you hear in the exhaust caused by unburnt fuel igniting (and is normally heard when running, it wouldnt be possible to break a kickstarter with a backfire), a kickback is the mixture in the cylinder firing before the piston's over top dead causing the engine to cycle backwards and kick the kickstarter up into your foot as youre trying to kick it down (it hurts, and can sometimes launch you off the bike..)
I always get a kickback on myWR if i'm holding the throttle even slightly open when i try kicking it.. The starting conditions for the engine must be that the throttle slide is as closed as possible, otherwise too much air is admitted to the cylinder, you can't compress it with your leg effort unless you kick really really hard and you might find that kickstarter coming back at you.. Check that nothing is stopping the throttle slide closing fully, like an excessively wound tickover, stuck throttle cable etc
I always get a kickback on myWR if i'm holding the throttle even slightly open when i try kicking it.. The starting conditions for the engine must be that the throttle slide is as closed as possible, otherwise too much air is admitted to the cylinder, you can't compress it with your leg effort unless you kick really really hard and you might find that kickstarter coming back at you.. Check that nothing is stopping the throttle slide closing fully, like an excessively wound tickover, stuck throttle cable etc
No, a backfire is when the engine ignites and pushes combustion gases 'back' through the carb. This is usually associated with a kickback. It is usually caused by a very rich pilot or choke, or timing that is off.
Decel pop is caused by a lean condition, and occurs with the throttle turned off, when de-cellerating.
Posted 05 February 2012 - 05:01 PM
jcartier314, on 05 February 2012 - 02:48 PM, said:
Doesn't matter where the choke is it will still backfire... And the carb was done when the top end was done and it backfired before that anyways so I don't think it's the carb..
It's either the timing or the carb, assuming you have your valves in spec. Carb is much more common, and most people do not 'clean the carb' correctly, and leave debris in the passages, and don't clean and lube the hot start and choke plunger.
If your choke plunger was leaking, it would do exactly what you are describing.
Posted 05 February 2012 - 05:23 PM
alright a mechanic went over the carb but I shall check it out and see if the choke or anything like that is leaking
Posted 05 February 2012 - 05:28 PM
do you guys think there is a chance that it could be the keyway on the magnito? or something like that ?
Posted 05 February 2012 - 08:08 PM
Not on a 2008. Now, your Stator could be overheated and intermitent, but that's pretty rare on a YZ.
Posted 17 February 2012 - 12:55 PM
OK so my 2009 YZ450F which has an air cleaner and FMF 4.1 Carbon Fiber Slip On pops on decel. I resealed all the pipes with high temp RTV and its still the same. Should I rejet it?
It has 15 hours on it and I just bought it dealer told me the carb was all stock.
It has 15 hours on it and I just bought it dealer told me the carb was all stock.
