09 Big Bear 250 Backfire

[pastorjeep]

Hey guys, I just picked up a 09 Big Bar 250 that was completely taken apart. The owner was trying to fix a no spark issue. I put it all back together and sure enough no spark. I have had a Timber wolf, 3 Bear Trackers, and  Bruin 250 over the years so yeah I like these things. After a ohm test on the stator sure enough dead pickup coil, so I ordered a fleabay special. I have put one of these on a couple of the other quads with good results. Anyway, the stator came today and I installed it right away. Quick check and sure enough nice spark. Hook everything up and no start, but a nice backfire through the carb. Clean carb,, no start. Check compression (120psi) no start. Hit it with starting fluid and it back fires real bad, you can hear the starter one way clutch complain. Try pull start and it really jerks your arms. I pulled it back apart and checked flywheel key. Lined up timing marks and they are spot on. Set valves to .004 intake, .005 exhaust. It seems like it is firing on the wrong time. But there is no way to do that, that I can see. Could the new stator pins be backwards? Would it spark but out of time? I am stumped. It doesn't even try to start just bad backfire through the carb. Ideas? 

[Gwbarm]

You have checked everything, it does act like a timing issue, if stator and CDI are a mismatch that could cause a timing problem, example if new pickup coil is not putting out the right voltage to the CDI, it could be sensitive to that, you can check the voltage its putting out by using a peak voltage adapter for your meter, I kind of doubt that's what it is, you are getting a good spark.. I would check to make sure piston is actually at top dead center on the firing stroke. Also put your hand over the carb intake while cranking to feel if you have good suction and no weird feelings there.

[Mech]

Good run down there Pastor. Sounds like you've checked everything..

Kicking back against your hand when you are using the pull start really points to bad ignition timing. Other things that can cause a spit out the carb, such as a tight valve or a lean mixture, doesn't cause that kick-back.

It could be the new stator isn't the right one for the cdi unit..  or the cdi unit has a problem.

Can you try another cdi unit ? Borrow one perhaps.. Do you have a buddy that will let you fit your cdi to their bike ?

[pastorjeep]

Thanks for the replies. Little update, I switched the pins on the stator and it fired right up...sorta. It still has the hard to crank syndrome like the valves are out of adjustment. The starter really complains trying to turn it over. If I keep hitting and releasing the starter it will hit a sweet spot and fire right up and idle decent with a little choke. But only runs for a minute or two and backfires through the carb and dies. It blew the carb off the boot. When I cover the carb with my hand you barely feel any intake suction. Stumped for the moment. 

[Mech]

No vacuum is a bit strange and you'd normally say cam timing, but you've rechecked that already.. 

Air leaks into the manifold, or anything that will make it run lean can cause backfires out the exhaust on over-run, and out the carb any time, but mostly as you accelerate. Lean running though doesn't generally cause kick back on the pull start or starter. It can occasionaly, but randomly, because of very slow burning lean mixture still being in there when a new charge of fuel comes in, but it's not a main symptom like it is in this case.  The symptoms and circumstances suggest it's bad ignition timing still.

Most yamaha have a feed into the cdi unit from the starter. When you push the starter button the cdi unit does something to facilitate an easy  start on the starter motor and to prevent kicking back. It may kill the first firing or retard the ignition timing  while your thumbs on the button. When you want to start it with the pull start, you are meant to use it with the decomp and so the kickback shouldn't occur.

Sooo..  Do you have a timing light  Does the cdi unit you have there have a feed for the starter.. Oh, and by the way.. some of them feed 12v in when the starter is operating, and others have 12v going into the cdi and then a wire going out to the start button.. Both systems detect starter operation, but you need to figure which way the bike is wired, and which sort of cdi unit you have.. 

All that said, that only relates to the kickback..  the short run before conking out is possibly something else, a fuel supply problem perhaps..  You need to watch out that it could have more than one problem..haha..  Not very helpful I'm sure but people get caught out and confused trying to treat two problems as one..  If you have a timing light, check the timing, then try to start the bike and keep watching the timing light, if it goes off while the engine is still spinning over then the cdi is failing for some reason. If the cdi isn't dying, then suss the bikes wiring regards the starter feed, and then once it's starting properly start to figure why it dies after a few seconds.. fuel perhaps.

[pastorjeep]

Mech, you are definitely on to something. The hard starting and kickback turned out to be a bad starter. So now it spins right over and cranks but will only run on full choke and still leans out and backfires after running a few minutes. So I put a new intake in it. It was a cheap one from the bay and it is poor quality. I think the main issue is a poorly jetted carb which is obviously not the original mikuni. I have no info about the carb from the po. 

[Gwbarm]

It's really bad so many people are throughing away their original good carbs because they can buy aftermarket replacement cheaper than a kit. I found a seller on Ebay that sells brand new Mikuni carbs for about 200 if you are interested. I am on a quest to replace all the carbs on my bikes with original only one more to go. I have bought 3 bikes in the last year and all had aftermarket carbs on them.

[Mech]

Ok..  That's good.

So.. running for a few minutes and then starting to run lean is not a jetting problem. A jetting problem happens the second the fuel starts trying to flow through that jet..  not after a few minutes running. You need to check the fuel supply is getting to the carb. The best way to do that is to undo the drain screw. That tests the tap, filter, and the float needle are all allowing fuel through. It needs to keep running even after the carb is empty. If t starts slowing it could be a dirty filter in the tank, or a vacuum because the tank vent is blocked. If the tank vent is blocked it will likely take 15 minutes to show a problem, if it's the float needle blocked it will take abut a minute to three depending on how blocked and how  you are revving it up and stuff.. Rough figures but you will get the idea.. 

If you get the fuel supply problem sorted, then the carb(and jets) might not need too much.. we hope.

Edited July 2, 2023 by Mech

[pastorjeep]

It only runs for one to two minutes, maybe less. I did try to rev it a little and it revved fine and then backfired and cut off. So I will go through the whole fuel system again this week. We have a worship team in town from Pretoria South Africa and hosting them is a round the clock task right now. Thanks

[Mech]

That definitely sounds like the fuel isn't getting into the fuel bowl quick enough..  If you pull the fuel hose off the carb and run it into a bottle it should flow petrol out full diameter of the hose, or very close to. As long as there is plenty to there, then the blockage is nearly certain to be the float needle or seat is playing up/maladjusted..  Unless this thing has a fuel pump ?