this was driving me crazy. my bike was back firing today while i was accelerating going downhill and when i would reach the end of my powerband of the flats. i had to go to about 1/2 throttle to make it stop. i pretty sure it was coming from the pipe and not the carb. any ideas?
heres my set-up:
50cc kstar w/hi-comp head
16 del (jetted to rich right now)
case matched
boss pipe
15/45 gearing
GPS 41.9
low end sucks right now
jetting?
air leak?
damn it!
it is almost always timing - check and see that your stator plate hasn't shifted - also be sure that your points gap is enough.
thanks martin, ill check in the morning.
every time I have had this happen it is because my points have slipped and they are just barely opening
Maybe the plug bridge gapped, it happed do me with break-in dust before. Down a huge vert hill.
on my old 2-stroke dirtbikes, it was the points that caused backfire under those conditions: as said above - rub pad worn/readjust or plate had loosened.
If all seems good, clean everything as you might be getting a weak ground (where you don't want it) across some oily crude, etc. which can also cause this prob.
I had the same problem, and it was my timing. I've switched to CDI on that bike and have had no problems with backfiring yet.
and if all your electronics are good then its carbon build up in the cylinder glowing red at high revs causing a misfire. This occurs when you run too rich.
hmmm, have you tried a new plug? Sometimes carbon buildup on the plug will make the bike misfire. You said you were running rich.
Eric
Timing!
Thats about the only thing that does that backfire thingy...I did say 'about' so put the flame throwers away...
i put a new plug in and backfiring continued
Only time I've ever had backfiring problems was right after I de-rusted my gas tank and hadn't dried it out thoroughly. Apparently water in your gas can make you backfire too.
im trying to figure out a good way of doing the timing now, im done with the eyeball technique
business card is the exact thickness, cut a strip off and it should slide out with just a bit of resistance
I have had this problem a couple of times. It was always the spark plug with a piece of carbon stuck in the electrode. The plug itself could be bad also.
ahhhh! ok, seriously, i'm making this fucking video RIGHT NOW! AHH.
replace the condenser, points and plug wire/boot/plug, ground everything back to the same location to insure a bike is electrically sound.
anther reason bikes backfire a bit down hills is that when u are going down hill ur running richer, and if ur already too rich ur fouling out. fouling out at speeds sounds like back fireing.
on a side note u can't really time a puch so far wrong that it will backfire. (i'm sure someone has done it but here me out) Point being that if you set the magneto on the woodrift key and set the stator at about right in the middle of the mounting slots then set the point gap to spec like .4mm-.5mm or so the timing should be good enough to run the bike w/o back fireing. by default the allowance you have on a puch won't let it really backfire.
So basically adjusting the timing is probably not ur problem unless its really slipped alot (like magneto slipped of the woodrift key) its more likey caused by faulty electrical components and or bad grounds/connections.
intake leak. check your intake gasket, or bolt tightness.
i bet a wire in your magneeto is touching your flywheel and shorting out at high rpms.
Time to look at what a backfire is...
I understand it is unburned gas exploding due to compression (like a diesel)...so in other words, no spark for a moment. So that to me sez its a setting with your electrical system. A wire frayed and touching somewhere, perhaps, look at your condensor wire.
or...flavor this...A stator retarted/advanced too much and or points unclean and not gapped correctly...
BUT if you are running extra rich, you could be fouling plugs because of too much oil clogging up your spark plug gap...turning your plug black...
Just a couple ideas...
Hows your plug chop lookie?
It might be that the bike is not loved. Did you whisper sweet nothings to it recently? The pinto ran great with a little bit of love before.
its not the pinto, its my maxi. but now that you mention it i did cuss it the whole ride...
i am so surprised nobody has said anything about HOT EXHAUST PIPES yet. When you were going down that hill, your engine didn't need all that extra throttle. it was already spinning crazy fast, however, you kept the throttle on which kept the carb dumping all this extra fuel into the engine. since you probably have points, they missed a few times, causeing some unburnt fuel to exit the exhaust port, and when it touched the inside of the hot exhaust, BOOM! Also happens when you are full throttle and pull a decompression valve. It stopped when you let off the throttle becuase first of all, you slowed a tad, and second, less fuel going in to be wasted. well, thats my theory anyway.
if its backfiring under deceleration, its the same reason a car does it... gas vapor in the exhaust igniting. there could be quite a few reasons for it igniting, but its not your timing, if it were timiing, you would be hearing little pops at the top of your power band. what i think is happening: you run at 40, let off the gas, that leans you out hella, the mixture is so lean that its not igniting (but its building up in your exhaust), it runs lean for a few cycles... then your pipe load of fules become a rich enough mix of vapor to finaly ignite (after quite a few cycles), sound good?
Doubtful, since he said up top that it was downhill and when it was wound out on the flats. It's not a hot exhaust pipe.
Jay, the symptoms you describe sound like slipped points to me. Had this happen on my Yamaha MG1 on a very long ride last year; the powerband felt like it was higher than usual (still quite rideable, but not as much torque), and down longer hills or when really wound out it would start backfiring pretty regularly. Pulled off, checked the points, they had slipped so they were barely opening. Took a minute or two to set them, and everything was joyous once again.
tl;dr my vote is slipped points
it's probably that damned brie.
might have gotten stuck somewhere in the intestines, then riding at full throttle shifted it loose...
once it got loose it was only a matter of time before BACKFIRE!
Points Jay. My yellow ped is doing the same thing. Just need to set the gap. Neal has done it a 1000 times, he can show you.
ill gie ya something else that smells like that brie
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