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Old 08-15-2013, 03:36 PM   #1
Enduronut357 OP
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why does my dt400 jerk when accelerating?

it jerks bad when accelerating lately, what could the cause of this be? its almost like it wont accelerate anymore when it jerks like this
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Old 08-16-2013, 05:53 AM   #2
brucifer
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Hmmm...chain skipping over the sprockets? Jumping in and out of gear? Spark plug fouling?
Any more details you could give us would help.
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Old 08-17-2013, 02:58 PM   #3
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ignition miss...
change the plug , check the timeing
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Old 08-17-2013, 03:51 PM   #4
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Quote:
Originally Posted by brucifer View Post
Hmmm...chain skipping over the sprockets?
have had this happen on a yamaha - the swingarm pivot hole on the frame can oval out and when you really get on the gas, the chain loosens/skips teeth... when you pull on the swingarm by hand it's fine, but when you get lotsa power to it it moves....
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Old 08-17-2013, 09:35 PM   #5
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Quote:
Originally Posted by stainlesscycle View Post
have had this happen on a yamaha - the swingarm pivot hole on the frame can oval out and when you really get on the gas, the chain loosens/skips teeth... when you pull on the swingarm by hand it's fine, but when you get lotsa power to it it moves....
Yeah that happens all the time,ovaling out the swingarm pivot hole.
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Old 08-17-2013, 09:41 PM   #6
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Originally Posted by Foot dragger View Post
Yeah that happens all the time,ovaling out the swingarm pivot hole.
I've never seen that but I've seen seriously worn and loose bushings. There's also a very obvious sound to a skipping chain.
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Old 08-18-2013, 04:07 AM   #7
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Yeah that happens all the time,ovaling out the swingarm pivot hole.
yeah, i never said it was common - but i do have a mx175 with this dilemma - took me a while to figure it out, as you couldn't repeat it by just yanking on the swingarm... i removed swingarm pivot nut and the hole in the frame was ovalled.. chain would loosen throughout a race like it was stretching, but it wasnt...the dumb swingarm pivot kept inching it's way forward on the drive side... probably was caused by loose swingarm pivot nut sometime in it's history? either way, the op post is kinda vague and what is meant by 'jerks bad when accelerating lately"
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Old 08-18-2013, 06:28 AM   #8
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It jerks pretty good because it is a 400cc two stroke
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Old 08-18-2013, 06:32 AM   #9
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It jerks pretty good because it is a 400cc two stroke
Thats what I was thinking,they're not the smoothest bikes made,jerking is probably a sign its running right.
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Old 08-18-2013, 06:37 AM   #10
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Originally Posted by stainlesscycle View Post
yeah, i never said it was common - but i do have a mx175 with this dilemma - took me a while to figure it out, as you couldn't repeat it by just yanking on the swingarm... i removed swingarm pivot nut and the hole in the frame was ovalled.. chain would loosen throughout a race like it was stretching, but it wasnt...the dumb swingarm pivot kept inching it's way forward on the drive side... probably was caused by loose swingarm pivot nut sometime in it's history? either way, the op post is kinda vague and what is meant by 'jerks bad when accelerating lately"
Ive had some old rat bikes that were wobbled out in many ways,would probably take while to oval out the frame there but with a loose swingarm axle bolt I can see how it would happen.

I had a 72 250 DT that would pop the headlight and the oil tank out upon hard landings,wire can only do so much. I just kept taping and wiring it together till I traded it for an old truck.
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Old 08-18-2013, 09:02 AM   #11
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Originally Posted by stainlesscycle View Post
yeah, i never said it was common - but i do have a mx175 with this dilemma - took me a while to figure it out, as you couldn't repeat it by just yanking on the swingarm... i removed swingarm pivot nut and the hole in the frame was ovalled.. chain would loosen throughout a race like it was stretching, but it wasnt...the dumb swingarm pivot kept inching it's way forward on the drive side... probably was caused by loose swingarm pivot nut sometime in it's history? either way, the op post is kinda vague and what is meant by 'jerks bad when accelerating lately"
i had a YZ 490 that had rusted swing arm bearings... and probably ridden that way for years. the swingarm pivot bolt had worn significantly from twisting in the frame & the hole in the frame was seriously ovaled to the point where it was ready to go. lift the bike and the back wheel could move 1/2" to either side, it was so bad.
previous owner warned me that it was a squirrelly ride. no shit. i fixed the frame, new bearings/pivot bolt and hot damn it was a different bike.

never rule out weird shit on an old bike
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Old 08-18-2013, 09:04 AM   #12
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Originally Posted by Enduronut357 View Post
it jerks bad when accelerating lately, what could the cause of this be? its almost like it wont accelerate anymore when it jerks like this
Sounds like not enough fuel.
Check the fuel flow from the petcock.
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Old 08-19-2013, 07:19 AM   #13
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To be honest, most people with far more experience than I are jumping to some ideas here without enough information.

The first question is, is the jerking coming from the engine or the drivetrain? You should be able to tell. Jerking from the engine will be misfiring, backfiring through the carb or general lumpiness. Drivetrain issue would surely be just as obvious, the whole worn swingarm idea is interesting and possible, but not what I'd first look at.

Potential problems:
- gradual plug fouling causing the misfiring and the "jerking". This could be a tuning (carb) issue, an oiling issue, or a bigger engine (worn crank seals and burning trans oil OR an air leak causing overheating) issue.
- not getting enough 2T oil. unlikely, but possible.
- fuel flow. possible. check tank cap vents, clean petcock, replace fuel lines, add an inline filter.
- sure. the whole swingarm pulling and messing with the chain thing is a possibility but i wouldn't look at it first.

I think the OP needs to tell us more information about this. Does it happen immediately after starting? OR after the engine is nice and hot? At what stage of acceleration, rpms and throttle opening? so much more.
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Old 08-26-2013, 07:33 AM   #14
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Originally Posted by dhallilama View Post
i had a YZ 490 that had rusted swing arm bearings... and probably ridden that way for years. the swingarm pivot bolt had worn significantly from twisting in the frame & the hole in the frame was seriously ovaled to the point where it was ready to go. lift the bike and the back wheel could move 1/2" to either side, it was so bad.
previous owner warned me that it was a squirrelly ride. no shit. i fixed the frame, new bearings/pivot bolt and hot damn it was a different bike.

never rule out weird shit on an old bike
Ive had old junk bikes as well. I bought an old YZ400 as a parts bike once,the owner was demanding extra money as he had actually made it run if you kicked long hard and fast enough,when the top end came off I could see where he had used a big coarse file to rasp the piston down to size after some seizures,ovaled bore,broken ring. But yet it ran and he was dam proud of it.
Yokels will do ANYTHING to get 1 last hour of ride time on a bike,as long as it doesnt involve $. Smash a stator into place by hammering american thread screws in? Yes if it works it works.
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