strange behavior in 5th on yzf 426

7 replies to this topic
  • no1toker

Posted October 04, 2012 - 09:21 AM


hi guys had the bike a few weeks not really rode it until today! basically 1-4 gears are fine pull like a train 5th at quarter throttle if fine but any more than that its power then like quarter of the power then back to full power and so on, its had a light horn indicators put on before i had the bike! it seems like brain of the bike is not letting spark correctly or summat in fifth...
could this be because its been converted to b able to use on the road and jus ment to have half power in 5th so that the engine doesn't get killed ??? any advice will help loads

  • 72degrees

Posted October 04, 2012 - 10:24 AM


A rev limiter should cut in at the same rpm in any gear. Never heard of a speed limiter. I can't even see how a loose connection in the ignition would only affect it in 5th either. It will be under higher 'load' in 5th I suppose so any weakness in spark might show up as misfire. If it's specifically linked to throttle position it ought to carburation related. Could it be some strange glitch involving the TPS? Fuel supply from tank to carb flowing freely (thinking fuel starvation when giving it the beans). But once again why only in 5th? Has it been put on tall gearing for road use?

  • no1toker

Posted October 04, 2012 - 10:27 AM


same gearing and runs perfectly in all other gears its just fifth! its boggled my brain tryin 2 think what it could be

  • grayracer513

Posted October 04, 2012 - 10:43 AM


There are two possibilities. First, 5th is the biggest load you can place on the engine, and in the case of marginal ignition integrity, spark that would break up under a load is more likely to act up in 5th than in any lower gear.

However, the more likely case is this:
Your bike runs perfectly fine in any other gear regardless of the load you put on it. It runs OK in 5th, too, but only if you avoid going over a certain amount of throttle. If you go beyond that, you start getting a skipping, jerking thing happening that feels something like the chain jumping over one tooth and down onto the next one.

If that describes your problem, what your problem most likely is, at minimum, the 3rd and 5th gears on the main shaft have worn locking lugs on their facing sides. As built, these lugs are slightly undercut so that the driving load holds them together. As they wear from shifting, the undercut disappears, and becomes slightly reversed, and the gears push away from each other under a load. Since the shift fork cannot be pushed back to a neutral position as with a car trans, it does not jump out of gear and stay there. Instead, the fork flexes enough to allow them to separate, then slams the gears back together in to the next cut out that comes by, and this invariably bends the fork as well.

Time to split the cases, is what it looks like.

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  • 72degrees

Posted October 04, 2012 - 10:52 AM


Dodgy gear dogs? Yes that could do it. On a Morini 6 speed box it has been known for wear on dogs and selector to allow dropping out of 6th in to a false neutral. Highly inconvenient when you are just about to pass someone at the end of Park straight at Cadwell park, I can tell you.

  • no1toker

Posted October 04, 2012 - 11:42 AM


thats a b******d! does seem abit like that :foul:

  • dirtynasty

Posted October 04, 2012 - 02:04 PM


Yeah, and Yamahas aren't known for robust transmissions. The R6's had massive issues with second gear exactly like you described. When you got on the gas hard it would skip and the bike would jump to the point where it's dangerous. The solution? Yamaha denied that they &%$#@!ed up in their manufacturing process, blamed the riders for abuse, yet released a re-designed shift drum that for the most part solved the issue and charged the owners for this new part.

Hell, just browse any used bike ad site and look at older, abused looking Yamahas. I bet you'll find a LOT of them have transmission issues. The FZR sport bikes I frequently see needing new transmissions.

  • four-twenty-sixer

Posted October 04, 2012 - 02:42 PM


Gray you are exactly right. Just had this same problem on mine not to long ago. Had to replace a broken shift fork and 3rd and 5th gears on the counter shaft. Wasn't to bad, about 200 bucks total after replacing a couple bearings and seals.

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