what causes 1st and 2nd gear to stick when MXing?

jaguar

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#1
When I was on a tight MX track the other day sometimes it took herculean effort to shift up from 1st or 2nd. I assume that riding hard in lower gears is causing some stiction in the gearbox from the heat.
If it is taken apart then what parts should be looked at for possible replacement?
 
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#2
It’s time for new oil (it always is) so drop the side cover off. Check that the lever isn’t binding & the splined shaft isn’t bent. You’d be surprised how many times it turns out to be external to the gear box. What you are sort of suggesting is the only times you are having trouble is perhaps at higher revs than normal?

Have a look at the arm on the shaft that pulls the shift drum around. The little pins that live on the end of the shift drum are ok & rotating with the lever ok.

After that (internally) I would expect damage to the drum itself or the dogs engaging into it. There could be some gauling on the sliding components particular for that shift. Bearing failure would tend to affect all shifts but it’s not unreasonable that if one side of the dogs sliding shaft has a lot of movement it may bind up. Look for anything blued up.

First to second are usually the gears to get munched first but hopefully the reason turns out to be external. Keep us posted.
 

dirt bike dave

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#3
Is it possible your clutch is worn or out of adjustment? That can cause difficulty in shifting. Maybe you get your clutch and motor hotter when MX riding than on the trail.
 
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#4
I have problems sometimes shifting into 3rd and 4th. Has been that way since the bike was new. Most of the time it will shift but sometimes it won't, what the hell? The bike only has 300 miles on it and has had 3 different kinds of oils at varying levels. nothing seems to cure it for me. 00 220
 
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#5
KdxChode

When you say have problems shifting do you mean it is stiff like above?, or finds a neutral in between gears? or what I suspect doesn’t quite make it & stays in the lower gear? - which would indicate possibly a shimming problem. Gearboxes are neat things but can cause all sorts of problems if misaligned

Gears don’t quite make it in, or jump back out, which undercutting can often help.
 
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#6
Sometimes it won't upshift. It just wants to stay in 2nd or 3rd. I can always downshift fine and 1st to 2nd is always perfect without hitting nuetral.
 
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#8
Kdxchode: Just wants to stay in 2nd or 3rd? Do you mean the lever feels free to move but it doesn’t quite make the shift? Again I would suspect on such a new bike a manufacturing shimming problem. Otherwise the lever isn’t travelling quite enough to encourage the 2 gears together & convince the detent arm into the next hollow of the shift drum. Now this may sound silly but if your gear lever is at a funny angle -like fairly high, you may simply just not be regularly lifting your foot high enough or it may rub against the case slightly to prevent full travel. Make sure the lever bites onto the spline tight.

Jag you made it sound like your lever gets stiff? So with the engine not running it shifts fine? If so would check that clutch isn’t dragging. Failing that would expect some slop in the change drum or the shaft the forks are running on.
 
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#9
KdxChode been thinking more about your problem last night & dipping into an excellent reference book Kevin Cameron’s ‘Sportbike performance’. It’s aimed at tarmac bikes but the principles are the same & written in a real world manner.

It occurred to me that the detent arm spring could be weak or the arm binding, - so the shift drum can bounce back again as the detent spring isn't forcing it into the next gear. It may also be that the dogs (these are the meshing teeth on the side of the gears themselves) could be badly formed so that they don’t want to engage & bounce back out again pushing the fork back to the original part of the shifting drum.

Still would put money on the external causes first though.
 
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#10
I never payed that much attention to what the lever felt like when it wouldn't shift. I will make a note of what happens after my next ride and post it. My shifter has plenty of clearance though so it isn't that, and it only happens sometimes. When it does I usually have to downshift and then it starts upshifting fine.

Funny thing though...When I first got the bike I was having problems downshifting and figured out that I was trying to shift the metal frame guard that sticks out. Got more used to the bike and don't have a problem with that anymore.