Lost third gear

[tom_yota]

1988 CR125R
I bought an abused bike in feb, and rode it once and noticed that third gear didn't engage, it was as if the bike were in neutral

I tore the motor apart and noticed that the shift fork had a grove worn into it. what was happening when third gear is selected, is the fork isn't pushing the dog teeth on the gear far enough into the other idle gear so the bike is essentially in neutral (no power being transmitted to the output/counter shaft).. I filed minimal material off of the shift fork, so that the gear sat flush again in the fork in hopes that it would engage itself again. While the cases were still split, i tested the shifting and activation of third gear and it worked flawlessly.. However the first time I rode the bike, I shifted into third and my "missing gear" problem was back. The wierd part is that the gear will take load while engine braking, however I can't apply power.. When i roll on the throttle I'm guessing there is enough lateral thrust to disengage the dog teeth from the face of the other gear.. I'm assuming this was an on-going problem with the bike before I bought it, because the dog teeth have been slightly rounded on the leading edge. The gear face that they insert to shows no wear.

I have been looking for a while to find used parts (since I first diagnosed the problem in feb), and finally have come across a guy parting out his bike..

My question is, do you think a new/used shift fork will fix my problem? The guy is willing to sell me all three shift forks, the shift fork rails and the gear select drum.

My secondary question is what would cause the gear to not sit flush on shift fork?

Sorry I have no pictures because I never thought to take any while I had the cases split, and the motor currently assembled in the bike. The motor, tranny and clutch all work MINT otherwise. I just wish I had third gear, the distance between second and fourth is pretty big while trail riding, and it's annoying having to shift 2-4 every time.

edit. I should have put this in the Mod/Performance/Repair section, can a Moderator please move this for me?

 

[whenfoxforks-ruled]

When the gear is engaged, its because there is a slight under cut to the "dogs" on the gears. It may appear square edged, and may have worn like that, some even have rolled edges worn into the corner. All very bad. The forks do not hold the trans in gear, they just move the gears. Engine braking a 2 stroke, stop that. Oh yes, the dogs, so anyway, when the dogs are bad, the gears are actually forced apart under the load of the engine and rider. If you have a really small machinist square, or a comparator, its real obvious. Its fix able, but expensive. Every gear that you find with a bad dog, the mating gear should automatically be replaced also, they are mated together. Change 1 and it will destroy the other. 1 has a dog exposed, and the other type has an undercut slot. Some are mated exposed to exposed. And its a REALLY good time to replace the 4 trans bearings and seals. Get a micrometer and measure your shafts where the bearing and seal ride. Under racing conditions, I have seen this issue blast the gears apart like they were made of glass, you got lucky. And no more engine braking on a 2 stroke! Vintage Bob

 

[tom_yota]

Thanks Vintage Bob.. For the record I haven't been engine braking, just stating that the gears do positively engage under reverse load, just can't take forward load.. WHen I kick it into third gear, using the clutch or not i can feel it positively engage, but they can't take thrust..

Thanks for all the info.. I assumed the forks held the gears in place, didn't realize the slight undercut.. Is there any kind of mexineering way to fix these gears? I'll ask buddy if he has a tranny available..

I'm assuming thats why the shift fork got messed up.. The thrust is pushing the gear outwards against the fork..

I'm gonna buy some gears, but not install them untill I need to tear the motor apart for some reason, most likely top end..

 

[whenfoxforks-ruled]

If you take an indexing head and milling machine, knock a few thousandths under cut, on all the dogs, exactly the same degree on each one, exact spacing, and its mate, it will work killer! Been there and done that, I think it was the 82 cr250 came with bad dogs from Japan, blew apart as stated. The second set, I cut, then Honda fixed it! It is at the point of kicking out of gear, even you can disintegrate the trans at this point. It spins pretty darn fast! I would highly recommend not using it any further. How are new parts availability for your model? Used tranny parts is a real shot in the dark, but doable if they are still good. Vintage Bob

 

[tom_yota]

My buddy has a mill, I'll see if he can help me out..

I'm not too sure about the availability of new parts, I'm going to call and iquire with the dealership tomorrow.. But I will most likely buy the used set of gears, and just swap over the 27 and 29 tooth gears on to my counter shaft.. I've checked a few online sources and the individual gears cost about $85 a piece.. thats making my budget bike too expensive for my taste haha

 

[whenfoxforks-ruled]

I would run the entire used tranny, if its usable. It may be in better shape? 85, I thought 45 was horrific! Vintage Bob

 

[tom_yota]

I was thinking that using just the gears I need would be a better idea, since I know that mine (aside from the two fudged ones) are in good shape and hold.. I won't have to reassemble the motor, and run it again before I find out that maybe second gear doesn't work in the donor tranny.. another reason to get the entire tranny is spare parts.. Maybe something else will go all FUBAR on me..

if it was $45 new I'd buy it, save me 3 hours of driving.. Gotta love the "currency exchange" between Canada / USA..

 

[whenfoxforks-ruled]

The Canadian exchange on parts, pretty confusing to say the least. Why can't a buddy from the states order it from Service Honda and send them to you, then the 85 is about correct???? Inspect the parts very carefully, install the gears that mate together, and hope for the best. Vintage Bob

 

[tom_yota]

I just sgot off the phone with the Honda Powerhouse Dealership near me, they have no problem at all ordering those parts for me.. to the tune of $94.62... EACH!

I have no buddies in the states..

I'm most likely gonna make the trip next week sometime to get that parts tranny.. I'm gonna bring a straight edge with me to check that there is undercut in the gears I require..