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Posted By: Ducknaldo T120 unit gearbox problem - 12/19/08 5:07 pm
The gearbox on my Bonnie has a problem.
I suspect it´s a worn camplate, but could also be the forks.
Maybe some of you has had the same problem and could enlighten me.
Normally it works fine, but when I really give it some throttle from standing still it jumps into neutral.
Really makes me look like a fool when trying to impress the girls:-D
What´s the most likely cause for this?

Posted By: Ducknaldo Re: T120 unit gearbox problem - 12/19/08 5:09 pm
Forgot to mention that it´s a `69 if that makes a difference.
Posted By: Jack Adams Re: T120 unit gearbox problem - 12/19/08 6:16 pm
Testaterone? And a well worn gearbox. Jack
Posted By: Ducknaldo Re: T120 unit gearbox problem - 12/19/08 6:32 pm
I kinda know something is worn, it´s merely the question of what.
Posted By: John Healy Re: T120 unit gearbox problem - 12/19/08 10:26 pm
Worn dogs on the gears, bent or worn shifter forks and worn cam plate fork roller groove in that order - Or worn fork roller groove, bent or worn shifter forks, or worn gear dogs in that order... It is the chicken or the egg.

The dogs on the gears are straight cut, and when they wear a bit, they can be prone to wear some more finally ending up with the bike not staying in gear. They typically put a lot of pressure on the shift forks and cam plate when they "blow" apart.

Now it could have started with rusted or weak shift plunger springs in the inner cover, or a worn, sticky or weak cam plate detent plunger or it be as simple as just too much testosterone and a sticky tire.
Posted By: Ducknaldo Re: T120 unit gearbox problem - 12/22/08 11:59 pm
Thank you for the more precise answer, John.
No offense Jack:D
I´ll post some pics when I get the darn thing apart.
Posted By: RF Whatley Re: T120 unit gearbox problem - 12/23/08 5:32 pm
Duck Doo -

The gears are going to look "normal" to your eye. After all, we're talking about 35 years of very slow polishing. The only way to tell if the gears need to be replaced is to compare them side-by-side against new ones from a dealer. You'll easily see then that the dogs are shorter and have a generous radius that the new gears don't have. Both the gears in the mating pair need to be renewed. Not the ratio pair, but the engagement pairing.

As John hinted, the gear misbehavior could only be a symptom. Plan on replacing all 5 coil springs in the gearbox as the cheapest insurance you'll ever buy.

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