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Pops out of 4th gear

Posted: Fri Jul 24, 2020 11:19 pm
by Ducman71
Hi everyone,

Bike is a new to me '77 900SS...took it out for a proper ride for the first time today after having spent the last few weeks sorting a number of small issues that I found by just riding it around the neighborhood after I received it. Everything was going great until I got to 4th pops out of 4th gear shortly after it is engaged, even at low load or engine braking (downhill). This happens whether I got to 4th shifting up from 3rd, or down from 5th. All other gears work fine, and it shift cleanly. It actually shifts fine into 4th and feels fully engaged when shifting, it just won't stay in gear. Ideas?


Re: Pops out of 4th gear

Posted: Sun Jul 26, 2020 4:49 am
by ducadini
Fourth gear is the direct connection between inshaft and outshaft.
Is that engine VERY recently dismantled ? then probably the shimming of BOTH shafts (and of course the selectorshaft) was neglected.
When shimming the clutch must be fitted (or a suitable substitute) and the sprocket (also or substitute) and tightened (a bit to take out play).
Only then can you see how the sliding fourth gear slides into the dogs of the outshaft.
If things get worse, the bearing in the case and the needles in the outshaft are going awol, damaging the outshaft (and later the inshaft).
Anyway, should be looked into as I don't see any other way to correct this.
Trying to get rid of it fiddling with the gearselectorboxscrew doesn't solve it.


Re: Pops out of 4th gear

Posted: Tue Jul 28, 2020 5:55 pm
by Ducman71

Thank you much for the reply. The cases have never been opened; the original lockwire and lead seal are still in place. Have you ever seen a situation where the shimming/end play of the shafts is wrong from the factory? And if so, wouldn't this issue have presented itself many years (and many miles) ago?

A few days ago I tried various positions of the eccentric screw on the selector box, as well as trying different shifting techniques. The results were interesting. On multiple occasions, I was able to get 4th to fully engage and stay in gear at a wide range of load conditions, including hard acceleration. This makes it seem like it is a selection issue (either the external box or the internal forks or gear spacing) rather than rounded off dogs on 4th. On the other hand, adjusting the eccentric screw caused shifting problems elsewhere with other gears. Also, with some adjustments it seemed that 4th would slip into neutral between 3rd and 4th (downshifting after 4th slipped into neutral would result in 3rd being engaged), while other times it seemed that 4th would slip into neutral between 4th and 5th (downshifting after 4th slipped into neutral would result in 4th being engaged). Finally, I noticed that after an upshift, it would sometimes be necessary to lightly push down on the shift lever to get the selector to snap into the next detent, and without doing so it would not upshift to the next gear at all.

Because of all of these inconsistent results seem to be related to the position of the eccentric screw and the laziness of the selector wheel to engage the detent after upshifts, I think my next course of action is to open up the selector box and see what is happening in there.


Re: Pops out of 4th gear

Posted: Wed Jul 29, 2020 6:36 am
by BevHevSteve
Im thinking the shifter spring Is about to or has already given up / broken.

Re: Pops out of 4th gear

Posted: Wed Jul 29, 2020 11:53 am
by Ducman71
I suspect that is at least part of what is going on as well Steve. I've had a shopping cart open on another tab for several days now, including a return spring among other items; order coming your way soon...

Re: Pops out of 4th gear

Posted: Thu Jul 30, 2020 10:06 am
by Ducman71
Well, it's not the selector return spring; the spring is correctly installed and feels strong in both directions. The only things I noted in the selector box were that everything was basically dry (no grease anywhere) and the selector wheel itself was not turning smoothly (it's shaft was binding slightly in the guide bush that it sits in). The fingers on the fork and the pins on the selector wheel all look perfect, no nicks or wear. I cleaned up the bore of the selector shaft bushing a little and greased everything liberally, and on a bench test it now selects each detent precisely and smoothly, and it self centers/returns just fine. So....

Is there a way to test for proper engagement of each gear by manually turning the selector drum shaft while unloading the transmission (moving the rear wheel) to confirm is each gear is engaging at the proper time (i.e. at the proper rotation angle of the selector drum)?

Re: Pops out of 4th gear

Posted: Sat Aug 01, 2020 3:07 am
by 900tlc
Sounds like its time to check the internal gear box,drive dogs probably rounded off and selector fork worn.
I had the same problem years ago on a GTS,you can mess around with the selector box and keep blaming that.But taking a corner at speed and losing drive,does nothing for confidence.Best also to get a professional to do it.I stripped the top end down,and handed the engine over,after the rebuild the gearbox had new cogs,which were up rated,and the bike I know is still going 30 years later.
I just hope Im wrong,Ducati gearboxes never get better,just alot of expense.