1983 XJ750 Slipping into Neutral at High RPM

As is in the title, my bike, which has previously been running well, is now having a transmission problem. When I accelerate and try to shift to second at anything higher than 3500 RPM the bike refuses to shift (makes a horrible grinding noise and won’t up shift) and if I am in second and reach too high of an RPM (4000ish) it slips back down into neutral. I have read about the primary chain guide disintegrating but the symptoms of my bike seem different. What needs to be done to get my bike working again? As far as I know the case has never been split before if that helps. Thank you all!

 

I would think you have a gearbox issue... The dogs which lock onto their adjacent gear sometimes wear
out. The corners firstly round off, then your left with not enough lead in to actually hold on.

 

Dang, that was my biggest fear as it seems to be the most pricy thing to fix. Would trying to find a gearbox out of a used motorcycle be a suitable solution, or will I have to just replace the afflicted gear?

 

There is an article in the tech section that addresses this. I have a 1981 SECA 750 with this same issue affecting my 3-4 shift and it was caused by weak springs and worn indexer. Not a difficult fix but do verify that your clutch is releasing and not hanging or sticking, personally I'd replace the frictions & steels anyway. Replacing the springs & indexer is not particularly difficult and only requires the removal of the side cover and some good hands. Chacal has the needed parts and upgraded ball bearing detent roller that will really make it shift like butter. I did my own ball bearing detent and that made a world of difference.

 

Thank you for the information! When I visually inspect the indexer it appears to match what I see in the service manual. I don’t know exactly what would make it look worn though, so I am open to learn. As for the plates, I replaced those last summer and once this problem occurred I inspected them again and they still seem to be in good condition. I didn’t replace the springs, but they still felt stiff and looked visually good. If you think that old springs could be the issue, I could try that, I just don’t want to parts cannon this thing. My clutch is also behaving correctly, and has no constant pressure against the springs. I have not heard of the ball bearing detent, is this a fix that I can do without splitting the case?

 

The only case splitting you would be doing is the side cover as everything but the shift drum is easily accessed from this point. If you're new to the XJ site I'd like to suggest spending some time in the tech section that can be accessed by clicking on the XJ4Ever banner at the top right of the screen. While it's not a substitute for the original Yamaha XJ factory service manual it not only fills in the gaps but also has tips & suggestions from the long time pros who really know the bikes, take what you read there as gospel and don't cut the corners. Having been on two wheels since before I was in the 3rd grade I know bikes well enough to understand the parts cannon thing, problem is the bike is a relatively small target and many things can get missed when the cannon is fired. There are a number of members with long gray whiskers here who have been working on these since the Nixon administration and if you have some time go back on the earlier threads & posts to soak up everything you can. Personally I started out on the Rupp's from the mid-60's that were powered by the K92's then the larger 2-stroke RMT and eventually a Kaw KDX-175, a Honda 250 scrambler before biting off more than I could chew with a Honda CB-X sport, the common theme with all of these was learning how to fix them myself in between the scrapes, bumps & bruises during the 1970's and 80's... it proves a caveman like myself can do it.

If the springs are weak from time they won't hold the roller down in the "star" detent and it can "walk" it's way up from vibration and the slight gear lash until it gets stuck between to gears as everything is then unsynchronized sometimes clashing between the drive dogs and chewing the edges off the gears themselves. This is the first place I'd look as the springs are the only retention mechanisms. Side cover isn't difficult to remove but do pay attention to the physical relationship of the shifter input shaft so you get the toe & heel back in the same spot. As you begin to pull the cover off keep a moderate pressure on the shifter input shaft pushing it in so it doesn't come out with the cover, once open snap a few pix with your phone if available so you have a reference to work with should confusion set in. Be prepared to change your oil as some gasket material may make its way down and do have the correct gasket on hand, you can use sealers but I don't recommend anything other than aviation Form-A Gasket which comes in a brush cap bottle and is designed specifically for oil submersion. Take your time with the repair and if the spring is broken do NOT reassemble until ALL of the pieces have been accounted for.

 

One other thing which causes this is a bent or damaged shift fork. That is easily replaced if you find a spare and if you do you'll probably find a spare set of gears and dogs.
Still gotta split though

 

Hopefully that's not the case no pun intended.

 

Thanks again for the responses! I will try @Brhatweed fix first and hope that fixes everything, and hopefully that will prevent me from having to split (though this poor bike [40k miles] is probably overdue for that kind of service). I do see the updated high performance pawl arm from Chacal, and the two torsion springs but I would like some clarification whether the tension return spring (the linear one) would be a recommended purchase since mine still seems to be in good condition

 

Replace them both and sleep well.