Topic: CB750 popping out of 4th gear ******** SOLVED

[HondaMan]

Sorry to be late to the party!   
There is a shim on the end of the mainshaft that Honda used to control this issue. Back-cutting sort of solves it by pulling the gears together: the shim on the end pushes the slack out, toward the clutch side. I think I have a bunch of info on this in another post where one of us was having a similar problem, a search about "4th gear" should bring it up?

Essentially, Honda used shims of .5, 1.0, and 1.5mm (or none) to push the stack close enough together to let the dogs engage their min of 3mm. But after about 40k miles or so, 3mm isn't enough. Good ones started at about 5mm deep, and last forever. So, if you fall into this business again, consider messing with the thickness of the end shims on the mainshaft: that's another way of solving it.

Backcutting is also a good one, just tricky (or expensive) to implement! It's MUCH better if you're racing, too. (We used to call it "Pull-cutting" back in the day, for the way it pulls the gears toward each other.) 

[MCRider]

Sorry to be late to the party!   
There is a shim on the end of the mainshaft that Honda used to control this issue. Back-cutting sort of solves it by pulling the gears together: the shim on the end pushes the slack out, toward the clutch side. I think I have a bunch of info on this in another post where one of us was having a similar problem, a search about "4th gear" should bring it up?

Essentially, Honda used shims of .5, 1.0, and 1.5mm (or none) to push the stack close enough together to let the dogs engage their min of 3mm. But after about 40k miles or so, 3mm isn't enough. Good ones started at about 5mm deep, and last forever. So, if you fall into this business again, consider messing with the thickness of the end shims on the mainshaft: that's another way of solving it.

Backcutting is also a good one, just tricky (or expensive) to implement! It's MUCH better if you're racing, too. (We used to call it "Pull-cutting" back in the day, for the way it pulls the gears toward each other.) 

Mark: Back when I built the bike in my avatar, or rather watched others build it while I did their menial jobs for them, the mechanic told me he was shimming the transmission. I think it was 2nd and 4th gear. That's probably what he meant, what you are speaking of. His name was Dick Suarez and he worked for Pops Yoshimura for at least a few seasons. Could make more money in a dealer shop/ scenario. But couldn't keep his fingers out of a racebike.

Wish I'd looked into it for current build, but I did have it "pull-cut" by APE, so I guess that's good enough.

[brandEn]

That's good info Mark. Where do you get the shims though?

[MCRider]

That's good info Mark. Where do you get the shims though?

That was my next question too. But shaking some old cobwebs lose I think my builder got them from Honda stock. May have carried a 268 (CB77 305 Superhawk) part number.

My memory was he just rummaged around in some parts boxes till he found what they wanted. Shimming the 305 trans was part of their production racing protocol. IIRC

[HondaMan]

For the Honda shims, check the parts fiche under the post-1975 bikes (750). They may still be around, as they are used in many other trannies, too. Honda sometimes changes the numbers to a generic number (with letters in the center 3 digits), but the first 5 and last 3 will look familiar, because those describe the function and thickness.

I've used garden-variety (hard) shim washers that fit. Don't use something soft (i.e. bendable by hand), but use something that pliers won't leave a mark on (hardened) when squished by hand.

About 2 years ago I worked on a K7 box that had one all torn up: Ace Hardware actually had some that fit, but were thinner. I used 2 of them to make the same thickness (I think it was 1.0mm) that was originally in the box. The engine is in Canada now, reportedly working fine, in a hi-HP K7 Wiseco 836cc cafe' build with Megacycle cam.

[flybox1]

That's good info Mark. Where do you get the shims though?

I got one off Amazon.  Perfect specs.   I'll find the listing and post it in the morning.  It's on my work pc.