Topic: 76 cb750 Transmission/Clutch help

[Sweep]

Very fresh motor, except I have had my transmission problems.  I had to pull the motor last season because the bike wouldn't shift up from 2nd I believe and it turned out to be a worn shifter fork.  This time it wouldn't shift up from 3rd so I pulled the motor again and went over the gears, replaced the shift drum, neutral switch, and all 3 forks with better used ones that were notably thicker at the tines.

Before I buttoned it back up I made sure that it shifted nicely through the gears and it does.  I put it all back together and it still shifts nicely but the gears aren't engaging because I can easily push the bike.  The clutch doesn't seem to have an effect at all, in fact I can't seem to get it adjusted.  It is just too tight for some reason.

I'm thinking mal adjustment of the clutch due to having a wrong model year cable could have done some damage but I'm not sure what.  Could the clutch have jammed something somehow?  I'm not too clear on how the clutch works unfortunately, I'm assuming it just disengages the drive from the gears but the gears and drive engage on their own without it.

I'm looking for some ideas or reason I shouldn't pull the motor again.  I obviously don't want to do this again, it's a load of work.

[Sweep]

I guess I'd like to simplify the question.  If the shift drum is shifting and the gears are turning then what can be causing them not to engage the final drive?  When I took apart the motor I never touched the top half of the works, just the bottom area around the shift drum and forks.

I could swear the middle fork engaged the top gears when I put the cases together but if in the event that it somehow did not would that cause these symptoms?

[HondaMan]

Ummm...are you saying that the bike is freewheeling, regardless of the gear selected?
 

[Sweep]

Ummm...are you saying that the bike is freewheeling, regardless of the gear selected?
 

Yes I am, sorry if I've been unclear.

I'm looking at schematics and a picture of the top end laying upside down and it seems to me that if the center shift fork did not insert correctly the gears would not push to the inside of the case to engage the final drive gear.

[HondaMan]

This bug is unusual, to say the least...the power comes from the crankshaft to the mainshaft (those are the gears in the upper half of the case) via the primary chain, then that shaft puts the rotation (power) into the clutch outer. The clutch plates couple that rotation into the mainshaft, where the splined-on gears get it to drive the gears on the countershaft. There are several "idler" gears, not splined to the mainshaft, which couple to the splined ones when they are slide sideways into the splined ones, by the forks. This is how the power is transmitted to the countershaft.

The countershaft has a large gear at the very end, near the center of the lower case, which couples directly to the final drive gear. The combination of gears that pick the rotation from the mainshaft, then couple that rotation to the countershaft, determines how fast this final gear spins, and it is always coupled to the final drive.

In other words, it is a "constantly meshed" gearbox, where power transmits from the dogs on the sides of the gears into the holes on their adjacent gears, to the shafts.

So...about the only thing I can think of that would let it freewheel would be that the middle fork is not engaged on the mainshaft, and the gaers on that shaft happen to be just barely in between two gears (2 & 3, probably). I seem to remember having done this once on a 350 twin (similar gearbox design) years (and years and years) ago. After that little incident I always run through all the gears upon reassembly, before I tighten the crankcase screws down on the sealant.

Sorry, I can't think of anything else at the moment! 

[Sweep]

Thanks for that explanation, very helpful.  I took apart the gear shifting mechanism and neutral switch, hoping that the switch wasn't placed right but when I took it off I am unable to get below 3rd gear (not entirely sure it's that gear but the neutral detent is in the 1 o'clock position.  I now have the motor detached and ready to pull again tonight and am feeding a buddy and his family in order to get the motor out. 

The drum will shift up freely but will not select a gear lower than the 3rd gear but when that is engaged the bike no longer free wheels.

I'll let you know what I find but I'm going overboard on testing before I buckle it back up again.

[HondaMan]

Thanks for that explanation, very helpful.  I took apart the gear shifting mechanism and neutral switch, hoping that the switch wasn't placed right but when I took it off I am unable to get below 3rd gear (not entirely sure it's that gear but the neutral detent is in the 1 o'clock position.  I now have the motor detached and ready to pull again tonight and am feeding a buddy and his family in order to get the motor out. 

The drum will shift up freely but will not select a gear lower than the 3rd gear but when that is engaged the bike no longer free wheels.

I'll let you know what I find but I'm going overboard on testing before I buckle it back up again.

The "below 3rd gear" situation is probably due to your removing the neutral switch. It doubles as an alignment device with that ball detent on the end. If you pull it out, then try to shift down into 2nd, the whole shift drum pushes outward from the center of the engine because it is moving 3 forks at that moment: the sideways torque then jams the shift detent roller against the sides of the "bird cage" and the drum won't turn. There won't be damage unless you stomp on the lever at that point: it just gets stuck and won't shift down.

I've seen that with broken neutral switches before. 

Is it possible that you might have installed the L & R shift forks backward? They are marked L, C, R on their bodies.

[Sweep]

Thanks for that explanation, very helpful.  I took apart the gear shifting mechanism and neutral switch, hoping that the switch wasn't placed right but when I took it off I am unable to get below 3rd gear (not entirely sure it's that gear but the neutral detent is in the 1 o'clock position.  I now have the motor detached and ready to pull again tonight and am feeding a buddy and his family in order to get the motor out. 

The drum will shift up freely but will not select a gear lower than the 3rd gear but when that is engaged the bike no longer free wheels.

I'll let you know what I find but I'm going overboard on testing before I buckle it back up again.

The "below 3rd gear" situation is probably due to your removing the neutral switch. It doubles as an alignment device with that ball detent on the end. If you pull it out, then try to shift down into 2nd, the whole shift drum pushes outward from the center of the engine because it is moving 3 forks at that moment: the sideways torque then jams the shift detent roller against the sides of the "bird cage" and the drum won't turn. There won't be damage unless you stomp on the lever at that point: it just gets stuck and won't shift down.

I've seen that with broken neutral switches before. 

Is it possible that you might have installed the L & R shift forks backward? They are marked L, C, R on their bodies.

The forks seemed fine but either way I removed and went over the drum and forks and followed the manual to put it all back together.  I think you hint at the problem, which was I installed my neutral switch out of order and after installing the forks.  I think it was probably never seated properly and caused the issues.  The motor is back together now and moves through all the gears.  I'm going to sleep on it and then test again tomorrow and then get it back into the bike.  Thanks for the help!

[Blitzburgh207]

I know I'm digging up a very old post here, but I just reassembled and went to pull out of the garage and experienced the same problem...  I spent some time shifting through the gears before torquing the case halves back together so I thought I did it right but apparently not. 

I started the bike, backed it out of the garage, pulled the clutch, shifted down to first (and heard the distinct "clunk"), let the clutch out, and nothing happened.  This is heartbreaking.

Not sure if you're (Sweep) still active on here, but if so, how did you solve the problem?  I really hate to pull the motor again but I'm going to do whatever it takes to get this rebuild back on the road asap.  Any help is appreciated!