Topic: 1973 CB750 Gearing question

[Leanier]

Hey, I just got my CB750, did a top end rebuild and now it runs perfect, except that it pops out of 4th gear under heavy accel, (bent shift fork im pretty sure).
I just have a quick question about gearing:
At 70mph my rpm's are at about 4500 or so in 5th gear, and i get it up to 106mph, and it hits redline (9k).  Shouldnt it be able to go alot farther?  I feel like this isnt just changed out sprockets, due to only being at 4500rpm or so at 70mph.  Could the tranny be changed? or is that a normal rpm range for changed out sprockets in 5th?
Thanks!!

[754]

If you have stock 48 tooth on the rear wheel, it sounds like you have a 17tooth on the front (stock is 18)

 Lots of folks use that combo..

[bear]

Hey, I just got my CB750, did a top end rebuild and now it runs perfect, except that it pops out of 4th gear under heavy accel, (bent shift fork im pretty sure).

The common cause for this problem are worn dogs.

Cheers,
Brian

[Leanier]

Whats the best way to fix the worn dogs?  Get new tranny internals? or can you get them re ground or something?

[bear]

Depending on wear you can have them built up and then undercut them. Not a big job, no parts required.

Cheers,
Brian

[Leanier]

Ok, so I Just looked at my sprockets now, and it has the stock 18/48 sprockets on it, any more ideas why it tops out at 106mph, has plenty of power, but the hits the redline there?  (lots of the teeth on the sprockets are broken and damaged.)
Any ideas?

[754]

if the teeth are broken or hooked on front, replace it quickly.. it will destroy chains and/or cause breakage which will not be pleasant..
 Are you sure clutch does not slip? Something does not sound right rpm-wise for that gearing..

[madmtnmotors]

Your clutch is starting to slip, the wind resistance at higher speeds is producing greater load. Adjustment at the clutch hub may help, I replaced the friction plates in mine, reusing the steel plates. About $35.00 from Parts-N-More.

+1 on replacing chain and sprokets with broken teeth. 100mph with broken teeth will soon send a broken drive chain through your lower case.

[Old Scrambler]

Check the carb jets............Most K3s came with 105 mains and the bike tops out at 105-6ish. Go to 115s or 120s and it will do 120ish........after you get some good sprockets.  The 105s are good for high altitude power.

[Leanier]

Seems to be the clutch causing the top out at 106 for sure.  I ducked low under the bars going with the wind and managed to get it to 111, so looks like a new clutch plates it is, Thanks for all the help!

[madmtnmotors]

I hope you have replaced the ragged chain and sprockets if you're pushing the ton...

[Leanier]

Yea the chain and sprockets were replaced the day I noticed them, Thanks!

[Leanier]

Ok.. so, fully changed the transmission out for a used good one, fixed the 4th gear issue.  Also installed brand new clutch plates, clutch feels great now, very grippy.  However.  The bike still tops out at 100 mph on the speedometer, at 8k rpm.  feels like it could go alot faster if it werent at redline.  I have no idea what could be up now.. stock 18-48 sprockets.  Clutch grabs right away in every gear with zero slip.  I dont know..

[scrapvalue]

Is it just indicating redline, or is it reved out as far as it will go?
Do you still have the stock point set up or do you have electronic ignition,(Dyna or other).
I have found that with the electronic ignition it will rev out further.
I've had mine to 115 mph indicated. The speedometers on these bikes is not exact. They usually register higher then what you are actually going.
If you have a gps, mount it on your bike and check the speed that way. It will give you a more exact reading.
My gps says I am going 55 mph when the speedo is showing about 59mph.

[Leanier]

The tach is indicating redline, right at the edge of the red zone, being 8k, it would easily rev higher but I dont want to damage anything in the engine by over revving.  I'm going to try new clutch springs next I think. just to be sure it isnt the clutch.

[madmtnmotors]

Ok.. so, fully changed the transmission out for a used good one, fixed the 4th gear issue.  Also installed brand new clutch plates, clutch feels great now, very grippy.  However.  The bike still tops out at 100 mph on the speedometer, at 8k rpm.  feels like it could go alot faster if it werent at redline.  I have no idea what could be up now.. stock 18-48 sprockets.  Clutch grabs right away in every gear with zero slip.  I dont know..

What year is your bike and what year transmission did you use? I'm pretty sure some model years used different transmission ratios than others and might benefit from a sproket tooth count adjusment to match the transmission ratio. Haven't researched the scope of the difference between model years, the trans ratio differences might not be enough to matter. Refer to Old Scrambler's post regarding jets. You might be getting all that's there and need finer tuning to find more.

[Leanier]

The bike is a 1973, I think that the transmission that is in it is from a 1974.  It could easily go faster if it wasn't already at its redline.

[Leanier]

Does anyone know what else could cause the clutch to slip.  I replaced the springs and friction plates, and adjusted it.  No help.  The bike is at 5k or so at about 75-80mph (or around there), then 8K RPM at 95mph... Definitely doesn't seem right.

[scrapvalue]

I don't think you have a clutch problem.
If it was slipping that bad, you would feel it under acceleration in lower gears. Especially in 3rd and 4th.
I would suggest getting a smaller rear sprocket. Drop at least 2 teeth. You will loose a little low end pull, but receive a higher top speed.