Topic: 1975 Cb750 k chain break - now shifts badly

[cado007]

1975 Cb750 k  chain break - now shifts badly
I recently acquired a 1975 cb750 k - PO had a chain brake - it cracked
case he took it out and had it welded and now it shifts rough.
He got frustrated and sold it to me cheap. It is pretty sweet -
pristine tank and good condition original pipe. I did the normal thing
cam chain valve new plugs new leads. It did not have oil so I dropped
oil pan and lowered bike several times to each side to see if I could
get debris out of gear if that is what problem is. I put new oil in
and the tried driving. It does not like to go into first makes noise -
will start and run in 2 nd all day if you pull up with your toe hard
you can get it in 3 and 4 th. I was running on an aux bottle with just
back brake so I never made it to 5th. Anyway I do have a spare 76
cb750 k motor  but this one runs awesome even with the pods on it. He
gave me the air box which will be going back on and all original
parts. Anyway do I need to remove lower case like this thread:
http://forums.sohc4.net/index.php?topic=105038.0.  or is there a
simpler remedy to try. It would be my first time opening a 750 case
although I have done a cb350  I do have Hondamans book to haha -
Anyhow any advice is appreciated thanks Damon

[HondaMan]

Ya never know: you might get lucky and discover the shifter drum's center screw came loose, which caused the mis-shifting in the first place, which maybe helped cause the case-crash...

Pull off the (left) transmission cover and see if the top of the shifter drum is loose: the screw tended to come loose in the K4/K5 series engines because it wasn't staked exactly correctly in many engines.

[cado007]

Thanks Hondaman!  will do

[scottly]

Another common mistake on the 750 is the neutral switch on the bottom of the engine. It fits into a groove in the drum, and keeps it from moving from side to side. If it isn't fully seated it will cause shifting problems. Does the neutral light work properly? If the switch isn't seated, the light will stay on, even when the bike is in gear.

[cado007]

Thanks Scotty - I will check that .
The neutral light does come on in neutral and goes off when in gear
It did while driving go into what I think was a false neutral in between 2 and 3 and light did not go off but it acted like in neutral.
To actually get in neutral I have to pull shifter up with my hand. I literally just got the bike so I have only done it once successfully and tit was  some work to do.
I will look under and see if neutral switch is oriented like your photo.
Thanks

[scottly]

To actually get in neutral I have to pull shifter up with my hand. I literally just got the bike so I have only done it once successfully and tit was  some work to do.

Do you mean the lever was hard to move, like something is binding?
If the switch checks out ok, the next step is to pull the shifter side cover. When pulling the cover, press on the end of the splined shaft with a thumb to keep it in place, while pulling the edges of the cover with your fingers.

[Don R]

 Is the clutch dragging? Put it in gear and roll it back and forth to see if it tries to turn the engine or even slow down the roll.

[cado007]

So per Honda Man's suggestion I removed cover and the screw did not look loose - the one close up part phot looked maybe bent or not sure if that is how it is. I went ahead and adjusted clutch . So now I can get it into neutral easy - I put it into down what should be 1st and it still spins ( like it is in neutral) I can pull up into second which seem to work and 3rd  -- but that's about it ( don't want to yank too hard with cover off. So any insight or is the next step removal and removing lower case( which I have never done but I am willing). Also I did compression test 1 168 2 150  3 156  4 170.  - 2 is a little out of 10 percent but other than me running for 10 mins I don't think it has be ran in over a year and I did test cold. I do have a spare 750 engine but this one seems good if I can sort out transmission.  thanks for any advice Damon.  also the neutral switch looked correct and I will try rocking back in forth in gear

[scottly]

The bright silver spring on the linkage looks much stronger than the stock spring? I don't know if that's the problem...

[rotortiller]

You should be able to fairly easily rotate the shift drum through all the gears while spinning the rear wheel by hand using a screwdriver with the shift mechanism removed (ratchet and wheels). Also check the stack up at the shift drum wheels and neutral detent. Could be a bent shift fork or damaged drum, this will tell you however it is unlikely. The drum should have no real end chuck in and out to speak of.

[scottly]

Notice how thin the wire is on this stock spring compared to yours. The end of the arm that engages the pins on the drum has to be able to drop down after each shift to clear the pins in order to engage the next pin. You should be able to push the lever down with your finger until it hits the shift stopper under the drum.

[cado007]

Hi Scottu,
I do see that spring difference  - I can easily push down with my finger tho

Rotortiller I can't pull the shift drum in or out ( is that end chuck?)
Also it turns with screw driver maybe one rotation and stops with rear wheel spinning and shifter disengaged \It looks like PO had this off trying to check so maybe he changed spring but - the problem seems to be (spring notwithstandin) that shift drum does not turn smoothly
I just tried again and now it drum will not turn with screwdriver
I did put in gear and rock and it did seem to turn engine it rolled freely with clutch pulled in - I think I was in 2nd
I attached pic of welded chain break area - again I believe the PO believed some debris fell in the break area and is causing the shifting issue and even tho he removed engine to have welded he did not open lower case.
I am new to this though so I could be wrong and I am learning - thanks for all the tips.

[bryanj]

Debris inside is possible as is excess weld material, it is common to find extra neutrals when trying to turn drum with screwdriver

[scottly]

Is the small roller riding on the edge of the thick disc on the end of the drum? In your first pic, it looks like the roller is sitting on the inside of the disc, or is it an optical delusion?

[cado007]

hi Scotty
the roller is in the right place must have been the photo.
I made some progress I tool the mechanism all the way off and I could turn the shift drum while with screw drivers spinning wheel pretty well
I put the mechanism back on and I could get it into neutrall with the light coming on pretty regularly it seemed to go into first properly too and seemed to go thru the gears but I was pulling up with my hand for  4 th and 5 and spinning back wheel
should I maybe just try to drive it like this and put some miles on it.
I rode it up and down driveway before removing cover and I had to start in 2nd and basically could just get it in 2nd and 3rd .
It seems like it has gotten netter - remeber I have only had this a few days
thanks

[bryanj]

Frequently to get the higher gears both gearbox shafts need to be turning

[HondaMan]

Hi Scottu,
I do see that spring difference  - I can easily push down with my finger tho

Rotortiller I can't pull the shift drum in or out ( is that end chuck?)
Also it turns with screw driver maybe one rotation and stops with rear wheel spinning and shifter disengaged \It looks like PO had this off trying to check so maybe he changed spring but - the problem seems to be (spring notwithstandin) that shift drum does not turn smoothly
I just tried again and now it drum will not turn with screwdriver
I did put in gear and rock and it did seem to turn engine it rolled freely with clutch pulled in - I think I was in 2nd
I attached pic of welded chain break area - again I believe the PO believed some debris fell in the break area and is causing the shifting issue and even tho he removed engine to have welded he did not open lower case.
I am new to this though so I could be wrong and I am learning - thanks for all the tips.

It's not unusual for the drum to get "stiff" when trying to turn it 'by hand', so to speak. The tranny gears just aren't spinning like normal, and their mass tends to try to prevent shifting when the engine isn't turning.

Your shift drum appears to be properly staked! The improperly-staked ones didn't have the notches extending into the top plate very well (or at all), which let the center screw come loose over time.

About that closeup of the welded area in front of the sprocket: is the upper case welded onto the lower case there? I've seen that happen when someone attempted a repair by just welding the hole closed again. Then the cases can't be separated anymore.

[cado007]

Thanks HondaMan
well.  Should I just put it back together and to try to put some miles on it and see if things loosen up?
The weld is in the top away from the case seam
Thanks Damon

[HondaMan]

Possibly: I once repaired a K1 engine here from a local rider who rode it over 25,000 miles with only 1st, 3rd/4th/5th gears because the "L" shift fork got badly bent when he nudged the bike off its sidestand in his garage with the bumper of his car one night. The bike fell over with the shifter on top of a paint can, and was left there for weeks before he noticed it. When Spring came, it had no 2nd gear because of the bent fork, so he just rode it that way for 20+ years before bringing it to me to fix. All I did (besides a top-end rebuild, it got rusted inside from an 8-year unused period) to the tranny was to replace the bent "L" shift fork, and he rode away happy, still riding it around here. All 5 gears worked again, then. I suspect yours might have this same fork bent: they are getting hard to find these days.

[scottly]

I made some progress I tool the mechanism all the way off and I could turn the shift drum while with screw drivers spinning wheel pretty well
I put the mechanism back on and I could get it into neutrall with the light coming on pretty regularly it seemed to go into first properly too and seemed to go thru the gears but I was pulling up with my hand for  4 th and 5 and spinning back wheel
should I maybe just try to drive it like this and put some miles on it.

Sounds like you may have fixed it be removing the mechanism and replacing it? Yes, take it for a ride and see how it works!
PS, it's always easier to find neutral when the bike is still moving than when it's stopped.

[rotortiller]

Rotortiller I can't pull the shift drum in or out ( is that end chuck?)

  Yes.

I made some progress I took the mechanism all the way off and I could turn the shift drum while with screw drivers spinning wheel pretty well

  That is a very good sign, likely once you got the shift mechanism sorted out it solved the situation. Fingers crossed for a good test ride! That weld job has a concern if it was done without splitting the crankcase halves due to potential debris entering the gearbox, just like someone mentioned.

[cado007]

I had some help to spin the rear wheel and everything is going into gear with both the screw driver and gearshift - it easily goes back into neutral
One thing I noticed is that it does not always go into 1st - it will go into 1st pretty consistently if I pull gently away from bike on the little pulley with red arrow.
does this pulley thing look ok - is there an adjustment that will help or  is it worn out and causing the issue, If so I could try ro order from cmsnl- I believe I am close - I have to wait for a new gasket before I can test and lastly those stator wires just hang on the top?
I also only have 1 dowel in the bottom left if that is correct what is best way to get gasket to stay back own when installing.
Thanks
Damon

[rotortiller]

does this pulley thing look ok

No it does not look right, the pulley for the neutral detent should roll on the edge, not the arm sliding on the edge.

[cado007]

Any idea what to do to fix?

[rotortiller]

Check the manual for the correct stack up of parts first. If that does not work shim it out at 17 with a suitable washer. It may be that item 7 is not tall enough? Maybe wrong part from 13 elsewhere on the diagram, not sure if they can be #$%*ed up and swapped. I am not there to evaluate shimming, you will have to decide if it's acceptable or not.