1972 CB750 k2 grinding gears in neutral?

[ejohnston]

I recently changed the oil and now am having some issues...
whenever I put the bike in neutral with the engine running I can hear a clicking noise like a gear is barely engaged. when the motor is running, in neutral, and the bike is up on the center stand, the rear wheel turns slowly and gradually builds up speed.... (with the motor not running I can smoothly shift it into all 5 gears and it even goes into neutral fine.)

When I try to drive the bike and I put it into gear it will pop in and out of gear unless if I ride the clutch and ever so slowly release the lever, then the bike will ride flawlessly in that specific gear.

I think the clutch is not adjusted right but I think it is a completely unrelated problem because clutch adjustments shouldn't affect neutral at all should it?

Is this for sure a transmission problem and is there anything I could do to diagnose the problem or should I just take it to a professional motorcycle mechanic?

Any sugestions as to what could be wrong? and is there anyway I could diagnose the problem by taking off the clutch cover and install a new set of plates?

 

[cyclebuster]

you may have a bent shift fork. the wheel spinning is normal, from the oil pushing against the gears. make sure your clutch is adjusted by the book.

 

[ejohnston]

@cyclebuster, could the shift fork be fixed or diagnosed without having to crack the engine case? would a good place to start the diagnoses process be ordering a new set of clutch plates and change those by the book?

 

[cyclebuster]

no it needs to be pulled and inspected. The clutch plate thickness has no bearing on the operation of the gear box. and adjustment should have no bearing either, but set it to specs anyway, then, put the bike in first gear fully and get it rolling. accelerate hard and let off a few times. then go into 2nd and do the same, all through the gears. If it pops out of just 2nd, there is an internal issue. if it follows through every gear, your detent may have failed.

 

[ejohnston]

no it needs to be pulled and inspected. The clutch plate thickness has no bearing on the operation of the gear box. and adjustment should have no bearing either, but set it to specs anyway, then, put the bike in first gear fully and get it rolling. accelerate hard and let off a few times. then go into 2nd and do the same, all through the gears. If it pops out of just 2nd, there is an internal issue. if it follows through every gear, your detent may have failed.

I just did your test and I think it is the detent because it pops out of every gear equally and some times it doesn't actually pop out of gear will just randomly start to grind and then spontaneously correct itself. Does the detent require the case to be cracked in order to be inspected/replaced?

 

[dirtdigger]

Take the shift lever cover off. All the detents are under that cover. Take a look for anything out of the ordinary. The plate on the end of the shift drum tends to get loose and can cause shifting problems, the plate is part of the detent setup. The screw in the center that holds the plate on can work loose even though it is supposed to be staked to keep it from backing out. Take the cover off and take a picture before you take anything out and we can take a look and see if anything looks out of place.

 

[ejohnston]

Take the shift lever cover off. All the detents are under that cover. Take a look for anything out of the ordinary. The plate on the end of the shift drum tends to get loose and can cause shifting problems, the plate is part of the detent setup. The screw in the center that holds the plate on can work loose even though it is supposed to be staked to keep it from backing out. Take the cover off and take a picture before you take anything out and we can take a look and see if anything looks out of place.

thanks, I attatched a photo and a video of me wiggling that piece so you can see if that amount of play is normal

 

[ejohnston]

[video]http://s276.photobucket.com/albums/kk40/erjohnst2/?action=view&current=IMG_2985_zps9c4adc9b.mp4[/video]

 

[dirtdigger]

Cant seem to find the video. Just by looking at the picture I dont see anything wrong. If the plate is loose at all then it is too loose, it should be tight. The arms with little wheels are detentes that ride on the little pins. If the plate that holds the pins in is loose it will cause the detentes to be in the wrong spot or cause really sloppy detente engagement. The part the gets me is you said you changed the oil then this problem started.

 

[ejohnston]

Cant seem to find the video. Just by looking at the picture I dont see anything wrong. If the plate is loose at all then it is too loose, it should be tight. The arms with little wheels are detentes that ride on the little pins. If the plate that holds the pins in is loose it will cause the detentes to be in the wrong spot or cause really sloppy detente engagement. The part the gets me is you said you changed the oil then this problem started.

hmmmm not sure why the video link didn't work, Today i am going to try to tighten the detent so there is no play at all. Ill let you know what happens, and changing the oil may not have been the culprit it may have already been loose and trying to drive it after the oil change just tipped the scales, I had let it idle in neutral before the oil change but it hadn't been driven for over a year.

 

[ejohnston]

Cant seem to find the video. Just by looking at the picture I dont see anything wrong. If the plate is loose at all then it is too loose, it should be tight. The arms with little wheels are detentes that ride on the little pins. If the plate that holds the pins in is loose it will cause the detentes to be in the wrong spot or cause really sloppy detente engagement. The part the gets me is you said you changed the oil then this problem started.

here is a few more pictures with the gear shift side plate off. I added arrows so you could see the play in the "gear shift positive stopper"

there was also a slight amount of play in the gear shift side plate as well but not as much.

Also I dont know enough about how this mechanism works but I noticed my parts are slightly different than they look in my 72 clymer's manual (as circled) so im not sure if this is how its supposed to be or has someone put on an aftermarket part from another year?

 

[dirtdigger]

You have all right stock parts in there, I have never actually seen any parts like they show in that manual, I believe the picture is very early production 750 because clymer copied the photos from the original factory Honda manual. That side play in the positive stopper is not critical and it is not a close tolerance item. I had to look twice at your photos because the shifter arm is on the wrong side of the positive stopper tab but I am assuming you moved it that way for taking your pictures. I honestly dont see anything wrong. Sorry to say but it is looking more like you have an issue in the transmission itself.

 

[ejohnston]

Thanks for the help and yes I moved the shifter arm for the picture, so yeah the more I think about it, thats what I have concluded as well. One last thing do you think think it would be cheaper to find a different, running motor with a working transmission or paying an honest, average speed mechanic 20 bucks/hour for a rebuild?

 

[dirtdigger]

If you are mechanically inclined you can do the work yourself as long as you pay attention and have a decent manual. They are not terribly hard to get apart there are just a couple of things to look out for when going back together. I would be more inclined to find someone that has had a 750 apart before rather than a guy that works on harleys or cars. If you can find a good running motor it may be in your best interest to do so but unless you see it run for yourself you wont know for sure what you are getting.

 

[ejohnston]

Is there a way to inspect/have access to the shifting forks without taking the motor off? (I coudn't tell from my clymer manual but (like through the oil pan)) ive worked on a bike before where you could but this is my first time messing with a cb750

 

[dirtdigger]

Oil pump would also have to be removed but you wouldn't have any real good look at anything in the shift forks. Case will need to be split. You can split the case without having to remove anything on the top end.

 

[ejohnston]

You can split the case without having to remove anything on the top end.

this is the best news Iv'e heard all day! Ill have to buy a honda shop manual, the only thing my clymer manual shows is going through the top (during a complete teardown)

thanks again for your help!!!

 

[dirtdigger]

Flip the motor upside down and you can do any work to the transmission etc and not touch the top end, crank, etc. Let me know if you need any help. Just to throw this out there, and I cant stress it enough do not use RTV silicone on anything on the 750 motor, get some good Hondabond or Threebond #1104. RTV will soften from exposure to oil and will break off inside the motor and plug the oil passages to the cylinder head and ruin the cam and cam bearings...it has happened to many people and I try to tell anyone that is taking there 750 apart. You will need this for the case halves when you go back together.

 

[ejohnston]

haha man thanks for that pointer! I actually coincidentally bought a tube of RTV silicone today but luckily now that I know, i will dodge that bullet and buy some prober silicone... and will do, im going to start that project this coming weekend and probably will run into a question or two, so thanks again for your time and ill let you know what I find...

 

[ejohnston]

I finally got it cracked tonight but being so late haven't had time to inspect it to much yet... I definitely see the results of the problem but haven't found the cause yet. My next question is what all should I replace? all new shift forks? shift fork drum? and the gear that is ground down on the side? any way im sure I will know more once I have time to study it more I just want to make sure I actually fix the problem instead of putting in new stuff for the same problem to keep persisting.


(sorry for the bad photos all i had was my phone camera)

also any tips on cleaning it? it okay to spray the whole bottom case with water and soap or is that too risky for rust?