Topic: CB750 2nd Gear Troubles

[cbbrat]

Hello all,
New to the forum. I recently purchased a 1973 Cb750. Runs great. However, yesterday a problem developed. At first,  when in 2nd gear around 5k RPM, it would babble and power would climb no higher. Then the problem developed into something else. Now, anywhere in 2nd it will maintain rpm but will grab with a loud clink. Hard to describe as I'm new to bikes, so any questions or help will be appreciated.
Thanks in advance

[ekpent]

Sounds like maybe the beginning of the dreaded 2nd gear loss. You may have a broken ring clip on the shaft causing some gear movement or the shifting fork could be worn in conjunction with the rounding if the gear engagement dogs.
  You say only second gear is the issue under a load. Has it popped completely out of gear yet ?
  If that may be the case after other troubleshooting there are things related here in the google search.  Here is an older epic 13 page thread with some good info and pics etc. http://forums.sohc4.net/index.php?topic=59158.0

[cbbrat]

Dreaded 2nd gear loss? That doesn't sound good. It only does it in 2nd gear and has not popped out completely. Is there anyway to avoid total loss or is a replacement in order? I honestly have no clue what to do, so any tips are greatly appreciated.

[goldarrow]

Could be Shift forks, gear dogs or shift drums, or others.  Be prepared to split cases.

[ekpent]

 I just updated my post with a thread link. Normally when that problem occurs  the bike will slip into a gentle false neutral when riding or accelerating in second under a load.
   It may be something as simple hopefully as a clutch adjustment or a shifting mechanism inspection.
  I don't want to freak you out yet as there may be a simple cure.  That thread though is a pretty good starter.

[cbbrat]

Thank you for the replies.
The bike does seem to go neutral, then grab with a hard jerk and mechanical noise. It will ride fine for a minute and do it again.

[ekpent]

That may be the gear dogs not fully engaging and remaining locked together.

[cbbrat]

Ok. In your opinion, what would the first and consequent course(s) of action be?

[goldarrow]

Thank you for the replies.
The bike does seem to go neutral, then grab with a hard jerk and mechanical noise. It will ride fine for a minute and do it again.

Sounded like my k5, jerks hard and frequent, then popped out of gear.  I had to replace shift drum, and may be forks too. Can't remember.  It's in my k5 build thread

[goldarrow]

Ok. In your opinion, what would the first and consequent course(s) of action be?

Oil change, adjust clutch, change clutch, Don't think you can do much else

[cbbrat]

I'll have to look over the threads. I changed the oil and only have approximately 500 miles since. I have a clutch cable coming in the mail. I suppose I won't lose my marbles just yet until I adjust the clutch cable. Lol   

[ekpent]

Here is a thread dealing with a bad main shaft clip I also alluded too but this problem is more pronounced when the bike is tilted to the left and not necessarily  2nd gear only related. Still a good read for you.   http://forums.sohc4.net/index.php?topic=86429.msg972479#msg972479

[cbbrat]

Again, thank you. I'm proud to own an old non-harley. I hope to work towards becoming proficient at knowing this bike. As I said before, all tips, tricks and advice are much appreciated.

[scottly]

Thanks for the link to the circlip thread, Eric. Saved me a bunch of time.
Brat, if the trans pops out of gear into a false neutral under heavy acceleration, it is generally caused by worn dogs. If the trans seems to jump in and out of gear and/or seems to lock up for an instant (the clink sound), it may be a circlip problem. 

[cbbrat]

Circlip problem? Sorry, new to the working on bikes world.

[hotdog]

The circlip referred to "pops" out of its location groove on the transmission shaft. Usually a symptom of gear dogs bring worn. If your gear dogs are worn, under load the transmission dogs will be forced apart, ultimately causing the bike to find a false neutral. When this happens it can "pop" the location circlip from its groove as well as bend shift forks.
Sadly it doesn't get better and the cases need to be split to repair the issue.

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[70CB750]

My K0 started doing this more than a year ago.  First it would jerk in second during acceleration after deceleration but eventually it developed into jerking anytime I shift to 2nd gear. 

Since I wanted to finish my second bike first (being without a running motorcycle is not acceptable)  I avoid the second gear.  For turns I downshift to third, when shifting up I shift twice from 1st skipping 2nd.

It takes a while to get used to it and 3rd does not accelerate as fast as second, but it is quite doable as temporary solution.  The K0 will need a complete frame up, but I want the K3F on the road first.  \

That said, I would ride the season and got myself ready for a transmission rebuild during the winter.

[calj737]

Have a look at this schematic. It might help you to visually understand the components being referred to. Not sure if this is the correct model year for your bike, but it will be similar.

http://www.cmsnl.com/honda-cb750-four-k3-usa_model481/partslist/E++18.html#results