Topic: Transmission bucking issue ...

[ThePrussian]

So I guess I am here for some confirmation ... I have never intro'd myself officially sooo ... hi

Bike: 72 (mostly) cb750K2 bare steel cafe from hell, 27k+ mi

Getting my bike running after this winter's sleep has been quite an adventure.  I was chasing what I thought was the engine missing, but in the end appears to be transmission.  It basically skips, really quickly, in first and second.  There is a lot of clatter when it happens and for the longest time I couldn't tell if the engine was winding up or not, its just sort of a *bang* ...  *bang bang* upon acceleration.  Bike doesnt move when it happens and needless to say it feels deadly when leaned over.  When I say it doesnt move I mean the power momentarily cuts out.  After going through everything it took a while but now I can tell in the frac of a second that YES the motor winds up.

Its a really hard thing to describe but I'm hoping someone will be like "oh yeah THAT."  Its basically like the clutch slipping but very harsh and mechanical when it happens, and quickly, machine gun esque.

SO, since the ENTIRE everything has been gone over (because I thought for the longest time my 1+4 coil was missing) I assume it is now time to pull motor and split the cases.  (because I will get asked; new coils, points, condensers, static timed dead on, cam chain adjusted, valves adjusted, carbs cleaned, filters cleaned)

I plan to leave the top end as it is, the bike runs real strong. 

Does this machine gun like slipping sound familiar to anyone?  Anything I should be looking for in particular?  ie Have my dogs gone for a walk or should I be looking for a replacement shift fork?  Trying to get an idea of what Im going in for from someone w experience.

Is there a chance in hell that its the steel clutch plates?  I replaced the friction plates only last year. 

Before this started I would occasionally go over a square edged harsh bump and it would fall out of second into neutral, also it likes to randomly miss second and go to neutral from the moment I got it.

I have been reading this forum all morning and found a ton about tearing it down but would like to have a better idea of the WHAT and WHY before I crack it open.

Thanks in advance to anyone w some input

[calj737]

One simple thing to rule out before you replace the transmission is the sprockets. Have you inspected/replaced them? They're on the outsid, so it's a quick check. If you find nothing wrong, I can only see a split case in your future.

[flybox1]

+1 on sprockets/chain
also, drop your oil pan, and look for metal shavings.

[ThePrussian]

One simple thing to rule out before you replace the transmission is the sprockets. Have you inspected/replaced them? They're on the outsid, so it's a quick check. If you find nothing wrong, I can only see a split case in your future.

I wish it was, but those (both sprockets/chain) are new as of last year. 

[ThePrussian]

+1 on sprockets/chain
also, drop your oil pan, and look for metal shavings.

Pan was off last year and cleaned the screen ... there was def SOME stuff (small shavings) there, but seemed an acceptable amount for age and known abuse *cough cough*.  Did an oil change recently and magnetic plug wasnt clinging to anything huge ... again, just a small amount of stuff.

[calj737]

And you're certain you bought compatible chain and sprockets? 530/530 or 630/630. Haven't mismatched anything?

[Bodi]

Most likely it's the drive chain jumping. In the transmission bent shift forks or worn tips, or worn drum grooves/fork pins can cause what you describe. Broken gear dogs are the usual result if you allow this at high throttle.
I suppose badly stretched primary chains could jump but I've never seen that. Super worn bearings on the primary and/or main shaft could allow the primary gears to jump as well?
So if you completely rule out the chain, it's time to check inside. Removing the clutch cover gets you at the primary gears. You can see/feel the chains through the bottom once the sump is off.

[ThePrussian]

Thanks for all the replies!!

I will double check my chain when I get home.  Although if its the chain jumping I am not sure how I could tell for sure ...

[70CB750]

What gear is it happening in?

[ThePrussian]

So I found my receipt and unless bike bandit screwed me all my chain stuff is 530. 

What gear is it happening in?

Its only in first and second, and Im beginning to think its those two gears kinda falling into neutral for a second

[70CB750]

So I found my receipt and unless bike bandit screwed me all my chain stuff is 530. 

What gear is it happening in?

Its only in first and second, and Im beginning to think its those two gears kinda falling into neutral for a second

My 2nd is doing it for two years or so.  I am just shifting through it whit plans to rebuild once my K3F is safely on the road.  Worn dogs I suspect, my 1st works well.

[ThePrussian]

So I found my receipt and unless bike bandit screwed me all my chain stuff is 530.  Chain slack is at about 3/4", my awesome Emgo shocks barely move under my 170 lbs anyways.

What gear is it happening in?

Its only in first and second, and I'm beginning to think its those two gears kinda falling into neutral for a second, just kinda bouncing in and out?

Took it out again and its missing 2nd pretty consistently, I kinda have to hold it up to get it to go, can feel the teeth chattering before it engages.

Does this sound like shift fork to anyone? 

[Old Scrambler]

Yes to shifter forks..............so you may find a badly worn or notched drum..........and one or more worn gears.  Upon re-assembly, you may need a shim or two........

[ThePrussian]

Yes to shifter forks..............so you may find a badly worn or notched drum..........and one or more worn gears.  Upon re-assembly, you may need a shim or two........

SO I have been reading everything I can.  I don't see anything about shims, anywhere though.  Can you clarify? 

I bought a set of gears from a 76 off ebay w good dogs (sit ubu, sit) Currently looking for a set of good forks and a drum.  Stuff is cheap enough I am just stockpiling.  Whatever doesn't get used will become a lamp.

Can someone save me the headache and tell me where they have bought bearings from in the past?  Also, IF it seems ok, can I leave the right countershaft bearing in place?

It's all coming back apart this winter, my only goal of this repair is to make it to the Deadbeat event the weekend after this one. 

Keeping this thread going to stay motivated.  Banged out a C hook last night so step one is done

[flybox1]

Yamiya

[ThePrussian]

Yamiya

WOW that site just blew my mind, thanks for the tip!!!!!

I am leery of how long it would take to get stuff from Japan though ... gonna give my local stealership a try tomorrow.  This trip is coming up fast yeeeeesh

[calj737]

Delivery in about 3 days. Consider that a mind blowing reality!