Oops! Oh Oh... CB750 motor might be hurt

[Garage Guerrilla]

Riding in today, I was passing a transport aggressively with my 1978 CB750K when the transmission jumped into neutral. As I was "well into" the throttle and turning somewhere around 8500 rpm the tach instantly bounced off the top (11000+). The bike still runs, but there is an loud top end tick. Anyone have any suggestions on where to start?

 

[Garage Guerrilla]

What, no replies? Help a brother out!! ???

 

[Skyeye]

The first thing I would do is run a compression test. If you bent a valve then this would be one way to find out.

Scott

 

[Garage Guerrilla]

The bike does start and run well outside of the noise, a compression test is a really good idea! Thanks.

 

[Garage Guerrilla]

Good news! Dug into the bike starting with removing the tappet covers and when I removed the 8th one (of course) there was a bit of "hammer" damage on the underside. I immediately checked the tappet adjustment screw and it was backed WAY out. I reset all the valve tappet screw clearances and "voila" back in business!! I was extremely happy to see I didn't kill the motor. I can only guess that when it overrevved that it backed out the adjustment screw? No stripped threads, no obvious damage, and the screw was still in good shape. I however did take the bike out for a good rip and again it jumped out of 3rd into neutral on me (once). I am starting to think I may be missing part of a tooth or something on third though...

 

[Skyeye]

GG,

Good news indeed! When I got my CB750K it wasn't running and I noticed the same "ding" on the inside of the tappet cover. Unfortunately I had a bent valve because the previous owner just ignored the noise and didn't check the valves. It pays to take notice when things don't sound right.

Scott

 

[Roustabout]

I however did take the bike out for a good rip and again it jumped out of 3rd into neutral on me (once). I am starting to think I may be missing part of a tooth or something on third though...

Slipping out of gear is not totally uncommon for old 750's (especially in modified motors that make serious power and bikes with tired trannies). The dogs on the 750 are straight and not undercut so they don’t have a positive engagement. A performance upgrade was to have the dogs undercut. Your bike could have a combination of worn dogs and worn shift forks(or you have a lazy left foot ).

 

[Garage Guerrilla]

Slipping out of gear is not totally uncommon for old 750's (especially in modified motors that make serious power and bikes with tired trannies). The dogs on the 750 are straight and not undercut so they don’t have a positive engagement. A performance upgrade was to have the dogs undercut. Your bike could have a combination of worn dogs and worn shift forks(or you have a lazy left foot ).

All three are quite possible!

 

[Rocan]

All three are quite possible!

you should seriously consider checking over your shift levers and tranny equipment.

ALSO if i remember correctly you can modify the shifter plate for easier engagement and get rid of missed gears. try google- its really quite an easy mod and only requires a dremel and half a brain.

 

[Garage Guerrilla]

you should seriously consider checking over your shift levers and tranny equipment.

ALSO if i remember correctly you can modify the shifter plate for easier engagement and get rid of missed gears. try google- its really quite an easy mod and only requires a dremel and half a brain.

Thanks, I will take a look.