4speed in a CT70 KO original 3speed

[Mini Habit]

I just finished a resto on a 1970 KO 3sp, when going thru the engine I thought it would be fun to put a 4 speed trans and 4 down shift drum in it, all parts are nice Honda from a doner KOH engine I had with trashed cases. I wanted to keep it semi auto, sort of a sleeper thing, can't tell it's a 4sp unless you rode it, and keeping it true to original. I rebuilt the original 2-disc semi auto KO clutch. The engine runs great and does shift thru all 4 gears...but, when dropping from neutral to first there is a noise like the clutch isn't disengaging enough with a little gear clash, also when downshifting the shifter sometimes hangs in the up position and I have to bump it down with my foot.
I have built many of these engines, I'm certain the clutch is adjusted properly. Has anyone else tried this modification with any luck? I'm thinking possibly the 4sp drum has a shorter throw and not getting enough clutch action? Hoping for an easy fix, never had the shifter hang up like this does.

 

[OLD CT]

This mod works however you need to use brand new 4 speed shift forks and pins. Mine works fine.

 

[Mini Habit]

This mod works however you need to use brand new 4 speed shift forks and pins. Mine works fine.

It's the same 4sp 4down drum from the same H engine doner that the trans came out of, I did replace forks, pins and keepers with new OEM parts. I rode it again for a bit today and it seems to be getting better. Glad to hear that it should work, was afraid it was known not to, Thank You.

 

[Robert thran]

I’ve also done that mod but it was to a z50…it worked great

 

[Mini Habit]

I’ve also done that mod but it was to a z50…it worked great

Thanks for the reply Robert. I'm hoping for the best, and it's breaking in well.

 

[Deoodles]

I’ve done that build and it worked okay for a while but it never shifted great. Count on premature fork wear imo.

 

[kirrbby]

I don't know of any reason that it wouldn't work great. I have a 4down semi-auto Z50, that uses a ATC70 trans.
No reason for the shifter to hang up for sure. I'd check that out. Maybe a bent shift shaft. Maybe the little arm that goes on the shift shaft that disengages the clutch isn't indexed properly..? Maybe worn parts...the bits that sit on top of the clutch to disengage the clutch when you shift..?
If it doesn't loosen up and work great for you, pull the clutch cover for a look.

 

[kirrbby]

If the shift lever is installed on the shift shaft when you assembled the clutch side of the engine...
Sometimes it can partially spin the shift shaft...(like if the lever is hitting the table, or your build box). Then when you point the small lever to the center of the clutch and assemble the right side...you don't realize that the clutch shaft is partially spun because the lever is hitting the floor. If that makes sense. That might cause the shift lever to hang up.
Short version...check to make sure that little lever points straight toward the center of the clutch.

 

[Tripod]

Swapping out the three speed is a great mod. Even if its still semi auto i like it.

 

[OLD CT]

Nice, if you want to smoke a cigarette and drink a beer.

 

[Mini Habit]

It's fixed and works great, can't say for sure what happened but pulled the cover, reset actuator arm, and double checked everything, and took extreme care setting the cover, it was running a little rich and lowered the needle a notch and it pulls better after half throttle. First gear is short, I'm running a 14 drive and 42 driven, anybody have a sprocket combo they like?

 

[kirrbby]

14 45 is stock for a 4 speed...so you should already be taller than stock...too tall. Is your engine still 72cc? Stock Trailwings too?

 

[Mini Habit]

I may be expecting too much out of it, I've only ridden it on the road in my subdivision and not wide open yet, when the trails here dry out, I'll see how it feels off road, that's what I built it for. I haven't ridden a bone stock CT in a long while, yes it has new Trailwings it sure doesn't feel too tall though. I have another rider to scratch my go-fast itch, a modified Z50 soft tail with 140 YX. Thanks for your help, it shifts smooth now.

 

[Mini Habit]

This mod works however you need to use brand new 4 speed shift forks and pins. Mine works fine.

Your 'right about forks and pins, if you're down that low it's a quality thing. I'm curious about what carb you run on your 140. I have a YX that came with a Mikuni that's hard to tune.