1970 Rupp jackshaft/gear ratios

#1
Finally got the 1970 Rupp Scrambler out of the basement for a maiden test run. It has a H35 on it. (Non Stock Wheels etc) Running 41 chain. Ratios go: 10 tooth clutch to 14 tooth jackshaft in, 12 jackshaft out to 52 tooth rear sprocket. Despite the engine not running smoothly (I don't have a 20 degree manifold) and possibly the fact that the clutch is maybe fairly worn out, maybe not. The bike barely moved. Am I way off in my ratio's?
Thanks, here is a video:
 
#2
Your entire problem is you have a super worn out clutch as well as a worn out band brake. Engine seems to run fine if you set it and leave it alone.
You will never get power from the engine to the rear wheel with that worn out clutch, buy a new one.
 

SAT

Active Member
#4
I can’t run the video just now. 6:1 ratio should be runnable. Every clutch needs some TLC, or replacing. Most bike drive trains have some friction/ binding that needs attention.
 
#6
So after installing a new clutch and the Tecumseh 3.5 running decently (still breaks up a bit, but certainly good enough as a test) the Rupp still bogs. I don't think it is underpowered although I haven't had a Minibike with this small and engine since the 70's, I think it is the gear ratio...any ideas?
 

SAT

Active Member
#7
Engine still needs attention if it breaks up a bit. Could be several things. I would sort em out and check the compression. It could be as simple as a cracked intake gasket. I still think that powerplant should pull decent at 6:1. Get er runnin proper and think about 7 or 8:1, if thats what it takes. You could easily drop to 10t jackshaft out.
Just my thoughts on what I would do.
 
#8
Thanks SAT, I might try the 10t output. But I'm also wondering that the engine isn't quite right because I don't have the 20 degree manifold. Appreciate the help.
 

SAT

Active Member
#9
Thanks SAT, I might try the 10t output. But I'm also wondering that the engine isn't quite right because I don't have the 20 degree manifold. Appreciate the help.
Yup, I don’t see how that float carb could perform well sitting at a constant angle. Someone with direct experience hopefully will jump in on this problem. Why don’t ya make one? It would be a great little project.
 
#10
Egad, your idle speed is like, 3500 revs.

You need to adjust it so it's just barely ticking over; "thump-thump-thump"--and it's obvious the clutch is engaging ever so slightly, as evidenced by the wheel spinning when you lifted the rear. Not enough to make it take off on its own though. A lot of friction and wear there, just sitting there.

:eek:
 
#11
I realize the idle is super high, and I usually adjust them as low as they can go. I'm going to attempt to make a 20 degree manifold and hopefully get some more control over the carburation. Thanks.

Egad, your idle speed is like, 3500 revs.

You need to adjust it so it's just barely ticking over; "thump-thump-thump"--and it's obvious the clutch is engaging ever so slightly, as evidenced by the wheel spinning when you lifted the rear. Not enough to make it take off on its own though. A lot of friction and wear there, just sitting there.

:eek:
 

Top