1970 Rupp Roadster Project

[har_v1]

I'm looking for some input on a rupp project.
Would like to just get it going with minimal $ for a first time mini bike driver...don't want to overpower it.

Considering putting on a 3hp B&S flathead I have from a edger (or a 79cc predator) and since it's missing the TC driver, just go with a 5/8 centrifugal clutch and replace the driven with a sprocket.

Before I start buying stuff and trying to assemble I would like to get some feedback...

1) Someone welded a plate on the engine mount, moving the engine mount holes right (as you are sitting on the bike) about 1/2 inch. With the B&S shaft at 2.5 in I'm concerned that this may not allow for allignment with jackshaft. Not sure what the plate was put on for...but it might have to go.

2) Since the jackshaft / driven can be a pain to remove...I've soaked them and afraid of breaking something...any thoughts on using a belt centrifugal clutch for the driver..OldMiniBikes sells one in 5/8...but not sure what my gear ratio would work out to...

3) I could go with a centrifugal clutch with a gear and buy a gear for the jackshaft (thinking 10T clutch and 12T jackshaft...and with 11T jackshaft and 60T rear on the other side giving me a 1:6.5 ratio), but 1970 rupp jackshafts are not slotted for keys for the gear which would replace the driven = more problems.

Going for cheap, simple, without destroying the bike in case a 4hp Tecumseh ever falls in my lap...with $$$ for a TC, new seat (currently off a honda), paint job etc...

Thanks.

 

[capguncowboy]

If I'm not mistaken, your jackshaft is keyed on the driven end, but it uses a roll pin on the sprocket side. That would alleviate your issues with using a sprocket, but I suggest sticking with the TAV set up. The driven unit that BWMS sells will work fine -- probably better than the original clumsy design Rupp used. With a Torque Converter, your gear ratio is endless and changes constantly. I think I read somewhere that it starts out around 8:1 and can go as high as 2:1 depending on your engagement springs.

If that plate doesn't work, take it off. I'm not sure why it's on there. The 4 holes in the original plate allign the output shaft with the driven unit perfectly.

That should be an awesome project. Good luck. I'll be watching!

 

[har_v1]

Thanks Cap,
You are right about the jackshaft.

Problem with going with the TAV besides costs, is that my briggs is a 5/8 shaft (as is the 79cc predator) and TAVs tend to be for 3/4. Could go with 212 predator, but I think its too much hp for what I want for now. Are TAVs for 5/8 shafts available?
Can't find where BWMS sells complete units.

Would like to go with the briggs since 1) I own it and 2) pulsa jet carb (so no float issues on the slant mount)...
Any problems running pulsa jets on slant?

Not sure about what gearing to use with a 3hp and the 10" wheels?..want more torque / less top speed for cruising in grass/gravel.

 

[capguncowboy]

I'm not sure how well a 3hp will perform with a TAV, or if one is made with a 5/8" shaft -- but I do know that you can get a 5/8" to 3/4" adapter and use the 3/4" version.