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History of this Minibike

This bike was owned by my cousins in Abilene Texas where I took summer vacations in the '70s. I rode it up and down the long gravel driveway over and over. Almost every trip the chain would fall off - I never figured out why. This was my introduction to small engine starting and adjusting.

Around 2014 I asked about the bike on one of my trips to Abilene and discovered it had been stored in the old farmhouse. It was buried in a pile of trash. It was pretty rough but everything was there just like I remembered.

About the RUPP C-250

The C-250 is near the low-end of RUPP's minibike offerings for 1968. The 'C' indicates the 'Continental' series while the 250 indicates the motor size. The motor is the smallest of their product line at 2.5HP. 6" turbine wheels, 7/8" handlebars.

This is a clutch-less 1-speed bike, turn the throttle to go and that's what you get. The single rear brake is extremely weak and makes the bike fairly dangerous.

There are no fancy accessories on this bike, not even a motor-kill switch - that has to be done by pressing a metal bar into the spark plug. But it does have a minimal front spring suspension and painted fenders.

The C-250 was only available in metallic red.

Here is a brochure I found for it.

Motor

The motor is a simple 2-1/2 HP Tecumseh horizontal shaft. It's typically used on snow blowers.

Model H25-25142F (H25 engine)
S/N 8116-12362

This motor is particularly hard to start and the reason appears to be that it's hard to get fuel flowing at the beginning. Once it's warm it will start with a single pull so I don't think it's a weak ignition or something wrong with the fuel system. I think it could benefit from a bulb pump so I'll look into that.

Restoration

Restoration was not complicated as the bike is fairly simple. Disassembly was easy. Most of it is original. I did not tear the motor down beyond the carb, exhaust, and external cover plates.

The frame components were sanded/sandblasted, primed with an etching primer, then rattle-can painted.

The carburetor and the pull-starter was removed then the motor was cleaned and painted with a high-temp paint.

New parts were bought - tires, carb, throttle and brake grips with cables, brake pads, spark plug, seat, etc. Reassembly was uneventful.

New Parts

My goal was to only replace parts that were absolutely needed for operation and safety. It was more parts than I had hoped because I'm big on preservation, but I kept the old parts in a tub for reference.

• Seat
• Carburetor (631070A)
• Air foam (29961)
• Throttle grip and cable (53")
• Brake grip and cable (59")
• Brake pads. 2 identical pads, D-shaped 5/16 thick, 1 x 1.125
• Spark plug (RJ8C)
• Muffler (30911)
• Starter pull (590420)
• Foot pegs
• Tires (4.10 width x 3.5 section height x 6 hub)
• Chain (#35)

Paint

Apparently all C-250 bikes are metallic red. Since I had a sample of the original color I matched it pretty close to a Dupli-Color BCC0365 Claret red metallic.

Parts were sanded and de-greased then painted with an etching primer.

The problem with this paint is it's not very strong and scratches easily. Also, it's not gasoline resistant which is a big problem.

This color is now difficult to get and probably was a bad choice.

The wheel hubs were sprayed with a silver version of this same paint and they haven't aged well either. The wheels were chrome with rust spotting and I left them like that.

Brake

All I have to say about this brake is - HAHA. It is essentially useless, even after being rebuilt with new parts, except the original housing and sprocket. Riding this bike must be done with carefully planned speed and trajectory because there is no stopping with the brake. Make sure you have good shoes on because you'll need them. I'd say it's about 10% of the strength needed to actually stop this bike, and it takes incredible force just to get that.

Due to the danger of this weak brake system I'm considering changing the housing and the sprocket so it will lock the wheel.

Fuel System

I added a standard 1/4" inline filter after the tank and an inline valve. These are both stock products at any lawnmower supply. The valve allows me to run the carb bowl empty so it can survive not being used for lengthy periods of time.

Chain and Sprockets

The motor has an 11-tooth sprocket behind the clutch. The rear sprocket is about 8.5" diameter with 72 teeth and also operates as a disk brake.

The chain is a standard #35 3/8 pitch cut to length so the motor aligns to the middle of its mounting slots when the chain is tight. This is a very common chain - I bought a 10' with master link at Tractor supply for $22.

I think the reason the chain always came off back in the '70s was because I was adjusting the tension with the rear wheel and getting it crooked. The real way to adjust the chain tension appears to be by moving the engine since the mounting holes are elongated slots. This movement also requires loosening the chain guard which also has a slot. Since the rebuild the chain has never come off.

Seat and Logo

The seat I got was a stock 8"x13 minibike seat. It's not exactly like the original and too soft but it will do.

Of course the seat doesn't have the RUPP logo on the back so I made artwork in Photoshop. Arial Italic Bold font seems to be pretty close with some squishing. The over-all dimensions of the logo on the seat are 1" tall and 5.5" wide. Here is the artwork I made in .GIF form.

FAIL

Then I cut a matt-board template on my laser cutter. I couldn't get the template to lay flat against the vinyl and the whole project was a disaster, plus there was some unexpected overspray on the fender and that paint is discontinued. I was able to clean most of it off and re-clearcoat.

WIN

I actually considered buying a Cricut vinyl cutting machine to make an adhesive template but ended up just cutting one by hand with an exacto knife using the logo printed on a piece of paper. The result is not perfect but it's much better.

Gas Tank Logo Label

The original bike had a factory logo on the gas tank which I duplicated in Photoshop - here's the GIF. I took the PDF file down to the local color copy shop and got a sheet printed for 70 cents. Then I cut the labels off the page and sprayed them with several coats of clear. This isn't like the original foil label but the grey background is close enough for me. There was someone selling original labels online but they were pricey at $50. I sprayed the back of the label with Gorilla spray adhesive and that attached it nicely to the tank.

Motor Labels

I decided to splurge on some motor labels ($18) and it really finishes the bike off nicely

Problems

Some of these issues make the bike a safety hazard. In spite of all this it's a lot of fun! :)

• This motor is very hard to start -

I've found if I twist the throttle a lot, couple pulls while on full choke, then pull while choke off sometimes works. I can rarely get it to start without 25+ pulls. It seems like the system needs a fuel primer bulb to improve starting, maybe that's something I can add.

• At starting it might take off -

When starting along with a little throttle it will shoot out from under you. To get the engine running and keep it running at first it needs a little throttle. It's possible the clutch is not operating optimally but I'm reluctant to replace every part on this bike.

• The brake is truly useless -

I rebuilt this brake assembly with new pads but this brake is completely useless. A strong adult pulling the lever as hard as possible will only slow the bike down a tiny bit. The wheel won't lock with me riding it, but I'm bigger than a kid. The mechanism seems to be operating and complete. You have to plan your speed and trajectory very carefully to keep from running into a fence. I'm not sure if this is the way it was from the factory or there is something wrong.

• Throttle response is not linear -

I've tried various linkage hole positions and made some improvement but it's still not as good as I think it should be. There tends to be 2 throttle positions - idle and full throttle. I have not discovered how to set the governor lever.

• Unintended wheelies -

Even though it has the smallest engine available and is not over-powered, if you sit back on the seat and twist the throttle it will wheelie out from under you. I've done this twice but no injuries...yet.

• Kickstand twisting -

The kickstand is attached to the bottom using one of the 4 engine bolts. Now matter what I do it won't stay in position and tends to twist up under the bike preventing access to pull down. This seems like a simple thing but I haven't been able to come up with a washer/nut system that solves it.

Update: I got this fixed by moving the kickstand to another slot with its own bolt. Yay.

• Gasoline on gas tank -

The paint I picked is not resistant to gasoline. No wonder some of the tanks were chrome plated. I might redo the tank with a fuel-resistant paint but I'm sure the color will be noticeably different.

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