My First Project: Briggs 3 hp

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Hi folks,

I've been lurking here a bit, gathering info on what kind of engine I could put on my old mountain bike. I can't afford one of the Huasheng/Honda 4-stroke kits right now, and I've heard all kinds of things about the Chinese 2-strokes exploding after a few miles. So when I saw a 3 hp Briggs & Stratton motor on craigslist for $30, I decided to build my own. My goal for this project is to keep the budget under $100.

My plan right now is to get the motor mounted in the frame, and figure other stuff out later. The engine doesn't quite fit, so I'm going to cut some notches in the bottom of the engine for clearance, near the bolt holes for mounting the engine. Then I'll make some motor mounts, either by sand-casting them out of aluminum or by borrowing a welder and fabbing them from steel. I'll probably have to re-route the intake and exhaust too, maybe going to a float-type carburetor.

For the transmission, I'm going with a belt drive like a Whizzer, with a smaller wheel rim as the rear pulley. I can either use a centrifugal clutch with a pulley or use an adjustable tensioner to engage or disengage the belt. With a 3.2" engine pulley this will give me a speed of 35 mph at 3,000 rpm. Not the best gear ratio, but maybe I'll add a jackshaft or a gear reduction later.

So, what do you think? Should I go with a centrifugal go-kart clutch or make my own manual friction clutch?

 

[Elmo]

Make your own. More satisfaction that way.

 

[Jesse002100]

i would worry more about your old mountain bike before you put a motor on it. maybe get some bearings all around and get the bike safe before you go and do 35 on it. but anyways centrifugal clutch on these work great and are easy for anyone to get on and go.
i hope you can get the motor to fit in there its going to be pretty tight

 

[Dan]

Cool, 30 bucks. Score. Could run the exhaust st8 or almost right back an' away.

I would suggest the cen clutch. Easier and if you can find some beat and broken old go-cart or minibike some where, can get it (the clutch) for next to nothing with the go-cart/minibike for spare parts for future builds and/or a trailer.
Just a thought. Folks throw em out or scrap em.

 

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I'll probably do a centrifugal clutch with a fixed tensioner (made from a skateboard wheel). I saw a clutch online for $30, but maybe I can trade some B&S engine parts for one. I also need to find a way to mount the carb and gas tank. Maybe I'll get a float-type carb instead of the draw-up one I've got. Then I can get a teardrop tank and make it look more like a real motorcycle.

Right now I'm mostly worrying about how to mount the engine without breaking my budget. I need to machine away part of the engine's crankcase on the bottom (where it bolts down) and make some motor mounts. Then I need to get a BMX rim for the rear pulley and figure out how to mount that. I'll definitely give everything a once over before riding it (especially since the frame has been sitting outside for about 1 year!), but I think I'll hold off on fixing the bike gears and pedals until later. Maybe I'll go with the wide 3-piece crank kit.

 

[Dan]

Sounds like your well on your way and have it some what mapped out. I've not tried it but is on the 2do list, (gets longer an' longer, snork) use a pc of flat stock and make a flat bottomed "U". With the mounting bolt holes at the bottom and either Ubolts or muffler clamps up the seat and down tube. Would be custom made just for that bike and could be made with a vise and torch.

This is a great thread about using cans to make a tank. http://motorbicycling.com/f26/apple-juice-tank-11720.html Look really cool but more old world MB rather then MC. Just a thunk.

Looking forward to seeing your build.

 

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Looks like my other comment disappeared, so here's take two:

I sawed away part of the engine today so that it would fit within the frame. After looking at how everything fits together, I realized that the clutch will not line up with the rear pulley. This means I'll need to use a jackshaft unless I want the engine sitting very far to one side.

Any tips on making a jackshaft system, or any other ideas on making a workable drivetrain? What sources are good for parts? I think my local hardware store carries v-belt pulleys and bearings, but I'll have to wait til they're open tomorrow to check. I might go with a chain-drive clutch and primary and a belt drive secondary, or maybe all belts and pulleys depending on which is cheaper.

 

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Thanks for the tips on the gas tanks, too. I might braze something up like that, but I'll probably just buy a Chinese tank or use an ugly plastic jug for now.

 

[Dan]

ZukJay had a great idea about useing jackshafts from old blowers. How cool is that!? free and funky lookin'. http://motorbicycling.com/f36/belt-drive-build-15794.html

I am testing CPVC for a fuel tank now. Just a reducing T, so it almost looks like it was ment to be a tank. I like the sort of steampunk/DIY look. Just a thought.
The kit tanks are fine and I personally have no problem with em on my bikes. Sure cant beat the price. Those apple juice cans are about the coolest looking ones I have seen.

Lots of innovative stuff, folks are building. Really love the DIY.

We should produce a tv show, junk yard wars MB edition, but use Federal prison-ers. Could you imagine the stuff they would come up with!? Give em lots of junk, a bicycle and they already have the time, snork.

 

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More like "Prison Shank Wars" amirite? I'd definitely be up for some kind of motor-bike building workshop or get-together though. Is there one in the Denver area?

I went down to the hardware store today, and it looks like they have fan belts and pulleys in all shapes and sizes, as well as bronze pillow blocks. They don't have keyed shafts or keys though, so I will probably end up ordering a jack shaft kit from a go-kart store.

Or maybe, since the engine is really way too wide to make the pedals usable, I could use the bottom bracket as a jackshaft support and use the rear derailer to shift?

Just throwing out ideas right now. My first priority is to make engine mounts, which if I'm lucky I can start on next week.

 

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Got some photos of the engine sitting on the frame, without motor mounts. I'm thinking about mounting it tilted forward a bit, but I'm not sure if that would screw up the oiling system. Maybe I can modify the oil slinger to make it work? Or I could tilt it the other way, so that the cam and oil slinger are under the oil level.

Tilting it would give me better clearance for the spark plug and the carburetor, but I would have to get a special intake pipe either way.

 

[Dan]

I have an HF that is slanted pretty far forward but not as far as yours would need to go in that direction. (so far, no problems)

Was thinking a slant head might do it for ya. Briggs & Stratton slant head cyclinder top - Google Search I did not read any of these but hope it helps. Only other thing aside from this or tilting that I can think of, would be a step threw frame. (AKA, woman's)

Dunno. But this is half the fun. The work arounds and planning stuff.

 

[Dan]

Forgot to mention and dunno if you have seen them but they sell wide cranks. If still to tight, can heat em up and bend out.

Many dealers have them but link will give you a pic. BE and Spooky are my favorite guys for stuff like that. Search Results

 

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Good news! I found an old lawnmower sitting on the curb with a "FREE" sign. Not only does it have a float-type carburetor with an angled intake pipe, but it has a belt and pulley self-propelled system. Two birds with one stone! Now I have a lot of the parts I need start my build. There's also a belt-drive rear differential, so I could make a trike

Dan, I couldn't find any angled heads, but I can fit the engine pretty well with the plug off to one side. The old lawnmower also has an insulated plug cap, so I might use that if I can get it off. That will keep my leg from getting shocked too much!

How wide are the 3-piece cranks? The engine is about 1 foot wide! I could make it narrower by taking the fan shroud off, but then I'd need another way to start it, as well as a flywheel with no fan blades.

 

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Here's another picture of the intake on the engine. It fits pretty well for a Tecumseh carb on a Briggs engine! I'll have to get a different air filter to make things narrower, though.

 

[Dan]

Awesome DC! Very cool. Funny how just what ya need some times presents it self. Serendipity baby!

Wide cranks give you 9 inches out of the box, but can be made wider. Normal cranks are 5"

Keeping the engine level will also make oil changes easier to get the level right.

What is the bolt pastern on your head, or numbers on your engine? I have a buddy who has a scrap pile of old engines and will take a look. If ya wanted to, Tyreslider or Manic Mechanic could make you a slant head. Just sayin'

 

[jbcruisin]

Here's a picture of my cranks after I got them heated & bent out.

 

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Here's what the head looks like (I have a spare)

The engine's model number is 60102 if that helps. The bolt spacing is 4" by about 2 1/2".

I also found a slightly shorter plug, but it might be burned out.

JB, how did you bend the cranks? I don't have an acetylene torch, but I can buy a MAP blowtorch or borrow a friend's propane weed burner. Or maybe try my charcoal furnace. Did you need some kind of jig or vice to hold it?

 

[jbcruisin]

I tried with my small propane torch & it wouldn't get hot enough. I didn't think it would, so I went to a friend that teaches welding at a school & we put it in a vise, heated it near the threads, bent it out. Then we heated it out towards the pedal end & bent it back in.
Hope it works for you.
jb

 

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Took apart the lawn mower transmission tonight. It's a whole mess of weird pulleys, rusted bolts, and sticky bearings. I don't think I can use much of anything from it, since the shaft has a hexagonal cross section and the pulleys seem to be taper-fit. I'm not exactly sure about that, since most of the bolts are frozen and I can't get the two main pulleys off the shaft. I'll probably use a go-kart jack shaft and some pillow block bearings after all. At least I have a couple idler pulleys and a carburetor for free!

Speaking of finding things on the road, look what my roommate found walking around! We gave him a bowl of milk and a couch to sleep on, but he'll probably be going to the pound tomorrow since I'm allergic.