engine option for new mini bike

[robbie]

I found this bike at the scrap yard for ten dollars the other day. It seems pretty heavy duty, but I notice that the steering joint is on the cheap side. No bearings....

The engine and transmission are gone, so all I'm left with is the wheels and a few accessories. It has disc brakes and a hydraulic caliper on the front, but the rear caliper is missing. I think it uses #35 chain. It's definitely larger than 8mm. I didn't compare to my #40 equipment, but I think it looks smaller than what I'm accustomed to.

I'm planning to put on a utility engine and make it go 20 to 30 mph. I have a Briggs 5 hp and Comet 30..... but somehow I think that combo should go on a kart. I was thinking of using a Briggs 3.5 hp engine from a lawnmower, set up to run horizontal instead of vertical, with a Comet 30. (I actually have two Comets) Before the site crashed somebody was telling me that the 3.5 is a gutless engine, but I'm thinking that if I add the torque converter it might do a respectable job. I don't need neck snapping acceleration. I just want it to be able to climb gentle hills. I'm already getting satisfactory performance on the other mini bike with a Harbor Freight 2.5 and a jackshaft at 9.2 to 1. With a torque converter the Briggs 3.5 has to be at least that good, no?

One other thing, if I'm going to put forth the effort to set up a lawnmower engine to run sideways and add a torque converter, I'll probably switch over to a better carburetor. The lawnmower has always been a good runner, but the carb isn't my favorite. I've always had to move the throttle back and forth to the right places at just the right time to get it to start. Any suggestions for a better carb?

 

[Doc Sprocket]

Interesting frame- I'd like to see what you do with it. I'm also interested in the engine conversion- there's been a lot of talk without much results. I have a vertical 8hp I'd like to convert, but too many irons in the fire right now.

For the carb, Just grab a Honda (or clone) carb for a 2.5 and re-jet, I think. Basic carb, should do the trick.

 

[robbie]

I have a vertical 8, too. I'll probably put it together with the rear axle out of an old Honda ATV skeleton I have. I figured I would probably find a Honda carb for it on ebay or something.

I've been looking around on ebay for a carburetor for the 3.5. There's a vendor selling the Honda 5.5 carb (which I already bought for my Briggs 5 hp), but it seems a bit large for the 3.5 Briggs. What do you think of this carb?http://cgi.ebay.com/ebaymotors/Carb...7?pt=LH_DefaultDomain_100&hash=item27b7c21b3b

 

[robbie]

looks like I could hang the whole engine/jackshaft off the brackets just in front of the back wheel. Then I'd have to find a place to put the foot pegs and brake pedal. It probably ought to be built into the engine mount as a unit, and attached to the brackets. Then I could take the whole thing off with two bolts if I decide to change the engine later.

 

[Bluethunder3320]

I have a vertical 8, too. I'll probably put it together with the rear axle out of an old Honda ATV skeleton I have. I figured I would probably find a Honda carb for it on ebay or something.

I've been looking around on ebay for a carburetor for the 3.5. There's a vendor selling the Honda 5.5 carb (which I already bought for my Briggs 5 hp), but it seems a bit large for the 3.5 Briggs. What do you think of this carb?http://cgi.ebay.com/ebaymotors/Carb...7?pt=LH_DefaultDomain_100&hash=item27b7c21b3b

you do realize you will need to most likely make your own manifold. should be alright... but 100cc-150cc-200cc is totally vague.

the only win i see there is that its made for a easy cable hook up for a twist throttle or whatever u do.

 

[robbie]

Yeah, that's what I was thinking. You can buy those Chinese bike throttle cable sets on ebay, no problem. But you think that's the only win? The displacement on my lawnmower engine is about 130 cc.

I can make the manifold easily enough. I can cut out flanges, drill the holes, and add a piece of pipe in between. I have a nice piece of chrome plated 7/8" steel pipe to use.

I wouldn't mind using a carburetor like the one on the Harbor Freight 2.5 hp, but I was having trouble finding one like it. I don't think Harbor Freight sells parts individually, do they?

 

[robbie]

What do you think of this carburetor? http://cgi.ebay.com/ebaymotors/30MM...rcycles_Parts_Accessories&hash=item5d2cf0b88a

People talk about Mikuni as if it's worth using.... I don't have any experience with them, though.

 

[sideways]

Both those carbys are way to big for the little Briggs. Pull the carb off the 8HP Briggs and put that on it. Make your own manifold to replace the horrible factory one. You should gain a fair bit of power just from doing that.

Thanks

Hayden

 

[robbie]

I agree with you about the factory manifold on the 3.5. It's a nightmare.

Isn't the carb on the 8 hp way too big for the smaller engine? The displacement on an 8 hp briggs is 319 cc, according to a post I found on another forum. That's almost 2 and a half times the displacement of the 3.5.

Do you say that the 2 carbs I found on ebay are too big because they are for 2 stroke engines, which draw air on every rotation instead of every other?

 

[Bluethunder3320]

Oh, didnt notice they were 2 strokes.

Hf doesnt sell individual parts... Wish.. But nope.

Google honda clone 6.5 carb or honda clone 2.5 carb and see what you find. Maybe a honda gx carb would be even better.

 

[sideways]

I agree with you about the factory manifold on the 3.5. It's a nightmare.

Isn't the carb on the 8 hp way too big for the smaller engine? The displacement on an 8 hp briggs is 319 cc, according to a post I found on another forum. That's almost 2 and a half times the displacement of the 3.5.

Do you say that the 2 carbs I found on ebay are too big because they are for 2 stroke engines, which draw air on every rotation instead of every other?

I doubt the carb on the 8HP would be any bigger than 20mm, might even be 15mm or so. Briggs lawn mower engines are traditionally way under carbed. If the 8HP carb is less than 20mm, I'd use it.

I'm saying those carbs are to big because that's exactly what they are. You could probably put the 27mm one on your 8HP but no way for the little Briggs, it'l never pull enough air for those carbs to work properly.

It's like putting a 850 Holley on a 4 cylinder Camry engine.

Thanks

Hayden

 

[porsche930dude]

you wont be able to bolt the tav to the bottom of a mower engine, no bolt holes and the crank is likely too big

 

[robbie]

I'm not hip to the official names for torque converters and CVTs, but isn't TAV short for Torque-A-Verter, which would be a specific product? The one I have is a Comet 30, and it includes only the pulleys and a belt. I'll have to build a jack shaft with pillow blocks, rather than bolting it to the engine.

How big is the crank on a Briggs 3.5 hp? I haven't pulled it out of the mower yet. It's cold and windy out there today. I might take it apart this week.

 

[robbie]

I doubt the carb on the 8HP would be any bigger than 20mm, might even be 15mm or so. Briggs lawn mower engines are traditionally way under carbed. If the 8HP carb is less than 20mm, I'd use it.

I'm saying those carbs are to big because that's exactly what they are. You could probably put the 27mm one on your 8HP but no way for the little Briggs, it'l never pull enough air for those carbs to work properly.

These measurements don't refer to the venturi diameter, do they? I looked again at the Mikuni on ebay, and they show a photo of it with a measuring tape. It looks like they're measuring it at 30 mm where it attaches to the engine port.

I pulled the carb off the 8HP briggs and the diameter is 23 mm where it attaches to the engine.

 

[robbie]

You were right about the crank size. My curiosity was getting to me, so I braved the cold wind to get a measurement. It's 7/8".

 

[Bluethunder3320]

awwwwwwwwww.................

do you have the tools to turn it down to 3/4?

 

[sideways]

I'm not hip to the official names for torque converters and CVTs, but isn't TAV short for Torque-A-Verter, which would be a specific product? The one I have is a Comet 30, and it includes only the pulleys and a belt. I'll have to build a jack shaft with pillow blocks, rather than bolting it to the engine.

How big is the crank on a Briggs 3.5 hp? I haven't pulled it out of the mower yet. It's cold and windy out there today. I might take it apart this week.

The crank is usually 7/8" and about 1" long, has a bolt through the middle of it, that bolt once held the mower blades on. [EDIT]: ahh I see you measured it. [/EDIT]

There is almost always a boss on the shaft, I cut a slot in mine and hammered a screw driver into it, I used the screw driver to twist it off.

When you make the dipper make a few test ones out of tin first, then do your final one, I made my dipper out of 2mm thick plate.

If there's another Briggs mower at the tip on Tuesday I'l post a guide on how to convert it. I might take my 4HP apart again and show you what I did too.

What kind of starter does your engine have? If its weak then it will break, my engine has killed 2 starters and broken one rope so far... Mine has the side starter, a little plastic gear drives a gear built into the flywheel, it's a really bad system. I'm making a kick starter for it now, I'm down to my last side starter and they are rare.

Thanks

Hayden

 

[Bluethunder3320]

hmm... has anyone converted a horisontal shaft motor into a vertical? lol.

 

[sideways]

hmm... has anyone converted a horisontal shaft motor into a vertical? lol.

Don't tempt me!

That'd actually be much more difficult, I duno if you've ever seen the oil slinger but unless the cam gear teeth have the same pitch and you made you own little mounting bar thing then it'd be near impossible.

Anyways... lol

Thanks

Hayden

 

[robbie]

There is almost always a boss on the shaft, I cut a slot in mine and hammered a screw driver into it, I used the screw driver to twist it off.

What kind of boss? The shaft on my engine looks like a regular keyed shaft. I haven't taken the blade off yet. Are you saying that there is something on the shaft which can be removed to leave the shaft the correct size?

If there's another Briggs mower at the tip on Tuesday I'l post a guide on how to convert it. I might take my 4HP apart again and show you what I did too.

What kind of starter does your engine have? If its weak then it will break, my engine has killed 2 starters and broken one rope so far...

I'd like to see some photos of your project.

My engine has the classic Briggs starter with the square ratcheting shaft. That's not a problem, is it?