4 Stroke Engine basics

[Timbone]

I am well into a 4 stroke build and I am finding myself a bit confused. Soon I’ll need to decide on a transmission plan for the build. I guess the reason I can’t decide is that I don’t completely understand what I am working with.

All I’ve built so far s 2 strokes. My current ride can go up to 37mph turning a 36T sprocket on a 24” wheel. Cruise is 28 to 30 mph.

The motor I am using will be off a lawnmower. Brand spanking new. Never been fired. I’ve made all the adaptations for horizontal use.

This is 140cc. I’m assuming this thing was made to run constantly at about 3600 RPM. So I’d like that to be around 28mph. I’ve removed the governor so I’d like to be able to goose it and jump up into the 30’s.

I’m seeing centrifugal clutches grabbing at 2000 to 2200 rpm. With 9.1:1 reduction, that’s going to be kick in speed of about 16mph. Is that acceptable? Will it work?

I’m thinking the 4 stroke will be much smoother and dependable than the HT China motors I am used to.

I’ll build my own jackshaft, kicking the chain to the right side. Left side will be for the rear disk. Not even gonna fool around with a pedal side. If I do add rotating pedals it will be a trick to look something like a bicycle.

Thanks in advance for any help!

 

[Will'smotobikes19]

So you modified it from vertical to horizontal? You made a dipper and made the carb horizontal? tap and thread the hole the gov shaft was in and red loctite a bolt in it. You could possibly use one of the piranha 140cc carbs. Is it a honda engine? Does it have a cast iron sleeve? I just got a 158cc weedeater push mower from my uncle the only thing I'm worried about is if the camshaft is plastic.

 

[Will'smotobikes19]

I'm getting an old push mower from my uncle might put the engine on a bike since I have no use for it since we have a riding mower. Probably going to have to clean the carb.

 

[Timbone]

So you modified it from vertical to horizontal? You made a dipper and made the carb horizontal? tap and thread the hole the gov shaft was in and red loctite a bolt in it. You could possibly use one of the piranha 140cc carbs. Is it a honda engine? Does it have a cast iron sleeve? I just got a 158cc weedeater push mower from my uncle the only thing I'm worried about is if the camshaft is plastic.

Yes, I made the dipper and cleared out the crankcase so that it has clear travel. I also filled in some holes to keep the oil from leaking from the main bearing. The way I will orient the engine will allow me to thread in some clear hose and fill with oil to the level of sight.

This is a brand new Briggs & Stratton 140 cc engine, so it’s going to produce some power.

I don’t think the plastic camshaft is a concern.

My lack of knowledge concerns gearing and how that affects the centrifugal clutch. At 9.1:1 would a centrifugal clutch function well?

I really don’t want to drop $250 on a CVT as I am not positive it would snug up the engine properly. I am really out of my area here.

 

[Will'smotobikes19]

What size shaft is it? 1 inch? depends on the clutch when it engages what lb springs are in it. It will definetely go 28 like you said probably atleast 35-40 its bigger displacement than a 79cc and you removed the gov. If you know of it's 3/4 you could use one of the 212cc transmissions. I might try to use my 158cc vertical engine for a build too.

 

[Will'smotobikes19]

That plastic gear would be alot better if it was steel. I don't know if the vertical shaft hondas have a steel or plastic cam. You could shave the head too a few thousandths.

 

[FurryOnTheInside]

My lack of knowledge concerns gearing and how that affects the centrifugal clutch. At 9.1:1 would a centrifugal clutch function well?

I really don’t want to drop $250 on a CVT as I am not positive it would snug up the engine properly. I am really out of my area here

Same here, but surely the speed at which the clutch engages is determined by the spring weight (strength) and pad weight (mass)?

 

[Frankenstein]

Well I'd get a clutch you can put weaker springs on, but that's from somebody who weighs more than 200lbs, you are a much smaller looking person and 140cc is a decent chunk of power, 16mph may be OK, just expect that clutch to taper away faster since it's a bit of a high number all things considered.

The torque-a-verter would be a solid idea, it would really distribute that power nicely for you and it shifts automatically. Are you using this on the road or off? If you have any plans with traffic you really should use a larger gear and get engagement at a lower speed if you keep a single speed set up.

 

[Timbone]

What size shaft is it? 1 inch? depends on the clutch when it engages what lb springs are in it. It will definetely go 28 like you said probably atleast 35-40 its bigger displacement than a 79cc and you removed the gov. If you know of it's 3/4 you could use one of the 212cc transmissions. I might try to use my 158cc vertical engine for a build too.

Shaft size is 7/8”! I’ll have to start the engine up and carefully grind down to 3/4”.

Ok, these answers are helping! A 212 CVT would be nice. Not sure what bolting it in would look like.

A big consideration for my build is that I am working on a mono shock swingarm and I’d really like for the swingarm axle to also serve as the jackshaft.

These custom build are hard!

Thank you for the help!

 

[Will'smotobikes19]

Im trying to picture that swingarm with a jackshaft built in. Maybe one of the 79cc transmissions could bolt up the holes are slotted just with a 3/4 clutch instead of 5/8 there is a jackshaft built into the 79 transmissions. My next build is going to ve a 2 stroke instead of the gx120 build.