Dolmar 50cc two stroke 3.9 hp 14,500 rpm.

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phatcruiser

New Member
I was thinking about my chainsaw that has this motor. It would make an awesome friction drive power plant. Granted at 14,500 rpms it probably couldn't handle wot for an extended amount of time. But it would be fun while it lasted.

Has anyone used a high power chainsaw motor in a motorized bike? If so how long did the motor last?
 
The 14.5k rpms re no load. Some saw manufacturers post them for what ever reasons. Being in the lawn equipment trade for 20+ years I cant recall seeing a saw fail from loosing a bottom end. Almost all failures resulted from a lean run and a melted piston on the exhaust side.
Don't worry about blowing it up, worry about "smoking" it. A saw engine(decent quality) has got to be the most durable of the smaller 2strokes out there.
 
Something them some fail to realize is that horse power doesnt get things moving, its torque that gets the wheels turning and provide the thrust needed to get something moving so even though some of the very small 2 smoker engines on the marker produce desent HP at high RPMs they dont have any low rpm torque sufficient for getting a 165lb rider on a 75lb bike moving down the road.

Take a mini bike, put an old 3.5 HP Briggs engine rated at 3,600 RPM on it and then take an identical mini bike and put a 49cc 2 stroke engine on that one that is rated at twice the HP of the briggs @ 10, 000 RPM, the old Briggs will have you taking off from a stand still fairly quick with a 25-30mph top cruise speed, the little 2 stroke with maybe 8-9 rated HP will not even get you moving most likely and even if you get it moving it will not have enough power for squat. .... HP is not useful. Unless the torque it has with it is matched to what its being used to move or pull, torque is what gets you moving.

Map
 
If you want a fun two stroke engine I recommend the piston ported two stroke engines used on x1, x2, x6, and Cateye pocket bikes, they rev high and also make great torque when geared appropriately. My BMX using a Predator branded engine of that type (Mitsubishi TL clone) hooked to the auger 33:1 trans overdriven with a 16t going to a 14t for about a 29:1 overall reduction on a 20" wheel will spin the rear tire in a burnout about three revolutions every time I take off, or power wheelie if I lean back on it. It seems the piston ported motors were chosen versus the Cag reed valved motors for mid-bike pocket bikes because of the great torque they provide, possibly because they have a larger stroke than the mini pocket bike engines. Another way to gain torque is to increase the stall speed of the centrifugal clutch by lightening the arms with a couple strategically placed drilled holes which are marked in the castings on the clutch arms. By doing so, the motor is able to wind up to a point where it's near or in the powerband before applying power so the takeoff torque is much better and the motor can pull more strongly toward its load and gearing/drag limited redline.
 
I've got a few pocket bikes, and with a little work, and the right set up, they may be a little slower of the mark (with a standard clutch), but faster than a 3.5 brigs in a mini bike.
And if you gear them down a bit, scary fun.
I'd be guessing 40 MPH, out of one of my bikes.
 
You fellas missed my point.... I wasn't talking about pocket bikes or pocket bike engines.....

There are a lot of 2 stroke engines out there that are rated at higher HP that are used on 1/5 scale RC trucks and buggies that people have looked at for use on a motorized bicycle, these along with the average chainsaw engines are what I were referring to, and still by the way the torque you can getting from an old flat head briggs with the right set up will stil smoke some of the 2 stroke engines mentioned, maybe not the 3.5HP platform but use the 5 HP engine with the right mods and it will run circl s around the pocket bike 2 strokes, why do think they use them for junior dragster that will pull that small dag car and a 10 year old kid from zero to 80+ mph in qn 1/8 mile..... is all about torque.

Before anyone freaks on me about the example I just used about the junior drags......lol I know they are super modified and run on alcohol and are only based on the old flat head briggs, but it still a small 4 stroke that will deliver massive torque, probably not really the best example considering how modified they are..... but oh well just what came to mind on the subject.....lol

Now all that said, it is teue that pound for poumd a 2 smoker that is designed right does a great job if the gearing is right and the pocket bike engines are fine, but those aren't the ones I was reffering to in my original post, and when I said mini bike I was talking about an actual old school mini bike, not these chinese mini motorcycles.

Hope this explains a little better what I was saying..... its hard to make good sense when just typing this stuff out sometime.....lol!

Map
.wee.
 
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Map,

What you say about chainsaw motors makes sense. The operations manual that came with the chainsaw says to rev the motor out before you start cutting.

In my friction drive set up I always pedal from a stop before the throttle is applied. With a chainsaw motor it would be necessary to pedal to get things going but I do wonder if that would help enough to bring the motor to redline.

It's all hypothetical because there is no way I am taking my chainsaw apart. I have way to many fond memories of cutting wood to heat my house. This was before I moved to San Diego.
 
phatcruiser said:
Map,

What you say about chainsaw motors makes sense. The operations manual that came with the chainsaw says to rev the motor out before you start cutting.

In my friction drive set up I always pedal from a stop before the throttle is applied. With a chainsaw motor it would be necessary to pedal to get things going but I do wonder if that would help enough to bring the motor to redline.

It's all hypothetical because there is no way I am taking my chainsaw apart. I have way to many fond memories of cutting wood to heat my house. This was before I moved to San Diego.
Click to expand...

Woo... I have cut a lot of wood in my life also.... I still hav several chainsaws but dont use them much now days but as soon as I get the wood heater set up in my new shop Ill be cutting wood to keep the shop warm while I work next winter.

Map
.wee.
 
A good chain saw is priceless , I have a poulan 3700 I bought in 1983 that still cuts as good as it did new. The only thing that has been done is air filter cleaning and spark plugs, it has never been apart. I have cut wood ever year and it hasn't failed yet.
 
I have an Echo 315 I bought back in 1983 or 84 its just a small saw that was an old saw when I bought it from a friend who owned a pawn shop, no telling how many truck loads of wood I have cut with that little saw, those old saws like that were built right....

Map
 
The one thing I miss about my house is cutting wood. I found a stand of ponderosa pine that had been killed by fire. Just enough to burn the bark but the wood underneath was pristine. I cut on that grove for 3 years before I moved and had almost wiped it out. Nothing like sharpening the saw on the tailgate of the truck. The smell of fuel and wood chips. Coming home wiped out but happier then a groom on his wedding night. Oh man, if I ever move back.....first thing I'll do is buy a couple of permits and get chopping.

My Dolmar is an awesome chainsaw. It never skipped a beat. It would go and go and go. There's been days wher I was cutting and hauling from sun up to sun down and she just kept on going.
 
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