thats true, you'd probably have to get a bigger carb and exhaust and port it a bit to get to rev to the break point
has anyone heard of an engine breaking from high revs
- Thread starter Prasinos
- Start date
i don't believe that at all. pull in your clutch lever and crank the throttle all the way. see what happens...
with a 44 tooth sprocket, it'd be easy to wind it up to the max revs. i've done it with a 36, a 32, a 30, and a 28. the 30 and 28 was flat out downhill, i still had to back off the throttle, because it wasn't going any faster.
despite what a lot of people say, you can get these motors to top rpms when gearing lower. a lot of it depends on weight. i'm 135lbs, my bike's probably 60lbs, and i can pull away from a dead stop with a 28. slowly, but i can (although i usually pedal off the line.)
my bike accelerates to top speed pretty quick, and it won't over-rev on flat ground.
since we don't have any specifics of how his bike over-revved we don't know the real facts.
i'm sure if you sat there with it in "neutral" and just gassed it, it'd probably blow after awhile. same thing with a 44 tooth and a downhill.
and there's NO way he got to 40mph with that 44 tooth, under the engine's power. it's just not possible. it's a matter of gear ratio. even with the best carb and exhaust, the motor still won't turn that fast. it'll accelerate faster, but won't increase top speed.
with the limits these motors have on horsepower, the only way to get faster is lower gears, less weight, and less drag.
with a 44 tooth sprocket, it'd be easy to wind it up to the max revs. i've done it with a 36, a 32, a 30, and a 28. the 30 and 28 was flat out downhill, i still had to back off the throttle, because it wasn't going any faster.
despite what a lot of people say, you can get these motors to top rpms when gearing lower. a lot of it depends on weight. i'm 135lbs, my bike's probably 60lbs, and i can pull away from a dead stop with a 28. slowly, but i can (although i usually pedal off the line.)
my bike accelerates to top speed pretty quick, and it won't over-rev on flat ground.
since we don't have any specifics of how his bike over-revved we don't know the real facts.
i'm sure if you sat there with it in "neutral" and just gassed it, it'd probably blow after awhile. same thing with a 44 tooth and a downhill.
and there's NO way he got to 40mph with that 44 tooth, under the engine's power. it's just not possible. it's a matter of gear ratio. even with the best carb and exhaust, the motor still won't turn that fast. it'll accelerate faster, but won't increase top speed.
with the limits these motors have on horsepower, the only way to get faster is lower gears, less weight, and less drag.
I totally understand your hesitance to test it yourself but i can tell you that john is right. you can hold these bikes at wide open throttle for a few seconds when they are not in gear and its no big deal. Ive done it on my many bikes i've built. 2 strokes have no rev limiter but they can only rev as high as the port timing allows. If you dont believe me, think about old Moto GP bikes, which are ported for much higher rpms than our engines, they can be relined all day without damage(probably not ALL day).
as for your skepticism about my claim, listen i didn't want to believe it either.
His bike is a 15 dollar goodwill steel framed bike with knobby tires with
-New spark plug boot, removed both head gaskets that came with the engine.
-Port matched the exhaust, removed catalytic converter, and cut out about half of the baffle.
-Dellorto 12mm carburetor with a #57 jet...pantyhose air filter
-He weighs 150 which is the only part of his setup that is superior to mine.
I have an aluminum framed TREK bike with thin hybrid tires and
-36 tooth sprocket
-Port matched intake and exaust
-NGK B6HS spark plug with 8mm boot
-SBP expansion pipe
-16mm dellorto carb with a #66 jet
-pantyhose air filter
-i weigh 225 and have pretty broad shoulders
And as much as i hate to believe it, he keeps up with me under WOT crouched over the handlebars. My speedometer is very accurate and i've tested it driving next to cars. You don't have to believe it, but i have seen him go 40 miles per hour turning 9250 RPMs on flat ground, if it was not flat, then he went 40 uphill as well because i have seen him do it in both directions on one particular piece of road.
I guess he just got lucky.....
The only solace i have is that i still have more torque than him.
thought i've always been a speed man myself...
as for your skepticism about my claim, listen i didn't want to believe it either.
His bike is a 15 dollar goodwill steel framed bike with knobby tires with
-New spark plug boot, removed both head gaskets that came with the engine.
-Port matched the exhaust, removed catalytic converter, and cut out about half of the baffle.
-Dellorto 12mm carburetor with a #57 jet...pantyhose air filter
-He weighs 150 which is the only part of his setup that is superior to mine.
I have an aluminum framed TREK bike with thin hybrid tires and
-36 tooth sprocket
-Port matched intake and exaust
-NGK B6HS spark plug with 8mm boot
-SBP expansion pipe
-16mm dellorto carb with a #66 jet
-pantyhose air filter
-i weigh 225 and have pretty broad shoulders
And as much as i hate to believe it, he keeps up with me under WOT crouched over the handlebars. My speedometer is very accurate and i've tested it driving next to cars. You don't have to believe it, but i have seen him go 40 miles per hour turning 9250 RPMs on flat ground, if it was not flat, then he went 40 uphill as well because i have seen him do it in both directions on one particular piece of road.
I guess he just got lucky.....
The only solace i have is that i still have more torque than him.
thought i've always been a speed man myself...
Last edited:
haha if you sit and let these things rev with the clutch in they won't blow up they will rattle themselves apart!
http://www.youtube.com/watch?v=uCl-M3PgVRA
Slow Boat to China, jazz standard
I think the "max safe RPM" is whatever your motor will do on flat ground with the air filter on and any muffler. 2 stroke engines may not be at their maximum efficiency at WOT, but are probably best suited to it compared to a similar output/displacement 4stroke.
I think these engines are probably oversquare and made to run wide open. The stock carbs probably could be throated or milled out, but should probably limit the motor from self destruction the way they are. If your engine is not getting overheated and losing power, you are probably fine.
I don't overspeed mine down hills, I either let the throttle up a little to slow down, or pull the clutch lever for georgia overdrive.
Slow Boat to China, jazz standard
I think the "max safe RPM" is whatever your motor will do on flat ground with the air filter on and any muffler. 2 stroke engines may not be at their maximum efficiency at WOT, but are probably best suited to it compared to a similar output/displacement 4stroke.
I think these engines are probably oversquare and made to run wide open. The stock carbs probably could be throated or milled out, but should probably limit the motor from self destruction the way they are. If your engine is not getting overheated and losing power, you are probably fine.
I don't overspeed mine down hills, I either let the throttle up a little to slow down, or pull the clutch lever for georgia overdrive.
I think the best way 2 blow one of these motors up would be to jet it so they don't run so rich at WOT
At some point it is no longer the horse power its the aerodynamics. " he keeps up with me under WOT crouched over the handlebars." I think the important point is "crouched over the handlebars" I have a road bike that I built and I made the top tube longer than most and it is fast I generally go about six miles an hour faster than on my slug touring bike.
I have tread somewhere that after fifteen M.P.H. eighty percent of the drag is wind resistance. I may be off the mark by a bit but I know that by Scott clip-ons make a big difference. I don't have any arrow parts for my MB but I was thinking of putting a zipper faring for the front.
I was hoping that it will help my top speed but I am planning that it will keep me less soaked to the bone in ice cold water.
Mike, The Bike Guy
I have tread somewhere that after fifteen M.P.H. eighty percent of the drag is wind resistance. I may be off the mark by a bit but I know that by Scott clip-ons make a big difference. I don't have any arrow parts for my MB but I was thinking of putting a zipper faring for the front.
I was hoping that it will help my top speed but I am planning that it will keep me less soaked to the bone in ice cold water.
Mike, The Bike Guy
I still don't believe him when he saids hes doing 40mph with a heavy steel 26" bike with a 44t sprocket and knobby tires plus he weighs 150lbs. 40mph uphill, thats just LOL.
But anyway at almost 10,000 rpms i wouldnt want my motor exploding under my family jewels.
But anyway at almost 10,000 rpms i wouldnt want my motor exploding under my family jewels.
Maybe he got the pick of the litter or maybe he did some work on it. I've seen people do things with go carts and 3.5 briggs motors that were hard to believe at first (staggered flywheel keys, carb polishing, re jetting, velocity plates, and maybe even milling the head.) Size matters, if a smaller motor is beating you. 
PS the knobby tires add inches to wheel circumference, and at high tire pressure reduce contact area on the ground.
PS the knobby tires add inches to wheel circumference, and at high tire pressure reduce contact area on the ground.
i just wish i got as lucky as he did with that engine, with my upgrades and an engine that good i cant help but imagine.....
definitely no work on that engine but what i said, the kid knows nothing about these things so ive done all the work on it
definitely no work on that engine but what i said, the kid knows nothing about these things so ive done all the work on it
