Bike Running Horribly

[Frankfort MB's]

These motors are 2 strokes so most of the time the power is most useful in the upper rpm range

 

[Steve Best]

It has to do with engine power going up in a linear fashion and power needs going up exponentially with speed. With the 1.5-2hp our bikes make they pretty much peak out at 30-40mph because it takes SO MUCH power to go any faster. If you have an older engine that could only do 6000rpm you would need the tall sprocket to get up to 35mph. My 2015 Grubee seems to like to rev to 8000 so it will go 37-38mph with the 44t sprocket. Likely if I put a 36 or 32 on it wouldn't go much faster (42mph?) because of the rapidly increasing HP needs at speed. I have hit an RPM wall at 38mph because I have nearly doubled the torque resulting in much better acceleration and hillclimbing capabilities in the 20-30mph range. Ideally you would tune the sprocket size to give maximum speed at maximum power rpm.

I like the sprocketing I have now because I can start away from a stop if I have to, and acceleration is is very good. 38mph is quite fast on bicycle brakes. I don't want to give up a lot on the low end to gain a little on the top end.

Once you get your jetting figured out I am willing to share port maps and pictures of what I've got.

Boost bottle test done in the rain. My carb with "Velocity stack" intake and better filter.
Yup, that is a waterbottle with a copper pipe (heat the bottle with boiling water to fit) throat epoxied to jar lid.

Steve

 

[Frankfort MB's]

Pics not working???

 

[Jeffrey gagnon]

Ok so under load riding bike i do about 5500-6000 rpms 34 mph on a 32 tooth
With tire lifted off the ground can hit about 7000 rpms dont know the speed as my speedo is off my phone i would do about the same rpms with a 36 tooth at about 32 mph

 

[Timbone]

36 is. a big gear. Unless your motor is running perfectly, you are gonna have issues.

 

[Jeffrey gagnon]

36 is. a big gear. Unless your motor is running perfectly, you are gonna have issues.

Had a 36 just last week i went down to 32

 

[Steve Best]

Ok so under load riding bike i do about 5500-6000 rpms 34 mph on a 32 tooth
With tire lifted off the ground can hit about 7000 rpms dont know the speed as my speedo is off my phone i would do about the same rpms with a 36 tooth at about 32 mph

My Grubee revs a bit higher. Usually a function of port timing and pipe tuning with intake length being a minor function. The pipe is pretty much not tuned at all so it is all about port timing. The top of the exhaust port is 1.125" from the top of the cylinder. The transfers 1.250" from the top of the cylinder. The intake doesn't matter so much because it is from the skirt of the piston.

Pulled my head off tonight to get a better one made, oil and soot wiped off with a rag. No carbon on head at all. Damn I love synthetic oil!

Steve

 

[Jeffrey gagnon]

If my motor can hit 7k with wheel off the ground this is at about 3/4 throttle, shouldn't it be able to hit those same rpms under load with a 32 tooth approximately how far (1/4, 1/2, 3/4, or 1 mile) would it take to get up to max rpms just before redline im not intending to redline im just trying to figure out what max speed is

 

[Jeffrey gagnon]

My Grubee revs a bit higher. Usually a function of port timing and pipe tuning with intake length being a minor function. The pipe is pretty much not tuned at all so it is all about port timing. The top of the exhaust port is 1.125" from the top of the cylinder. The transfers 1.250" from the top of the cylinder. The intake doesn't matter so much because it is from the skirt of the piston.

Pulled my head off tonight to get a better one made, oil and soot wiped off with a rag. No carbon on head at all. Damn I love synthetic oil!

Steve

Ok so the part about tuned pipes i do have a couple ideals from what ive read so far about that is its all about the header length and your port timeing for exhaust stroke however im not really interestes in modifying ports im more interested in the tuned pipes from what ive read on some can help top end where others help low end torque i think i bought one for low end power not sure

Its the flexable one that gasbike.net sells i bought it cause of its looks, the expansion chamber, flexibility and thought it would give me a performanc boost my question is do you think theres a wayy to cut the flexible part shorten the header length get some kind of spacer add some heavy duty hose clamps both ways kinda a criss cross. Anyways can that be done to said pipe to get more top end it seems like the stock exhaust is better and im thinking if j could some how shoten the pipe i have to get more top end i know its like jetting one size at a time from what ive read its about 1 inch at a time untill you get peak performance

And what would be the really peak performance of my engine if it can do 7000 rpms at about 3/4 throttle with the back wheel off the ground

 

[Jeffrey gagnon]

Or should it be slightly longer then stock exhaust where is a good starting point at determining header length stock header is 9-10 inches or so should i go longer or shorter for top end speed