Weird change in power

[adrian101]

I got my bike back on the road today, I also received my new 36t sprocket so i put it on and went for a ride.

now i have a problem. I'm guessing its the carby but im not sure?

It won't go past half throttle without bogging down. Ive check for air leaks and so on but nothing.

I can get full throttle response when i'm going down a small hill as the bike speeds up and the engine is at a higher rev but as soon as i get to a flat area it starts doing it again.

anyone know what the problem could be. I have a speed carby.

 

[spad4me]

I am going to assume you have a 48 cc happytime engine.

Wayyyyyy toooo much gearing.
Not enough power.
You will need a tuned pipe, lots of porting, and port matching, a high compression head, and finally some ignition work.


What did your kit come with originally a 44, or 41 tooth sprocket.

 

[adrian101]

I am going to assume you have a 48 cc happytime engine.

What did your kit come with originally a 44, or 41 tooth sprocket.

i have a 66cc f80b engine (im guessing they call the china engines "happytime"?) lol
It originally came with a 44t sprocket.

It also had the problem before with the old NT carb before i upgraded when i had the 44t sprocket on it. When i placed the speed carb on it with the 44t the problem went away. Now that ive changed to the 36t sprocket it's back again...

I'm awaiting a tuned exhaust and high compression bullet head which should arrive either today or tomorrow.

when you say porting what do you mean? i've never had to do it to theses engines before?

what do you recommend for ignition work?

-Adrian

 

[motorpsycho]

a smaller rear sprocket will reduce the rpms of the engine when at speed. so say you're cruising along at 15 mph, and you open the throttle. the gear is too high and it will cause the motor to struggle when it tries to rev and pull the weight.
gonig downhill is a different story because you have inertia and gravity on your side. this makes it easier for the engine to rev.
a bigger sprocket will give you snappy throttle response at slow speeds. a smaller rear sprocket will take away that snappy-ness because it's made for top end speed.
think of it this way...try taking off in a manual transmission car in 3rd gear. once you get the car moving, mash the gas to the floor....the engine will be lazy, bog down and it might even quit running. because the gear ratio is too high.
take off in 1st gear and mash the gas...the engine will rev faster and the car will move quicker.

 

[adrian101]

a smaller rear sprocket will reduce the rpms of the engine when at speed. so say you're cruising along at 15 mph, and you open the throttle. the gear is too high and it will cause the motor to struggle when it tries to rev and pull the weight.
gonig downhill is a different story because you have inertia and gravity on your side. this makes it easier for the engine to rev.
a bigger sprocket will give you snappy throttle response at slow speeds. a smaller rear sprocket will take away that snappy-ness because it's made for top end speed.
think of it this way...try taking off in a manual transmission car in 3rd gear. once you get the car moving, mash the gas to the floor....the engine will be lazy, bog down and it might even quit running. because the gear ratio is too high.
take off in 1st gear and mash the gas...the engine will rev faster and the car will move quicker.

ahhh i see now. is there anything i can do to help the throttle response apart from change over sprockets again?

 

[srdavo]

ahhh i see now. is there anything i can do to help the throttle response apart from change over sprockets again?

yes...... pedal longer & faster.

 

[adrian101]

yes...... pedal longer & faster.

NOOOO NOT PEDALING LOL

 

[retromike3]

36 OK for me

I have a 36 rear cog and it works just fine for me. With my SBP tuned pipe I can go at a petty good clip, even going up some serious uphill grades. I do wind it out going downhill but I try not to over do it too much. Sometimes I just let the clutch out and coast down hill. Since I got the pipe at the same time as the cog I can't say much about not having the right gear. You might want to wait for the pipe to show up before you put the new cog on.

mike

 

[GearNut]

It sounds like you need an SBP shift kit and a bike with gears to shift.