RACERS OR RACE ENGINE SPECIALISTS. REAR SPROCKET KNOWLEDGE NEEDED!!!

Joined
Feb 15, 2019
Messages
57
Hey everybody. I currently have a 30 tooth sprocket on my bike and it still has tons of torque and power left to give. The low end torque is kind of lacking, but that is typical for the oko carb and is not my concern but once you get to about half throttle its like you hit the NOS bottle (I have no NOS or boost bottle) and hang on to your hat. At about 3/4 throttle the engine is screaming and I know it can handle a smaller sprocket. I still have a little fine-tuning to do on the carb but as is right now it tops out at about 43 Flatlands and 45 or so downhill and the engine is screaming.(too much)* I want to go faster and so does the motor. I was wondering what's the smallest sprocket anyone has used with sucess and what was your weight and speeds you had reached. I weigh about 180 lb and my bike is a 26in frame tank type has a pk80 race engine built by Fred over at CR machine along with a combustion chamber pipe, 21mm oko carburetor (45 IDLE, 2ND HOLE ON NEEDLE,95 MAIN), reed valve w windowed piston, and fully ported cylinder for optimal top end and 3rd transfer port done by Fred plus a couple mods I do myself to the piston which I am not at liberty to discuss because of competition. I am an experienced rider and have found some sprockets ranging from 15T-28T but I don't want to buy them all at $17 a piece. I was thinking of just getting the 20T and 25T but thought I would check the forums and see if anyone had some knowledge on this subject first... any help is greatly appreciated and thanks in advance. Pics are before I installed new carb and reed valves
 

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Street Ryderz

Well-Known Member
Joined
Oct 14, 2016
Messages
3,066
Hey everybody. I currently have a 30 tooth sprocket on my bike and it still has tons of torque and power left to give. The low end torque is kind of lacking, but that is typical for the oko carb and is not my concern but once you get to about half throttle its like you hit the NOS bottle (I have no NOS or boost bottle) and hang on to your hat. At about 3/4 throttle the engine is screaming and I know it can handle a smaller sprocket. I still have a little fine-tuning to do on the carb but as is right now it tops out at about 43 Flatlands and 45 or so downhill and the engine is screaming.(too much)* I want to go faster and so does the motor. I was wondering what's the smallest sprocket anyone has used with sucess and what was your weight and speeds you had reached. I weigh about 180 lb and my bike is a 26in frame tank type has a pk80 race engine built by Fred over at CR machine along with a combustion chamber pipe, 21mm oko carburetor (45 IDLE, 2ND HOLE ON NEEDLE,95 MAIN), reed valve w windowed piston, and fully ported cylinder for optimal top end and 3rd transfer port done by Fred plus a couple mods I do myself to the piston which I am not at liberty to discuss because of competition. I am an experienced rider and have found some sprockets ranging from 15T-28T but I don't want to buy them all at $17 a piece. I was thinking of just getting the 20T and 25T but thought I would check the forums and see if anyone had some knowledge on this subject first... any help is greatly appreciated and thanks in advance. Pics are before I installed new carb and reed valves
First let me get this straight,the engine is screaming too much at 3/4 throttle and tops out at 43,but the engine want's to go faster?Do you even realize at what rpm that is? That poor engine LOL,try to reduce the header length first and re tune for that but tbh that's not a good pipe for competition.
 

The_Aleman

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Joined
May 2, 2007
Messages
948
Back in 2010, MotorBicycleRacing and I saw Bairdco go GPS-verified 55MPH using a 28T sprocket, but that bike had 24" tires and he's a compact dude, weighed less than 150lbs. That's an 11.48:1 ratio and works out to a bit under 9K RPM. Last time I talked to Baird, he said he's gone even faster.

A 30T (12.3:1) on 26" at 43MPH is a lil under 7K RPM. Going to a smaller sprocket is gonna hurt the low-end even more, and may actually reduce your top speed. Those highly tuned chinagirls like to rev. I agree with Street's observation about your exhaust. If your engine is having trouble revving over 7K RPM, you may have some kind of exhaust restriction. Even a stock chinagirl will exceed 7K RPM readily if it's not overtaxed. I've gone 42MPH with a 34T with nothing but intake and exhaust port-matching, that's ~7500RPM. I weigh 215lbs.
 

JerboaJohn

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Joined
Jul 29, 2018
Messages
1,272
I have this 11T drive sprocket,, it'll give 10% more top end at expense of acceleration (vs. a stock 10T. I don't see em very often that's why I got it but hadn't used it
 

Spare_Parts

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Joined
Nov 29, 2017
Messages
1,754
That pipe is killing you. I have gone 54mph with 11/36 sprockets and 26" wheels. That was with the MZ65 pipe. Think 47mph was tops with the pipe you have.
 

FNTPuck

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Joined
Jul 2, 2018
Messages
510
something's not adding up here
They are correct. That pipe is better than stock, but not a top performance high RPM part.

The chamber on that pipe is small, and it doesn't have enough volume to scavenge at high RPMs. Compare that chamber to the mz65 or even better a converted dirt bike pipe and you will see the difference.

The exhaust will make or break any combo on a 2stroke, and a good pipe is the single biggest performance boost to a stock kit.
 

JerboaJohn

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Joined
Jul 29, 2018
Messages
1,272
no doubt, but youd think that pipe + nice engine would do more than a stock setup, or are you saying, the pipe is too stuffy to even let it run at least to a tuned stocker's speeds?
 
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