Gearing making my head spin

heftynoma

New Member
I calculated my gearing when building the bike to have a top speed of 55 and cruise at 50. I have only gotten my bike up to 42 and I don't know the rpm but I was assuming 7,000 just based on noise alone. I live at 800 ft above sea level just for context. Test runs were made on perfectly flat road basically straight. According to these graphs I'm only reving to 5 grand but it don't sound like it. I'm gonna get a rpm gauge and test again. What rpm do you guys shift at and cruise at.

Bike is a 80 passport with a lifan manual 125 for a pitbike.
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dirtbkr188

Active Member
The speed calculator that I use shows the "caution zone" for RPMs starts at 10K, you should be able to get to 8K easily with the Lifan 125. I entered your numbers and the results are very close to yours, my results are shown below.

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heftynoma

New Member
The speed calculator that I use shows the "caution zone" for RPMs starts at 10K, you should be able to get to 8K easily with the Lifan 125. I entered your numbers and the results are very close to yours, my results are shown below.

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Thank you for helping me. I'm gonna see if I can get a tach off of Amazon prime, but i'll take it out today and see if I feel comfortable pushing it another 5mph in each gear. I will report back later today.
 

Robert thran

Well-Known Member
Thank you for helping me. I'm gonna see if I can get a tach off of Amazon prime, but i'll take it out today and see if I feel comfortable pushing it another 5mph in each gear. I will report back later today.
Look for a “tiny tach brand” it’s just a little digital box you can mount with Velcro and easy to hook up ,,,you just wrap one wire from the tach around your spark plug wire..we used them on our sea doos for fine tuning , there water proof and last for ever..
 

heftynoma

New Member
Just got back from a 11 mile ride and got it up to 47. So I guess that's just 6k rpm. I'm still doing some carb tuning for under 1/4 throttle. Seems to fall on its face bog, then immediately picks the front tire up in 1st and 2nd. But anything after 1/4 throttle is smooth as can be.
 

dirtbkr188

Active Member
Can you post a few pics of the carb you have on the bike, if you don't mind? It might just be an air or fuel screw adjustment to try and eliminate an off-idle hesitation. Also, do you find that you run out of gearing real quick, and have to short-shift it into the next gear? possibly a 17T front sprocket to start with, being easier to swap out than a smaller rear sprocket.

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heftynoma

New Member
Pretty sure it is a 26mm. When i first rode it the needle was in the middle and it was running super fat. Pitch black plug. So I moved the clip up one notch from middle and that fixed 1/4 to full throttle. I have the adjustment screw pretty far in cause I assumed it was getting choked with fuel. Thank you for the help. By the way I've never had to use the choke to start it. I leave the kill switch off pump it twice and it starts first kick every time. I've done this in 25° 35° and today was 55° is that normal.
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dirtbkr188

Active Member
It appears to be a Mikuni-clone carb, it may measure 26mm at the intake flange, but it is usually tapered down to 22mm+/- at the carb throat. It has a fuel adjustment screw located under the intake and in front of the float bowl. Because it is a fuel screw, you turn it OUT to richen the mixture, and IN to lean it out. Do you know how many turns OUT from seated the fuel screw is?
 

OLD CT

Well-Known Member
Also pull the plug and check if it lean or rich. The knockoffs are usually jetted too lean, leaving you with a ''lean'' bone white plug.
 

bruces

Active Member
The exhaust at the cylinder has a big dent in it ,and if it’s stock for the passport it is probably undersized for the 125 you are using .
 

heftynoma

New Member
The snow finally melted and I went for a ride today. I unscrewed the fuel screw 3/4 of a turn and it smoothed out the 1/4 throttle hiccup I had. The exhaust is custom and the first inch is 1.25'' cause that's the biggest that will fit in the head. And the rest is 1.5''. That tiny dent is there so I can get a screw in cause it's to big for the factory stud.
 

KCCats

New Member
I have a Lifan 110 with that carb and a 125 exhaust
It has a 105 main and 17.5 Pilot
And it still has to have the choke on to idle
I'm old and have been doing this 45 years or more
Granted I mostly work on two strokes.
But if the screw is in Front of the slide isn't it air?
And behind is fuel?
Some of the Big 40mm rack carbs for Polaris have both!
 
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