2 stroke chinese miscellaneous


Local time
2:57 PM
Apr 20, 2008
i have a few miscellaneous questions about the 2 stroke engine. it all works great, first off!


1. How far out should the idler screw be turned? i think i have it set to maybe one and a quarter - one and a half turns from all the way in. seems to be working ok, but instructions said to 'turn it till it idled at 4,200 rpm'. what am i supposed to count?

2. ive read about the engine 'breaking in' and then going faster. i have the 44t sprocket, and i go about 30mph flat surface. for the first tank i did 16:1, now im on the 20:1 ratio. i took it easy the first tank, but now i open it up often.

(3 and 4 may go into a different section. if no one answers, ill post elsewhere)

3. if i have mountain bike tires as shown, about what air pressure should i hold in both? should it be a little less since ill be going so fast? i think they are 26".

4. i bought a Bell F-20 bike computer from Fleet Farm (or Farm and Fleet you crazy southerners. :] )
anyways, how do i know that the reading is accurate? put the mounting bracket by the tire and the magnet on the spoke, but does it matter how far up or down you put it? it has to be adjusted up and down to get the mount and the tire magnet close to each other.
i was cruising with one friend on a moped, and he said we were going 25 and thats what my speedometer said. was riding with another friend and when i said 25, he said 30.
the man with 2 watches never knows what time it is.
the man who mounts the cheap computer on his bike never knows how fast he is going?

thanks for any help!
I use the idler screw to adjust to where the engine runs about as slow as I can get it to run smoothly when the clutch is disengaged. You have to give it a slight amount of gas when you engage the clutch, but not too much or you'll burn the clutch.

Not sure exactly what your second question is. Everyone will have a slightly different ratio that they mix to. I "broke in" at 16:1 for about 3 tanks and I have been gradually changing it but I probably won't go much if any past 20:1.

I run my tires about what I would run them normally-but check they before EVERY ride and if they are low top them off before you ride. IMO riding with low tire pressure is a sure way to reduce the life of your tires/tubes. As far as I understand it the tire pressure is based on a certain weight the tire was ment to carrry, not so much the speed it was ment to go.

Usually the directions for the bike computer will tell you aprox. location of the magnent and pickup for the spedo. Most of the time it is closer to the outer part of the wheel. It bases most of the calculation on counting how many times the magnent passes by. Putting it close to the center of the wheel makes it come by a little faster but not enough to change things too much. Also-you usually have to program the computer for the correct tire size.

Hope this helps.

Idle screw should be turned until it sounds like it's idling and it doesn't bite into the clutch at low speed take offs.
Take it easy in the break in, be sure stop every so often to allow the engine to cool down, be sure to vary the RPM, but don't hold WOT open for too long.
ive noticed that since i adjusted the idle screw to about one and a half, it sounds pretty good. so i think im just going to leave that. when it came, we forgot to check it. so it worked ok, but it was out like 4. >.< i got about 20 miles to the half gallon tank. then i remembered to check it, and that was obviously the problem. now im easily getting 100-150 havn't quite checked yet.

lately ive just been taking little trips with it, instead of holding it open all the way, to try and hold it about 25mph. better on gas mileage, anyways.

as for the on board speedometer, it did have a chart with numbers, depending on the size of your tires. i put in the correct number, but im not sure how accurate it is since my magnet is mounted about in the center of the spokes, because that is where i got the best clearance. i know how to change the number, and im wondering if i put in maybe a slightly smaller number, it would be more accurate?

here is what the charts were.

Mountain Bike:
24" - 1888
26" - 2045
27" - 2155
28" - 2237
"The wheel value is the number of millimeters per one revolution of the wheel".

i think mine are 26", so i put in 2045, but since the magnet is more centered, it may not be accurate?

thanks again for any help on this last question.
The instructions that came with my kit said the idle screw should be 4.5 turns out. Once the engine warms up I stop and adjust the screw a bit to get a minimum maintainable idle. (Until it sounds "right" to me).

Tires: I always inflate my tires to 5 lbs under the recommendation printed on the sidewall of the tire. I think they ride smoother this way and can compensate for expansion in this Florida heat.

Speedo: The dimesion of the tire can vary up to 1/2" either way depending on weight and inflation. When checking with a tape measure, it's a "best guess" scenario anyway. I don't think absolute accuracy is attainable for long.

As far as positioning the magnet, one revolution is one revolution regardless of where you position the magnet on the spokes. (Think about it).

Just my opinion.
Gaskicker: you may have a different kit than mine, so the idle screw could be completely different... my screw comes out at about 5 turns. 0.o

when it was out at 4, the gas was leaking down real quick and it got into the air filter a little bit. i got terrible gas mileage because most was making it out unburned.

and 5lb sounds about right, i just wanted to make sure that they weren't in any real danger of being broken at full inflation going faster than normal bicycle speeds.

i could also stop being an idiot and look on the side of the tires where it says, 26". this was after i got the tape measure and guessed 26" anyways. :)

now that i think about it, you are right. i thought closer to the center of the tire would mean faster revolutions, but no matter what the spoke isn't gonna move. so 1 revolution IS one revolution. maybe that's why they didn't specify where. :)

thank you all very much! all my questions were answered!
Here is my response:

Engine idle is not set by number of turns out but set at an rpm that provides a reliable idle at the slowest possible engine speed. Note that if you idle too long at slow speed that fuel/oil can pool in bottom of crankcase and cause engine to stumble off line especially if your mixture if rich and most are at idle.

32:1 is what I use in all of my 2 stroke engines during break in and 40:1 thereafter. 16:1 is doing nothing more than spewing unecessary pollution and contributing to the bad reputation that 2 strokes have when you drive by people or other riders and smoke them out.

Tire Pressure: First of all, you should be more concerned that you have a quality tire and tire tube. If you don't have a tube that is one of those "no flat ones" then you should get them. They are safer and will save you lots of grief on long rides. I use tubes that are heavy duty and have slime in them. I also have used on some builds, tires that were flat resistant. My record is 4 flats in a span of 30 miles during a ride. That taught me quick to use quality tubes and tires. Pressure: depends on how you ride. My tires are rated for 65psi. I run them at 50psi in city driving and I lower pressure to soften the ride if I ride on dirt trails.
4. ... does it matter how far up or down you put it? it has to be adjusted up and down to get the mount and the tire magnet close to each other. ...

So here's the deal with speedometers. All the computer does is measure the number of times the magnet passes by the mount in a given time window. To make the calculation, it uses an internal reference clock to define a time window (each time your speedo updates the speed is a new time window), and a counter to keep track of the number of passes in each time window.

As far as where along the spokes to put the magnet/mount pair, it really shouldn't matter (much), and here's why: No matter where along the length of the spoke you put the magnet, it will ALWAYS pass the mount X number of times per second. Think about it. Pick a vertical spoke. As the wheel rotates, every point along that spoke is in this vertical line position at the exact same time each revolution, and only once per revolution (not counting upside-down). The only difference between being close to or far from the axis of rotation of the wheel is the linear velocity (not angular velocity) of the magnet. On the outside near the rim, the magnet would be moving at, say, 10 mph, but close to the axle, the magnet would be moving at, say, 5 mph. Over our range of speeds, this difference in linear velocity shouldn't make much of a difference.
skyliner70cc: thank you for your response on all of my questions!
i adjusted the idle a tiny bit more in, and it still idles good. i think i found the sweet spot. in and further and i think it would die.

many people use 16:1 to break in and 20:1 after that. that is what the instructions said, after all. im a little weary that you nonchalantly halved the amount of oil you use...

my tires are rated 40-65psi, so i just inflated them to 40 for now. if i feel they still are a bit flat, then i will go to 45. i just wanted to be sure they would not explode at high speeds.

likearock111: that definitely makes alot of sense. i wanted to make sure that your example of 5mph inner and 10mph outer wouldn't be a big issue when i was going lets say 20 or 30 mph. i think after 10-15mph it's pretty much all the same. thank you for your insight!

by the way, my name is Dan. i never introduced myself. i have the chinese 80cc (70cc) kit. i finished it last tuesday, and can't get enough of it! i put it on a Schwinn Mesa Mountain bike. with a few new brackets to accomodate for the thick frame, my dad and i were able to get it on no problems.

i live in Appleton, Wisconsin. not a whole lot of people from up here, but im down south a bit farther during the winter anyways. i just needed a cheap way to efficiently get around town, and building a bike kit was def the answer!

on my to do list:
1. get a "ngk-b6hs" spark plug (according to the Pre-build checklist i found here)
2. get... some sort of new boot and spark plug wire. need help here, too. i don't know anything about the types of wires or boots or how to replace, etc.
one of the inside clips of the boot broke when it did not fit on the cheap chinese spark plug. it works, but im 90% sure i can get 3-4mph by replacing the spark plug and the wire/boot.


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