Just bought an EX and was considering re-jetting the carb and possibly an offset key. I want to keep the stock muffler and air box for now. I read somewhere a .032 worked well on a stock motor, any other sizes I should try? Any ideas on how much offset for the key? Any problems with the offset keys shearing off?
Light mods for the Coleman CT200U-EX
- Thread starter MJL
- Start date
Can't see why a .031 or 32 wouldn't work fine. I just run an open air filter 140 e tube and currently a 31 with the stock muffler. Don't get a lot of ride time but the engine runs just fine. Originally had a 33 in it and when I made the change to a 31 I didn't notice any difference at all.
If you are going in baby steps why not start out with a 4 on the key?
If you shear a key on a mini bike engine it would be caused by some other catastrophic engine failure. It shouldn't be a concern just for timing changes.
If you are going in baby steps why not start out with a 4 on the key?
If you shear a key on a mini bike engine it would be caused by some other catastrophic engine failure. It shouldn't be a concern just for timing changes.
The key is merely a reference point and should not hold the flywheel in place! The taper of the two parts holds everything together. If you shear a key, you have done something wrong!
The stock airbox is very restrictive, so like SAS said, a .031 or .032 will be fine for you. Just remember, it's called "tuning" and not "setting". There are a lot of different factors. We just worked on a EX with a header and air filter kit and it was very happy with a 140 e-tube and a .040 jet! This was on an otherwise stock engine
The stock airbox is very restrictive, so like SAS said, a .031 or .032 will be fine for you. Just remember, it's called "tuning" and not "setting". There are a lot of different factors. We just worked on a EX with a header and air filter kit and it was very happy with a 140 e-tube and a .040 jet! This was on an otherwise stock engine
you really don't need to do anything to the flywheel imo unless you're gonna do billet flywheel,rod and springs....to me gearing it right for ride area is first and then intake,exhaust and jetting..if plan running stock muffler and airbox cut out a sguare opening in front and back of the airbox to open it up some and get a 32 jet if you want it on the quiet side..these bike are pretty quick and alot of fun with a few tweeks
I agree about the jetting, but to make things happen, an air box delete (better filter) & a new exhaust header is really what you will need to make the motor run properly. Even with the governor in place, these simple mods will show more of a difference & make more improvement from stock than you would expect they would.
Doing these basic mods, replacing the head gasket with a thinner one & changing the timing will feel like a completely different motor to you, without sacrificing any starting ease or reliability. Best of luck with your plans & hope you enjoy the journey!
Doing these basic mods, replacing the head gasket with a thinner one & changing the timing will feel like a completely different motor to you, without sacrificing any starting ease or reliability. Best of luck with your plans & hope you enjoy the journey!
Thanks! Right now, I am sharing the bike with my kids who are new riders, so I have to be able to tame it down easily. I'm thinking about trying some of the other governor spring holes for a 4000-4400 rpm max, then using the throttle limit screw when the kids ride.
Quiet is also very important as I ride on my neighbors' property and want to be considerate. I did see where someone removed the muffler and "throated" it a bit where it meets the block for better air flow. I might give that a try.
The problem I have with the intake is how much most of them hang off the side of the bike. With the kids driving, I'm sure it would get knocked off, lost, etc. I saw someone use a some type of U bend to get the filter back under the bike which looked good.
I did put a 60 tooth rear sprocket on and it works much better on our hills and tight trails. Not quite a wheelie machine, nor would I want it to be (yet), but it does lighten the front end up nicely when I get on it.
I've got some other projects ahead of it, but I would like to find a cheap roller Baja and do a stage 1 212 with a TC and seat springs, just for Dad!
Quiet is also very important as I ride on my neighbors' property and want to be considerate. I did see where someone removed the muffler and "throated" it a bit where it meets the block for better air flow. I might give that a try.
The problem I have with the intake is how much most of them hang off the side of the bike. With the kids driving, I'm sure it would get knocked off, lost, etc. I saw someone use a some type of U bend to get the filter back under the bike which looked good.
I did put a 60 tooth rear sprocket on and it works much better on our hills and tight trails. Not quite a wheelie machine, nor would I want it to be (yet), but it does lighten the front end up nicely when I get on it.
I've got some other projects ahead of it, but I would like to find a cheap roller Baja and do a stage 1 212 with a TC and seat springs, just for Dad!
Last edited:
just add little what CarPlayLB had mentioned earlier.
before installing the flywheel add some lapping valve compound to the taper part on the crank and flywheel and lightly spin the flywheel back and forth. you should have a full gray color of lapping compound on both pieces. then clean parts off and re-install the keyway and flywheel.
sometimes we here the phrase do one thing at a time. but really re jetting,good air cleaner,header,advance timing ALL work together to make your motor feel noticeably different......
MJL yes that will help too.... there strong motors. it won't come unglued at 4000 rpm's
before installing the flywheel add some lapping valve compound to the taper part on the crank and flywheel and lightly spin the flywheel back and forth. you should have a full gray color of lapping compound on both pieces. then clean parts off and re-install the keyway and flywheel.
sometimes we here the phrase do one thing at a time. but really re jetting,good air cleaner,header,advance timing ALL work together to make your motor feel noticeably different......
MJL yes that will help too....
there strong motors. it won't come unglued at 4000 rpm's
Last edited:
If you're concerned with the filter getting knocked off, use a UNI foam filter instead of a K&N. The mounting for the UNI filter will bend & flex if the kids bump it, without knocking the filter off the carb. The K&N has no flex, so it could be dislodged if bumped.
https://affordablegokarts.com/colle...rs-160-200/products/air-filter-uni-dual-layer
https://affordablegokarts.com/colle...rs-160-200/products/air-filter-uni-dual-layer
Got some parts ordered. A 140 tube, a .032 jet, a couple spare jets to drill if needed, a couple idle jets to play with, a 5 degree key and a Autolite 3932x plug (a bit warmer than the 3910x). I saw reports of the Autolite plug taking up an additional 1-2 cc in the chamber, which by my math, would raise the compression to 8.5-9.1x. I noticed it seemed to run even leaner when I removed the air box cover. So I might play with the stock cover a bit as well. For anyone interested, I plan on making the changes one at a time, as much as I can, and test ride in between. It'll be kind of a "sleeper" engine...
Last edited:
Put my plug and .032 jet in tonight, with test runs in between. Right off I noticed the new plug made it idle better than with the stock plug. It usually idles very low just fine, but tonight, it wanted to die even when warm with the stock plug, probably due to the 20 degree temps. The new jet allowed me to restart a warm engine without choking, a nice improvement. Bottom end power and throttle response seemed a bit better, nothing earth shattering, though. Still seems to run better with the air cover on than off, so there still may be some room for more gas with the cover off. Etube should be in the next day or two.
Last edited:
Throated my muffler this morning. Took one Dremel grinding stone and about 15 minutes. Plugged the hole with a rag before grinding, then blew it out real well with compressed air afterwards. No huge gains, but the combination of the mods is starting to add up for sure. Also added a thumb throttle to make it a little easier to hang on tightly while on rough trails.
Attachments
Lapped my flywheel and shaft and installed the 5 degree key. Can't say for sure, but I almost feel like I might have lost a little power and/or response. Do I need higher octane fuel to take advantage of the timing advance? I'm running 90 octane ethanol free. I'd have to go to 10% ethanol to get 93 octane. Or does it just need more fuel when advanced?
Made the head slapping realization today that I wasn't getting full throttle. The new throttle cable must have stretched a bit around the time I put the timing key in. I tightened it back up and it now runs like a scalded cat! I can jump the smallest hump by blipping the throttle, but the front end still says (mostly) on the ground under full throttle acceleration. Coming out of turns is so much fun, I made lap after lap around our 4 acre pasture. Took a plug reading and it was white, so still some room for more fuel. My e-tube still has not come in, hopefully it shows up tomorrow. That should about do it for me, any more and I'm liable to hurt myself! Interestingly enough, the engine is running better than ever, but I seem to have lost some hill climbing ability. My thought is the stock clutch is slipping. Does this sound like the case? Overall, I'm very happy with the improvements.
Last edited: