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Discussion Starter #1
I just got a 90 KX100 that was sleeved and bored to 54.5mm. The guy I got it from couldn't get it to run after boring it. I got it to start and rev without a load but I don't have enough power to use second gear. I took it apart and the exhaust port is cut larger in the sleeve than in the jug? I'm not sure if I should start over with my 80cc spare jug and having it sleeved and ported? I've been looking for a 90 kx100 sleeve but can't seem to find one. Are there any places out there that still have any of these? Thanks
 

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an odd sounding problem. The exhaust port should have been cut to match but no one here knows how bad the mismatch is. If porting is really screwed up it will not perform.

So it ran and you just took it apart?
Did you try adjusting the clutch cable? The motor can rev to the moon but if clutch slips then it will hardly move.
 

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Discussion Starter #3
the top of the exhaust port in the sleeve is about 3/8ths of an inch higher than the aluminum in the port. I thought about taking my dremel and cutting it to match. Not sure that will help. I'm wondering if the sleeve was for a newer model
 

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the other ports inside are probably more important. Do those match?

Look at the big picture. If they are all wrong you have a serious problem.

3/8" sounds way off. If all the rest of the ports are off at the top like that then that is the problem. The port windows would be tiny if I am correctly imagining what you are describing since you provided no pictures. The sleeve job would be botched and most likely the cylinder is junk cause it would be cheaper to buy another one.
 

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Discussion Starter #5
The other ports match. I read that the ports are tapered on the 100 and that boring to 54.5 will change the port height and timing. It did run before punching it but was low on compression and started a bit hard and could have had more power. Its much worse now. It sounds like the engine is misfiring when under a load. I have the stator set on the middle timing mark which is the stock setting for an 80cc but I have no info on the 100 to know if it should be different. I'm going to open the port up tapering it back so it has a gradual slope into the exhaust port.
 

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boring usually always lowers port timing height which can hurt performance. The bigger the bore job the worse it gets.

Stator timing will be the same.

Too lean? Too rich?
 

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Discussion Starter #7
Thanks for the responses. It's running really rich. When I put the engine in last week it had 155lbs. I only ran it 5 minutes or so but my friend that had the bike just told me on the phone he didn't know if the rings were right side up or not. Could that cause a run ability issue? I looked for a marking on them but there is a water spot where the ring gap is so I cant tell. I want to get a new piston and rings to get rid of this variable but am worried I may still have issues due to the ports. This bike also started out as an 80cc I found out so I'm wondering if there are any other parts of the bike that are different. I already swapped the main jet for the 100cc one but I don't have a clue if the jet needle is the same or not. Maybe the clip is in the wrong spot on the needle. I'm thinking fixing the exhaust port and checking every spec on the bike is a good starting point along with new rings. Where can I find specs for this bike? Were there any stock kx100's in 90? I have a book that covers 83-90 60-80cc but no 100cc info. I am also running low octane gas and was thinking about trying super.
 

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i think someone on here did this 10 conversion and posted the carb specs.

Low octane may cause knock, especially at high RPM.

Rings should have numbers printed on them and those face up. You would probably have to remove the rings to see them. Dont know how much diff it makes but they say to install them that way.

The exhaust port would not kill performance. But I would open it up.
 

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Discussion Starter #9
Thanks wish I had read about jetting before removing the jug. I'm going to open the exhaust port, get a 100cc jet needle and play with the jetting from there. If all else fails I do have an 89 kx80 engine but I hate to go that way if I don't have to.
 

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Discussion Starter #10
It was a jetting issue. Running ok now. I opened the exhaust port first and it made very little difference. The bike seems to be all low end and mid range power. Is there any way to make it have less bottom end and more top end power?
Also is there a way to get a harder hit out of the power band? I noticed the 80cc bikes have a 49 tooth rear sprocket and my bike has a 54 tooth. The bike is in real nice shape but has an exhaust jimmy rigged from a newer bike which makes the air filter box not fit and there is hay wire for the mount buy the tank so I'm thinking I will buy a pipe and baffler. Which pipes preform the best with this bike (90 kx100). I know I can only get one made for an 80cc in this year.
For those of you who are wondering I didn't run the bike without a filter but the one I have on it is exposed so trail riding or racing will have to wait till I fix the exhaust issue. I have a little less than $100 in this bike so far so buying a few parts does not hurt much.
Thank you for the help nd4speed. The knowledge I got was worth a lot. I'm not new to 2 stroke bikes but never had an issue like this before.
 

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small displacement motors usually have little low end power. Having a larger than normal rear sprocket is gearing for low end power.

Power band is affected by pipe shape. I would look for cheap as possible pipe like a OEM one. But do what you want. I would chose a pipe made for the displacement size 100cc.

May airbox boot was ripped and I only recently got a parts bike with a good one. I removed the airbox and clamped a UNI POD filter directly to the carb- completely exposed on all sides. Depends on how often you want to clean it and on your riding conditions- lots of water = air box

A jerry rigged pipe could have been an important piece of info unless you are positive it was for a KX100.
 

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Discussion Starter #12
I know the pipe is from a mid 90's bike. Not sure if it was an 80 or 100cc. I do need to put my air box back in. I go through alot of water. I can't find a 100 pipe for that year bike. I called my local dealer and they said 95 is the first year the 100 was made from the factory. I noticed the exhaust seems to bolt up from the newer 95 and up bike but the pipe at the back of the expansion chamber and the baffler pipe has some bends that doesn't allow use with the air cleaner box.
I have a gas shielded mig welder and was thinking I could cut the bent section out of a 100cc pipe and replace it with a strait section of pipe the same size. Will modding the exhaust this way cause another issue or would I be better off using an 80cc Pro Circuit pipe and silencer that direct fits the bike.
The trails in my area are like a black water race track so if I suck in water my jug would be a done deal.
 
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