Help - 2 stroke rebuild issues

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3/16/2021 5:56 PM

Hi there,

Daughter and I had a winter project of putting a new top end in her KX100. Seemed to go pretty smoothly and last weekend fired it up for the first time. Started after a few kicks and seemed to be doing its thing just fine.

After about a minute of just idling with a little throttle blipping, it just shut off. We tried to start it up and it wouldn't go, and then she noticed gas pouring out the carb vents. Turned the gas off, and parked it.

Googled the situation and it seemed to indicate stuck float in the carb. So tapped on the outside of the carb, tried starting again - just wouldn't fire. Lots of kicking, pulling the plug, checking to see if it's saturated and smells like gas - nope.

Checked for spark. Yup it has spark.

Pulled the bottom plug off the carb - no gas draining out. Ok sounds like we have a fuel delivery problem.

Took carb off, cleaned it thoroughly (longer story there but that's the important part). Confirmed clear passage into the float bowl, put it back on, tried to start again.

Still no fire. Lots of kicking. Tried bump starting. Rechecked spark. Yup. Spark plug still does not smell like gas. Pulled drain plug from carb. Fuel pours out. I assume this means I have gas sitting in the bowl but does not seem to be getting pulled into the cylinder.

When I kick, it really sounds like nothing is happening, then will occasionally sort of puff. But never sounds like it wants to start.

At the kickstarter, it really does not seem to have hardly any compression. It is not even close to blowing my finger off the spark plug hole when cranked.

So I bit the bullet and just pulled the cylinder off to have a peek.

I notice a lot of oil residue in the top end, and the rings are lined up on the index bumps (or whatever the right word is) but one thing that seems suspicious is that the rings do not "pop out" of the piston grooves. They seem wedged in.

It seems maybe that the rings not sealing might cause very low compression which perhaps does not cause enough vacuum to pull the gas in when I kick it. Does this sound reasonable?

What's the fix?


3/16/2021 6:10 PM

Sounds like the rings are stuck and not sealing/creating vacuum. The rings should float nicely in the ring grooves and compress/expand easily. Take a pic of the piston and cylinder and post.


3/16/2021 6:13 PM

They are definitely not floating nicely. The rings are wedged in the grooves pretty solidly.

Will see if I can get some photos embedded here.


3/16/2021 6:44 PM
Edited Date/Time: 3/16/2021 6:46 PM

Depending on the piston brand you went with some have directional piston rings. Look for a little letter on the piston ring and have the letter face UP. Double check the instructions but the marking or letter should usually face up.


3/16/2021 10:06 PM

Well, I'm an idiot.

I decided to take everything apart, and figured I better go ahead and hone the cylinder again. When I finished, I started to look closely at the various ports in the cylinder...and noticed there was a ton of residue from the honing process that ends up on the "non-cylinder" side of the edges of the ports.

I cleaned the cylinder after I honed it before, but probably not particularly well, and definitely paid no attention to anything that go into any of the ports.

So my theory was that the honing residue contaminated the cylinder, piston, ring grooves, and rings. Fortunately, it only ran for a minute before dying.

I cleaned the crap out of the cylinder, rings, and piston, and threw it all back together. Definitely had more compression this time around.

Started first kick dizzy

Lesson learned. Sometimes I wish I'd just pay people to work on bikes instead of insisting I do it myself and go through all these learning experiences. sick


3/17/2021 8:57 AM

garasaki wrote:

Well, I'm an idiot.

I decided to take everything apart, and figured I better go ahead and hone the cylinder again. When I ...more

But hey you analyzed, diagnosed, and solved the problem. Sounds like a a lesson learned along the way. I feel like this win story belongs under the category of why 2 strokes are better than 4!



3/17/2021 9:42 AM

What type of hone did you use? Generally speaking you shouldn't hone a Nikasil coated cylinder, it can remove the coating.


3/17/2021 11:19 AM

kieran_macca wrote:

What type of hone did you use? Generally speaking you shouldn't hone a Nikasil coated cylinder, it can remove the coating.

Not really. Nikasil is harder than most hones so it's usually a pointless act. Certain hones can chip the plating at port edges. A good brush hone is about the only type an average mechanic should be using on plated cylinders. Otherwise use scotch brite if you need to do anything.

After "honing" any plated cylinder - wash in warm soapy water then wipe down with ATF after dry until rag comes back clean. Assemble dry & ride it medium hard after one warm up.............


3/17/2021 1:01 PM

I used a ball hone. Stayed on the light side of it (time and speed). Seemed to leave a very nice finish afterwards.

This time I did use warm soapy water, much of it, and brake cleaner then WD40, compressed air blowdown, wiped down, assembled dry shortly afterwards. Same process for cylinder and piston and rings.

It's started first kick on 3 separate occasions. Once last night (late at night so I shut it down pretty quick), and I've let it idle twice today followed by a long cool down period.

So far seems to be moving in a positive direction. I definitely had a bit of a freak out yesterday.

I've learned a lot through this screw up though.