Piston Scoring/Loss of Top End Power! HELP!!!

[Ecarta]

Why am I scoring pistons? Why am losing top end power?

This might be a bit long. But I want to give all of you all the background info and photos to help you troubleshoot this best way possible. OK here goes…

One year ago, Feb 2021 I picked up my first 2 stroke, a 2004 KDX200 in very good condition. Not sure how long it sat, but it did sit for some time. There was a lot of spooge in the exhaust system. Cleaned it up, rebuilt carb and it was running fine.

Had little problems all summer. But after 30 hours of riding in Sept 2021 I noticed it was down on top end power. Out of curiosity I removed head pipe to take a look at the piston. There was significant scoring. (see pic of old piston). I removed the top end and the cylinder was also scored but not as bad. (Sorry no pic of that)

I bought a new Wiseco Piston/rings and sent it, and the cylinder to Powerseal USA for re-plating. When it returned I rebuilt the top end, with all new gaskets. Then I followed the Wiseco break in procedure to a “T”. I went on 2 or 3 rides about 5 hours. Still seemed down on top end power. But I noticed a light coating of oil on crankcase behind cylinder. Ended up being bad base gasket. About a half inch of gasket was like “sucked in”, but yet it was blowing out a ton of air and oil mist/mix.

I thought, “Air leak, that’s why its down on power.” So, I rebuilt the top end again, with all new gaskets. The base gasket was no longer leaking. But it still seemed l down on top end power same as before, Top speed 55-60mph and its screaming!

Removed head pipe looked at piston...Its starting to score again!!! WTF???!!! Cylinder starting to score again too..all at just 10 hours. (see pics). I also noticed some milky trans fluid as well. So now I am in the process of a complete engine rebuild, I plan on replacing crankseals/bearings and water-pump seals/bearing.

However, upon removing the right side engine cover, I noticed the “the governor lever thingy” at the bottom of the KIPS advancer shaft was bent/smushed.

Now is it possible that's why I was down on top end power?? And if you notice exhaust side of new piston in pics, there is a horizontal line that matches up perfectly with the one KIPS exhaust valve, that matches the width of the scoring. Maybe KIPS valve was stuck to close and interfering with piston because that lever thingy was bent at bottom of KIPS advancer shaft??

What else could be causing my piston scoring and loss of top end power? Are they related? What are your thoughts?

Please be respectful, I am new to all this and still learning! Thanks!

[John_S]

Wow I’m sorry to see this. You mentioned that the base gasket was no longer leaking after you replaced it. An engine with a slight air leak can run fine grunting through the woods for a long time but would amplify the issue much quicker if you were doing long wide open runs.

Did you do a leak down test after the new gasket set?
Do you know where the timing was set? Could hurt top end power if too far retarded.
Do you know the main jet was the correct size with plug chops or working your way down from too rich. A lean main jet would definitely be down on power and rpms and get the piston real hot on top speed runs.
What oil ratio are you running?

[bufftester]

Your KIPS governor not operating correctly is the cause of your power loss. It is supposed to operate the kips at ~6000 rpm. If it doesn't, you will get exactly what you describe. As for the scoring, that is a different issue. I would check the KIPS main valve to see if it is protruding into the cylinder at all (it shouldn't).

[SS109]

I would say debris caused that scoring. It is on more than one side of the cylinder so it couldn't be just the center power valve causing that problem. I would say the engine is sucking dirt in from somewhere. Check how your air filter fits, the air box to air box boot joint, air box boot to carb connection, and your carb to intake manifold connection. Is your air filter properly oiled with no tears? Also, it looks like on the cylinder that the ports are not properly chamfered or am I seeing that wrong?

Yes, the KIPS governor pin would be the reason you have no top end power as the power valves are not opening. Make sure to get a new pin and follow the proper installation procedure (to the letter!) for connecting the KIPS linkage or you will bend/break the new one.

[Ecarta]

Wow I’m sorry to see this. You mentioned that the base gasket was no longer leaking after you replaced it. An engine with a slight air leak can run fine grunting through the woods for a long time but would amplify the issue much quicker if you were doing long wide open runs.

Did you do a leak down test after the new gasket set?
Do you know where the timing was set? Could hurt top end power if too far retarded.
Do you know the main jet was the correct size with plug chops or working your way down from too rich. A lean main jet would definitely be down on power and rpms and get the piston real hot on top speed runs.
What oil ratio are you running?

Thanks John S for your input!
-I did do a leak down test on new gasket kit -Held 6psi with piston BDC. Only went down to 5psi after an hour until I disconnected it.
-No not sure where the timing was set. Just learned about stator placement after I removed it! lol How does that affect power?
-Havent done any plug chops yet, I just know that jets are stock and clean, how do you do plug chop anyway? What are you looking for?
-I did engage in regular high speed runs on back roads to get to trails. Like 7 miles each way. On old piston I might be 90% throttle for like a minute or two at a time. Until buddy of mine told me to avoid that on 2 strokes. On new piston I might wind it out to WOT for 5 seconds max, until I pull in clutch, let is coast for a bit, then wind it out to 90%, pull in clutch coast, repeat.
On old piston I ran 40:1, after replate/top end rebuild/new piston I have been at service manual recommended 32:1 Figured that would give more lubrication.

[Ecarta]

Your KIPS governor not operating correctly is the cause of your power loss. It is supposed to operate the kips at ~6000 rpm. If it doesn't, you will get exactly what you describe. As for the scoring, that is a different issue. I would check the KIPS main valve to see if it is protruding into the cylinder at all (it shouldn't).

Thanks bufftester for your input!
Yes I was wondering about that, thanks for confirmation. So we know that is at least part of, if not the source of the top speed power issue! Yes I am going to reassemble the kips valve in cylinder and see if it protrudes at all. It didnt seem like it did from glancing at it during removal. But now that Im looking closer, I have to rule that out. I mean were talking thousandths of an inch of protrusion might be enough to cause issue, you agree?

[Ecarta]

I would say debris caused that scoring. It is on more than one side of the cylinder so it couldn't be just the center power valve causing that problem. I would say the engine is sucking dirt in from somewhere. Check how your air filter fits, the air box to air box boot joint, air box boot to carb connection, and your carb to intake manifold connection. Is your air filter properly oiled with no tears? Also, it looks like on the cylinder that the ports are not properly chamfered or am I seeing that wrong?

Yes, the KIPS governor pin would be the reason you have no top end power as the power valves are not opening. Make sure to get a new pin and follow the proper installation procedure (to the letter!) for connecting the KIPS linkage or you will bend/break the new one.

Thanks ss109 for your input!
Yes, it does look like debris could be cause for scoring. But I am usually pretty good at keeping filter oiled, especially after replate/rebuild. I always grease that rim of filter so it seals tight into airbox and screw thumbscrew all the way. I have inspected the carb to reedcage boot multiple times and it looks perfect. The other 2 areas I have also inspected but not as closely...I will inspect them again in greater detail.

About the ports, you are correct they are not chamfered. Didnt know that was a thing until last week..lol..just had a former motorcycle mechanic do a quick inspection of cylinder and pistons. He works at local AutoZone had some customers waiting... Anywho, he did take 5 minutes to check it out, he felt the ports and was like, "They need chamfered". So good eye! OK so how do I do that with ruining the cylinder??

And yes I will follow KIPS linkage assembling directions to a "T"

[SS109]

-I did do a leak down test on new gasket kit -Held 6psi with piston BDC. Only went down to 5psi after an hour until I disconnected it.

Sounds good.

-No not sure where the timing was set. Just learned about stator placement after I removed it! lol How does that affect power?

KDX's are set at the zero setting from the factory. Advancing it gives more low end power while retarding it gives more upper rpm power.

-Havent done any plug chops yet, I just know that jets are stock and clean, how do you do plug chop anyway? What are you looking for?

Mr. John_S has plug chops down to a science! Here's his very own thread on doing them: viewtopic.php?f=105&t=21813 Thanks again John_S for your awesome write up sharing this valuable info!

-I did engage in regular high speed runs on back roads to get to trails. Like 7 miles each way. On old piston I might be 90% throttle for like a minute or two at a time. Until buddy of mine told me to avoid that on 2 strokes. On new piston I might wind it out to WOT for 5 seconds max, until I pull in clutch, let is coast for a bit, then wind it out to 90%, pull in clutch coast, repeat.
On old piston I ran 40:1, after replate/top end rebuild/new piston I have been at service manual recommended 32:1 Figured that would give more lubrication.

Please don't believe that running your bike at high speed is somehow bad for it. If a bike is in good running condition, and properly jetted, you can run any liquid cooled 2T at highs speeds for long stretches with zero issues. If it couldn't be done then the old air cooled 2T street bikes from the 70's would have been seizing left and right. I regularly rode my hopped up RD350 on long road trips and never worried about it. I've ridden my KDX, holding 50-55mph, running a 40:1 mix, for miles without issue as well and so has all my buddies riding all the big brand bikes and some running 50 or even 60:1 mixes.

Question, are you running a quality motorcycle rated 2T oil?

Yes, it does look like debris could be cause for scoring. But I am usually pretty good at keeping filter oiled, especially after replate/rebuild. I always grease that rim of filter so it seals tight into airbox and screw thumbscrew all the way. I have inspected the carb to reedcage boot multiple times and it looks perfect. The other 2 areas I have also inspected but not as closely...I will inspect them again in greater detail.

Check real close on the air box to air box boot. Some have found they don't seal well to each other and have had to use sealer between the two. A bright flashlight can help in locating any gaps. Oh, forgot one, also make sure the air box boot is mounted completely on to the intake of the carb. It can be easy to get it only part way on yet looks good from one side. Check both sides visually.

About the ports, you are correct they are not chamfered. Didnt know that was a thing until last week..lol..just had a former motorcycle mechanic do a quick inspection of cylinder and pistons. He works at local AutoZone had some customers waiting... Anywho, he did take 5 minutes to check it out, he felt the ports and was like, "They need chamfered". So good eye! OK so how do I do that with ruining the cylinder??

Here is a video I found showing an easy way to do it by hand. How To Chamfer Ports On 2 Stroke Scooter Cylinders There are many videos on youtube covering the same topic, going in to more detail about why it needs to be done, but most like using power tools. I prefer the hand method as there is less of a chance of slipping and doing damage to your cylinder. Of course, do what you're comfortable with.

And yes I will follow KIPS linkage assembling directions to a "T"

[Ecarta]

-I did do a leak down test on new gasket kit -Held 6psi with piston BDC. Only went down to 5psi after an hour until I disconnected it.

Sounds good.

-No not sure where the timing was set. Just learned about stator placement after I removed it! lol How does that affect power?

KDX's are set at the zero setting from the factory. Advancing it gives more low end power while retarding it gives more upper rpm power.

-Havent done any plug chops yet, I just know that jets are stock and clean, how do you do plug chop anyway? What are you looking for?

Mr. John_S has plug chops down to a science! Here's his very own thread on doing them: viewtopic.php?f=105&t=21813 Thanks again John_S for your awesome write up sharing this valuable info!

-I did engage in regular high speed runs on back roads to get to trails. Like 7 miles each way. On old piston I might be 90% throttle for like a minute or two at a time. Until buddy of mine told me to avoid that on 2 strokes. On new piston I might wind it out to WOT for 5 seconds max, until I pull in clutch, let is coast for a bit, then wind it out to 90%, pull in clutch coast, repeat.
On old piston I ran 40:1, after replate/top end rebuild/new piston I have been at service manual recommended 32:1 Figured that would give more lubrication.

Please don't believe that running your bike at high speed is somehow bad for it. If a bike is in good running condition, and properly jetted, you can run any liquid cooled 2T at highs speeds for long stretches with zero issues. If it couldn't be done then the old air cooled 2T street bikes from the 70's would have been seizing left and right. I regularly rode my hopped up RD350 on long road trips and never worried about it. I've ridden my KDX, holding 50-55mph, running a 40:1 mix, for miles without issue as well and so has all my buddies riding all the big brand bikes and some running 50 or even 60:1 mixes.

Question, are you running a quality motorcycle rated 2T oil?

Yes, it does look like debris could be cause for scoring. But I am usually pretty good at keeping filter oiled, especially after replate/rebuild. I always grease that rim of filter so it seals tight into airbox and screw thumbscrew all the way. I have inspected the carb to reedcage boot multiple times and it looks perfect. The other 2 areas I have also inspected but not as closely...I will inspect them again in greater detail.

Check real close on the air box to air box boot. Some have found they don't seal well to each other and have had to use sealer between the two. A bright flashlight can help in locating any gaps. Oh, forgot one, also make sure the air box boot is mounted completely on to the intake of the carb. It can be easy to get it only part way on yet looks good from one side. Check both sides visually.

About the ports, you are correct they are not chamfered. Didnt know that was a thing until last week..lol..just had a former motorcycle mechanic do a quick inspection of cylinder and pistons. He works at local AutoZone had some customers waiting... Anywho, he did take 5 minutes to check it out, he felt the ports and was like, "They need chamfered". So good eye! OK so how do I do that with ruining the cylinder??

Here is a video I found showing an easy way to do it by hand. How To Chamfer Ports On 2 Stroke Scooter Cylinders There are many videos on youtube covering the same topic, going in to more detail about why it needs to be done, but most like using power tools. I prefer the hand method as there is less of a chance of slipping and doing damage to your cylinder. Of course, do what you're comfortable with.

And yes I will follow KIPS linkage assembling directions to a "T"

Great info ss109! Especially about the timing....also Ill be sure to do plug chops after I reassemble and follow John S's directions!

I am using Lucas Oil semi-synthetic 2 stroke oil. IDK Is that any good?

And ok yes I'll be sure to check both sides of airbox/connection on reassemble.

[John_S]

Great points from SS. I’ve heard good things about Powerseal but it was their job to do the chamfer, right? I would at least contact them and show them what you’ve got going on. This could’ve caused or contributed to the problem. I’ve always read how important it is on a replate or bore job to chamfer the ports so the rings don’t expand and catch on the sharp edge.

[SS109]

I am using Lucas Oil semi-synthetic 2 stroke oil. IDK Is that any good?

That's the same oil I've been running for years now. Good stuff.

[TwistedRoot]

My '02 200 cylinder looked almost as bad. I think the petcock filter came loose and there wasn't anything stopping sand and dirt from getting in there. Im glad I used Powerseal for the replating and am now using an inline filter to keep it super clean. Some don't agree with the in-line filter but I have had zero issues fwiw.

[SS109]

My '02 200 cylinder looked almost as bad. I think the petcock filter came loose and there wasn't anything stopping sand and dirt from getting in there. Im glad I used Powerseal for the replating and am now using an inline filter to keep it super clean. Some don't agree with the in-line filter but I have had zero issues fwiw.

Yup, no reason to not use a fuel filter. It's not like our bikes drink so fast that a filter restricts the flow. I do prefer the paper element filters, though, or anything that can filter down to about 50 microns or less.