Help confirm my problem... assuming I'm running lean

Hi KTM thumpertalkers,

Been a while since I've used TT, but now I'm stumped.

For the last few years I've owned a KTM 200 EXC which I bought new in 2010 (I think, I can't remember :goofy: ) . The bike has been 100% great up until 6 months ago when it became hard to start... and then one day died and would not start at all. No big deal time for a top end. The hours seemed low compared to what a lot of KTM riders are quoting for rebuilds.

Anyway the old piston :

DSC05701_zps74485aa9.jpg

That is the exhaust side of the piston.

So I ordered a new piston and cylinder assembly and installed it. Bike started up nicely and was all good except I found that my powervalve was gummed up. I removed the cylinder to have another go at the power valve and look what I found with my new top end:

Scratchesnewpiston_zps0780054f.jpg

The new cylinder:

Scratches1_zps6cf660d8.jpg

The bike was warmed up generously, obviously a clean airfilter and was ridden how you would break in a bike. (Gentle but in varying gears and varying throttle openings, not WOT!) Approximately one hours engine running time.

So what is going wrong here. Help is much appreciated as I miss riding and probably can't afford to do this repair again too soon!

:rolleyes::(

Mark

If clearance is set right and ring gap is set, then I would put my money on jetting. It kinda looked like to me the wear was more on bottom of piston IDK it could just be picture, but in that case you could have bad crank bearing.

Post u picture of the inside up underneath of both pistons, along with the dome of them.

I will get photos of the new piston later today if you want them still, as it is not here at home with me. I have to admit that I did not check the gap in the rings, assuming that brand new rings, piston and cylinder would be correct from factory... This could have been where I went wrong .

As for the clearances, you mean shimming up the cylinder with gaskets to the right height as well as that of the powervalve? Done and done.

I've attached photos of the old piston:

Never mind the marks in the center of the crown, they were put there by me. The underside is not burned as you can see and there is no other scuffing on the piston other than what I've photographed.

Bike is jetted stock and I have only played with the throttle stop and idle (air) screw to get a smooth reliable idle.

Again thanks for the help.

Running lean can be caused by more than just jetting. I always do a "leak down test" on engines when doing a top end this will tell you if engine is sucking air from anywhere. As far as clearance I was referring to the gap between piston and cylinder wall. I have never ordered a new cylinder from the factory, but you should always check clearance and ring gap it's just good habbit. Just for your info what you set with the gaskets is called "X dimension". Now for my opinion I would reassemble check all stated clearance, ring gap, leak down etc, and then you can check jetting. Depending on how many hrs you put on it before you dissasembled might need a cross hatch put on cylinder.

Running lean can be caused by more than just jetting. I always do a "leak down test" on engines when doing a top end this will tell you if engine is sucking air from anywhere. As far as clearance I was referring to the gap between piston and cylinder wall. I have never ordered a new cylinder from the factory, but you should always check clearance and ring gap it's just good habbit. Just for your info what you set with the gaskets is called "X dimension". Now for my opinion I would reassemble check all stated clearance, ring gap, leak down etc, and then you can check jetting. Depending on how many hrs you put on it before you dissasembled might need a cross hatch put on cylinder.

Tims380 thanks for your help, exactly the kind of advice I was hoping to get. I will check and double check all these things and make them my habits too in the future. Will report back later this week on what I find.

It doesn't seem to be excessively lean as the bottom of the piston doesn't appear to have oil burnt onto it. Really high temps will do that, that's why I asked for the pictures. I'd look towards clearances next and make sure you have a good crank. After that it will get a bit more complicated...

Hi, thought I would update this...

Pulled the bike to bits again, all piston/cylinder/ring gap measurements were all within spec.

Rode the bike today (public holiday for ANZAC day) and it went very well. Going to order yet another set of gaskets and cir-clips and tear the top end down again at 10 hours or sooner to have another look.

Went to find parts to build a crank seal tester but could not find what I needed, so will order a motion pro one, I'm starting to get worried about the crank and its bearings to tell the truth. Wonder what I will find :busted:

Once again thanks for your input.

Mark :ride:

Just a thought but there is a wrong way to install the rings. The letter goes up. If you install the rings upside down they do not seat well. This is assuming you used Vertex / OEM, the pictures look like it. Also, since you have it apart post up your jetting specs.

Joe.........

If you are running a screen type spark arrestor make sure you are cleaning the screen, or just get a turbine core. A clogged screen or exhaust restriction will do the same thing you are describing here. I've had it happen to me.

Just a thought but there is a wrong way to install the rings. The letter goes up. If you install the rings upside down they do not seat well. This is assuming you used Vertex / OEM, the pictures look like it. Also, since you have it apart post up your jetting specs.

Joe.........

Hi Joe, the bike was not apart at time of your reply, I might tear into it again this weekend and pay particular attention to the carby and crank (which seemed fine to me upon previous inspection). Will post up anything unusual. Jetting is stock so don't know what it is, will check the manual.

If you are running a screen type spark arrestor make sure you are cleaning the screen, or just get a turbine core. A clogged screen or exhaust restriction will do the same thing you are describing here. I've had it happen to me.

Might be on to something. I have not touched the exhaust system at all, the EXC's come with an arrestor standard, do they not? On top of this the pipe is ridiculously dented and bent and no longer seals at the exhaust port at all.

(Entirely my fault from crashing obviously) :busted:

Thanks for your replies.

Got a bit of a shopping list it seems, shame as I was wanting to upgrade my 4X4.

List so far: New pipe and silencer

Full gasket set

New handguards

fork seals

Grips

Brakepads

chain and sprockets

Was dreaming of re-springing the bike too before all this happened

Mark

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