Whats up with this KTM piston?

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airmobile101

Lifetime Member
Apr 17, 2012
#1
Guys, Im not a 2-stroke guy and I don't know shit about them. Long story short. My neighbor down the street races this 2004 KTM 105SX in the Missouri Harescramble series. He recently acquired this bike by trading his CR85 for it. It was in bad shape when he got it,but he wanted a KTM. Anyways, the guy he got it from claimed it has a new top end. It very well may have,but it didn't last long. 10 hours tops! After sinking 500 bucks into it, we made it into a nice little bike.

Yesterday we ran into trouble. He was riding it in the woods behind my house and it just dies and wont start. He kicks it over a few times and claims he feels zero compression. I kick it over and claim the samething. So I slap a compression gage on it and we only get 5 psi total out of 10 kicks with the throttle being WOT. And the fun begins...........

I rip off the cylinder and find it looking like this. The ring collapsed and the top of the piston looks like it has collected aluminum or whatever on top of this piston. Its gouged good on top also. The cylinder has one extreme tiny score mark near the intake port. Sooo......... after making a few calls, a local dealer actually had a piston for cheap so we went and picked it up. Cleaned the cylinder with a scotch bright pad(didn't have a hone that size) and slapped it together. Did a compression check before firing and we achieved 160psi. Fired it up and she runs great. But....... Im a little concerned on how he is doing his pre-mix. He takes a 1 liter of Motorex and dumps it into a 5 gallon jug of fuel and runs it like that ??? WTF???? Isn't suppose to be mixed 40:1 0r 30:1 PER GALLON????? And yea.......... its like a disco smoke machine!!!!! lol

Anyways, here is a pic of the old and new piston. Give your thoughts on the piston and pre-mix. I would have ordered a hone,but we are on a time crunch since he has a race this weekend.
 

chronicsmoke

Banned!
Lifetime Member
Apr 27, 2012
#2
Lol I run 32:1-40:1 in my 250r.. Thats alot of oil! :suspicious:

He has it mixed at like 18:1 lol

Hows his jetting, Air? Was his plug blacker than toby's arse?
 
D
#3
Looks to be detonation with the introduction of foreign material which is probably the top ring land pieces bouncing around in there after severe detonation. Is this modified anymore then you know? Seems to e a LOT of oil rich which makes it fuel lean. Could be it needs more octane. I would first teach him 4oz to a gallon of oil, and see if anything else is modded. Premium minimum but could need more octane
 

airmobile101

Lifetime Member
Apr 17, 2012
#4
I did clean the carb when he first got the bike. I pretty sure its all stock with zero mods. I dont recall the main jet size off hand. And yes, the plug was black as hell. As a matter of fact, he fouled a plug a few weeks ago and the thing wouldnt start. I wasn't aware on how he did his pre-mix until he mentioned it last light. I asked him if he was crazy:crazy: He response was, thats what all the 2 stroke guys told me to do @ the race. Im thinking,"Yea..... they just fucked you." lol
 

alanbmx

Active Member
Apr 12, 2012
#5
with that much damage to the piston the head has to be magled! That will cause hot spots, causing detonation and you will have the same thing with the new piston real soon. Jetting is real important on two strokes, you can run 50 to 1 all day long if your jetting is right. Crank bearings and seals can cause major problems too. I a crank seal is blown you will run lean.
I would replace the head, check the end play on the connecting rod, install a new piston and rings then check the jetting and get the plug to be chocholate brown
 

airmobile101

Lifetime Member
Apr 17, 2012
#6
I cleaned the head up. But your right, it wasn't pretty. There was no end play on the crank. He will be home soon since today was his last day of school and its a half day for him. Im gonna make him drain that fuel that in there now and we are gonna start from scratch with a new mix of fuel.
 

alanbmx

Active Member
Apr 12, 2012
#7
make sure it is "premium" fuel at least 91 octane. Alot of guys go cheap because it is a 2 stroke. Not a good Idea. Compresion ratio does not care if it is a 2 stroke. All dirt bikes need premium
 

alanbmx

Active Member
Apr 12, 2012
#11
sloppy cranks and hot spots on the head are death for 2 strokes. hat to say I told ya so but.... Get out your wallet KTM's are not cheap:zx11shocked:
 

airmobile101

Lifetime Member
Apr 17, 2012
#12
Yea......... lol, lesson learned the hard way. But, thats how you learn I guess. The kid has no money to fix it. His mom is absoultely broke since she just bought a new house. Sooo......... I think we are gonna kick it in the basement for now. I priced everything, and its gonna run around $450.00 just in parts. The only top end component that still is good is the cylinder from just doing a visual. I didn't check it for out of round or anything like that. I might help him out a little so we can get this thing going again,but Im gonna get a service manual for it first before I tackle anything else, then I guess I need to get the tools to split the cases.
 

airmobile101

Lifetime Member
Apr 17, 2012
#13
Well.......... did better when I seached the internet better. Got everything enroute for 375.00 shipped! Wish me luck, never rebuilt a 2-stroke before. Mom actually footed the bill.lol Things you do for your kid ;)
 

Burke

Troll Slayer
Lifetime Member
Apr 18, 2012
#14
2 strokes are much easier... just make sure you don't have any air leaks or you will burn it down within minutes
 

alanbmx

Active Member
Apr 12, 2012
#16
crank seals, will give you air leaks, but 2 strokes are pretty easy, keep everything clean and get the jetting right before you go riding, buy a few jets and check that plug.
 

Burke

Troll Slayer
Lifetime Member
Apr 18, 2012
#17
everything has to be sealed... intake, head, cylinder, exhaust flanges... everything... ghetto way to check is to start it up and spray the areas where there are gaskets with carb cleaner and see if it revs up a bit.

real way is to plug exhaust flange & intake and put 7psi of air in the motor and it should hold for 30 minutes

there is a thread on another forum on how to make your own tester... if you want I can dig it up and show it to you
 

airmobile101

Lifetime Member
Apr 17, 2012
#19
Update......... I got the engine tore done and I was mistaken about it being crank bearings going bad. It was the lower rod bearing. Sorry for the mix up.