2 stroke burn / carbon pattern interpretation

[KDXer]

Would be very interested to hear the cause if/when you find one.

[cluffie]

Pulling the head off a ktm 150 at 24hrs which is trailriden only.
Are there any experienced engine builders out there that can comment and what are people's thoughts on this and the way it is running?
95 fuel,motorex 2t oil at 40:1
The rider is young and feedback is hard to gather. As long as it starts and runs they just ride.

[cluffie]

As the 150 head is the 2 piece insert design and can sit in any position,is there any prefered sparkplug electrode orientation for a 2 stroke motor? ie across, towards back or front. I remember when we were boat ski racing the mechanic used to index plugs a certain way.

The oem position when i pulled it apart was with the electrode facing across the motor.

[Nutty]

Originally Posted by cluffie

As the 150 head is the 2 piece insert design and can sit in any position,is there any prefered sparkplug electrode orientation for a 2 stroke motor? ie across, towards back or front.

The open end of the plugs' earthing arm should point to centre of the rear boost port. On road racing 80s and 125s I index plugs. For everything else it doesn't matter.

That head has too much SC Cluffie, it should be 1.00-1.05mm. Worth a check. Can't wait for mine to arrive...

[riles]

Cluffie, for reference here is an old pic of the piston top when I did a top end at 75 hours on my 2013 KTM 150. Same as you, 40:1, well jetted, Dave did some minor cylinder mods and set the Squish at .95, about 10psi more than standard. It was a good motor

I sold it to my neighbours teenagers who are still riding it.

Nutty, That's interesting about the rear boost port. Some years ago I read about indexing plugs and ever since I try to put the open end East/West but often it doesn't line up. Let us know how the new 150 goes. There are still some small bore fans around.

Cluffie, is that two piece head an aftermarket item?

[riles]

Specs for interest

[cluffie]

Thanks guys. As for indexing Nutty, i thought it was as you said. As the head is 2 piece it's easy to screw the plug into the head while it is off and see where the electrode ends up. Then mark it with a texta and put the head insert in to suit. i'll set it up towards the rear boost port.

Riles,stock head. The plugs vary a bit and i had 3 spare plugs so i screwed them all into the head to see where the electrode ended up. 2 plugs were the same and one was different so i now have a spare plug that will line up to the rear boost port.
On a 300 i wouldn't worry about this but as it's a smallbore and 2 piece head why not it's so easy.
75hrs for a piston is exceptional Riles. Did you do rings before 75hrs and were you happy with this amount on hrs or would you change the service interval?

[riles]

Cluffie, i think 75hrs was the first top end. No rings before that. I kept a check on compression, the next one was at about the same hours. The neighbour I sold it to came back last year and said can I do a top end for him which I did, it had done 80+. Bottom end is still fine, so is the cylinder and the ring gap always measures about the same so I am not going to do them before 75 hours now. Everyone has their opinion on top ends but I'm giving you my 150 experience since you asked.

[cluffie]

Thanks Riles, 75-80hrs sounds safe from your experience. I'll keep an eye on it and do some comp. testing up until then.

[jaguar]

I think the clean areas of the piston indicate there is lots of fuel/air mixture flow over it from the transfers and/or boost. Ideally the direction of the clean areas should be rearward. If one side of the piston is cleaner than the other then the transfers aren't opening equally.