need help reading piston burn pattern

[Strange Magic]

oil in the chamber. That is burnt oil on the piston.

Your exhaust port is black and not grey.

[MadBill]

From the first pic, it looks like hard piston/head contact has been occurring on the right side, which is odd, considering that both head and piston have presumably been lathe-turned. (unless the head was faced off-square after the quench area was machined) A bent rod could cause it and wouldn't help ring seal.

[bladefrost]

Hi Bladefrost

"down on power" compared to when, and what engine condition and parts?
Did it run well when you first put that piston in? Any compression or leak down tests?
When you put in the cam did you "degree" it?


I'd dyno test the fuel system. 1/2 gallon of gasoline per hour per HP must come out the fuel line, thru the petcocks and filters.

Down on power compared to other bikes on the same class. It didnt run well any time at all. I also degreed the cam when i put in.

Dont have access to leak down and compression test.

[bladefrost]

My piston after smoothing polishing and contouring
https://www.dropbox.com/s/oxcgjldy6sog2 ... 0.jpg?dl=0

My cylinder head
https://www.dropbox.com/s/g9hlspkvjvpwh ... 3.jpg?dl=0

Excuse the appearance of the parts as i am on a tight budget. The head surface needs reconditioning

[bladefrost]

From the first pic, it looks like hard piston/head contact has been occurring on the right side, which is odd, considering that both head and piston have presumably been lathe-turned. (unless the head was faced off-square after the quench area was machined) A bent rod could cause it and wouldn't help ring seal.

No head contact sir. Just very close til it touches the head. Bikes response is very poor. I think it has problems elsewhere. Idles at 3k rpm. Seem very high. At 1k rpm it will die. Also hard to start hot or cold. Need to give it throttle inorder for the engine to start.

[mag2555]

You posted that the motor has 12.5 to 1 compression, how close is the top center of that piston to the chamber when checked with clay!
I would be certain to say that if that piston top was thick enough to cut it down to what's needed for a 11 to 1 compression that once the motor was rebalanced and rejected you would make a bunch more power as that dome is too tall!

What compression are others running with this motor and head, also did you , or someone port this head?

[user-9613590]

3000 rpm idle (before 1000) sounds like leaking valves..

But smooth out the piston,check the valve seal(or better yet get a new performance type valve job done),Check/new piston rings.
Not to mention proper intake(no restrictive "foam filters")

[BrazilianZ28Camaro]

Piston seem much better now, can you radiuse the spark plug pocket making it a convex shape? That would probably help too.

[Powertrip]

You have received excellent advice from all, you just need to follow up. Your piston looks much better, as mentioned I would take a flat file to the piston in the sparkplug area, eliminate the pocket.
There is an ideal quench clearance for every combination, and just touching the head at high RPM is not it, that is the way you ruin crankshaft bearings. The fact that your combination likes less timing at high RPM would indicate the quench is too tight.
You have not addressed my question on the carburetor float bowl angle, if too severe it will be impossible to jet correctly.

[MadBill]

I don't like to sound judgmental, but the first picture of your head showed a beautifully finely-machined gasket surface, but in the second one it looks like the head's been shipped cross-country by mule train, loose in a burlap sack full of claw hammers. What happened?

[bladefrost]

You posted that the motor has 12.5 to 1 compression, how close is the top center of that piston to the chamber when checked with clay!
I would be certain to say that if that piston top was thick enough to cut it down to what's needed for a 11 to 1 compression that once the motor was rebalanced and rejected you would make a bunch more power as that dome is too tall!

What compression are others running with this motor and head, also did you , or someone port this head?

I have the same compression to other guys on the same class. It was ported already when i bought the head. I just need to improve the short side radius on both the intake and exhaust. Havent checked the dome to chamber clearance. These guys i bought the parts from have done a lot of this kind of engine and they race it too.

[Strange Magic]

Your piston now is very pretty. Now what are you going to do about your problem?

[bladefrost]

3000 rpm idle (before 1000) sounds like leaking valves..

But smooth out the piston,check the valve seal(or better yet get a new performance type valve job done),Check/new piston rings.
Not to mention proper intake(no restrictive "foam filters")

If i advance the cam it would idle below 1k rpm. Does it still suggest leaking valves? Or is it the cam too aggressive for the engine displacement? Seem it has low vacuum at idle rpm

[bladefrost]

Your piston now is very pretty. Now what are you going to do about your problem?

Thanks. Did test run today and i can feel it has more torque. Problem is i cant nail the throttle quickly because it would bog specially when underload it would seem to misfire. Need to slowly roll on the throttle. That explains also why i have flat acceleration

[bladefrost]

Piston seem much better now, can you radiuse the spark plug pocket making it a convex shape? That would probably help too.

Thanks. I test run today and i feel more torque but the bog is still there. When under load when i quickly accelerate it would misfire. Also on center stand i cant nail the throttle quickly without the bog. I will try to make the spark plug relief better next time coz i already installed the engine when i read this message.