2 stroke burn / carbon pattern analysis - photos

[blowtus]

On a 2000 ktm380exc. Pulled the head off ready for a re-ring, and am going to try to improve combustion characteristics via head shape and 'grooves' - search for 'singh grooves', I can't link here...
Thought I'd take a bunch of photos and see if any smart folk can do some interpreting of what's going on for me. That carbon pattern on the head looks pretty damn ugly... the red line I drew in points to the exhaust port.

I'm basically fighting atrocious fuel economy that can't be resolved through 'normal' means. Head has previously had squish tightened up, running mobil1 racing 2t at 50:1, jetting is pretty close but still not perfect.


members.iinet.net.au/~cdriver/headcomp.jpg
members.iinet.net.au/~cdriver/pistoncomp.jpg

 

[tony91]

Clicky.....


members.iinet.net.au/~cdriver/headcomp.jpg

members.iinet.net.au/~cdriver/pistoncomp.jpg

 

[letsride24-7]

Switch to AMS-OIL.....

 

[Pete Payne]

The jetting is realy rich looking . Lean that bad boy out .

 

[IndyMX]

The jetting is realy rich looking . Lean that bad boy out .

Yeah, no doubt!

jetting is pretty close but still not perfect

How do you figure?

 

[adam728]

I agree, jetting looks way rich. That's were I would start anyway.

 

[blowtus]

Lets just assume I understand how to jet a bike ok? :) 380's are pretty finicky on jetting at the best of times, this one is a pig and spends a fair amount of time on tight and technical stuff. I've had it lean to the point of pinging, but rarely rich to the point of blubbering.

 

[jason33]

well try a new oil its not burning clean-i use klotz 40:1
your plug should be a tan like most of the head -not black,did you use crappy oil for a lil while??
then check the jetting
oil and fuel go through the jetts not just fuel,smaller jet =less oil/ also less fuel!!
therefore ,your still not getting more fuel to oil /air its still the same just less
do you under stand what i mean??-- it still will not burn the oil because of the fuel-to oil ratio /air
"unless you have a oil injected machene"!! then the jet will let less/more fuel to oil
and your air screw adjusts the air
your plug looks oily not sooty from lack of air
hope this helps everyone- :cool:

 

[blowtus]

the spark plug is a bpr6es, projected tip version of a 2 range hotter than stock plug. Never ran crappy oil. I was on a different oil last rebuild and it did the same. I'm going to try a bit of an oddball cut in the head to get some more turbulence happening at low rpm and try and get the mixture burnt.

 

[Pete Payne]

What are the specs on the head?
Squish clearance , step in the head , Squish width . head volume , trapped vol . etc.

 

[blowtus]

clearance is 55 thou at the outside, 4 degree taper, 31cc head volume. I'll measure the width when I head down to the garage later - can you explain step in the head and trapped volume?

 

[Pete Payne]

The step in the head is the distance from the flat gasket surface to the begining of the squish band at the bore edge.

Trapped volume is the volume within the combustion chamber of an assembled engine ATDC. Normaly it is measured with fluid from a graduated burret in cc's,

 

[blowtus]

Step in the head is 90 thou - it's an o-ring head if that makes a difference. Can't effectively measure trapped volume, but the deck height is 0 and the piston is flat topped, so assume negligible difference to head volume.