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  #1  
Old 26-10-2006
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2 stroke burn / carbon pattern interpretation

anyone around here much chop with interpreting burn / carbon patterns?
Pics of my foul smelling, fuel consuming 380 engine. Pulled the head for a re-ring and perhaps a little bit of fiddling...
No, it's not running particularly rich - that carbon pattern on the head looks kinda ugly though...

http://members.iinet.net.au/~cdriver/headcomp.jpg
http://members.iinet.net.au/~cdriver/pistoncomp.jpg
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Old 26-10-2006
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MMMMMMM burn patterns


From what I have been told the perfect burn pattern on top of a
two stoke piston will look like the map of Australia and that looks
far from it.


Has your piston/rings been in contacting your powervalve?
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Old 27-10-2006
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I'm wondering if the squish band is even, maybe the head was milled at one stage and not done evenly.
The dark patch behind the plug is interesting, highlights the importance of indexing the plug.
I think most people would find a carbon build up in their 380 two stroke, it'd be real hard to ride the thing at a level that is going to keep the chamber clean.
If it hasn't been pulled down in a long time, the build up, whilst not being ideal, is probably acceptable, it could take a lot of experimenting to get it clean.
Maybe try looking at a cleaner burning oil, but I bet your already using quality oil anyway.
Where is the dark patch in relation to the exhaust port and the transfer ports?
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Old 29-10-2006
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Quote:
Originally Posted by asa572
Has your piston/rings been in contacting your powervalve?

Well,was it?
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Old 29-10-2006
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I wouldn't be too worried, re-alining the ports angles and hights will to eliminat the issue will not change the power too much.

I would keep trying some new oils... and not loose any sleep.
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Old 29-10-2006
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not losing sleep over it but would prefer a clean running bike if possible - ridiculously fuel inefficient currently.

asa - believe you were right, there is a very light scuff on the pv. apparently this is not abnormal though...
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Old 29-10-2006
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I know what you mean about the fuel range! Guess thats what you get when you add 180cc to a 250cc engine and fuel tank!

Love that power though! The ddl or del needles worked best back in 98.
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Old 27-10-2006
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Here's a few basic tidbits on combustion characteristics in a 2 banger:

- Oily carbon deposits like your 380 display's indicate very poor burn characteristics (significantly lower temperatures) in certain areas of the combustion chamber and there can be a number of contributing factors. Without a range of information on riding style and tuning parameters of the engine I can only offer you a small amount of information.

- Looking at the cylinder head photos it seems to have a strange chamber shape, indicating it has probably been modified. It's unusual to see a convex surface originating from the plug location within a hemispherical chamber.

- Squish band angle (basically; difference between piston dome angle and the outer area of the cylinder head (flat part) angle) is impossible to determine looking at photos. Without going overboard I can point out it is critical in a two stroke to get this right, exceeding MSV (maximum squish velocity) or not achieving a decent MSV can cause all sorts of combustion problems, including uneven flame propagation.

- The Spark Plug in your engine is not running at the optimal core temperature.

- 380cc engines are similar to the 500cc engines that I work on, they don't rev like a CR80s so the lubrication requirements are not as severe.
That is unless you do FINKE style desert runs at WOT for significant amounts of time, the oil ratio can be run allot leaner than 25:1, my own CR500 is run at 44:1 on m800.

***IF YOU CHANGE THE FUEL/OIL RATIO THE BIKE WILL NEED TO BE REJETTED.***

- Check your crank seals, the drive gear side seal is immersed in oil from the gearbox. The vacuum created on the intake cycle can draw oil past this seal if the crank bearings are gone or the sealing surfaces of the seal are damaged.

- Bike is usually very smokey when you do this crank seal, and the gear oil seems to vanish as quick as you can fill it up !!!

It is likely that allot of your fuel consumption and combustion residue problems can be corrected if the jetting of your bike is optimised for your riding style and type of riding.

- However if the above suggestions don't help, the combustion chamber should be looked at in more detail by an engine tuning specialist.

Hope this info is of some use,

CR500rocket
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Old 27-10-2006
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Interesting stuff, cheers.
The head was machined for tighter squish tolerance and the bowl was shaped to maintain a reasonable CR - what do you mean by conxex? The head is basically flat at the plug, and then moves smoothly into the curve - the lighting in the photos does look a little as though it doubles back on itself though, is that what you meant? The squish band is 55thou high at the outer edge and has an angle of approx 4 degrees. Compression is about 11:1.
Do you think the head shape as I have described would be causing problems, or is that not enough info?
What would spark plug indexing achieve for me? Should I point it at this low heat / inactive combustion area?
The crank seals and main bearings (along with the top end) were renewed about 3000km ago.
I have fiddled *a lot* with the jetting, nothing ever seems entirely perfect but I have it quite good.
The oil is mobil1 racing 2t at 50:1, the bike gets put through a lot of slow and reasonably technical riding.
The spark plug is a bpr6es, a projected tip version of a 2 range hotter than factory suggested plug.

Oh and the red line on the head pic indicates the exhaust port.
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Old 27-10-2006
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... coming from a Husqvarna background myself, have you had a chance to check the stator output voltage/amperage. I know that after i had mine rewound on the WR250 and WR360 there was a noticeable improvement in the way that it ran - both cleaner and smoother. An Iridium plug also seemed to help it burn a little more evenly ...
Also a few of my friends have KTM 380's and they are almost (but never quite 100%) happy with their jetting. Perhaps it is a trademark of that model of bike?
just my $0.02c
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