dbw - dirtbikeworld.net Members Forums

dbw - dirtbikeworld.net Members Forums (http://www.dirtbikeworld.net/forum/index.php)
-   Tech Talk (http://www.dirtbikeworld.net/forum/forumdisplay.php?f=30)
-   -   2 stroke burn / carbon pattern interpretation (http://www.dirtbikeworld.net/forum/showthread.php?t=25562)

Charlie 26-10-2006 09:51 PM

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...


asa572 26-10-2006 10:10 PM

MMMMMMM burn patterns:rr

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?

Battered Sav 27-10-2006 12:24 AM

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?

CR500rocket 27-10-2006 04:38 AM

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.


- 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,


Charlie 27-10-2006 10:17 AM

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.

pk 27-10-2006 06:18 PM

... 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

Charlie 27-10-2006 06:43 PM

didn't check the output, but did swap stators, cdi, coil, and carby :) no difference.
You're right that they're a finicky engine. I picked the wrong bike to go all 'clean and happy running enthusiast' on hahahah. Been a good learning experience it has...

CR500rocket 27-10-2006 07:44 PM

I have to agree that an iridium or even a racing plug with the smaller electrode should give you improvement. These types of plugs take far less ignition energy to ionise the gap due to the smaller surface area of the electrode, allowing the intensity of the discharge to be significantly higher.

As for the squish band you have the right idea, closing it up is usually an improvement from the factory who allow rather large tolerances for component mismatch.

The angle of the squish band area on the cylinder head is calculated from the piston dome angle.

This chamber angle should be slightly greater than the piston dome angle to keep the charge velocity constant as it is accelerated towards the central combustion area.

Depending on the engine and riding type, generally charge velocity can be anywhere from 20-35 meters/sec

You should expect some carbon build up if you have done 3K on a top end !!! However its unusual (from my experience) to see an area of the actual combustion chamber with a dead zone where so much carbon has built up.

I must point out that I don't actually work on KTMs and I'm not familiar with this specific engines quirks. However the same general tuning rules do apply to any 2 stroke regardless of the brand.

Charlie 27-10-2006 07:51 PM

the pistons are flat top, so I guess 4 degrees difference qualifies as 'slightly greater'. I tried iridium plugs with no improvement.
I guess I'll go ahead with my crazed plan to cut a slot through the squish section, from the squish band outer edge towards the spark plug... as per http://somender-singh.com/

Results / photos of disaster / tales of wonderous improvement should be forthcoming in a few weeks.

big fella 28-10-2006 09:46 PM

From the sounds of it you have run the bike 3000+ kms and found it like that?

What oil are you running? I have seen some oils build up even worse than that. The pattern just shows the the port hights and angles aren't perfect.

The intake charge simply hose's off the carbon, where the carbon is build up is where the intake gases missed or not as strong.

Clean it off and rebuild her with a new piston (it will be due!) and try running Castrol A747 for a few 1000kms and see what you get.

All times are GMT +11. The time now is 03:53 PM.

Powered by vBulletin® Version 3.8.9
Copyright ©2000 - 2020, vBulletin Solutions, Inc.