KX250 -06 increasing compression

[Anssi]

Hello,

I have a -06 KX250 with about 25-30 hours on it. I'm approaching the first top-end change and was thinking about having the head machined for a bit more compression because I would like more grunt. I hear the stock heads have pretty low compression to take into account the crap fuel people put in the bikes.

I intend to use Shell V-power 99 octane ( http://www.shell.com/static/fi-fi/downloads/tt/v_power.pdf , the PDF includes the information in English as well as in Finnish and Swedish) fuel mixed with 2% Motorex Cross Power 2T ( http://www.motorex.com/index.cfm/fuseaction/show/path/1-418-443-772-785-799 ).

The real question is, how much to shave off the head? A riding buddy had 2mm taken off his -05 YZ250 and that bike is running great. Obviously I would rather not take too much and have the bike ping itself to death.

How about advancing the ignition? Any experiences with the KX? How do you generally do this? Twist it a tiny amount, fire the bike up and check for detonation and repeat?

 

[KX02]

Maybe you could just try a high compression piston from Vertex or Wiseco instead of shaving the head.

 

[john3_16]

I'm wondering if 99 octane will be enough if you jet it sharp with increased compression....From what I understand KX's run pretty strong and already need some extra octane on the stock engine...Just bumping up the compression without porting may change the powerband characteristics in a way they you may not like..

I know it's not a KX but my 03' CR250 has a ported and decked cylinder and it needs VP C12 to keep from detonating...And it's over $7 a gallon...Just take that into consideration on wether or not bumping the compression will be worth it to you.

 

[KDX CRAZY]

Not sure what other mods you have done , but if not rather change pipe and reeds first , also get the ports done by a professional . Increasing compression as mentioned will change the power delivery and you may not be happy , also pinging will be a big possibility .
If increasing power on a 2 stroke was a simple as skimming the head then everybody would be doing it .

 

[SADude]

Jetting, sprocket selection and Flywheel weight if you are a trail rider. KDX Crazy, I assume you bought your bike from Russell Campbell Racing?? I spoke to Geoff Wolhuter a couple days ago and he runs the following jetting 160/48 clip in the top location on the needle and runs a 300g flywheel weight. Sprockets 13/48 for Tail riding enduro WFO style. Obviously this is set up for less than 1000ft above sea level and running the 95 octane pump gas that we get here in KZN.

If you bike is jetted properly and geared properly I don't feel there would be any need for increasing the compression. Compression on a KX250r 2006 is 10.5 (low speed) – 9.1 (high speed):1 and on a YZ it is 10.6 (low speed) – 8.9 (high speed):1. The YZ probably has the best bottom end off all 2-strokes that is why they get used for trail riding more than other MX bikes, I would say the KX competes favorably.

I agree if you want more then a pipe or reeds would be where I would start, before major engine mods.

 

[kawamaha]

A riding buddy had 2mm taken off his -05 YZ250 and that bike is running great.

the question is - is this the whole story? or has he reshaped the combustion chamber or changed some other stuff. maybe it could be done if the YZ has a flat piston. with a curved piston I can not imagine to shave off 2mm. but I would not shave more than 0.1mm without an exact instruction of an expert :coocoo:

 

[KDX CRAZY]

SADude

No I didnt buy from Russel but I have had the bike dyno'd to get the best from it and have 155 main 48 pilot needle 2nd to bottom , 35 hp now .

 

[skipn8r]

I'm approaching the first top-end change and was thinking about having the head machined for a bit more compression because I would like more grunt. I hear the stock heads have pretty low compression to take into account the crap fuel people put in the bikes.

........

The real question is, how much to shave off the head? A riding buddy had 2mm taken off his -05 YZ250 and that bike is running great. Obviously I would rather not take too much and have the bike ping itself to death.

I don't know about Kawi parts, but Honda heads are only about $55. I can have my head milled locally for $20. So even after milling, I can go back to stock for a total $80 investment. 2 mm seems like a huge amount to mill off a head. I measured my stock squish clearance at 0.060" and it went to 0.070" after a new Wiseco piston. So I had my shop take 0.020" (0.5 mm) to get down to 0.050" squish clearance. I haven't gotten the jetting sorted out, but it feels really good so far. Part of my reason for taking at least a small amount off the head is that I'm at 5,300 ft elevation.

I say experiment and go back to a stock head if you don't like it; it's cheaper than experimenting with a reed cage, a pipe or porting.

 

[Anssi]

Thanks for all the replies. With these comments I suppose he must have meant (or even said, maybe I misinterpreted since I had no reference point) .2 mm.

However, I am absolutely certain that the only thing that has been done to that YZ has been shaving that .2 mm or 2 mm off the head with no other work.

 

[Anssi]

I did some testing. Set the ignition timing to as much advance as the marks allow. Mid-range was improved noticeably.

Backed out to halfway between right in the middle of the marks (where it was stock) and full advance. Got worse. Then to one quarter from full advanced -> got better again, but I think fully advanced still was best (allowed me to get out of turns faster and clear jumps easier).

I also started to measure my squish clearance, but the solder I had was only 2mm (0.08") and that barely got touched by the piston. It was hard to really see where the flattened spot was, but my micrometer showed about 0.067" (damn you and your archaic units of measure). I will check this again with thicker solder.

BTW, my manual says stock compression is 10.2 - 8.9. There is no mention of allowed squish clearance.

If I confirm that 0.067", could I maybe have 0.4 mm taken off the head to get the squish to 0.05" and increase compression a bit?

 

[skipn8r]

If I confirm that 0.067", could I maybe have 0.4 mm taken off the head to get the squish to 0.05" and increase compression a bit?

In my experience, yes. It seems that the minimum recommended clearance I've found is 0.035" and I suspect that's the minimum to avoid mechanical issues (someone else chime in here). You should be mechanically safe down to 0.040". Don't know where you'll run into detonation issues.

 

[Anssi]

Ok, I got some 3mm solder and did the measurements again. The squish clearance was about 1.73mm (0.068") on both sides.

One interesting thing is that the squish band seemed to have very little angle (between it and the piston). Even with the micrometer I could not really see the clearance getting larger towards the plug (until of course the squish band ended).

I have the extra stock piston that came with the bike and the top is a bit convex. Does that mean the piston is domed, i.e. are "flat top" pistons truly completely flat? This of course figures into the squish band angle.

Any ideas on how the angle reflects on what I'm about to do (take 0.016" or 0.4 mm off the head)?

 

[Anssi]

I had .3 mm milled off the head. There was little noticeable effect on the power delivery. I was able to retard the ignition from the fully advanced setting without the ill effects I suffered previously.

I will now still take off 0.2 mm to bring the squish clearance to about 1.2 mm. If that also brings the compression to too high a level, I will run a thicker base gasket or just have the head machined for a bit less compression.

 

[Anssi]

Well, this is turning into a monologue, but here goes.

I had an additional .2 mm shaved off the head for a total of .5 mm. The squish clearance is now 1.17mm (this time measured by simultaneously putting two pieces of 2mm solder into the cylinder, one front, one back from the spark plug hole, both gave the same reading).

Also, the engine now runs as it should. Very cleanly, and has the bark that it should have. I was able to retard the ignition timing back to stock. There was not a hint of detonation with even the advanced timing, though.

Also, the stock jetting seemed a bit rich (all through the head mods and timing changes) so the needle position is now one step leaner than stock but I had to turn the air screw in from 1.5 turns to 3/4 turns to compensate for the change in low throttle openings. (I didn't experiment too much, but 1.5 was lean and 3/4 ok). The Shell V-Power 99 fuel that I'm now using and the bike seem really responsive to changes in jetting.

I might put in Boyesen Pro Series reeds but I think that is as far as I will go with engine modifications in the near future. Perhaps a pipe next year.

 

[kiwijohn]

Hey man....

I have an 06 as well, but I'm all about woods and fun riding so I'm definitely not after any more power! the KX is awesome!

But since you tried timing changes, which one do you think would make for a fast woods bike mod?

I could do with more bottom end because a high rpm KX250 in the woods is just scary!! I was thinking about retarding the timing a bit to smooth out the bottom end, but a stronger midrange would be good too..... any thoughts?

John

 

[Anssi]

Well, my problem was that the squish clearance was out of whack at 1.7mm. The bottom end and mid-range were really weak for MX, even on dry clay. Then when the revs got high, it hit like all get out.

Advancing the timing gave me some bottom end to work with. The timing is so easy to adjust (just the 5 or 6 bolts for the cover and three for the stator plate or whatever) that if I were you I would just experiment. It takes like 2 minutes to make the change.

 

[skipn8r]

Also, the engine now runs as it should. Very cleanly, and has the bark that it should have. I was able to retard the ignition timing back to stock. There was not a hint of detonation with even the advanced timing, though.

Sounds like a good result :cool: .

 

[kiwijohn]

Advancing the timing gave me some bottom end to work with. The timing is so easy to adjust (just the 5 or 6 bolts for the cover and three for the stator plate or whatever) that if I were you I would just experiment. It takes like 2 minutes to make the change.[/QUOTE]

Cool, thanks man. I'm setting up a test track with a couple of guys this weekend to set up our suspension and jetting so I'll try the timing in both directions (retard and advance) to see which works best... will let you know.

My bike is strong pretty much everywhere... I've got a 42 pilot, 158 main and 2nd clip. Was too rich and blubbery before. Now it's crisp and lively. I might add a flywheel weight, but I'll try the timing mods first.

Thanks again .


John :cool: