Looking for stock port timing specs on KDX 200 and 220 or other enduro 2stroke?

[sosomechanic]

I'm tinkering with some cylinders on later 250 MX bikes with the aim being a torquier low end and much more linear power delivery. I know that for an engine somewhere near 200 cc's the two KDX engines are said to be not too far on either side of that description. I've recently turned down the cylinder base 1.5mm on my '03 RM 250 and made a 19 ounce flywheel weight ( that's not a typo, the 10 ounce I made first wasn't enough) to tame it for eastern woods use. I'm really interested in comparing my current port timing to some of the enduro bikes to see how far I'm off before cutting chips on a couple of my friends bikes. Losing significant peak horsepower and overrev is rarely noticeable in our riding conditions and isn't much of a concern. If anyone has mapped or run across the port timing specs for these or perhaps some similar enduro machine between 200 and 300 cc's I'd appreciate the 411. Thanks, Michael.

 

[skipro3]

You might try eric gorr's site. I hear he's THE MAN when it comes to this stuff. Crappy info, I know, but what I really wanted was to ask if you reshaped the head to compensate for the trim on the cylinder base and how did you determined how to do that?

 

[sosomechanic]

I did modify the head on my lathe. I set the lathe to the same angle as the stock squish band and initially just cut deep enough to widen the band to about 70% of the bore area. This required a little over 1mm of material removal in the squish band area and the squish clearance was about what I wanted ( about .070") at this point, so I tested it like this. Cranking compression was 225 and I was getting significant detonation ( I run premium pump gas). I pulled the head and removed some material from the bowl area in a until I had the compression around 195 psi. This seems to be about right, it has the odd occasional ping at very light load ( the pilot is a bit lean) but showed no sign of detonation when I did a plug check to test the main jet earlier today.
Thanks for the tip on Eric's site. I have his excellent book and used the general info in it along with guidance from a brief conversation with him to decide on the porting changes. His site doesn't have any specific info. that I've found regarding typical enduro port timing. Of course it may well be that the low/ mid port timing reccomended in his book ( exhaust 90 degrees, transfers 118 degrees) closely approximate the port timing on enduro machines. I just don't know. My cylinder is in the ballpark of these figures and is definitely easier to ride but is far from "electric" or smooth.
I suspect that the cross sectional area of the transfer port windows and ducts is a bit large for the kind of torquey and smooth low and mid I'm working toward even if they have similar timing to an enduro cylinder.... Wish I had a EXC or KDX cylinder in hand to inspect.
One other item of interest is a divider plate I made to go on the reed side of the slide in my carb. Just maybe it will help with low end torque and traction. With a 40 pilot (stock is 50) and the stock needle set full lean the idle to 3/4 throttle jetting was so rich it would barely manage to pull itself up onto the main. I have the largest diameter needle and leanest pilot (a 35) coming this week to try to get in the ball park with it. I've heard of rich midrange jetting hiccups in some cases so I'm going to texture the bottom side fairly rough to help prevent a laminar flow that would pool the fuel and possibly degrade jetting constistency. That's the theory at least.
Still looking for enduro port timing, anyone?

 

[skipro3]

I was going to suggest a divider plate as I was reading your reply. How's that working for you? I had RB Designs modify an air stryker carb (36mm) to replace the 33 mm one I had stock on my KDX220. It's like having a two barrel carb now. I can feel with the throttle control as I cross the divider and open the carb past half way. It really improved the low end snap from closed to 1/8th throttle, but it did take a little fooling around with the air screw. My friend's KX500 has a pilot of 38, so a 35 isn't out of the question on your 250. He had the same work you discribe done by Eric and couldn't be happier. The bike's power is very linear with the control in power from off idle all the way to the moon.
How's that flywheel weight working out? Did you need to increase the spacer to accomidate the extra volume that must take under the ignition cover?
I've seen someone on e Ba y advertize a CD with porting spec's on the KDX for sale. You may want to check that out. I've eyeballed the 200 vs the 220 ports and they look way different from each other. FYI

 

[sosomechanic]

I have no idea how the divider plate will work out once I have some jets that will carburet cleanly with it. I can definitely say that it provides a good increase in vacuum signal strength from idle most of the way up the needle because the jetting was WAY rich once I dropped it in. The RM still has some raspy, somewhat loaded up feeling jetting below 1/8 throttle ( without the divider plate) that I haven't been able to clear up with pilots or air screw adjustment. Hoping to avoid cutting the slide because they are expensive. I'm optimistic that the stronger signal provided via the divider plate will clean up the jetting here and let the engine pull cleanly/with more torque off the bottom.

The flywheel weight definitely helps, the 10 ounce I tried first just wasn't very impressive. We ride on lots of gravel, hard pack clay power and gas lines, and other slick stuff. The 19 ounce ( version two) definitely made a noticeable improvement in controlling wheelspin. If I can get the porting , exhaust and carb setup dialed I might go back to the 10 ounce one later or somewhere in between. There is very little clearance under the flywheel cover on the RM. The 10 ounce was about 5/16" thich and I had to machine a spacer using a tracing from the gasket. The heavier weight is 5/8" thick and just barely clears with the 3/8" thick spacer.
Thanks for the tip on the KDX cd, I'll look into it. Later, Michael.