Port Timing Question

[andrew]

I have a '96 KDX200 which I purchased second hand some years ago. I was told it had been ported for more top end, but I'm unsure exactly what that entails. What I do know is that the base has been milled, increasing compression. That same action has lowered the cylinder away from the KIPS actuator, so that the teeth are only half engaged on the main KIPS shaft.
On a recent rebuild I put in a second base gasket to raise the cylinder so the KIPS actuator engages properly. I know this has lowered compression slightly, but can you please tell me how much effect this is having on my port timing.
I have noticed that it seems a little bit "doughy" right off the bottom, but when the powervalves open it reacts quickly. Prior to this, it seemed a lot more lively off the bottom, with not quite as hard a hit on top.
Is this my imagination, or can this minute change in port height affect it that much?
I'm not sure whether to leave it be, or to take that extra gasket back out.

Thanks,
Andrew

 

[Rhodester]

I had Eric Gorr shave the cylinder base on my '95 (H) and it only raised compression 8 lbs. on my tester. I hoped to focus a little more power toward the bottom end also to clean up a hesitation problem I was having. It seemed to help the hesitation in the low end a little, but the power band seemed to be the same. It didn't seem to be enough to effect my KIPS actuator that noticably.

I used to have an ATK 406 that had stackable shims instead of a head gasket. I don't know how thick the shims were (paper thin), but it did have a noticable effect on low end snappiness (is that a word?) and top end run out. 1 shim had a real snappy bottom end and 3 to 4 shims would soften the bottom end snap and allow it to run out on top better.

Try searching the Mods and Performance forum for threads concerning porting and you may find your answer. I think a very small change in exhaust port height can have a big effect on power band focus. Lowering overall port height focuses the power band on the bottom end and raising port height (especially the exhaust) focuses the power to the top end. Lowering compression softens the bottom end while allowing for more top end run out. Raising compression adds to bottom end response while limiting top end run out. You can see that the base gasket trick has the port height and compression tendencies fighting each other.

You can also improve run out by retarding your timing (at the expense of snap off of the bottom) or improve your bottom end snap by advancing the timing (at the expense of top end run out). I advanced my timing to a little past (more advanced) of the most advanced mark (as per Eric Gorr) and it did improve the snap off the bottom. I noticed no change in the engines ability to run out on top. The reason is that the KDX200 only has a 7.9:1 compression ratio to begin with, so the run out penalty is virtually nil. I had no over heating or detonation effects either. I specifically told Eric I wanted to run 91 octane pump gas when I had him trim the cyl. base. I think I went from 134 lbs. of compression to 142 lbs.

 

[JasonWho]

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[andrew]

Rohdster, you said
"Lowering overall port height focuses the power band on the bottom end and raising port height (especially the exhaust) focuses the power to the top end. Lowering compression softens the bottom end while allowing for more top end run out. Raising compression adds to bottom end response while limiting top end run out."
Then by adding that gasket, what I have done in effect is to firstly raise the port height, focusing power to the top end (at the expense of the bottom); and secondly lowered compression thereby softening the bottom end response and getting more snap and top end run-out.
That seems to go with what I have just experienced, so thanks for that. Still interested in any comments as to how much effect one would get from the addition of the one gasket. Is it really that touchy??!

Thanks heaps,
Andrew