KX250 cylinder head squish

**F5 Dave** · 15th December 2015, 19:04

Its a case of the lesser of two evils. Your squish on a 250 will have to be more than 0.8 at thinnest point. I`d aim at 1mm. But you also have to run no more than say 14:1 on what passes as premium here. Maybe less. But a std bowl at that clearance will give a sky high compression.

So either replace the head. Machine the head (best option for performance) or correct with extra gaskets either above or below. Below will raise the timing a little but at least it won't blow up. Above, well I wouldn't risk two gaskets unless I knew it was common practice. And I'd trust Eric Gorr to know a heck load more than me.

**ktm84mxc** · 16th December 2015, 14:05

As a side note 80's KTM's used barrel base gaskets of varying thickness from 0.1mm= 1.5mm to set the squish band or as kTM called it Dimension X . All engine gasket sets came with 5 base gaskets of varying thicknesses . The cheapest & easiest is to get base gaskets to get a 14=1 compression ratio .

**J.A.W.** · 16th December 2015, 14:33

Originally Posted by rudolph

I have a 1990 kx250 that i have just rebuilt the engine...

Here's the Eric Gorr chapter on Kawasaki KX tuning issues.. http://www.eric-gorr.com/images/docu...TuningTips.pdf

**rudolph** · 16th December 2015, 19:09

Originally Posted by husaberg

Did you tell him you had Decked your cylinder too much?
I wouldn't suggest extra base gaskets just a thicker one. IE to correct the additional clearance you had created.
Eric Gore suggests two head gaskets for these models.
https://books.google.co.nz/books?id=...%20250&f=false
PS. 1.5mm is not a squish clearance. Its the width of a bus. (1.2mm seems about right though)
What puzzles me is you said the head was modified before you got it so it obviously ran well enough before you decked the cylinder.

I rang nz cylinders and they sed they only gave the cylinder a very light skim, nothing major,

I got the bike in parts, the engine was rooted, guy had the wrong base gasket from a 1992 witch was blocking off that port under the reed and he had put so much silicon over it it was stuck all over the cylinder walls,

He claimed it only had 2 hours on a top end rebuild but he had to take it apart to fix the main bearings, it had a new piston that was wrecked so I don't think it went very well and it died.

But yes I want to aim for 1.2mm

**rudolph** · 16th December 2015, 19:17

Originally Posted by J.A.W.

Here's the Eric Gorr chapter on Kawasaki KX tuning issues.. http://www.eric-gorr.com/images/docu...TuningTips.pdf

Thanks, I just had a read of that, he talks about changing the port timing by shimming the cylinder but I like the hard hitting power

**husaberg** · 16th December 2015, 19:46

Originally Posted by rudolph

I rang nz cylinders and they sed they only gave the cylinder a very light skim, nothing major,

I got the bike in parts, the engine was rooted, guy had the wrong base gasket from a 1992 witch was blocking off that port under the reed and he had put so much silicon over it it was stuck all over the cylinder walls,

He claimed it only had 2 hours on a top end rebuild but he had to take it apart to fix the main bearings, it had a new piston that was wrecked so I don't think it went very well and it died.

But yes I want to aim for 1.2mm

Raising the cylinder raises both the ex port timing and transfer duration so it will hit harder anyway.

All the Kawsakis back then were hard on main bearings. No idea why?
With the piston being toast it was likely due to a poor cylinder coating or mismatched piston.
If money isn't a concern I would re do the head profile to match a more modern design with a lower comp and decent squish.
Any decent engineer can do this as long as you draw him exactly what it should be.
Just copy a more modern design like a KTM or Yamaha. I'd suggest you try an find a small firm in an industrial estate as most big ones will just give a silly price to get rid of you.
there is one guy on here that might be able to help you.

http://www.kiwibiker.co.nz/forums/me.../19368-RMS-eng

Try googling something like Klemm Reaserch (not sure if it is the same crowd as klemm vintage now)they used to share openly what they did and they used to specialise in them. Gorr also offers very great advice.
As Dave said the carb will be toast by now. The needle jets wear oval and can't be replaced, To correct this you will likely need a thicker needle. Again google should be able to help you with alternative needles.
When the Keihin needle jets wear it will be far too rich in the midrange and it will be very peaky like a 125.