KX250 cylinder head squish

[rudolph]

I have a 1990 kx250 that i have just rebuilt the engine, i checked the piston to head squish and it has only 0.43mm including the gasket, i had the cylinder bore re plated and they refaced the cylinder deck so that must have decreased the clearance a bit, also the head has been planed at some stage in its life.


Dose anyone know a good place in auckland that could re mrchaine the combustion chamber?

[husaberg]

I have a 1990 kx250 that i have just rebuilt the engine, i checked the piston to head squish and it has only 0.43mm including the gasket, i had the cylinder bore re plated and they refaced the cylinder deck so that must have decreased the clearance a bit, also the head has been planed at some stage in its life.


Dose anyone know a good place in auckland that could re mrchaine the combustion chamber?

Put in a thicker base gasket..... sorted.

[J.A.W.]

I have a 1990 kx250 that i have just rebuilt the engine, i checked the piston to head squish and it has only 0.43mm including the gasket, i had the cylinder bore re plated and they refaced the cylinder deck so that must have decreased the clearance a bit, also the head has been planed at some stage in its life.


Dose anyone know a good place in auckland that could re mrchaine the combustion chamber?


Dunno 'bout Auckland Rudolph, but if you want a reusable custom thickness copper gasket, Lani is a great guy..

http://www.coppergaskets.us/index.htm


Or, for machining the comp chamber, Dave is worth a call.. http://www.twostrokeperformance.com.au

[rudolph]

Thanks for the replys, a thicker base or head gasket is out of the question, it turned out my head had been planed way too much, I bought the bike in parts, I don't think the last owner had any luck with it the way it was.


I may have found a 2nd hand head. but otherwise I will talk with that 2 stroke guy in the link :)

[husaberg]

Thanks for the replys, a thicker base or head gasket is out of the question, it turned out my head had been planed way too much, I bought the bike in parts, I don't think the last owner had any luck with it the way it was.


I may have found a 2nd hand head. but otherwise I will talk with that 2 stroke guy in the link :)

Why are they out of the question $1 worth of gasket material some grease and a pair of scissors.
Its how a lot of factory bikes set the squish. we are only talking an extra 0.4-0.5mm here at most.

[SS90]

Why are they out of the question $1 worth of gasket material some grease and a pair of scissors.
Its how a lot of factory bikes set the squish. we are only talking an extra 0.4-0.5mm here at most.

As he said "the head has been machined at some stage of its life" I would guess that means that the original squish area has now been machined to almost nothing, and it is now left with a sharp edged bathtub head dome, and a narrow squish area.
If so, no amount of base or head gasket thickness will fix the problem.

[husaberg]

As he said "the head has been machined at some stage of its life" I would guess that means that the original squish area has now been machined to almost nothing, and it is now left with a sharp edged bathtub head dome, and a narrow squish area.
If so, no amount of base or head gasket thickness will fix the problem.

But that is a guess from you. As he has just said the squish is now .4mm rather than about 1mm only .6mm at most has been removed are you not able to follow that?
Unless it was an idiot that machined the head the squish step would not have been removed at all. He has not stated that this has occurred he said he had had he had machined the cylinder deck. As it was running before then without any incidents As he never mentioned any it only needs the height relationship corrected.
The head on a 1990 KX250 is not bathtub shaped at all regardless, its a very conventional squish with a width of about 50%.
I note Eric Gorr actually recommends to use two head gaskets on this era bike with permetex sealer. But what would he know...........

[F5 Dave]

So you've checked it with solder on an assembled engine? Another head would be easiest option, but anyone competent with a lathe could fix it and probably improve upon the shape. About 0.9 or so is ideal for a 250. The problem is that that clearance will make the compression too high. So some will need to come out of the bowl.

A cleaner running bike will be the result. But a 1990 will likely have a groove worn in the carb slide which will have more bearing so save to replace that.

Plenty of articles to read but you probably just want to ride it.

If you do find the Eric Gorr 2 dirt bike book it is well worth the money.

[rudolph]

This is my head, you can just see the step from the gasket face to the squish band on the upper right

http://i354.photobucket.com/albums/r401/castinbronze1rudolphtrc/IMG_1508_zps2sinuirq.jpg (http://s354.photobucket.com/user/castinbronze1rudolphtrc/media/IMG_1508_zps2sinuirq.jpg.html)



This is an un planed head, it has a much bigger step

http://i354.photobucket.com/albums/r401/castinbronze1rudolphtrc/s-l1600_zps567jcnlv.jpg (http://s354.photobucket.com/user/castinbronze1rudolphtrc/media/s-l1600_zps567jcnlv.jpg.html)


The KX uses a steel head gasket,

[F5 Dave]

Yeah my GG needed almost 2mm planed to get it the right clearance, not uncommon. So you measured it on the bike with some solder down the plug hole?

[rudolph]

Yer I poked solder down the plug hole, then removed the head and layed a bit across the piston and put the gasket on and bolted the head down and turned the motor over.

[F5 Dave]

Is that a central straight plug? If so real easy to turn more of a step and take some area out of the chamber to get the comm to where it should be. But you should really measure the chamber with the piston held at the top, a burette and some light oil (fork oil is good).

[rudolph]

Is that a central straight plug? If so real easy to turn more of a step and take some area out of the chamber to get the comm to where it should be. But you should really measure the chamber with the piston held at the top, a burette and some light oil (fork oil is good).

The spark plug is at an angle so carn't hold it with a mandrill in the lathe.

But yes the chamber would need to be CCed with a burette but if this new head mesures up ok with solder I will run with it.

[rudolph]

I have been talking to Gaudenz Gisler at Gisler Moto and he says don't put extra gaskets under the cylinder and I should aim for 1.2 to 1.5mm squish.

[husaberg]

I have been talking to Gaudenz Gisler at Gisler Moto and he says don't put extra gaskets under the cylinder and I should aim for 1.2 to 1.5mm squish.

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=kuYFuUqzNBMC&pg=PA191&lpg=PA191&dq=squish+clearance+250&source=bl&ots=wM3eS1yMfi&sig=-FsGB6oI_6W1VP3waDtUr3IO4GM&hl=en&sa=X&ved=0ahUKEwiyz5ne-NzJAhXBnpQKHUnABzQQ6AEIWTAM#v=onepage&q=squish%20clearance%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.

[F5 Dave]

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]

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.]

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


Here's the Eric Gorr chapter on Kawasaki KX tuning issues.. www.eric-gorr.com/images/documents/KawasakiModelTuningTips.pdf

[rudolph]

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=kuYFuUqzNBMC&pg=PA191&lpg=PA191&dq=squish+clearance+250&source=bl&ots=wM3eS1yMfi&sig=-FsGB6oI_6W1VP3waDtUr3IO4GM&hl=en&sa=X&ved=0ahUKEwiyz5ne-NzJAhXBnpQKHUnABzQQ6AEIWTAM#v=onepage&q=squish%20clearance%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]

Here's the Eric Gorr chapter on Kawasaki KX tuning issues.. www.eric-gorr.com/images/documents/KawasakiModelTuningTips.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]

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/member.php/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.

[F5 Dave]

Yes Chris (RMS) would be able to help I'm sure. He's out in Tits if yer near the west and knows his older MX stuff.

[rudolph]

I got the other head today, going to send it back, its total junk, it has no squish band on one side as it had been badly faced