Maintenance & Technical > KX500 Original

Squish & Compression & Head gasket thickness

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mun:
Doordie is right.
Less squish velocity > less turbulense > slower mixture burning > more time to detonate. This is theory.
However i think that you can try with two gaskets with on you risk. If isn't work, go back to one gasket.

When you have too much squish clearance, you have less power on every rpm. Squish clearance is one most important thing on tuning.

quincyman:

--- Quote from: IridiumRed ---Two headgaskets on KX500 -

Eric Gorr's  book, "motocross & off road performance handbook", volume 3, page 193

"if you want to get a smoother low end pulling powerband with more over-rev, install two thick head gaskets (11004-1186)..."

I think he mentioned it somewhere else in the book, might be wrong about that, but he definitely said it there

its THAT quote that has me confused.  Two thick headgaskets = a lot more squish clearance = less squish velocity

Now, the compression ratio has fallen due to the greater volume in the combustion chamber, and that decreases the octane required, BUT the loss of squish velocity should require MORE octane

I just wondered, for the KX500 in particular, how those two effects interacted.  IE, in terms of octane required, if the drop in compression ratio more than offset the loss of squish velocity...



And the reason I ask is, I'd like my motor to run well on premium pump if possible, w/o having to mix in anything else.... its hard to get where i live
--- End quote ---


I have the same book you are looking at. I made this mod years ago because I ride my KX500 in tight woods situations and wanted more tractable power.  It still is a beast but more controlable. I was also sick of buying kick starters. I broke 3 in a 2 year period. Doubling the head gasket did as the book suggest. Lowered the compression so I no longer break kick starters and made it more tractable. You will have to change the jetting a bit. I just lowered the needle one clip and all was well. I don't know much concerning the science of squish but do know the bike is a better woods bike. I also sent my flywheel to Aloop to have a permanent weight welded to the flywheel. This also made it more woods worthy. If I ever go back to desert or sand dune riding I will buy a stock flywheel and use only one of the thick head gaskets.

Well that's my 2 cents on this.

doordie:
You loose about 6-10 % overall-power with a squishclearance over 2mm,sure it?s make it more like a "woodsbike" ,but if you turn the throthle wide open for a while it will for sure "pinging". :cry:
Better to reduce the advance timing to make it not so "peaky"!
IF you want to get less compression you have to "digg-out" some material in cylinderhead,but still have the right squishclearance in range 1,5mm. :wink:

//doordie

KXcam22:
I have to agreee with Doordie & Mun. Squish clearance is extremely important for both performance and a properly running engine.  For those seeking a softer starting/running engine I would suggest a flywheel weight would be more effective.  All theory aside, the function of the squish band is to force the combustible mixture into the top portion of the head for ignition by the spark plug. Any mixture left behind in the squish band (caused by too much squish clearance) is not ignited and represents lost HP and efficiency.  The optimum shape for a head should have a minimum squish clearance and then the combustion chamber is adjusted (machined) to provide the correct compression ratio.  As an example, my last 2-stroke roadrace bike (RD350LC) had a .200" squish clearance (huge).  I had to mill the head .100 and toss the .080" headgasket to get me down to .020".  But wow what a difference it made!!  I was lucky in this case that the original compression ratio was low enough that I did not have to machine the chambers to reduce compression.   I doubt I would do the 2 headgasket thing.  My KX is a woodsbike and I love the crisp throttle response the optimum squish clearance gives me (adjusted with Doordies help of course). Runs nicely on 94 pump gas.  Cam.

IridiumRed:
hey Guys

I posted a long reply about this and subsequently it didn't get posted... argh

Short version - will replicate the longer version later if necessary

I rebuilt my motor, full rebuild, used the nice "top end rebuild" sticky (thanks guys!!)

I measured the piston to head clearance a bit differently though

I assembled the top end as it would be with the motor fully together, except - no rings on the piston (less drag), no base gasket, no sparkplug (again less drag) BUT with a piston ring on top of the piston

Then i could spin the crank over by hand (using the ignition/stator rotor), and feel for when the head/piston interfered/locked up

Then i used feeler gauges under the cylinder block, at opposite ends (to eliminate "rocking" or tilting)

I kept using thicker gauges until no interference could be felt

Then i looked at the thickness of the piston ring, and the thickness of the feeler gauges, and from that I could determine what base gasket thickness i needed to run.  

Ended up at .055 (1.4mm approx) = .045 ring thickness - .020 feeler gauge (where the interference JUST went away) + new base gasket thickness of .030 (made my own)

Took me a while to think up how to do this (probably not new idea, just new idea to me!), but it was easy to do, very repeatable (within .001")

Bike runs great!  180 psi compression.  But getting the powervalves clean and reset (great idea about flipping the pins on the main/center powervalve), I can kick it over in tennis shoes, no prob, starts 1st/2nd kick.......

What do you guys think of that method?

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