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Squish Band Question

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What is the desired effect when altering squish? I had been having issues with my 2002 RM 250 and excessive spooge, and have gone through everything from jetting, leaky crank seals, reeds, packing, pressure tests and even premix flash points! Whew! So I guess the question is really: Can and or will manipulating the squish band effectively help ignite the 'excess' fuel/oil in the combustion chamber, and help with spooge??? Thanx guys, I appreciate ANY advice.

The bike runs awesome, but I cant seem to get that last bit of spooge to tune out!

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What sort of mods are done to the bike. My 2000 CR125 had big problems with spooge too and went through the same thing. Need to know i little more information though:

-What kind of oil are you running and at what premix ratio? ( you can run up to 20:1 without spooge if jetted correctly, most don't believe me)

-What octane of fuel?

-Has the top end been freshened up or a compression test been done. How much compression did it have?

-What were the stock jetting setting and what has all been changed?

-Has a plug chop test been completed and which what results (running the bike wide open for a length of time then pulling the clutch and killing the engine. What color is the plug?)

Hope to help you out and get your oil pucking problem to become a thing of the past!

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DirtRider has run 20 to 1!! My kinda guy! The engine NEEDS oil to achieve ring seal!

That said a common issue causing some of these engines to run poorly, make an oilly mess etc is a leak between trans oil & crankcase. Primary drive side seal being the most common but I have seen other leaks, simple test is to route a hose from the trans vent to a glass of water, then rev the engine, if it blows bubbles we have a leak.

Back to squish, the tighter you make it the better it will run, more power sharper responce etc etc, right up to the point where the piston starts tapping on the head, then the engine blows up. For a low maintence bike I would not go tighter than 1mm. If your banging bars with Bubba and your factory trained mechanic has it open every week monitoring carbon patterns on the piston, then you squeeze until the carbon shows traces of contact, then add a couple thou next week and monitor it every week

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Interesting. How much time does the bike spend in the powerband? I was wondering if maybe (assuming everything else is fine and dandy) the exhaust scavenging isn't being worked 100% and it's rebreathing a little of it's own combustion products (when a stroker's "on the pipe" resonance in the exhaust literally sucks combustion products out of the exhaust port). Aftermarket exhaust? Or something unrelated like over-oiling the air filter a little?

Squish is a good thing in an "easy fix for a real-world problem" kind of way. I've read that the mixture that's trapped may be subject to incomplete combustion so when people ask "What's the ideal squish clearance?" the ideal answer would be "zero!", But as Dave Hopkins suggests, most of us prefer riding to wrenching and the guide I was given a long time ago was 0.1mm per 10mm bore. So a 66mm bore equates to a .66mm clearance.

Perhaps it's a bike-specific thing. I'd suggest posting a link to this thread in TT's RM section to attract other owners.

Hope you nail it!

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I had not heard the "0.1mm per 10mm bore" but it makes sence. There was a time I was racing a 250 Husky and tinkering with squish every week, I found at .8mm I started to get a witness mark on the piston (bore may have been 76mm?) so .76 would have been just a bit snug for comfort but very close. Other factors come into play, those engines (this was early 70's) had long cylinder studs from case up thru the head and most of the Jap engines had cylinder bolted to case and head to cylinder each with short bolts so the expansion rate from crank to head was somewhat different

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Maximum squish velocity should be considered with this mod. Less clearance may be better from a standpoint of eliminating end gasses, but it can also drive the msv so high you could bring on detonation. If you "correct" the squish clearance you may also want to adjust the compression back down by modifying the chamber. As a general rule, heads with very wide squish bands usually were set up to run with wider clearances, those are the ones to be careful with.

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