Nobody here can tell you, there are way too many variables that are involved and not enough info in your post to make an accurate call.
IF your YZ has completely stock porting and head dimensions, you should be able to safely reduce your squish clearance to 1mm and still be able to run pump gas. This should bump your power everywhere, especially the midrange hit. All you need to check this is some solder wire and accurate calipers.
Doubtful. I removed about 12 thousandths and tried 92 octane. Worked o.k. till about 1/2 way through the 1st practice lap then started pinging. I pitted and have ran a mix of race gas/92 pump at 50/50 since. Makes a nice little improvement in power.
Mikie, its likely that the piston will contact the head if you reduce the squish clearance .020 inch. The trouble with older engines are variables like worn or stretched rods and bearing freeplay.
A quick and easy way to make a modern dirt bike pump gas useable is to use 2-3 base gaskets. That will increase the squish clearance and reduce the compression ratio. Two gaskets for 80-125cc and three gaskets for a 200-500. Use some type of copper-coat gasket adhesive on both sides of all of the gaskets to reduce the chance of leakage.
Eric, I already did the deed, I took .020 off the head, put it back on the cylinder and checked the squishband with solder through the spark plug hole, it was about .035 so I matched the angle and turned .007 matl. off to give me a squish of .042.Is that enough clearance?
I forgot to tell you that I just rebuilt the motor (new every thing) It is stock except for wiseco piston and FMF GNARLY pipe power core silencer and an 11oz. flywheel weight.I just sent the suspension off to MX-TECH to be revalved for harescrambles,Jeremy already shortend it 2inches ( yes I am short)I want to make it more harescramble worthy.
Yes .042in. is usually enough but consider this, you have no way of staicly measuring the clearance when the piston gets hot and starts to elongate, or the rod swings farther to TDC because the bearings are getting worn. Then of course there is the question of expansion.
I think that .060 inch is a safer choice, that is the magic number that Yamaha uses to set-up newer models like the 1999 and later YZ250s.
When I had my suspension shortened, by Factory Connection, they advised me not to shorten the front and rear by more than 1 inch (I ride MX hare scrambles, and enduros). Please post a review of your 2 inch lower suspension and let us know how it works. I particularly interested in how it effects handling in whoops and in sweepers.
I can only tell you that compaired to the stock 98 YZ 250 suspention the shortened version is WAY BETTER! That could be because it was revalved at the same time it was shortened and I am only 5'6" I never raced MX after the shortening but I did race some harescrambles and I could not believe the improvement,both in the suspention and my finishes.big confidence booster.
Eric, I took .018 off the sqiush band to give me a grand total clearance of .060 with the original .020 I took off the head surface did I actually raise the comprestion enough to gain performance? If I did is it possible to run pump gas? Do I need to check head volume? Do you know head volume for stock 98 yz 250 or what I should be shooting for? I am not apoised to running race gas if I have to, I ran it in my 93 kx 125 you ported for me(fast).
Steve125, I think you may be right, I wish I would have checked head volume before I removed any aluminum! I ended up with a 22cc head volume minus 5.53cc for the piston dome and I get 16.47 but the sqiush band is now 9mm wide. What does it all mean? I don't know enough about these types of mods yet. I'll take Eric's advise and see(hear,ping?) what happens. mikiemoto