Another MM700 Clutching/Jetting Question

[deepnsteep]

MM700 Jetting and Why

I've got a '98 MM 700 that I just got PI end dump pipes for. Stock porting and a milled head. I don't know the exact specs on the milling, but it was done by Cooke City Yamaha some years ago...before I bought it. PI recommends 132.5(1) and 131.3(2/3) on the mains, 62.5 on the pilots, 2 for needles, and 2-1/8 turns on the pilot screw.... The guy I bought the pipes from had 55 pilots written on the spec sheet for the area I'd be riding (8300-10000).

Looks like stock clutching should be close.....0.8g in both holes in stock weights, yellow-white-yellow primary (2 shims, short cap), green yamaha secondary with 43 degree helixIf I remember right, I have 19:40 gears with 9T drivers.

Not too worried about the clutching, but would like to know what the tighter squish in the head does with jetting and why.....

Thanks for any input!

deep

 

[ModMMax]

I am familiar with the head modification you have. I had one on my 97 and in fact bought it at Cooke City Yamaha. It is my understanding that the stock head was milled and the squish recut to near factory. The reason is sort of like this. Every 1000' of altitude costs a 2 stroke engine about 3% of its power. It is possible to compensate for this loss by reducing the head volume, thereby increasing the compression. And compression is power. The squish is something else. The dome shape of the head chamber has a very small clearance between the piston and the head near the edges. If this clearance is too small, fuel will detonate around the perimeter of the combustion chamber. Causes heavy badness. Recuting the squish to near factory specs means this perimeter area has sufficient clearance to not cause detonation. My last MM was built to run at 7500+ ft on premium fuel with a gallon of C12 in every tank fill. Not a real exact science but it worked good and was reliable. I did some testing with it at 1800 ft on 50/50 premium/C12. I have used both 62 and 55 pilots. You should start with the 62's. The pilot circuit seems to contribute to mid range jetting so start at 62 and if you find that it is loading up in the idle circuit and low mids then you might try leaning the pilots. All the modded 700 non PV engines I was around seemed to develop a low end hesitation. Most often the problem was caused by mods to the air box. V-force reeds seemed to help cure that reluctance the engine had when coming off an idle. I finally cured mine but it took it while. Good luck.

 

[deepnsteep]

Thanks for the info Max.....I will have to measure the squish when I get it out of storage and see where they've got it.

Have any suggestions on the mains?

Thanks!!

deep