milling 2 stroke yamaha 90 heads,, how much

I have a 2 stroke yamaha 90 with hydrotec heads, they appear to be milled about .125 what is the max that I can mill without hitting water jackets. I want some more compression.

The other questions

I have the hydrotec heads and boyson reeds, and 165 jets. It seems flat at the top end. What jetting would you have thought??

What max spark do you recommend.

I am running 100 octane.

I have been running 3.0 loopers for the last decade and don't have a good feel for the smaller engines. I have it on a avon Rib using it for sport and skiing.

what is the comp now? last i knew 165 is there biggest jet. how fast are you spinnin her at wot?

Does that engine have a squishband, and have you checked the squish height? Usually unmodified production engines, due to manufacturing requirements such as keeping the scrap rate down, have pretty loose squish heights. It might be that just milling the head so that the squish is down around .032-.035" will gain you a little compression AND a lot better turbulence in the combustion chamber (mixes up the air and fuel better), AND a lot better resistance to detonation, even with the slightly higher compression. Win, win win. Depending on your engine, you might have to mill the deck of the block instead of, or in addition to, the head in order to get to the right squish dimension and proper match of piston crown and squishband.

When the rules allow you to do this, it's an excellent first improvement to make to any loop-scavenged 2-stroke, and can be good for most other engines with a squish area (specifically including any auto engine with closed-chamber heads, although they are far more of a hassle because you almost always have to mill the deck of the block, rather than the head).

The head can be milled a lot more than .125

Finally back to this project. I measure 145 on the hydrotec heads... (cold and starter cranking)..

Robert

Had a Yammie triple in my shop with milled heads like that. Made 190 plus compression. Had to use both hands to pull start that beast!

Jeff

how much were those heads shaved? I figure about 160-165 is where I would like to be. I measured the squish at 0.039 so there is not a whole lot to work with there.

If I mill a set myself,, what dimensions should I shoot for on the squish band. Before I bought these from Hydrotec I talked to them about more compression and they said that they did not like taking more off because of porosity in the heads and the water jacket. I think I will ask them again if they will build me another one. Their price is not a whole lot more than buying a new stock head.

I have a thing about trying to keep motors always running so I don't want to pull this head to mod, I want to mod another head and swap.


robert

It had an aftermarket head.

Jeff