I got a project saw

[ironman_gq]

I am not very experienced with making mods to chain saw motors. With race motors that is a different story. The motors that I tuned over the years were a matter of evolving with fairly predictable results. It appears that most guys who are raising the compression for their chain saws are aiming to get the squish area between .020'' to .030''. I do not hear anybody say that the larger bore motors need a completely different ratio or is the purpose just to be the ball park with a increased compression ratio? Race motors some times produce much less HP with an increased compression and some times they will destroy themselves with too much compression. With chain saw motors they seem to be much more forgiving and respond well to a little more compression is this true? Thanks

Relatively speaking, chainsaw motors aren't high performance motors. They're built to run hard for a long long time (2000+ hours) and for their size they really don't make a lot of power. A 90cc chainsaw motor might make 6-7hp where as a a similar sized dirtbike motor might make 25-30hp and last 50-150 hours between rebuilds. Being that they are relatively low performance they are fairly forgiving in what you do to them as long as you don't exceed certain points (less than .020 squish) and don't have mechanical interference internally (ring hanging in a port, piston hitting the head) and can keep them reasonably cool.

 

[user 122190]

Owner is back. I expect a call sometime this morning about breaking it in. Since this saw had become a major disappoint to him when it was lightly straight gassed, this will seem almost new to him. Sounds like the breakin should be run moderately hard (periodic WOT with light loading) for short durations. Although I have no reason to doubt it will run just fine for a good number of years, until we get it broken-in and tuned, I'm a bit anxious.

 

[grizz55chev]

Owner is back. I expect a call sometime this morning about breaking it in. Since this saw had become a major disappoint to him when it was lightly straight gassed, this will seem almost new to him. Sounds like the breakin should be run moderately hard (periodic WOT with light loading) for short durations. Although I have no reason to doubt it will run just fine for a good number of years, until we get it broken-in and tuned, I'm a bit anxious.

Bump

 

[user 122190]

The saw has been used but could not hold an idle and bogged out under load. Today was my first access to the saw and I confirmed what I suspected. I had the owner hold the carb, and it starts easier, the idle is greatly improved and the full throttle sounds good. When putting it back together, the carb was a bit floppy because the flange was stripped on the left side such that the carb would not hold tight to the intake boot. I was aware of this when putting it together but wanted to see how everything else worked. I'll order the new part and get this last problem tidied up.