A little machine work to kill several birds with one stone. The 390/2188 is another one of those 90cc saws that has too much exhaust duration. By cutting the squish and base, I lose exhaust duration, gain intake duration, and raise compression in a single step.
If you look closely you will see that I cut the cylinder extensions while the jug is mounted this way.......saving time.
I like to check squish clearance after I cut the band to determine exactly how much to cut the base.
.046"......so I take .028 from the base to end up at .018".
Next step is to indicate the mandrel so the base will be cut dead square. I use a live center with a flattened point, that way I can tap the jug around to get it just so.
Then to check our new clearance.
.019......that will make the squish velocity pretty high on this engine. The band is not very wide, so I can run it a little tighter that on some other engines.
Transfer height on this engine is 120 degrees from TDC. Here the transfers are roughed in......lots more to do to them.
Almost got tunnel vision and forgot to take pics.
Here I'm reshaping the intake port......I don't like an overly wide port so I just widen out to the edge of the factory bevels. Proper shape and opening height is my goal here.
Now for the exhaust. On this engine I don't widen, or alter the height of the exhaust port at the cylinder, but the outlet is worked over pretty heavily.
Remember, we wanted less time/area on this engine. To widen the exhaust port would defeat the trouble we just went to to lower it by machine work.
Matching to the muffler...
Creating a scavenging step.
This is just the sizing......finishing comes later.
The carb adjusting screws have locks that require special tools to adjust. I defeat those so that the customer can properly adjust their carb with just a tuning screwdriver....