cutting the bottom of the port is just common sense if it is in the exhaust streem ,as in porting ,you make it smooth
and free flowing if its a ridge smooth it out if it a depression ,fill it. my pistons on all three ryobi's goes past the bottom of the exhaust port to smooth this out you must grind it either flush or lower than the piston topif you grind down too far
the piston skirt will uncover a small area in the exhaust port that will be open to the crank case ,connecting the crank case to the exhaust port .if that happens you will get less than good results, this is something that I came very close to doing ,I got carried away grinding by the time it was smoothed out and polished I had gone well past my target.I didnt realize that there was a risk until after reasembling it and didnt want anyone to make a mistake based on my advice .
the bottom of the port has nothing to do with exhaust port timing ,and very little to do with duration, the top of the port controls open and close of the exhaust port the piston top controls 1\2 way on the duration. the port is almost to wide now,its very close to the transfer ports and one ring end,. if made too wide the ring could conceivably expand into the port and catch on the port as it travels up breaking the ring
the thing about the carbs ,yes I understand that you can only put a larger carb on to a certain point .after which you're too big, however, if you cant get the old gasses out then there is knowhere for fresh gasses to go.the stock carb,which I am running is around 9 mm at the venturi ,very small ,most guys do replace the carb with a larger one .the 13.5 zama that I refered to is the same one poco 242 runs on his toro 25 engines without any problems. I for whatever reason cant get one to run lean enough to use ,it maybee its too big, but poco has used them on ryobi's before with success, so I thought it was worth mentioning
I was allways taught that the area from the bottom of the piston pin bosses to the bottom of the piston is the skirt
rc pilot; I still think some of your problem stems from your exhaust port being to restrictive 128 degrees is exactly what aero nut was talking about,
yep,sometimes I get a little enthusiastic ,its one of my" endearing character features"or flaw if read another way.if I have good results with a tunning trick I like to share I dont horde them like a miser and gold,I thought thats what this forum stuff was about.
rc ign; do you have any other problems with any more of my statements ,have I explained myself sufficiently,after re-reading this thread I can see where you could get confused if you skimmed it .or took it out of the context with which it was presented .however results are what matter I picked up 300-350 rpm's doing what I described.if I'm wrong please tell me so I can make the appropriate corrections. but I dont want to split hairs Idont have time for that .