Usually port timing for a particular engine works in conjunction with port area....so we have a parameter known as time-area which determines the port ability to flow within a given time period.

Having done the time area numbers on my 34cc engine I used the following durations (for a power peak of 7500 rpm)

Exhaust: 160 deg

Transfer (twin transfers only): 125 deg

Intake(piston ported): 160 deg

The factory timings (for stock power peak of 5600 rpm) were

Ex: 140 deg

Transfer: 110 deg

Intake: 135 deg.

My timings may seem radical but are necessary because I am also looking for piston & ring life. I could go wider on the port but that then I would be crossing safe limits of about 65% beyond which ring life reduces.

If you have a rotary valve setup then you can safely go to upwards of 210 deg before experiencing significant blowback. This is what I have found on my 125 cc engines. Using a pipe with a rotary is almost always mandatory if you are going to open the port very soon though.


I dont know much about reed valve engines havent worked on them.. Bryan, if you are going to retrofit a reed valve from a larger engine to a smaller one (which is turning many more revs than the larger one) what are the guidelines, or thumb rules if any?