I have a Poulan 25cc weedeater conversion going, I have a Frank Bowman ring in the works for it but I was hoping someone with more conversion experience could also help.

In this engine @ TDC the piston is .025" away from the top of the jug. I could shorten the jug by .005" so the piston would be that much higher in the jug.

Would it be worth the trouble? . I have verified with my depth micrometer set that I can reduce the gap at TDC to .015" and still have no interference from the spark plug as the top of the jug is hemispherical. But would this also change the port timing?

Just an idea, I have 5 of these engines so I'm not that worried about experimenting some.

If the base gasket is on the thick side....try removing it first and use Yamabond to seal it back together. Just make sure to clean the mating surface good. Capt,n

Yes it will change you port timing, in most cases for the worse, and the gains in compression won't fix the fact that you reduced your exhaust timing and will get less RPM. My opinion would be to leave it. New ring, advance the timing, larger carb, open exhaust and you will have a nice engine. If you want to run in the 8500-10000 rpm range, then experiment with opening and raising your ports.

PS-please post your experiments and numbers though, we all want to learn at your expense-HA!

If you lower the jug, you may also need to cut the ports higher to keep the timing the same on the those. I'm not sure of 5 thousandths will matter much or not though, I'm way out of my league here...

Edit - Didn't read the post above mine...

Heres a link to porting a homlite.There all about the same.You might want to PM av8tor as he did a couple of his.Also check out porting in the search bar in the forums.
http://testfly.0catch.com/homelite/e...lindermod.html

Here's the thread on a Homelite 25cc I did just for fun: http://www.rcuniverse.com/forum/m_26...tm.htm#5736440

Some of it won't apply, but it talks about the lowering the cylinder/port timing issues. Raising the compression ratio on these little engines is always a good idea in my opinion, as they are quite low stock. It's no big deal to check the exhaust timing and raise the port to increase it. It often needs it anyway. Use around 150 degrees for the normal 7500 to 8500 rpm range. (They sometimes come with exhaust timing as low as 120 degrees.) The transfer port timing is usually ok without changes. On an all out effort engine I change the transfer timing too, but they are tricky to port.

If that is a Featherlight engine, you might contact aeronut, as he has had some successes with porting them and gaining quite a bit of power over stock I believe.

Good luck,
AV8TOR

Well I got the numbers for the timing figured out, I think I did it right.

Intake port is open 116 degrees of timing.

Exhaust port is open 134 degrees of timing.

Transfer ports open for 85 degrees of timing.

with a 16X8 APC prop I get 7200 RPM (CDI), compared to my Evolution 26, Same prop gets 7800 RPM.

I'll se what kind of increase I get with the new Frank Bowman ring.

Yeah, that exhaust timing is a little low.

Two important things on these little engines are to use a free flowing exhaust; (I use two 1/2" i.d. outlet tubes and NO baffles in the muffler), and a larger carb. 11mm is a good size carb venturi for these size engines. I don't recommend any porting or other mods until these two things are addressed first, assuming of course that the engine is in good shape to begin with and doesn't need rebuilding first....

AV8TOR