Pumping effect: basically on a two stroke, the crankcase acts like a fuel pump and since the air is involved too, sort of like a supercharger as well. The higher the compression ratio in the case, the more the effect though higher ratios also mean less volume so you lose on volumetric efficiency so around 1.5 to 1 seems to be the magic number.

The link:

http://edj.net/2stroke/jennings/

Many thanks for that information. I'll be busy for a while going through all that.

Kindest regards,
Lars.

What would happen if you ground the bottom of the exhaust port on the sleeve say 1mm to match the opening in the case?

You can get a worse milage. The small edge works like a gurney strip, any flow wil be bended up instead of direct to the port.

I'm trying to mod my older TN12 up. The piston/sleeve/Conrod are all good. The piston reaches the top of the sleeve with 1.5-2mm left before the piston stops. The rod also doesn't have any play.

From Steve Pond's article, I see that front port is smoothed out, but what about the exhaust port? Where would that be modified? What about the little groove on the crankcase, should I smooth that out a little bit? And what about the crankshaft inlet? How much material can be taken off a .12 piston safely?

I want to do this and am not worried about damage, but I'd like to do this correctly, or close, the first time.

I'm looking for better top end results. Not the best. :)

If you are looking to improve top end with a simple proven and reversible solution, place a shim under the sleeve´s edge and the seating area on the cranckcase, that way you´ll increase total exhaust timing giving you more top end. start with a 0.10mm shim. Don´t forget you´ll have to re-shim the combustión chamber accordingly, that is remove same amount of shim 0.10mm

AFM

the exhaust port? Where would that be modified: for top end you would have to cut the top edge to increment exhaust duration, but that has to be done in degrees and very tricky if you are not experienced.

And what about the crankshaft inlet? here is where you get the most notorious engine improvement. Induction timing works like this: earlier opening = more low end . Later closing = more top end.
Also keep in mind that the greatest boost in power is gained by increasing total induction duration, and increasing inner diameter of passage in crankshaft.

How much material can be taken off a .12 piston safely? stay away from this option.

AFM

I cleaned up the front port, bottom of the sleeve, a tail groove on both side ports, I cleaned up the crank groove, and roughed up the inlet port on the front of the crank. I actually used a file in there, just a couple rubs nothing crazy. I kept reading about turbulence for better mixtures.

If it runs like crap oh well. :)

Modding
 

I would like to thank all that have posted info on this thread. It has helped a lot. I built an angle finder like Stefan posted using a 5 mm pinion gear glued to the back. I also made a piston stop to get TDC . I first Modded a Sportwerks .26 . I ground away the bridge at the bottom of the intake runners to match a Novarossi I had. I then sharpened the bottom of the sleeve. I didn't touch the ports on the liner but I ground on the crank to make it close at 65 degees after TDc with a 215 duration. It runs awesome I tried it in my driveway and I thought it was going to tear the driveline out of the car. I have a lrp 30 x I was going to do and it doesn't need alot . Maybe a couple degrees on the closing of the crank as it closes at 62 degrees. I wonder how thin I can go on the Crankshaft before it would break? It is 14 mm and the hole in it is 10.5 mm. I was thinking about going to 11mm. Fuel mileage isn't a concern.

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This tread is 100 years old. If you go through a lot fo treads you will find all answers on you any raised question.
On this question I will answer for you-do not cut bottom edge of exhaust windows. If you cut and it will open while piston in Top Dead Center your engine will have decompression of crankcase, it will mess up all your timings, millage and power. What is the reason you want to cut it> do you think if you make smoother pass for better flow it will run better? It doesn't work that way, it is not sewer pipe with shit flowing in it, there are many processes involved, including sound wave, so that "smoothing" will do nothing in terms of better flow. But decompression will kill your engine performance, specially with tuned pipe. But I can guarranty you, it will work much better without tuned pipe.:nod::D

BTW, too many new faces here and most of them didn't read all our materials in 2004-2006 time frame.
Here is trick for you-if you want to change exhaust timing, or transfer timing and don't feel comfortable to mess with sleeve, you can do it change without touching sleeve. You can cut piston correspondent to port segment. Search somewhere in my tread Palmaris Engines ( http://www.rctech.net/forum/onroad-n...ne-thread.html ) and you will find good pictures there how it is done. I did many many times. I know it is a lot of reading and searching, but it worth it.

Hey brother, do you know difference between explosive and combustion? Nitromethane is explosive, but very poor combustible material and Stefan is almost right. Dynamite is explosive, but try to light it up with matches-good luck with that! and stop reading Wikipedia, it is social source, not scientific:lol::lol:

Glad to see you back EdwardN :)