The thing you have to realize when you get into porting and modifying an engine is this:

To really learn anything--you have to ruin the engine.

By that I mean:
It takes some SERIOUS dedication to port an engine and document what actually worked and what didn't. You have to only do one little modification at a time and then put it back together and run it. Tach it. Document what you did.

Then you tear it down and grind on it some more. Only do a little bit and only work on one specific area at a time. Then put it back on the test stand and run it.

It takes a helluva lot of time to really get one worked over enough to gain anything significant. If you just go in willy nilly and start grinding here and there--you'll trash the engine real quick and you won't learn a thing.

You can always increase the intake duration by grinding on the CLOSING side of the crankshaft opening. But DON'T screw with the opening side.

You can make gains by machining the mounting face of the backplate a bit. This will push the cup on the backplate into the crankcase further. Thats called Stuffing the Case. If you reduce the volume in the bottom of the crankcase--you effectively speed up the intake charges moving through the engine. When you speed up the charge--you get HP. How much you can stuff the case by machining the backplate is up for grabs. Eventually, it's going to hit the crankpin and the rod. You put a thicker gasket on it to bring it back out or the motor goes bye bye. A few thousanths off the backplate could make a big difference.

There's a thing called Swept Volume. Thats the amount of crankcase volume UNDER the piston. If you decrease the total CC of swept volume under the piston--you get HP. The reason why is because when the piston goes up and sucks in a fresh charge--the smaller volume under the piston will be under greater suction from the piston. More suction--more fuel gets pulled in. It accelerates the incoming charge and puts more in there. Then when the piston comes back down--that big charge is looking for a way out. It gets squeezed down in a smaller crankcase because you decreased that volume. So, the piston comes down into a smaller space and compresses a larger charge. When the transfer ports open-- WHOOOOOOOOSH -- big ole charge gets pushed up into the combustion chamber. Makes a bigger bang when the piston comes back up. Stuffing the crankcase and drilling out the crank shaft can work REALLY well, but there's a limit to both of them. You can only stuff the case so much. You start getting realy nice results from bigger charges and faster transfer velocity. So, you drill out the crankshaft to pull in more fuel. You can pull in more fuel because of the larger suction pressure under the piston when it goes up on the compression stroke. But, drill out the crankshaft too much--and you just UN-STUFFED the case because you introduced MORE volume to it. First you took a few thousanths off the backplate to lower the case volume. You got some nice numbers so you thought you'd drill out the crank and let in even more fuel. But, the amount of material you take out of the crank just adds to the overall volume of the case again and the charges slow down again. Paperweight. Unless you do something else to counteract it. Increase the compression. Incrase the duration on the exhaust. Stuff the transfer ports. And on and on and on and on and on. Everything you do has an effect on something else.

You can increase your exhaust duration by raising the top of the exhaust port.

You can drill out the hole in the center of the crankshaft to move more fuel. But, when you do that, your slowing down the velocity of the charge. So, you need to do something else to make up for it. Everything yo do will help it in some way--and hurt it in another way. It's a very hard act to balance. Take a little off the transfer ports or chamfer the edges--and it might run like dog poop. But, then if you go in and drill the crank out a bit--it will benefit those transfer ports and all of a sudden you get 300RPM. Every modification you make will have an effect on something else. Thats why you only do one modification at a time. You need to know if you hurt it or helped it. The only way to do that is to grind on it and then run it. Then grind on it and run it again. Baby steps. 1 thing at a time.

But--and this is the absolute most important thing--you NEED a degree wheel. You need a degree wheel and you need to document what duration you have on the intake and exhaust and the tranfer ports. You need to know where your starting. Whats the duration NOW--while it's stock?

Once you know the duration, you can start to grind on it.

There are guys here who can tell you how far you can take a sport engine. Go to far, and it's a paperweight. There are techniques that need to be observed as well. Do you just want top end speed? Do you want low end torque to spin a super big prop at lower RPM? If you port it out to run like a raped ape--it might not idle worth a darn.

In all the trials and grinding that you do--you must realize that you'll be one of the lucky ones if you end up with a running motor when your done. I ruined several motors before I figured out how much you can do. You have to consider the motor expendable. You have to be willing to trash it. If you aren't willing to sacrifice it--then leave it alone because 9 times outta 10 a guy will trash the first 5 or 6 motors he ports and modifies. You learn stuff. You figure out what works and what hurts. You make gains and then you loose. So, you go back in and grind on something else to try and get back what you lost. Eventually--it makes a nice paperweight. But, you DO learn stuff.

If your really serious about porting that little engine--get a tuned pipe first. A tuned pipe is bolt-on power. Bolt it on and cut the header to length and you'll gain more HP than you ever thought possible. There's techniques to tuned pipes too. You can tune for all out top end RPM--but you'll sacrifice midrange and idle. Or, you can tune for just a nice overall boost in performance and be happy with it. It will idle nice and transition nice and it will make a lot more power than stock. Eventually, you'll be tempted to hack the header off a bit more and get a few more RPM out of it--thats when the idle goes down the drain. Easy fix. Buy a new header and start over.

I don't know if he's still on here--but Dave Agar knows a lot about this kind of stuff. And there are a couple other guys who know all about the duration and degrees, but I can't remember their names. FlyBoyDave is a gearhead. He might be able to tell you how much you can get away with. And William Robinson is another good engine man--but I don;t know if he's strictly a 4-stroke God or if he knows about porting 2-strokes also.

I used to have a nice spreadsheet saved on my harddrive--but you know what happens when the worms get into your harddrive. You loose all the wedding pictures and holiday pictures on the disk and your wife cuts you off in the bedroom for about 2 weeks. [sm=cry_smile.gif] Then you go out and buy a CD burner and put all your pictures on disks about once a month. Too little, too late.

Good luck.