Hi Master_Blaster, I see you’re in a dilemma about what to do. I’ll try to offer some advice. There are several problems with an oversized sleeve and piston in a stock barrel. When a barrel is bored to accept an oversized sleeve, the ports are cut and the timing is slightly changed (can be for better or worse, depending on the engine). The exhaust port in particular is changed because it is angled downward, and it is nearly impossible to move the exhaust port high enough to regain the port timing of the original engine. You can try using a base spacer and additional gasket, but the top of the barrel may have to be decked, depending on how much you raise the barrel.
An exceptionally large piston causes more heat to be built up in the engine especially at higher revs because of the swept area it gets in an undersized barrel. This can cause problems with the Blaster because it is an air-cooled engine. The larger piston also weighs more, and this causes slower revs, and it limits maximum revs as well. Also, on some two-strokes with small crankcases, increasing displacement can be bad news at high rpms. This is because there isn’t enough crankcase volume to effectively fill the cylinder, and this can cause the mixture to lean out and power to be lost.
But on the positive note, oversized pistons in the stock sleeve can yield neat results without the lost port timing of the big-bore. As horsepower is a function of piston area, more power output can be seen by boring out the stock sleeve and fitting the corresponding oversized piston. Just make sure to properly bevel the port windows when doing this, or the ports will act like scrapers to remove oil; the result, the new piston rings won’t last very long.
On a side note, the stock Blaster cylinder head is in a VERY mild state of tune. The squish band clearance is too loose to be of help. If you’re looking for some cheap horsepower, you can mill the head slightly to lower the squish band and put more mixture where it can be burned. If you have a shop do the barrel porting, make sure they give some thought to the cylinder head. If the shop knows what they’re doing, you can tell them what kind of riding you do, and they can cut a head and modify the barrel (if necessary) for the correct squish velocity. Just make sure they work on two-strokes and have enough experience to do this type of work.
Time for stoker engines. I think these can be best explained, with the advantages and disadvantages by reading this article from Group K
http://www.groupk.com/tec-y-strokers.htm. It’s a little long and slightly dated with a focus on watercraft, but the theories and issues they faced are very real.
So I guess in the end, 01blaster_freak has a very good solution; porting on the stock barrel and removing casting flaws definitely helps output. A tuned/aftermarket expansion chamber and silencer work with porting to help output as well. If you’re interested in doing your own porting you can follow what the great Rick MacDizzy did here
http://macdizzy.com/1989hopup.htm He really gets into the porting on Parts 6 and 8. Hope I’ve helped.
