big bore

[Flash]

Alright just curious, has anyone had any experiance with big bore kits in 2 strokes? My understanding is that port height and shape and angle are very important so me thinks this isn't good, as increasing the size of the bore will mess htis up.

The reason is that the AMA have now passed a rule that a 125 bored to 144 can now race against a 250f!! All of a sudden there is a lot of big bore talk in the states, and of course this has got me thinking :-k :D

[cornerspeed]

I have read about 'big' boring when I had my RGV250 (to 300) and its a MASSIVE (and expensive) prosess. I read from a guy that done it and he said it was all worth it in the end as the power incress was awesome

[Wobbly]

From my ramblings from time to time, it sounds like it's a case by case basis. Depends on how much wall thickness there is in the port walls and the geometery used in the ports themselves. In any case, once you've had the bore done you'll need to spend a fair bit of time re-profiling the ports entrance into the combustion chamber to get the entry angles set again. I'm not exactly sure how you go about measuring the entry angles to begin with, except for by eye - its not like you can trace it out like you can with basic port timing.

But, you enjoy playing with porting tools right? :lol2:

[Harry525]

I would have thought that taking a 125 to 144 would be replacing the barrel completly, not just making the hole in the 125 barrel bigger. Even if you were to bore out the 125 barrel there are guys out there that do porting for a living and they're very good. Don't forget the go kart scene is pretty big and they are porting small 2-strokes all the time.

[Harry525]

My first bike was a TS185ER. I pulled the barrel off and took it to a kart shop of porting. The bloke asked what I wanted (more bottom end please) and he said yep no worries. Two days later it was back together and guess what? More bottom end!
Cost? $80

[Flash]

There is a lot of these being done at the moment in the states I can get the 144 "Kit" for just over $900 US. Which includes porting etc. I have even considered the stroker kit as well 167cc(big block :lol: ) but the end of the day the cost it well, lets just say I'd could almost buy a 250.

I've got a Go Kat back ground and there is only a few that I would trust with porting anything!! Go karting has died in so much in the 5 to 10 years that there is no class going that is allowed to have port mods done!! They only run Yamaha's, Rotaxes, ARC's and Leopards.

[motard_mike]

i used to ride a rz250. i did the ports put bigger carbys on it and shaved the head. it went a little better. then i put the 350 barrels and head on it, again shaved head and ported barrels. it turned the bike into an absolute beast, i wish i still had it. i didnt change the stroke or the rods just bolted on new barrels and pistons. it had heaps more top end and a little better down low. this was not just because of bigger displacment but the bore/stroke ratio changes. for me it was a very cheap project (under $1000 total). i would find out what the compresion ratio of the new setup is aswell. both low and hi rpm.

[Flash]

It comes as a full set up with the compression sorted as well as the porting. So they say!!