Originally Posted by
speedpro
The CR piston with the cutaway is probably OK. It will increase the port timing but the timing stock is horrendously conservative, as suits the bike's original purpose.
In my opinion it would be better to use a full skirt piston and take the piston port down to achieve the desired port timing. This will result in increased port area as well which it needs. Once the piston port is where it should be you will probably have gone into the reed valve area by busting out the bottom of the barrel. I had great success in completely removing the reed valve and filling the whole area with devcon. The end result was 200 degree piston port timing, plenty of port area, nice smooth intake port matched to a round slide 32mm Mikuni.
You are going to have to plan the whole thing before you start machining and grinding. If you machine the piston port down and then drop the cylinder because of a short piston causing excessive transfer port timing you end up with excessive piston port timing. A bit of devcon would fix it but I prefer to have as little filler in the intake port as possible, in case it falls out. The exhaust ports in those cylinders is a bit limited as well. The main problem with the ports though is the old style transfer ports and the angles entering the cylinder. If you correct the roof angles towards modern numbers it will raise the port which isn't a bad thing and scavenging will improve markedly, helped by a good pipe.
In order I would look at:
Rod and piston combination
Transfer porting
Transfer timing or T/A if you want to get that serious. T/A is Time/Area.
Exhaust timing or T/A
Piston port timing
The exhaust port is already too big at the pipe flange. If you do port it to get the port timing, only take out the minimum amount at the cylinder face and don't remove alloy further down the port. With the single exhaust port there is going to be a limitation on the blowdown T/A. Blowdown is how much the exhaust port is open before the transfers open. Raising the transfers will increase the transfer T/A but reduce blowdown T/A. Raising the revs will increase the blowdown T/A required.
It's going to do your head in at the start.
Do you have an update for the 750?
Bookmarks