Sprockets

[tz375]

At the drag strip the Phat Trakka has been as fast as 107 on 15-50 gearing. On the street it felt more relaxed with 15-48.

I don't know what's supposed to be on a T250, but that one sounds from the description as if it is geared too tall.

[Tripleking]

That web app looks useful: http://www.gearingcommander.com

Stock sprockets on the GT750J are 15 / 47
Stock sprockets on the GT750M are 16 / 43
Max optional rear sprocket per the parts manual is 51

Have lots of hills where I live.
My GT750M has the stock gearing.
The bike pulls out from a stop going up hill like its a slug slowly accumulating speed while you wait for 4.5k rpm, then its zoom ignore those square roads signs with numbers on them.
It runs fine below 4.5k rpm, starts easily hot or cold, & idles nicely around 1.3k rpm -- it just doesn't produce much power.

I was thinking about switching to 15 /51 sprockets.
That would give me a speed at 4.5k rpm in the 5 gears of: 21, 33, 44, 53, 66 mph.

Alternatively, I'm think about switching to Jemco 3-into-1s.

Your experiences sound eerily similar to mine. I will check my sprockets tomorrow.

[smokin_blue]

I don't know that I can solve your issues but I can possibly donate a point of view.

If you go to my site http://www.street-unique.com" onclick="window.open(this.href);return false; and look at Smokin Blue you will find the bike I will reference here.

for my first full custom I built a 1974 GT250. In doing so I left the porting stock and built pipes for it based on a design I came up with through the help of the Two Stroke Tuners Handbook by Gordon Jennings. I designed them to peak at 7K rpm. Like any (non power valved) bike with pipes it was a dog off the line at low rpms. Twist the throttle and wait for it to wake up. Mine would start to light the pipe about 4K, get some steam at 4.5k and by 5k it was into the pipe. By 8k it was falling flat. That was fun but of course you always want more.

From there I bought a T350 engine which remember is only a 315cc. I had to bore it so it ended up at 330cc. Since I had fully rebuilt the GT250 lower end I bolted the T350 top end down to the GT250 bottom end. That is possible going from a T top to a GT bottom. However another period book I have says the to bolt a T350 to a T250 requires you to clearance the cases on the T250 bottom. They apparently used the 350 clearances when they made the GT cases if that is all true.

Now, with that configuration and the same pipes it became an even more fun little scoot. I gained much more low end torque (all relative as you are still off the pipe and fighting the pulse timing) and it came off the line much better. Then when it came on the pipe it had all the more power and torque that adding 32% displacement will do. Now my power characteristics as far as the pipes go never really changed.

All that said I am wondering if your issue lies more with the new power curve you have created in your motor with your porting mods. You may have gone a little too far. You have created an engine that is much more peaky and actually peaks right up near or above red line.

A couple other notes: I left the gearing stock on mine. For comparison with yours try a speed check. In never ran a speedo so I would use 4th gear as my speedo. 4k in 4th was 40, 5.5k in 4th was 55 so if I was tooling along and wanted to check my speed I would drop two gears, check the rpm and then click back up.

So my thoughts are you may have pushed the porting a bit too far and now you are fighting for top end speed/gearing to match your power but you are going to really drop your low rpm power. You might want to consider a different top end and either leave it stock or not go quite as far on the porting. From there invest or build a set of pipes to amp up the fun.