GT250 Cafe - Build Experiment

Suzukidave · Post by **Suzukidave** » Sun Jan 03, 2010 9:21 pm

Lane , i think Tom has pulled ahead in the amount of holes we can cut in

but joking aside the extra holes are due to the reeds and the way they prevent back flow thru the carb . The two side passages from the intakes to the transfers on Toms cylinders will allow a proper drawing chamber to now pull mixture straight from the intake and the hole in the floor of the intake now will allow 360 deg intake without putting holes in the pistons as well as on the piston down stroke mixture in the crank area will now be pushed up thru this hole into the intake while the reeds are closed and forced out thru the rear boost port . On mine i had to take a different approach as with the 750s twisted cylinders you cannot cut in the two side passages to the transfers so i cut in the 2 passages in the intake floor that open next the bottom of the transfers , not as ideal as Toms setup but will also allow the 360 deg intake with a boost to the transfers and also allow a flow to the intakes rear boost port.

tz375 · Post by **tz375** » Sun Jan 03, 2010 9:54 pm

Nice description Dave.

Tom has a huge amount of reed and port area and that may decrease velocity through those ports, but in theory that isn't an issue. Those crankcase ports are somewhat like the RG250 gamma design, but the boyeson transfer ports and huge reeds are different.

With teh right pipes and porting it could theoretically make as much as 50hp, but for a street motor it should probably have 28mm carbs and RZ reeds. My RZ350 makes 56 and it's got stock carbs and Spec11 pipes. So 28mm will flow enough to support 60hp at least.

A pair of RG250 carbs might be the go, or a set of RG500 carbs. No need for anything bigger until it passes say 60 HP at 12,000 IMHO.

Great project. I can't wait to hear it run and see the dyno chart.

Tom Garcia · Post by **Tom Garcia** » Mon Jan 04, 2010 6:26 am

rngdng wrote:
Where are you guys getting your ideas?

Lane

It's these voices inside my head....make them stop!

Actually, I looked at a lot of CR125 cylinder pics...as well as some misc Apriliea/Honda/Rotax bits... Wish you could put big sweeping transfers on these things.

The number of ports in the liner is actually still stock - 1 exhaust, 2 transfers, 2 (bridged) intakes, and 2 boosts. The two boost, in stock configuration, are ducted to the transfer tunnel area where the Boyesen ports are now. When I hogged out the intake side for the reeds, it just exposed everything.

The stock holes are all opened up but no new holes... I thought about adding the exhaust subports like on the 75 and earlier cylinders but....

Tom

rngdng · Post by **rngdng** » Mon Jan 04, 2010 7:07 am

I'd never seen the ports aroud the liner to the transfers. Interesting.

Lane

ja-moo · Post by **ja-moo** » Mon Jan 04, 2010 10:45 am

rngdng wrote:I'd never seen the ports aroud the liner to the transfers. Interesting.

Lane

Those are called Boyesen ports ( I think someone already mentioned that) Like in a carb, fast moving air creates a vacuum. The low pressure in the intake port from the moving air in the transfers will open the reeds for additional mixture. That's what those ports do.

Tom Garcia · Post by **Tom Garcia** » Sun Jan 10, 2010 2:51 pm

And sweet stainless screw on spigots for the chambers....

Thanks Mark! (MinSC)

Tom Garcia · Post by **Tom Garcia** » Sun Jan 31, 2010 4:25 pm

With the arrival of the replacement for the missing shift drum roller bearing, I was ready for some progress on the kitchen table....

Nice to finally have some progress after a couple hectic weeks at the office. Guess I better get myself busy moving the ring locating pin on the pistons. After that, just getting the heads done...and the pipes....

Thanks for tuning in!

Tom

water cooled · Post by **water cooled** » Mon Feb 01, 2010 5:51 am

Tom, its amazing what you have accomplished. Looks great. Cant wait to see it in final form! Nice job.

Kevin

Tom Garcia · Post by **Tom Garcia** » Sun Feb 07, 2010 1:44 pm

Work has been a biotch... (At the office now)...

Did manage to get the jugs on to set the base gaskets for trimming... Will need to take them off to do the piston install and head domes...and drill the reed blocks...

Tom Garcia · Post by **Tom Garcia** » Sat Feb 20, 2010 10:01 am

Finally got a chance to use the drill press at work and move the ring locating pins on the pistons. Only had to move about 6mm around but a very critical 6mm indeed... Forgot the other cylinder and a reed assembly at the house....will need to try and drill the reed mounting holes over lunch...

tz375 · Post by **tz375** » Sat Feb 20, 2010 8:02 pm

Tom,

How did you get the old pins out, or did you dremmel the heads off and leave the cut off stub in the pistons?

Tom Garcia · Post by **Tom Garcia** » Sun Feb 21, 2010 8:18 am

Dremel'd them off under the magnifying light.. I ground down a set of pliers that fit in the ring groove first..but that didn't work..

Tom Garcia · Post by **Tom Garcia** » Sun Feb 28, 2010 1:23 pm

Well...I felt a little better late this weekend now that I located Allan's master cylinders and wound the Kaw low speed coil for Dale... Not feeling so guilty...

Opened up the pistons, tapped the cylinders for the reeds, a little JB Weld putty here and there....

Still need to cut the heads...modify the clutch/cover....ignition/cover...

Tom

tz375 · Post by **tz375** » Sun Feb 28, 2010 11:43 pm

Tom, what are those huge reeds off? They look like they could handle very high flow numbers. Will the reed petals lift enough at low speed to make any bottom end power or are you planning on using Boyesen two stage petals?

It's looking very impressive BTW.

Tom Garcia · Post by **Tom Garcia** » Mon Mar 01, 2010 6:51 am

The reeds are off a Polaris XL/XLT 600/700/780 triple... Same reeds I had on my H2 Special. They have plastic petals...