50cc carb guru needed

[00cbr_f4]

Hey guy just looking for some carb options for a school project. So here is some quick background. the motor started as a 149cc Briggs with a custom made sleeve. 50cc honda piston rod and crank. Honda racing head and cam. this is a efficiency competition so typically we would run at 2700-3200rpm and throttle full on or off. so im no sure were i should start thanks for the input guys.

[Kurlon]

What Honda did you steal the piston/rod/crank/head out of? Honda 50's run 13mm carbs, generate about 1.6hp to the rear wheel, and spin to about 10k RPM. Full throttle at 3200rpm... not much flow, I'd want a smaller carb I think to improve atomization. I'll try to see if I can find a stock 50 dyno, knowing where peak torque occurs would be a good place to target for max efficiency I'd think.

[00cbr_f4]

we have a dyno on site so we will check the curves often to make the best setup but we have had general problems with parts since the market is small and our project has quite a few constraints.

so a 13mm is good but a smaller carb would improve atomization? that make me think it would be easier to lean the mixture for the rpm right? does Honda make carbs small enough or is there and aftermarket maker that would be more suitable.

thanks for the help guys this small dirt bike stuff is all new to me.

[SSearchVT]

The venturi size for the carb depends on a few factors:
- The volumetric flow that the cylinder and exhaust can handle
- The operating speed of engine.

The smaller carb venturi will keep the intake velocity high at lower rpm's while a larger venturi's allow the sheer volume of air to flow at higher rpm's (jetting needs to be correct for both situations). One direction you might want to look at for very small carbs is the chainsaw/weed eater market. From the voice of experience - it's easier to start too rich and lean it out - spark plugs are cheaper then pistons. Good luck...

[Kurlon]

If you want to get real crazy, read up on Yoshimura's line of modified Mikuni flatslides. Instead of a tapered needle, they use a hollow tube with a series of holes drilled up it. The idea is you get better atomization by drawing the fuel up the tube to the middle of the flow rather than having the atomization occur just at the top of the emulsion tube. They don't make a carb small enough for your setup, but I don't see why you couldn't rework a 13mm unit in the same way if you've got access to a good machine shop. You'd need a new emulsion tube and hollow needle, and would need to tweek the slide to accept the new needle. You'll want to be able to key the needle so it always has the emulsion holes aimed straight forward and back.

[Kurlon]

http://www.yamahat135.com/forum/viewtopic.php?t=321

http://www.youtube.com/watch?v=APq5jz4T_08