Jump to content
Sign in to follow this  

carb sizing help needed please

Recommended Posts

Hi, all. Mike from Tacoma here.

 

I come seeking your help. I am not a thumper guy nor a 2stroke guy. Ducatis run in my blood.

 

However, I am in the final stages of developing and building a highly modified 1993 Honda RS125 factory road racer with a Kawasaki KX100 motor to set (hopefully two) 100cc Land Speed records at Speed Week at the Bonneville Salt Flats in August.

 

Since it's a dirt bike motor, I figure this is the place.

 

First let me tell you a little about Bonneville. There are two courses. 5 miles and 3 miles. All vehicles attempting records under 175mph run on the short course. Records are set by timing the vehicle's average speed over the last mile. So it's basically a 2 mile drag race and a one mile cruise at wide open throttle. For a perfect run, I will need to hit my maximum speed at the exact moment I cross into the third mile and then hold it for a mile. Challenging? Yep! Fun? Ohhhhh yeah!

 

Arlan at LED Performance in Gresham, Or. is building the motor. He's a very experienced builder. Mostly super fast quads, moto-x  and dune bikes. However, neither one of us has any experience building a 100cc motor to run flat out for 3 miles at 4200' elevation.

 

We are wondering about carb size. The KX100 came with (I believe) a 28mm Keihin. We are thinking boring it to 30 mm. Too much larger and I think we may be lowering the airspeed thru the venturi causing a loss of power. The motor has been fully ported with the ports raised to make max power about 13000 rpm. That's what I'll need to reach the 120mph goal. The current record is 106. He's building the pipe next week. Any thoughts? The elevation has us making educated guesses at best.

 

Also, any thoughts on reeds?

 

Thanks a ton in advance!

 

Mike

Share this post


Link to post
Share on other sites

Do your math to figure out the pumping efficiency. Size the carb to match. Since you are going for max power and not rideability, an oversize carb a non-issue. Seat of the pants, 30mm should be fine, if not small.

Share this post


Link to post
Share on other sites

Thanks!

 

OK, the math says 30 should be about right. A little larger would be OK at the top  but I'm thinking a conservative carb size  will help me pull the super tall gearing. I've got 34 and 35 rear sprockets and 13, 14, 15 and 16 fronts to play with. Not so sure she's going to be able to pull that 16t. We shall see.

 

I'm at sea level now. How many jet sizes smaller do you figure I'll have to go to run at 4200 ft?

 

Any advice on reeds to run at that RPM?

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:

Sign in to follow this  

×