air flow

No Mike, I never talked to Superhunky. EC spoke of him from time to time. I believe they met when EC was doing some editorial work for Dirt Bike magazine and they went back a long ways.
 
Brian, It's been a long time since I flowed a 38 SBN and I don't have the data, but from memory I believe it was right around 140 CFM, Mike says he's flowed them maybe he can confirm that number or dispute it one. Jack, the alky double stack will move enough fuel for a big thirsty motor, a lot of the early HR conversions were done without doing the necessary mods to get enough fuel past the needle and seat, that's where the bad rep came from and why the grose jet became popular as a solution to that problem even though they would eventually start leaking and cause a tune problem, but with the cross hole drilled in the bottom of a stock alky seat and the appropriate mod to the carb body, fuel starvation there is no longer a problem. I built an HR196 for a GX390 running 25% nitro that worked fine, nitromethane will definitely test the carbs ability to move enough fuel. Ya Mike, I've calculated all of Tillotson's V. to TB ratios and have found them to be very consistent from the hl's to the HR's. The hl360A and the HR191 have exactly the same ratio of 1.12 to 1 in fact along with the Mikuni 34, the largest is 1.25 to 1 in a HR190A that has a venturi the size of a 181 and a shutter that's .020 bigger than a 191's. I'd say you'd always want a throttle bore that can out flow the venturi so that it doesn't become a restriction point in the carb, that's the whole point of the ratio, but once you reach that point then going any bigger isn't necessarily going to be better, I prefer to reduce the profile of the butterfly and shaft to pick up flow and velocity. Whether a larger V to TB ratio is gonna be helpful is really gonna depend on what your flowing into. If your flowing into a manifold that's gonna taper down to a 1 inch port, then smaller is better so that your manifold taper is shallower and can be shorter, both are good for flow. If your dumping into a big reed cage more ratio can be a good thing as you can pick up some flow without reducing the shutter and shaft profile. Jon
HI there I c its a very old post. I'm building a gx 440 motor want to put a sbn carb on. If you could help me plz with needle size and pop pressure to start with. And if you can stil rember the jet sizes. I'm running 109 race feul. A lot of people put them on. Im in south africa and struggling to find some one that can give me the right info. Thanx
 
Yes , this is an old post. For that engine I'd start at about 15 lbs pop-off. As far as jetting for gas is concerned I can't help much there we run alky for most all of our racing . I would start with the stock jetting that comes in the carb, you have some latitude with the needle adjustment and it may tune out. If the engine falls on it's face or bogs when you hit the throttle it's lean off the bottom, it's gonna be easy to see if it's too rich, you'll see the black smoke. Actually, when you get it tuned for the best performance off the turn, you could see a little black smoke as the revs come up off the bottom, but it should clean right up.
 
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