Basically, you need a balance in the system. However, you need a pump that delivers enough fuel, no matter how hard on the throttle you are riding. So you need to size your pump a bit more than the normal usage so you don't ever end up in a starvation situation. If your pump delivers more than you need in most situations, you have to have a way control that over-delivery about 75% of the time. The "T" in the system allows that. The fuel injection systems have a similar mechanism, with a pressure regulator to keep the pressure constant. Even if its just a dribble through the overflow, the carb always has the right fuel level to perform at its peak.

Its impossible to get a fuel pump system to deliver the exact amount at the right pressure. There always needs to be some mechanism in the system to even out the flow based on demand. Carb floats can account for some of that variation, but smoothing out fuel deliver usually results in a better running engine in more conditions. The "T" and overflow port is a simple and very effective way to balance that flow.

Cheers,

Hey Angelix
i have mamaged to find a Mikuni DF52-136 here in NZ and hope to have it here sometime this week. it's a dual outlet so i'll just add a T piece. At 30l/hr I will go for the 2nd T piece for a return line to the tank. Thanks for your help.

For those interested and maybe wanting or have to fit a new fuel pump, here is what and how I fitted this Mikuni vacuum pump. Works really well and made a big difference to the fuel delivery.

Clive, do you have a fuel petcock or did you add a vacuum fuel valve ?
(I assume as the carb is not complete it`s a work still in progress)

G.

Hi G
yes using a fuel petcock - the original one on the sport tank. Carbs incomplete?

here`s what I mean (breather and fuel overflow tubes)


G.

I went to order an underpressure fuel valve from a Monster at my Duc dealer this morning (ca. €33). He confirmed to me that the vacuum fuel pump doesn`t close when the engine is off. Hence gravity pushes fuel from the tank to the carb (as long as the fuel level in the tank is higher than the float chamber). Not a problem if you close the fuel petcock and/or have properly working float needle valves. With the fuel valve there`s no need to worry about this. That makes it convenient.

G.

Hi G
i'm not connecting up the overflow or breather lines on this - any reason i need too? my sport is my track day bike and so far have done 2 runs on it. at the moment getting the jetting sorted on the carbs now it has pod airfilters & a fairly free 2-1 exhaust - started with 137.5s now up to a 145 main so far and its really starting to pull hard. sounds fantastic too. before the rebuild it had the weber which i swapped for these mikunis, perhaps too quickly in hindsight.

one of my front discs has a slight warp, do you know where (if) any are available?