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Mustang505 wrote:Angelix
what vacuum pump have you fitted from Mikuni?

The one I went for is the MIKUNI DF44-18 Single Outlet 20 Litres per Hour.

I got it from Allens Performance who are the UK distributors for Dellorto, Mikuni and Keihin and at the time they confirmed that it was more than adequate for the FCR39s

The original Monster SS vaccum operated pump flows at 16-18 Litres per hour.

Theoretically 20L/h for an engine with 75-80bhp is tight (average comsuption is 230grams of petrol per horse power per hour), so I guess this application would be to the limit if you are racing, however for a normal average use with the occasional full open throttle it is more than enough.

I mentioned this issue to Allens performance, they told me it would be sufficient even for a higher power and FCR41s.

ducinthebay wrote:In general, vacuum pumps are pretty low pressure and fairly low flow. Easily enough for a 750-1000cc twin. Most snowmobiles use them. A "T" in the line will just allow for a more constant pressure and delivery of the gasoline, which is always good.

If you are running a vacuum pump and the FCRs, the floats can probably deal with low pressure adequately, but a "T" will even out the pressure and it should run better. Even if you aren't experiencing any overflow, you may be running a higher float bowl level than anticipated, which will make the bike run richer across the range. The stock pump, on the other hand, will probably overwhelm most any carb with out a "T" in the line.

Cheers,

I think I understand the principle, never though of that, and I will test & compare both solutions as I like to experiment.

I am working on the tank right now and it was my intention to remove the return and go straight from the Pump to the FCRs.
At this point I will leave the return fitted and simply add an "in-line" tap so that I can chose to run the set up with the return either open , fully closed or partially closed.

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,

I understand that, but, as far as I know, "vaccum operated pumps" are not designed to work with a return.

The type of pump is beside the issue
It is all about fuel supply

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.

Mustang505 wrote: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.

very nice, although I would not advise to secure the fuel hoses with PLASTIC ties.

with the heat they will loosen and there is the risk of fuel spillage right above the engine.

I would definitively use the good old classic clips/clamps made of steel

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?

i'm not connecting up the overflow or breather lines on this - any reason i need too?

Breather line boxes on G`s pic above is to provide the carbs w still air f proper vacuum operation.
Normally needed, but lines could be roted to behind battery f e. Worked on my Monster.