Fuel pump

Hello mikkke. Did you use the insert that fits into the reed block manifold? Some people call it a stuffer. It supposedly smooths out the air/fuel flow. Some people say to use it, and some people say not to use it. I think it works better with it.


Hi guys, I have a Mikuni fuel pump (14 litre/hr) that I have not installed yet. Thanks to Curare and Beerace for making it clear, and to Mikkke for the photos. So let me get this right. I can tap into my Malossi manifold to get the necessary „vacuum pulses“ to run the fuel pump? I do not have to tap into the engine case? Because tapping into the manifold would be so much easier than the case. I drew pictures of all the plumbing with the appropriate „T“ pieces and then I discovered Mikkke’s photo. And my drawings were the same! This makes me very happy.

Thanks guys, Scott

hello Mikkke,

yes, if when you press the clutch the engine doesn’t go up in rev it is not a gear probl.(though your ratio it is too high anyway).

The mixture screw has its influence from iddle to 1/8 throttle so nothing to do with high rev. (you have to fix the mixture screw to have a fast and regular response from iddle. You have 1 1/2 turn out you can go to 2, but as I said leave it for the momentent is a fine tuning, you will think about it when you can get high rpm). For this reason it didn’t change anything when you changed its setting.

How does your engine respond in neutral? When you accelerate it is very responsive or not, try to give me as more information as possible or let me hear it!

Is your crankshaft standard ?

And are you sure now your manifold base is flat?

And sorry about the 65 (actually was a complete PX200 christmas
three. I solved the little problem and I already sold it. I thought you were not interested any more since we didn’t get to any agreement)

Thanks Mikkke, that’s the one. Ade

hello Mikkke,

yes this is the piece you need to conected to the manifold from where the pump get the pression.
But you don’t want to make an hole that big in the malossi manifold! Try to get a alu tube of 8mm so you make a slightly larger hole and you can fix the 8mm tube with some paste for cold alu welding. If you prefer you can get it hot welded.

You need also a small reduction to put in the circuit if not will not work well. At least in my experience.
And beerace talked about some problem he experienced in the fuel tap assembly. He resolved it blocking the fuel tap tube.
I have never experienced, up to know a problem due to the fuel tap tube and I leave all the time open, but if you have a problem remember it!

…it is difficult for me to explain in words, it is a small cylinder from one side about 8mm and one side maybe 3mm. You put it this inside one of the hose you will use to connect the pump.
Not everybody use this though, but whenever I use one I don’t have any prob. with pressure and other things.

I will take a picture.
I will send it tomorrow… if I still have one of these… I should.

hello sliphorn,

when you mount the fuel pump be careful where you put it.
Try to put it in a position not mutch lower that the tank fuel tap(I think this is the english term), so the hose that has to go from the pump to the carb is not in a much lower position than the carb itself. If you don’t do it when you change the jet or other thing to the carb, that makes the fuel go out, then good luck and good leg too, since with the kickstart you are not going to creat that much pressure for the pump to push the fuel to the carb, if the carb is much higher in its position then the fuel pump.
I hope this is clear. It is more or less like the principle of the comunicant vases, since the pressure of the pump at the engine start is not that mutch.
Remember this…

hello Adrian,

I have done just two of them in comb. with the fuel pump. And two for me is not enough to make a generic conclusion, but it worked no problem. If you fit it one you could share your experience so we are going to have three cases!
The boost bottle is anyway a closed circuit so it effects more the way the carb feels the engine case compression (in very basic words not totally correct though, just to have an idea) then the fuel pump vacum activity itself…but usually I prefer being pragmatic and having experience…and in the two cases where already the pump was functioning well, after fitting the boost bottle in another take-off vent closer to the carb and pointing up, I didn’t experience any bad effect on the fuel pump.
So please if you fit one let me know where you have fit it and how it was for you.

Thanks Sliphorn Did you order one?

Hello Mikkke, I didn’t order one yet. I’m waiting for the money because they are rather expensive, but I think worth it in the long run.


Mikke, can you put the diagram of the fuel pump fitting back on please? My computer died and I lost the diagram, I have to start again. Thanks. Ade.

Yes it is a black rectangle part that reduces down to an oval shape to smooth the air flow.


Excellent photo Curare! I’m so glad I discovered this forum. Now I know how to do it. I’m thinking of getting an oversize fuel tank (11,5 litre) from a gentleman in England by the name of Dave Hampson. If I do, it will be necessary to use the pump with the large fuel tank in order for it to function properly. By the way, I have a 21 tooth cosa clutch with a 68 primary.


The diameter is ok, it seems that it fit very well, do yo have to put it where I show on the picture joined? or elsewhere?

I gonna have a little ride right now to check if everything is ok, these are the first sun rays of the year, it was time!!!

Curare, did you saw on the net this Vespa with a 750 engine fitted? Those Italians are really nuts!! Aren’t we???

I can try to send you the file if you want.

;D ;D ;D ;D ;D ;D ;D

Hi Curare,

My gear ratio is an upgear kit by Malossi, 23x64

I did not do the plug chop already, I will try tomorrow or tuesday as I have a lot of work at the factory and when I get back home I’m tired and also I have to take care of my two little kids before they got older, you understand what I mean!

My exhaust is a SIP Performance.

I feel but I’m not sure that it’s a carb problem do you know how much the mixture screw need to be opened?

Don’t you think that if I ride in 4th, the engine don’t seems to take rpms, even when I press the clutch for a little moment, the engine don’t seems to take more rpm.

You are probably right but my beginner’s feeling make me feel that the carb is not well setuped

Don’t you think that if it was a gear ratio problem, the engine will sound good but with some difficulties to run well?

Give me your feeling I know that you are right!!

I will change it just as you say before, did you check the 65 primary drive?

So, dear Curare, I’m gonna got to bed as I wake up at 3.45 tomorrow.

Good night!


hello sliphorn,

yes you can get the vacum pulse from the malossi manifold, but not from the rubber one into which is connected the carb, but from the metal one, as close as possible to the inlet port.
Give a look at my net -address below (vesparacingteam…), clic on and you see a picture of a tube I welded on a manifold to be connected to a fuel pump. You can also weld it with same cold welding paste…or same screw in tube.

I use one some years ago whenI was riding a PEUGEOT 103SP (49.9CC) with cylinder kit, carb and exhaust and I remember that nothing special arrived when I mount my Polini boost bootle.

Curare, do you use cold weld? If yes which brand?

Thanks Curare, I will remember to fit the fuel pump in the proper position.


Thanks Juan, that’s a help. Ade

I post the picture in the forumpics, follow the link