Mikuni fuel pump 98830000

I have just recieved this artical today,unfortunatley no directions arrived with it.This isn’t the problem as it’s fairly straight forward.When looking at it I see 3 nozzles,obviously one in and one out.Is the other for oil?I will be running pre-mix so it will not be needed if that is the case.Can anyone let me know if there is a blanking kit or suggest the best way to blank it if not.Thank you.


nuttydon gave a perfect explanation, it is true about his saying about returning to the tank but davedaniel don’t get too afraid since experience in mounting shows result of good efficiency also the option of just a small reduction cilinder if you ever have the problem of too much pressure.[H]

It is worth mentioning that you should blank/ solder up the brass breather that is inside the tank sticking up from the filter/tap assembly. If you dont the pump is PULLING fuel down from the tank and can pull air down as it is less dense than fuel therefore easier. It does and has caused me problems in the past. Adrian Newnham has also had a topic on this a few months back and was fully covered then that is/would be relevant for both mikuni and dellorto.

Sorry for being thick,Nutty, but does this mean that the return would nead to be through the fuel cap or tank top?[?[]

Just back into the tank is fine. Normaly this is to the top of the tank as no pressure is generated pushing against the fuel already in the tank i.e against gravity. However with a bit of jiggery pokery, I’m sure you could fit a metal pipe near your current outlet (presumably at the base of the tank) and have it long enough that it goes straight up inside to near the top of the tank. This would need brazing. that way your smaller bore return pipe could run next to your feed pipe and be quite neat.

You could fit it through the cap but this may effect the breathing if the cap has a breather on it and would look crap and be awkward. A small pipe going straight in the top of the tank is easiest but again may not look to good and could lead to some awkward return plumbing.

Don’t get me wrong you may not need to do any of this return stuff but if you find you do need to after fitting then it is a lot more dangerous welding a used petrol fume filled tank than a new clean one.

Personally I would always use a return on any pumped system as it just makes sense not to risk fuel being pumped out your float chamber. Even your home plumbing system has a return and that doesn’t catch light.

Please shout again if this is not clear, I could perhaps e-mail some pictures.



I have tried to put on word the metod but for me it is difficult.
Try again to have a look at the address I gave you.
On the left column you have the voice : Equipment.
Under equipment there is :Carburettor.
You click and when it opens go down up to Petrol pump kit.
Click on and when it opens go down until you find a scheme.

Yes, you need to get the pressure from the cranckcase. You can make a hole on the manifold as close as possible to the crancases and weld (hot or cold metal welding) a 8mm tube from where you can connect a hose that will go to the pump.[H]


reed or rotary the pump works on the same principle.
Make the hole to fit the 8mm tube to the manifold as close as possible to the engine intake.[H]


mounting a petrol pump is a preatty easy affair it is easier if you see a drawing. You can find a mounting scheme at www.worb5.de/index_e.html
then on the left column you click carburetor under equipment area.
then go to petrol pump kit. You will find a mounting drawing. It is in german but it is clear.
You find there is also a small cylinder for pressure reduction. This is if you use pump stronger then 14l hour the pressure will have to be reduced if not will come out from the carburator side holes.
You need just a small ammount of pressure.

I am sorry I didn’t understand the separate oil problem question.
Look at the drawing then if not clear ask again. Me or other here will maybe capable of answering.[:roll:]

Thanks for all the advice,lads.A bit more work than I envisaged but I should get there.Much obliged.[:look:]

Cheers lads.The problem I see now is that I’m running rotary valve.The reason for the pump is I’m fitting a long -range tank therefore it needs it as the gravity feed won’t be adequate to draw the fuel due to the height of the tank bottom.Is this still usable with the rotary set up or do I have to use another style pump for this?[:shock2:]

Dave, If you get problems with flooding due to the pump being too high pressure/flow, you can always connect it to the carb via a ‚T‘ piece. One end connects to the carb, one to the pump and the third should go back to the tank as a return. This return pipe needs to be a lot smaller bore. That way there is a constant flow to the carb and some pressure at the T piece but if it is to much it just goes back to the tank. A usefull spin off of this is it will mean the fuel circulates so in traffic the petrol does not get hot in the feed pipe and vapourise. Obvoiusly you will neet to fit a return pipe spigot to the tank but that should be easy enough.

Just limiting the diameter of the vacuum pipe will not do it as when you throttle back the most vacuum is produce so the pump works hardest just when you are shutting off you fuel demand, this is normally when it would flood worst. The T piece return is the answer IF you have problems. Note most cars and fuel pump based bikes DO have a return flow pipe.

Good luck


Thanks for the link.My main point was the 3rd valve,I wasn’t sure on it’s purpose.I thought it may have been for 2 stroke but it obviously isn’t for that at all.I can’t find a diagram on the link and it seems the 3rd valve’s purpose is pressurisation as you say.What I needto know is does this require drilling into the casings or is there another method?[?[]

I think I’ve pretty much sussed it out but I would appreciate pictures for cross reference anyway.[:D]

Dave, as you say the two nozzles (in and out are obvious). The third nozzle must be plumbed into your crank case. You need to fit a small pipe (about 8mm) to your bikes crank case. If you look at the Worb5 site motors you will often see a little spigot coming out of the modified Reed case welded to the crank case. Each time you crank turns it creates a small vacuum in the crank which draws in the inlet charge. It is this ‚vacuum pulse‘ that pulls the diaphram in the pump and gives a squirt of fuel each time.

Welding is best but you could thread one in or perhaps use epoxy like Araldite etc.

Hope this is clearer


[:D] Thanks Bud.[H]