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Mikuni Vacuum Fuel Pump!

Discussion in 'Technical Help' started by doogalman, Mar 7, 2021.

  1. The vacuum fuel pump on my Monster 900 is possibly the fault that I’ve been trying to pin down for some time.
    After opening it up the clear plastic diaphragm is obviously in need of replacement. The black rubberized one is not so bad.
    Searching about I am struggling to find a rebuild kit.
    I am trying to avoid buying a second hand pump as I’d like the reassurance that it’ll last some time, as they are getting on now age wise.
    Has anyone a spare rebuild kit they would be happy to sell? Or have you replaced the pump with another of some type, vacuum or electrical?
    F29B70D6-1E16-41F3-BC1B-CBD95CF810C5.jpeg 6E9109D3-9870-416C-A83D-5D5EE0F449D5.jpeg
     
  2. Never the same as "in the flesh" but doesn't look that bad from pics.. Unless diaphragm split leaking or worn or one-way valves gummed up/suspect, then I would reverse clear item to remove "pant" - done this many times - and then give it a go. Are you happy that your vacuum tube is not perforated?




    Although non-std, and will need mounting differently, there were many (new) similar pumps to choose from, at a reasonable price but this could have changed now.
     
  3. Hi Chris.
    I have been trawling the web for hours trying to find answers about different pumps. But I can’t find any definitive data about the mikuni one that’s stock. DF-62 is the closest in looks and style but it’s a twin output job. Anyone know exactly what model was used on the monsters?
    As for the condition of mine Chris. The valves appear to be ok as is the black rubberised diaphragm. It’s the clear one that is the suspect. I read that if its bowed and gone a bit stiff/crackly then its performance is going to be diminished.
    CB753E7B-E3D1-4106-92AE-018BACAEA9F7.jpeg 215D66F0-697D-42CE-931F-CD122E7077F2.jpeg 4E84D250-60E3-474C-A0E3-F041D1F46160.jpeg
     
  4. Vacuum tubes all ok
     
  5. It is a grey area, we are a few years down the line and the "pant" on your clear diaphragm does look quite pronounced so of course, a replacement is the best route but as long as it doesn't look like splitting soon the reversing it is a lovely trick to get it working for longer. If you look at the way it pumps you will see which way it pants. I had a quick look at the replacements and things have changed. There used to be around 3 genuine Mikuni (Go-kart/Skidoo) pumps that were available new and cheap but seem to have dried up apart from the States. I couldn't find a service kit for yours either, which used to be easy to find, but must be kicking around somewhere, it's probably worth trying Allens :-
    https://www.allensperformance.co.uk/
    or any good carburretor specialist.

    There are several clone pumps from China now but i haven't tried any.
     
  6. #6 Chris, Mar 7, 2021
    Last edited: Mar 7, 2021
  7. So reversing the valves so it pumps the opposite way... interesting.
    Yeah the kits are scarce in Europe. Not really wanting to go stateside as I’m in Germany dobeoul likd yonavoidcthe hassles there.
    I have found some in France but the kit then runs at about £40 inc p&p. Very steep.
    I have a Mikuni importer in Germany to call tomorrow so see what they say. But they are not listing what I want on their site. They may offer some help so will see.
    Will look into the alternative pump options.
    Cheers Chris
     
  8. Allen’s are out of stock.
     
  9. well sort of! - it starts off flat, you know that, but it gets set in one direction which hinders it flexing back and slightly affects the volume pumped each stroke. By reversing it, the pump then has to make an effort to reverse the set and pumping is improved slightly. As said, i've done it many times on Elefants and Monsters and you can see the difference in the output. Even left "as is" the pumps soldier on ok, it's only when you run out of fuel or swap to reserve sometimes that a tired diaphragm shows it's weakness.
     
  10. Ah I get you now. Just flip it over.
    Will have a play in the next few days.
     
    • Like Like x 1
  11. yes, flip it over, that's a much better term!
     
  12. So the raised bubble side faces inward of the valve body.
    I will give it a go then if good I will order a snail-mail one from wherever.
     
  13. Small update. Very helpful lady at Germany’s Mikuni importer has today sent a rebuild kit in the post.
     
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