1984 GPZ 750 too rich, help please

Discussion in 'Motorbike Technical Discussion' started by Joe Prinz, Aug 4, 2003.

  1. Joe Prinz

    Joe Prinz Guest

    My problem: go passed 1/4 throttle, hit like 5000-6000 rpm and the
    bike stops pulling and lugs.. Check the plugs and they are black
    (engine killed during problem period to check plugs). Remove stock
    filter and the damn thing runs perfect. Pretty much thinking my
    diagnosis is right on.. its rich.

    I am in Colorado too.. (high altitude)..

    Pulled the carbs.. they do not look stock.. in fact after calling
    previous owner.. they are not.. they only have 2 jets and the needle..
    Mikuni CV's but not sure which ones.. the needle or secondary jets are
    110's and the mains are 37.5's. Needles have the circlip on the top
    highest position.. this is leanest right?

    So what do I do??? do I go to the next leanest jet??? Do I not use
    these carbs? I am on a budget so I'd rather fix this cheap. It goes
    like stink when you pull the air cleaner.

    The manual says stick secondary is a 125 mikuni round head. HELP!
    Joe Prinz, Aug 4, 2003
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  2. Joe Prinz

    Rob May Guest

    I think an aftermarket fuel injection system, even a cheap one like
    Megasquirt is overkill for his problem. But you're right on about the air
    filter. Replace it. Maybe consider a K&N or other filter that will flow
    more air. An aftermarket air filter and maybe smaller main jets will
    probably solve his problem for less than $30 or $40 dollars.
    Rob May, Aug 4, 2003
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  3. It's even nicer when they're old enough to go pillion and you discover
    that they *love* bikes.

    My 13 year-old son and 11 year-old daughter have both got their hearts
    set on a powered two-wheeler, just as soon as they're old enough.

    The Older Gentleman, Aug 5, 2003
  4. Joe Prinz

    Joe Prinz Guest

    Are you kidding? Remember I said I want to fix this cheap?

    I remove the cleaner and the cap to the airbox and all is well..
    remove the cleaner and install JUST the cap and the problem is there
    to a lesser degree.. its NOT the cleaner.. although I did order a K&N.
    I went from a 110 to a 100 today and the same problem, maybe a little
    better. All 4 plugs are black.

    Any idea what Mikunis these are with just the two jets? I am
    thinking to fix this I'll need to go to a 90 jet... is that insane at
    Denver altitude of 1 mile? I don't want a hole in a piston... if
    I whatch the plugs it shouldn't happen though.

    Any experts out there??? I will check float heights as well.. Could
    it be some sort of vacum problem where the sliders aren;t moving? I
    found 1 slider spring .5" shorter.. so made it longer.. it feels the
    same as the other 3 now.. it didn't before.. but this didn't solve it
    either obviously.

    Needles are all at leanest setting.. could they be wrong size???
    Damn thing runs PERFECT with no filter and airbox cap... revs to 11k+
    and wants to do more.

    Going to 90's tomorrow..and if need be frickin 80's.. eventually my
    plugs will at least be off-white.. I hope..

    Joe Prinz, Aug 6, 2003
  5. Joe Prinz

    Joe Prinz Guest

    You must not have noted that when the airbox cap for the filter is in
    place the bike still runs like crap.. so the K&N alone will not fix
    the problem. If you want to help me, read what I say please. So far
    adding fuel injection is the lamest "idea" I have ever heard.
    Joe Prinz, Aug 6, 2003
  6. Joe Prinz

    Slikboy Guest

    Someone once told me that plugs don't show much in the way of white colour
    (lean), because of superclean gas. They said that the good old days of
    checking a plug colour for lean are pretty much gone. Any experience on

    Slikboy, Aug 7, 2003
  7. Joe Prinz

    Joe Prinz Guest

    Sorry just getting frustrated. I have since found that one of the
    carbs did not have the choke closing completely. But I think that the
    part that wasn't closing was just the extra air, but not sure.

    I have also rejetted to way beyond what I think is acceptable as far
    as leaness.. down to a 90 jet and its still very much a problem. So I
    am thinking I have something hidden here. I am going to "soak" the
    carbs and clean the passages.. maybe bring them in and have them hot
    tanked. I'm thinking that the tubes the jets screw into with the
    little holes are clogged... but I cannot figure out how to remove them
    (mikuni bs34 carbs).

    Maybe my ignition is bad? But I doubt it since it'll go with no
    filter and no filter cap.

    My compression is ok per my guage.. at the bottom of the range
    120-125psi.. with a warm engine.. not sure how calibrated my guage is
    though. I should test it.

    In regards to the other post.. yes unleaded gas makes reading plugs
    difficult but it can be done. But when they are black its not hard to
    determine the situation. I do however need to replace them now
    because I can no longer read them..

    Joe Prinz, Aug 7, 2003
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