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running lean; temporary fix?

Discussion in 'Bling and Appearance' started by abvc, Jan 4, 2008.

  1. i think the bike is running bit lean. i ordered a jet kit for gpx250 from the us which hopefully would arrive soon (no news yet, maybe it's the new year thingy...). however what would be the options to temporarily fix it beside tweaking the jetting?

  2. Where is it running lean? I'm assuming higher up in the revs? Can you raise the stock needle at all? You could alternatively plug up your air box opening a little, run a slightly oilier air filter, etc etc :roll: ... but realistically just run good fuel in it and don't flog the shit out of it until you get it jetted properly.
  3. actually i need to do more test run to really find out where it runs lean. it surges when cruising above 80 km/h. yeah guess just wait. good fuel as in highest RON?
  4. Why do you think it is running lean?

    What has changed on the bike to make it run lean?

    Jets don't change, nor do engines require a richer mixture for no reason.

    The most likely reasons for leanness are:

    1) Cracked, loose inlet manifold rubbers or connections

    2) Restriction in fuel supply - this will normally cause the engine to cut out completely until enough fuel runs through again.

    3) Incorrectly fitted air filter - this is very dangerous since it will allow dirt through, which wrecks the engine.

    3b) Removal of restrictions (there for noise reduction) in the airbox cover.

    4) Some ignition faults appear to be fuel-related.

    5) Incorrect cam timing (from a faulty cam chain adjustment) can cause rough running


    Trevor G
  5. i changed the muffler. i also read the bike was designed to run slightly lean for emission

    from those points, they looked fine. the fuel looks fine, i did clean the air filter foam. haven't had cam chain adjustment so far.

    also , what would a drill & plug on the muffler joint do (my guess is extra noise)? it was mentioned as part of fitting
  6. What muffler did you fit?

    In practise the jap 250s seem to run OK on standard jetting. One size up on main jets is usually sufficient and some claim an improvement.

    An alternative, intermediate mod is to raise the needle slightly. Since most jap carbs no longer have slots on the needle and a circlip to position it, the acceptable mod is to use a 3mm (internal) washer under the top of the needle. You need to make sure that this fits inside the recess in the top of the throttle slide. A 7mm outside diameter washer is usually OK. The washer should be no more than 0.5mm thick.


    Trevor G

    PS The washers are not easy to buy individually - places like Bunnings or auto shops do not have small enough OD washers. I bought a box recently so can supply you with a couple for the cost of postage.

    PPS Raising the needle only makes the mixture richer over the 1/4 to 3/4 throttle range. You still need to go up on the mains if necessary.
  7. Get thee to the dyno.
  8. Run premium fuel or tape up the hole leading into the air filter. Both are temporary fixes
  9. Go get a performance filter then get dynoed, might as well milk it for all its worth in one step :D.
  10. That would exacerbate the problem. Freer flowing intake = even leaner.
  11. I think higher octane fuels actually lean the burn, generally higher octane fuels require more fuel, I think the difference in pump fuels is negligible.
  12. High octane fuels are formulated to be slightly slower burning and resist pre ignition - thats it.

    All other things being equal, only the density of the fuel will affect the mixture. Some high octane fuels, notably the no longer available "optimax" were formulated to be a much higher density fuel than most other fuels and especially ULP. As a consequence reports of carburetored engines, and some fuel injected (but without an oxygen sensor) vehicles experienced hard starting, fouled plugs etc.

    Current high octane fuels are of a similar density to ULP, in particular those containing ethanol.
  13. How do you explain the lean stumble, white pipe & plugs when I run 100 octane av-gas when jetted for petrol?
  14. This was my understanding as well. :?
  15. Easy. Avgas contains lead, hence the white pipe. That is why it is not available for use in road-going cars or motorcycles. Its use in that way is illegal, I believe.

    There appear to be two different formulations of avgas, "lean" and "rich". That is not a component of pump petrol.


    Trevor G
  16. Another temporary fix is to only ride the bike on absolutely stinking hot days! :LOL:

    (The hot temperature reduces the density of the air hence richens up the A/F mixture :) )

    What is missing in Fricken's limited post is that a properly resourced dynotune effort will end up with carby adjustments that give the bike a proper mixture across the board.

    :-k How many extra horses can you expect from a GPX250 with a performance can, breathier filter, a jet kit and some serious proper tuning??
  17. Nah I don't think it is a lead thing, I am comparing leaded petrol to leaded av gas.