The pursuit for a brown sparkplug



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    Alright, trying to set up this Athena kit. Currently running 280 main jet and this is what the sparkplug looks like after ~250km. Looks pretty good huh? Sadly, the bike won´t rev fully, no power beyond 3/4 throttle. Running lumpy, strange. Wouldn´t say bogging but something is certainly not right down there. The question is, should I go down to 270 or up to 290? What´s your opinion? Personally, I´d want to try a smaller since almost everyone with the kit runs 270. However, since I´ve not pushed the bike that much during break-in, and the sparkplug isn´t completely black from a very rich mixture, I´d say I might as well try 290. A safer bet for sure.
    And no Calum the pv is not pinned in the closed position 😉



  • It's not really appropriate to be judging the fueling on break in, you need to put that engine under load and perform a plug chop to really understand how the fueling is doing.

    Once it's bedded in, perform a plug chop and report back what the plug is doing then.

    I'd say that plug looks a tad lean to me.

    Easiest way to tell is to restrict the air flow into the bike, if it performs better then you know you need to increase the fueling.



  • This is mine (no kit though), a bit of deposits from excess oil, but otherwise tan as Hasselhoff!!
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  • Wow! I'm impressed! I'm now running 290 main jet. Can't feel much difference. My issue is, it does not pull after 3/4 throttle. The revs go up, exhaust getting louder, I brace for the real powerband but then nothing.. Dropping to 3/4 and she goes! But there is definitely power left to scavenge. My next alternative is to try a 270 main jet. And yes I've tried the choke, no results.
    Should I really be giving the piston, cylinder full throttle? I'm closing in to 300km.. Shouldn't be an issue right?



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  • I've tweaked the oil pump a bit for my Athena, will this dramatically lean the mixture? As I've said, I'm running far bigger jets than anyone I've seen used the big bore kits.



  • @irongamer727 What is your bike not doing?

    I dont get it.



  • @darty it's not performing well in the upper revs. The jetting is off, I'm steadily climbing in sizes (now 320) but the plug still looks a bit too lean.



  • And I have a friend running 240 with the kit. We are both running stock Air box and so forth.



  • Are you certain there's no leaks between carburettor and crank case? No head gasket leak, and no leaks in the carburettor itself allowing air in?
    I'd say there's definetely a leak somewhere allowing a lot of extra air in.



  • @glynn123 nope everything is sealing, not leaking etc.



  • You using the stock carb? Pretty sure they use a powerjet which should be the jet you want to be changing.



  • @calum yep there is a powerjet. I'll give the really big jets a try first. Because again, I've not heard anyone changing anything but the main jet.



  • @irongamer727 So the powerjet kicks in at wide open throttle.

    You should be doing a plug chop at WOT.

    Therefore, changing the main jet won't affect the colour of the plug during a plug chop.



  • Pretty sure the power jet isn't changeable on these carbs.



  • @calum I'm currently going WOT sixth gear and turn ignition off. I can thereby see the plug colour at WOT. And it still looks a tad to white for my taste.



  • The Powerjet is a fixed system on the TM28,

    I used to run 340 Main, 30 Pilot, only because of extensivly modifing the Airbox volume,

    And, it was rubbish mostly.

    I had to replace my main body Carburettor gaskets because of symptoms near enough the same as you.

    I would recomend throughly checking for Air leaks as this is almost certainly your problem!

    Dont just throw fuel at this and hope for results.

    Work backwards.



  • @darty how would you check for Air leaks then?



  • I do have the entire pice where the snorkel existed cut out. So there that as well.



  • @irongamer727 You donkey,

    Why dont you try and restrict Air by patching up a section of the Airbox,

    By doing so, i’ll bet a Roast dinner that it sorts out your lean condition.

    I’ve played around alot with this, you’ll stuggle to fuel this properly with the TM28