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Cutting out at sustained high RPM

Discussion in '2-Stroke Machines' started by Kurlon, Aug 27, 2017.

  1. Kurlon

    Kurlon Well-Known Member

    I'm using a SmartCarb, single circuit Lectron/PosiFlow/Lake Injector/etc style carb, so the needle is my only adjustment method and can swing the mix over a pretty good range top to bottom. Fattening the mix did help push the problem further up the rev range.

    No tach, technically my hour meter can give me a slow tach reading but too hard to notice on track.
     
  2. dave3593

    dave3593 What I know about opera I learned from Bugs Bunny

    Yea ok, the Lectron is one of the few the needle makes a difference at WOT.

    One thing that can make pinging start when it was not before is carbon build up on the head or piston.
     
  3. pscook

    pscook Well-Known Member

    If it's cutting out, like with a kill switch, I would investigate plug cap, coil, coil ground, and kill switch. Vibration at high RPM could affect any of those factors. It really doesn't sound like fuel, as starvation is a tapering of performance, not light switch. Same with mixture (rich or lean). It just stops pulling, or surges, but doesn't cut out abruptly. Cutting out, to me, is electrical in nature. Check all connections in the ignition circuit (including the kill switch), which should be like four wires on that bike.
     
  4. dave3593

    dave3593 What I know about opera I learned from Bugs Bunny

    When you had it on the dyno do you think you reved it as high as it is cutting out now? Was the power signing off? Which fuel and premix oil was in it then?
     
  5. Kurlon

    Kurlon Well-Known Member

    Dyno and prior run time at NHMS was on a +2mm overbore, lower volume / higher compression head and we were cutting off the runs a touch before what I'm hitting now. At that time I was running C12 + Yamalube 2R at 32:1. Ended up doing a new top end over the winter as that cylinder developed an external crack in the water jacket. So this top end has not been on the dyno yet. Build was setup purposely lower compression with a goal of a wider power spread, had been running it on 93 pump gas, again with Yamalube at 32:1 without issue, but also short track only, no visits to NHMS on it this year.
     
  6. dave3593

    dave3593 What I know about opera I learned from Bugs Bunny

    Bummer that can't be more directly relatable.

    So many questions and variables. Pinging, ignition timing, actual RPM, correct powerband.
     
  7. Kurlon

    Kurlon Well-Known Member

    And worse, the idiot turning the wrench is not 100% certain what he's doing! :D

    I was just chatting with a friend that I'd love to have a month off prior to this weekend, plus all the tools including a dyno so I could diag properly, then tear down however far is required to correct the issue, reassemble, retest and then jump on track and hammer with confidence. I need to find a kart guy or a two stroke GP guy to buddy up to this winter to learn from.
     
  8. dave3593

    dave3593 What I know about opera I learned from Bugs Bunny

    I just remembered something else that should be tried if you already did not. It is possible that vibration will make a kill switch intermittently ground out. I have seen a case where this would cause a bike to cut out but only at very high RPM. It was found only when the switch was completely disconnected.
     
  9. pscook

    pscook Well-Known Member

    Different bike (and 4t not 2t), but my CBR600F3 started cutting out on the track today. I knew it was electrical because it was random, but RPM dependent. Got back to the pits (after ceding 1st to an F2) and found the problem and fixed it. Funky electrical connections and RPM harmonics do weird things. Check every wire and connection in the coil circuit. Run the bike on a stand and wiggle every wire to see if the problem can be duplicated.
     
  10. Kurlon

    Kurlon Well-Known Member

    Got back to the small track today, and like an idiot ran the bike out of gas. It was deja vu all over again, bike just cut out without any warning like I had hit the kill switch. Total shut down, no sputter, no last gasp, we done. Push back to my pit, check, tank is dry. Fill it up, fires right back up and ran the rest of the day without a hitch.

    I'm now leaning strongly towards fuel flow issue under high demand.
     
  11. keypup

    keypup Well-Known Member

    I would double check that you are getting adequate fuel flow through everything up to the carb (tank/petcock/filter/hose/etc.) by measuring with a graduated cylinder over a given time and relating back to what the engine should be consuming.

    I have seen petcock seals swell under short and long term storage while closed (aggravated by ethanol in fuel), and when opened back up the seal drags into the valve orifice, slightly obstructing the opening and reducing flow - sometimes enough to cause issues at sustained high power but not visibly enough to cause concern with a qualitative inspection.

    Then begin looking at the flow path into the carb, to ensure the needle/seat are not obstructed/damaged or sticking - even the rubber tip on some needles can swell and cause issues though unlikely.

    I didn't catch what fuel you are using - pump or race fuel?
     
  12. lanehammond

    lanehammond Well-Known Member

    If it that style carb make sure the holes in the carb that pressurize the float bowl are unblocked.. on mine I switched to a uni filter that had a inner lip in the filter that blocked the holes off and had this same problem
     
  13. lanehammond

    lanehammond Well-Known Member

  14. Kurlon

    Kurlon Well-Known Member

    When I drained the tank, flow out the petcock was good, short of pressurizing the tank I don't think the hose could support more flow.

    Carb will be coming off by the end of next month and I'll be bench testing it for fill rate and seeing if I can verify the check valve balls are free and not gummed up.

    I started out running pump gas, problem persisted on Sunoco 260GT.

    2nd day at Boxshop, heavy rain, fighting a little lean condition at idle and sometimes felt like the slide wasn't dropping fully on it's own. Close the throttle and it'd continue to want to pull / rev, blip it and it'd clean up. Two blips later, stuck... Time for a teardown, inspect and clean.
     

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