Late model KTM 300, octane rating vs. partial throttle engine behavior

Discussion in '2 smokers' started by MotoDane, Apr 13, 2021.

  1. MotoDane

    MotoDane 1/4 liter carb eater

    Joined:
    Nov 27, 2013
    Oddometer:
    101
    Location:
    WPB Florida
    For the past two years I have been riding a 2004 300 EXC that I have completely rebuilt to factory new spec and better (even had US chrome redo the cylinder!) and a Keihin PWK38 carb with a JD jet kit. It has always been "busy" at speed, with current gearing above 65mph there is always some pipe bang/surging/bucking at partial throttle (give me full power or nothing) with a thin margin of happy throttle zone to maintain speed that gets thinner the faster you go. I have assumed this to just be the nature of the engine and accepted it, as it behaves like most other (smaller) aggressive two strokes I have worked with. Power valve tuning did help this, opening later to keep it off the pipe seemed to make it more well behaved at cruising speed. Everywhere else it is fantastic, excellent power and response, just doesn't like to sit still. The usual fuel is REC90 with 60:1 Motul 800, on a rare occasion Klotz super techniplate when I feel like smelling nice.

    This past weekend when I was at the track spectating at an FMRRA race I decided to put some race gas in it (because why not). I put around 2.5 gallons of Sunoco Maximal 116 mixed 60:1 with Motul 800 in the tank. Aside from the awesome smell, the engine dynamics have totally changed. I can now cruise happy at any speed without any of the aforementioned ill-behaviors and it seems all around more happy and responsive (definitely feels like it's making more power at WFO, but I have not dyno tested it yet). I am totally stumped as to why this made such a huge difference under vacuum, this fuel is not oxygenated, so I can rule that out. Night and day difference, anyone have any insight as to what is going on here?

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  2. Foot dragger

    Foot dragger singletracker

    Joined:
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    Your making up for something not right with jetting or carb by running race fuel. Something like that.

    Ive had over 15 KTM 2 strokes and once jetted they are good to go. 65mph is not going to be a good cruising zone for one of these bikes.
    It will go 65 and faster but holding it there..............its not what its made for.
    No 2 stroke dirtbike is happy at that fast of a cruise with out a whole lotta work.

    Really high octane is for really high compression,higher octane means a slower burn. Helps with high temps at high compression and full load.
    Its not like running nitro. Too much octane, over what an engine needs can make the engine run heavy,laboring trying to burn slower burn high octane.
    300 KTM doesnt have really high compression.
    -------------------------------
    I bought an old 200 EXC a while ago,a 2001. It wouldnt jet in,was rich no matter what I tried. A brand new Keihin carb with JD jet kit and bam it runs like a brand new bike now. The nozzle the needle goes up and down in wears out and cant be replaced.

    "some" older 300's had jetting problems that were seemingly un-solveable. 250's werent affected.
    2004 is a 17 year old bike,crank seals been checked?
    #2
    MotoDane likes this.
  3. MotoDane

    MotoDane 1/4 liter carb eater

    Joined:
    Nov 27, 2013
    Oddometer:
    101
    Location:
    WPB Florida
    I'm probably overstating the ill behavior at speed, in fact I am perfectly happy with how it runs and didn't think it could run any better until last saturday. And I do know I am asking too much of it to cruise at highway speed, totally not what it is designed to do, but you have to ride 20 miles of slab to do any sort of interesting riding around here. Gearing is 13/47, so that helps some.

    I am aware of the burn rate difference and wasted energy with the high octane fuel. I figured it was going to run the same or slightly worse/higher EGTs but did not expect the improvement that occurred.

    The entire bike has about 120 hrs on it since the total rebuild, down to individual components, the carb is a brand new PWK38, and the crank seals and all wear items were replaced while I was in there.
    #3
  4. Foot dragger

    Foot dragger singletracker

    Joined:
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    Yeah. I can jet mine to scream along at pretty high speed with a bigger main jet.
    But then going slow in the woods it doesnt really like the richer jet. Back in the day,they had these remote choke add on things.
    When screaming down a long straight section the rider would reach over and pull the choke on a little,and tune it for wfo running.

    The woods riding around here,there's really no place to get the throttle wide open much.
    #4