Carby Air/fuel ratio concerns

Discussion in '2-Stroke Engines' started by maxalliance, Jul 22, 2013.

  1. maxalliance

    maxalliance New Member

    Hey, my name is Jordan and I'd like to start by saying thank you to the committed members of this community who offer sound advice in a friendly manner. I've spent a lot of time reading through the forum and have been fortunate to have found all my questions already asked and answered in clear, simple terms (I'm a noob).

    This is actually my second kit. I bagged on my last one for over a year before it went and I decided to build a new one.
    I drove my last one pretty hard in pretty severe conditions so I'm really surprised it lasted as long as it did but now I am looking to make something more powerful as my last kit was second-hand and likely never ran properly.

    I've got a GT5-A [66cc slant] from here : and assembled everything myself onto a Nakamura Trailback.
    It was a very tight squeeze and unfortunately required drilling through the down-tube to install it (I ordered a large universal U bolt mount but it was still way to small).
    I purchased a boost bottle kit as well but there is NO way it will fit, even with my intake offset pipe, it's very close.
    I even had to take the top off the head and turn it around so that the spark plug pertruded toward the front of the bike.

    I also purchased the NT Speed carb and installed it right away.

    It took some tinkering to get it started at first but I was easy on it during the breakin and noticed that it already had more low end pull during breakin than my old engine (same model) ever did.

    This performance, however, was short lived. After the first 2 tanks (mixed 16:1) I leaned my mixture out to about 24:1 and swapped my stock plug for a BH6S.
    Shortly after it seemed to lose a lot of power in the low and but still pulled hard at 3/4 - WOT.
    I tried switching back to the old plug and it didn't make a difference.
    I spent a lot of time reading online and learned about 4 stroking. It seemed to match the description so I moved the C clip up on notch (to second from top) on my carb and didn't notice a difference. My plug still looked pretty dark/oily.
    I moved the clip up again (to the top) and now it runs perfectly (so it seems). Still 4 strokes a little now then and and idles high sometimes but otherwise runs and sounds great.
    My concern is that my plug now looks much lighter than before. I wouldn't say it's chalky but it's not as wet and is a medium-light brown. I don't know how light is too light though and find it hard comparing it to photos online.
    I've included a photo of it here :


    Again, WOT has never been a concern, it has always pulled smooth without 4 stroking. only at low -mid throttle did it bog.
    It runs great now but I worry that it may be too lean.
    Are there any other tricks or tests I can perform to find out of it may be running too lean? Do any of you use the leanest setting? What problems should I look for?
    I'm using the stock jet that came in my Speed carb and would like to avoid changing the jets as i plan to get a new carb in the near future. I am using the stock exhaust and have since switched back to the BH6S. I plan to get an irodium extended tip, maybe a 7. Should this help prevent wear/damage when running at its leanest? Which number would be best?

    Also, I've gotta ask. I know that many will say it's a no-no but I haven't read many reviews from people that actually owned them. I'm honestly considering getting a kit, even just to say it's got nos. I know there are concerns with leaning out your ratio too far by injecting dry nos into the intake manifold without added fuel however I read somewhere about running it through the carb air intake instead. Has anyone tried this or is able to provide some insight as to whether or not it may help wear?
    or this

    Thanks, - Jordan
    Last edited: Jul 22, 2013

  2. HeadSmess

    HeadSmess Well-Known Member

    plugs too cold, oils too high, mix is too rich. one or combination of all 3. i recomend going a 5 or staying with the 6.

    you want the thing to look "almost" new when you do a plug chop. only very light tan discoloration. "chalkiness" unless it was a new plug, is invariably the carbon starting to burn off as the plug starts reaching the correct operating temp. so a good sign. if it IS a brand new plug, oh dear. its melting :) too hot! unlikely to happen unless you have a 4 with a hi comp head...

    due to the lousy carbs, they get hard to start when the main jets just about spot on. you have the choice of hard starting with eventual full speed(dogs until they warm up) or easy starting with not so good a performance (dogs all round). a mikuni fixes this but raises all its own issues, like mounting.

    ive always liked the idea of squirting NOS in through the choke section of a mikuni vm18. they add fuel, not cut the air. so the mix would be richened appropriately with some tweaking. a suggestion ;)

    and once upon a time i dreamt of an alternative...hydrogen peroxide. it has a few things going for it. the water content increases CR. the disassociation of the water cleans internal parts. the extra oxygen provides, well...extra oxygen. hope someones added fuel to compensate? then, water produces steam which expands and increases cylinder pressure.

    and finally, spraying h2o2 and water in as a mist would cool and densify the incoming air.

    h2o2 is easily made, theres no law saying you cant use it "on the street", and in the lower percentages, ie 6% or so, is quite safe. 50%+ stuff needs special transport license :jester: yet the same place would sell 98% sulfuric acid without batting an eyelash...go figure?

    it does work, apparently. i havent gotten around to "disposing" of a lawnmower engine to verify it for myself :(

    just misted water alone produces results. that i have experienced. 454 big block. we did silly things to it :)

    other things to think of. lows and highs.

    get the jet size first. the main jet, for WOT. get this right then fiddle with the needle and the clip, etc.

    now. when one says "its losing it in the lows", im assuming this is full LOAD, with the throttle WIDE OPEN. not just poppin along a flat road with the throttle just cracked off idle. even, possibly, with some brake applied.

    i say this because these engines hardly ever get used at half throttle, and anyone complaining about bad "lows" on take off will invariably have the throttle wide open.

    and for this reason, playing with the needle is going to do just about zilch. a standard slide carbs mixture is set by throttle opening, not engine speed. it helps transition, but once again...most people just twist the throttle wide open. theres no gradual opening, unlike on a 1300cc that would flip if you tried twisting the throttle off... they fit CV carbs to big bikes for a reason. they iron out the steps in a carbs operation. make it smooth and linear instead of "notchy".

    engine speed does affect waves in the intake tract that can assist or conflict with carb operation at various RPM. this in turn affects airflow through carb hence mixture.

    the other end of the chart...when im doing half throttle/idle plug chops, i usually go downhill to get the maximum rpm. the mix stays the same for the half throttle/idle setting regardless of engine speed. so coasting downhill prevents me from trying to open the throttle past halfway, or off idle hence i get a correct half throttle/idle plug chop.

    and theres not much point worrying about half throttle chops with an engine under 5hp cus you wont OPERATE at half throttle!

    when its 50 hp, take the time to get the throttle as smooth and linear at all stages as possible or it can act like a bronco!

    if youre losing low end performance but the top end seems better or the same...check crank seals.
    Last edited: Jul 23, 2013
  3. jaguar

    jaguar Well-Known Member

    best way to tell if it is too lean is to look at the underside of the piston. It will be black (due to too much heat burning the oil there)
    maxalliance likes this.
  4. HeadSmess

    HeadSmess Well-Known Member

    just buy my new tool, available at all leading retailers. allows full unobstructed 360 degree view of the underside of the piston, without removing a bolt!

    pm me for details :)

    without it youre screwed. gotta pull it all apart. ha ha ha ha ha.

    ?HT? it wont start if its getting so lean that pistons start charring underneath...

    now i have to prove myself wrong. oh boy.
  5. Fabian

    Fabian Well-Known Member

    Problem is easy to fix:

    Get a reed valve intake system with a Walbro (diaphragm carburettor) adapter and a 14mm Walbro carburettor to make the adjusment of air/fuel ratio a simple procedure, then install a TTO cylinder head temperature gauge and a KOSO Exhaust Gas Temperature gauge which will allow you to accurately zone in on the perfect setting for best engine operation.

    It is also particularly helpful to get yourself a Jaguar CDI.