Carby If you tune carb with one model plug, do you need to retune if you switch plugs?

Discussion in '2-Stroke Engines' started by Fletch, Feb 7, 2011.

  1. Fletch

    Fletch Member

    I was thinking about getting some cheap plugs to have as new for tuning the carb, and then switching to a different plug after it is tuned, but I think the plug itself affects tuning right?

    Am I right that you should use the same model plug when tuning as you plan on using afterward?

  2. GearNut

    GearNut Active Member

    It typically does not make a huge difference, however I would never use cheap plugs when trying to tune an engine. The cheapness can make tuning more difficult especially if you try to read the color of the spark plug to help determine the state of tune.
  3. Fletch

    Fletch Member


    Thanks for your advice as always. I've got a box full of plugs (mostly the ones that come with the kits) that are all dark from running rich at one point while tuning. I have 2 good NGK plugs that are dark as well, and I can't justify buying another good plug just for tuning the carb.

    I'm trying to figure out how other people do it, because once the plug is dark it won't lighten back up right? At least not in a short test run? So should I start out intentionally lean in the future and then get richer until the plug is dark enough tan color?

    What is a good cheap plug I can find locally just to test? Actually...where can I find NKG BH*S or BPH*S plugs locally?

    Sorry about the barrage of questions! I was running rich on purpose for a "break in" and because I read some people say that they'd rather run rich all the time and extend the life of the engine..."oil dripping from exhaust is normal".... etc, etc.. Only I really soaked the fiberglass in my expansion muffler to the point it was leaking out. I opened the muffler to replace the fiberglass and left the **** thing at Home Depot last night looking for replacement screws! Ahhhhh! Man.... These bikes really are a constant project. I don't think I'll ever get one "done".
  4. wildemere

    wildemere Member

    I tune my HT's with a non contact themometer, a good hill and a speedo .

    For me its trial and error, the plug stays put.

    Plug inspections with new plugs are for racers at the ragged edge of tuning.

    They use a new plug with each tune-up change with power "chopped" at max RPM.

    Then an inspection of the insulater at the plug base with a magnifier lens.

    General plug tip colours are good for initial tune on general purpose engines however.

    For the happy time china girls-

    Find a decent hill. Run it up and record the top speed and temps next to the plug and 1" from the exhaust port.

    Repeat after a tuning change. Record the results.

    Try for around 180 celcius on the head and ~195 on the pipe.

    Higher temps on the pipe are lean.

    When the pipe is at a lower temp than the head you are rich.

    One change at a time then retest.

    These numbers are from tests at around 20c ambient.

    I sometimes see a 5 or 6 km/h improvement up my "dyno-hill" after tuning.

    The best improvements from my testing are:-

    Bigger carb (19mm)
    Shaved head (2mm)
    Ported transfer ports on case and barrel (the best increase)
    Ported barrel (intake and exhaust) Not so much
  5. GearNut

    GearNut Active Member

    For tuning purposes, the color of a spark plug will lighten up after it is black. It will burn of the oily gunk. That is why it is so important to choose a spark plug that is the correct heat range. Hot enough to keep itself clean but not hot enough to blister, and not too cold as to always foul out from carbon and combustion residue.
    Some folks use a 2 mile run as the run time that the spark plug needs to change color, some folks believe that it is a 5 mile run. I typically go somewhere in the middle, more than 2 less than 5 and it has always worked for me.

    I like the theory of the temperature tuning method. I have never known anyone that used it though. Now I gotta do a Celsius to Fahrenheit conversion on it.......