Engine Trouble Is My Engine Head Mounted Backward?

Discussion in '2-Stroke Engines' started by Waxxumus, Aug 4, 2015.

  1. Waxxumus

    Waxxumus Member

    How would one be able to tell if their engine head was mounted backward? Or will it mount in any position?

    Its one of the standard compression, non-slant head.

    I was looking at it, and it almost seems like its seated wrong.

  2. tzvii

    tzvii New Member

    Mount it so that the spark plug's open face is towards the intake to improve efficiency.

    EDIT: You also want the cooling fins to be running in the direction of airflow to improve cooling.
    If you can't get the spark plug aligned correctly you may use indexing washers.
  3. crassius

    crassius Well-Known Member

    they go any way except sideways - my kits come in with tallest part of fins toward back, but I turn them around just for looks
  4. jeffuehrer

    jeffuehrer Member

    I've had them mounted both ways and both ways worked.
  5. HeadSmess

    HeadSmess Well-Known Member




    it makes sweet f all difference really, on a tiny engine, but its true... it is noticeable as cylinder diameter goes up. big engines really benefit from it. try it on your harley or v8 some time ;)

    i recall a certain champion plug, cj8r or similar...and the r in that case meant the electrode was indexed 180 degrees from standard. NOT resistor. otherwise identical. out of one of those brush cutters where the plug entered the head at right angles and sat right up in a little conical combustion chamber. with standard cj8 was always a pig...

    good visual example...

    index the plugs so the open side faces intake... or closed end faces exhaust, as i learnt it, as a two stroke etc has multiple intakes...

    think that the flame front spreads from that point... so the exhaust is sorta "shielded" momentarily... and in two strokes, thats the hot side, so a bit of cooling helps.

    just as slant plug heads and offset plugs go opposite exhaust...

    and as for centre plug heads...line the fins up for airflow and try getting the gap to the back...
    Waxxumus likes this.
  6. tzvii

    tzvii New Member

    You are right that indexing the plug will likely only see a fraction of a percent increase in power on these engines. On larger displacement engines it can make a more substantial difference, though.

    Besides, fractions of a percent gains can add up! There is no reason not to index it; it is good practice anyways.
    Waxxumus likes this.
  7. Waxxumus

    Waxxumus Member

    my fins are tallest in front atm
  8. Waxxumus

    Waxxumus Member

    Well, I was told I hit 28 mph today, 800 miles into life of engine.
    Gets REAL hot. Like touch it for a moment and get 3rd degree burns. Im pretty sure its too lean.
    Thinking about either moving my jet back to .070 from .065 or enriching the mix of oil/fuel with more oil.
    Like going from 32:1 to 30:1 or 28:1. HMMMMM. been running a spark plug too cold for the enviorment anyway. soooooooo. IDK YET. tuning is an interesting process.
    Thank you all for your info. Much appreciated. You are all very helpful.
  9. tzvii

    tzvii New Member

    Adding more oil makes the air/fuel ratio leaner. The terminology can be a bit confusing; a richer gas/oil ratio (more oil, lower ratio) makes the air/fuel ratio leaner (less fuel for the same airflow, higher ratio) with all else the same.

    Since adding more oil leans the air/fuel ratio, it will generally make the engine run hotter (assuming that there was sufficient lubrication at the previous mixture).

    I say generally because the highest combustion temperature in an engine occurs a bit richer than the stoichiometric air/fuel ratio. Since small engines usually run at a richer ratio than stoichiometric, the combustion temperature will increase as the ratio is leaned approaching the ratio yielding maximum temperature, then decrease as the ratio is further leaned.

    TLDR: adding more oil to the mix will generally make a small two stroke engine run hotter not cooler with all else the same.
    Last edited: Aug 11, 2015