Engine sound in relation to stroke?

Discussion in '2-Stroke Engines' started by TINKERER, Mar 9, 2009.


    TINKERER Member

    Today was my second run on my bike, I have about 20 miles or so on the engine, my question is why does the engine sound so good, like weeeeeeee like a real 50cc moped when i first take off and then about mid rpm range it starts to go brahhhhhhhhh. ? I'm using an 66cc HT with stock pipe , ngk b6hs gapped to .20 and im running 92 octane with a 16:1 ratio full synthetic.

  2. cooltoy

    cooltoy Member

    You have to burn at least two tanks of gas to seat the rings.They seem to run much better after that.
  3. Old Bob

    Old Bob Member

    Sometimes two strokes will slightly four cycle with a lighter load and if running a little rich too.

    TINKERER Member

    bike is about 80lbs and I'm about 180 so 260 a light load? these things are no work horses.
  5. fetor56

    fetor56 Guest

    20 Mls is nothing....figure on 200Mls for consistency(check your carby needle setting though)
  6. emdude

    emdude Member

    DITO's on the prior responses.

    I'll ad:

    The weeeh and then baaaah...very descript by the way, I wish I would have though of that.....is as has been pointed out that the the engine is four stroking. On little 2 strokes like this it's caused by some limiting factor. This can be anything from restrictive exhaust to too small a carb to poor port timing of any of the cylinder ports. See prior post on this subject by me.

    My engine ran that way out of the box. But I attributed it mainly to the fact that it was running WAY too rich. I also suspected that the base gasket in the cylinder was partially to blame because I had inspected the engine before ever installing it and found that said gasket substantially blocks the transfer ports. After one tank of gas I decided to inspect this and the blockage was GONE. Simply eaten away. That didn't take long. But I now that others had found the same blockage in their engines as well. It would partially account for the increase in power when the engines are "broken in". Power will increase as the transfer ports (gasket) open up.

    Based on my experience this is nothing to worry about since the blockage disappears soon enough.

    After quite some tuning and experimentation I've come to the following conclusion...at least for now... concerning these engines:

    Ports and carb are all ok. The EPA exhaust that came with mine was a bit restrictive and I removed the entire inner tube that goes into the muffler from the back. That did make some difference The biggest problem regarding top end four stroking was caused mainly by the fact the the carb is set up way too rich. And I live at 1600 feet altitude which makes the problem even worse. I went through quite a few steps to arrive a leaner carb mixture. Initially I "wired" the main jet and then went the solder a re-drill route. Again, see prior posts by me.

    From what I can tell based on my engine there is quite a bit of performance in these little things. I have my carb mixture set up so that when the engine is fully warmed up it NEVER four strokes at WOT. The WEEEEE continues and the engine pulls HARD until it reaches a point at which the vibrations from the engine become so severe that I have to back off the throttle for fear of having engine parts removing themselves from said.

    At the same time I can tell there's still some power at those higher RPMs left, but it would require crank balancing to to able to use it.

    Basically it boils down to that within a given set of parameters, ports, timing, carb size and exhaust, and these are all given unless you change them, you have to lean the mixture out juuuust enough to get the engine to rev to it's limit with out four stroking. That's the best you will be able to do and from what I can tell is actually quite impressive for a little engine like this.

    The problem is the juuuuust lean enough. You still want to er on the richer side. One indicator of close but still too lean is with WOT at moderate RPM on a straight away or a slight hill, roll back the throttle but not too quickly. if there is a slight power surge just as you roll it back, it's still a bit too lean.

    Also, with the engine fully warmed up and at the lowest idle it will tolerate RIP the throttle open fast. The engine should spin up in RPM QUICKLY. Any hesitation, even if it recovers and then spins up, it's still a bit too lean.

    And, none of this tuning can be done with any success if there are any air leaks around the carburetor or any part of the intake....I had plenty.

    Hope this helps a bit.

    TINKERER Member

    Thanks emdude, I really enjoy your informative posts.
    I checked my needle setting and it is at the second notch from the top, Would it be ok to move to the top position and run leaner? remember I'm still using a 16:1 ratio with full syn oil.

    TINKERER Member

    Upon further inspection, I realized the c-clip was on the third from the top setting, (I have a spare carb and that needle was on the second to the top), So I moved it up a notch and went for a ride what big difference but still 4 strokes at mid range, (then my new pipe came from Spooky Tooth, Its in another thread)


    I bolted that on and went for a ride it helped alot, but im going back to the stock pipe and will be moving the c-clip up to the top and hopefully that works.
  9. BSA

    BSA Guest

    FYI I see in your location you have written frozen tundra. If you are in a cold place NGK BH6s is too cool a plug. Go for a NGK B^HS or equivalent. Mine was B6HS and ive been operating in and around 30-40 degrees F and the plug is black and iVe had a hard time starting. Noticed my plug was oil fouled aswell.