Port Timing

Discussion in '2-Stroke Engines' started by 210061741, Nov 16, 2009.

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  1. 210061741

    210061741 Guest

    Ok i ran my bike for a wile with no base gasket.
    Ran good but lost a little low end.
    Wouldn't rip until in the higher rpm range.

    A coupla days ago i added 2 base gaskets that were .045 inch thick each.
    This shoulda raised the jug by .045 from stock.

    I lapped the head and jug so i could remove the head gasket to keep the compression high.

    I't hasn't been running so good.
    Just dosen't rip and hit the high RPM's like it used to.
    Allthough i seem to have better low end and idle.

    So the question is what really is optimum port timing for the HT?

    An .045 inch change in either direction makes a big difference.

    And thats not alot of material if you started grinding the ports.

    Fortunatly i didnt do that.

    I would like to trim the piston skirt and give it a try but geese. it would be nice to know what will happen before i do it.
    Has anyone done a port map for the HT ???


    Now i think tonight i'm gonna go back to 1 base gasket and see where things are at.

    Iv'e had muffeler issues also with the expansion chamber and that could also be affecting things.

    These motors are touchy.

    With a little luck you can get it rocking down the road like you wouldn't believe.

    Then change somthing a little bit and poof.

    She aint runnin so great anymore.

    O well least i work out this configuration on my old motor before my new one gets here.

  2. AussieSteve

    AussieSteve Active Member

    Fitting 2 x 45 thou base gaskets would have decreased the compression, not just altered the port timing. Leaving the head gasket out would have alleviated this a little, but a head gasket isn't 90 thou thick, so you still had reduced compression. I'm surprised that you say your bottom-end improved. With lower compression, it would suffer too. That might have been the result of the altered port timing, but I doubt it.
    Overall, though, you need to take both into account.
    Personally, I've decided to buy a Rock Solid Engines billet aluminium head to increase overall power, (16:1 CR), but to stick to the stock carb and exhaust to keep some bottom-end. (I'll probably jet the carby down, but I'll see how it goes.) Then a smaller rear sprocket to bring up top speed a little. This should suit the 66cc engine since they don't like to rev too high.
    ... Steve
  3. 210061741

    210061741 Guest

    I had also sanded down the head and jug quite a bit.
    Enough to get a smoothe uniform surface on each.

    I pulled the head and jug off yesterday but i still left both gaskets in the base.

    It was tough to peddle so i'm sure i still had pretty high compression in the head.
    I was thingking maybe too much.

    The base gaskets squished down quite a bit.
    You can see a good indentation where the mating surfaces are at.

    Also if you read the tuning guides they all say if you mill the face of the head or jug you would need to remove 2X as much stock from the combustion chamber dome to keep the same compression.

    I'm sure to have removed about .030 off the head and jug.

    So i don't think i actually got the jug lifted that much due to the gasket squishing.

    Also i really sealed the engine up good now.

    I think for a long time my engine was pulling in some extra air.
    Progressivly i had to move the E-Clip down on the needle. "over a month or 2"

    It started out running great with the needle in the middle groove.
    Eventually i ended up moving it down and down and down until i got to the last groove.

    Then it idled a little rough but had good power and reved high as i wanted.

    I put the head gasket back in to lower the compression a little.
    I have the E-clip on the 2nd groove from the bottom of the needle.
    I think im' gonna move it back down to the richest setting and give it a try again.

    It runs and pulls ok but it isn't real smoothe.
    It misses alot.
    Normally it would 4stroke and miss a little till it warmed up.
    Then it would smoothe out and rock.

    Now it feels like i cant get the carb to settle in right.

    I'm starting to think that since i sealed it up soo good now it's not getting that extra pull of air so it just won't run right.

    I'll have to play with the carb some more.

    I got a new PK66 coming today.
    But i don't want to mount it till i have this 48cc running right again.

    Maybe i shouldn't waste my time on the 48 for now but i wanted to work the new motor over before i swapped them.

    Got a billet intake coming too so ill have to throw that on my 48 and get rid of my boost bottle.
    The boost bottle did work well for a long time.
    That could be it too.
    I've increased the volume of the crankcase without changing the bottle at all.
    Thoose bottles are real picky in that the CC of it has to be just right or the thing won't run good.

    Boy it gets tough to troubleshoot when you change soo many things.

    O well sooner or later i'll figure it out.
  4. 210061741

    210061741 Guest

    Still no luck with this.
    I tried everything.
    I have no choice but to pull the gaskets and hope it runs like it did.

    Unless my CDI is going haywire or magneto i dont know what is goin on.
  5. 210061741

    210061741 Guest

    Finally got it pinned down.

    Pulled out them 2 base gaskets and WOW she's back.
    Had to back off the trottle today it's running so strong.
    Revving so smoothly and high.
    Going from hardly peddeling to top speed very quickly.
    No dead spot no lagging just pure adrenilane power.

    Believe it or not the thickness of the compressed portions of the gasket i took out was
    .050 inches thick.

    That much of a change in the ports makes a HUGE difference.
    Also i trimmed off .090 on the intake side of the piston skirt.
    Didn't help with the 2 base gaskets in there but it is even stronger than it was when i had no base gasket before.

    This shows me that porting is key to getting these motors to premium condition.

    I also pulled off the boost bottle and replaced it with the billet intake.
    Definatly didnt hurt anything.

    I think the boost bottle would help the most with a reed system and would really only add volume to the crankcase.

    So it is safe to say that the intake port belongs lower and that the piston skirt should clear the intake port at TDC.

    So we are back up and running.
    Gonnar ride this 48cc anothe week or so until i have the new 66 i got ready.

    Be sure to watch my threads about it... It's gonna be a well documented Happy Time.
  6. Old Bob

    Old Bob Member

    Raising the jug LOWERS the intake timing and RAISES the exhaust timing, that combination is a power killer.

    Higher exhaust timing raises the rpm where peak torque occurs.

    If you are looking for more power across the rpm range, don't change the port timing but make ports wider and reduce squish band.
    But before you do anything read A.Bells book and Gordon Jennings book.
  7. 210061741

    210061741 Guest

    Yep both very good books.

    If i red them again i'll have them both memorized word for word.

    I'ts still fun to play around.

    Sometimes what you end up with theoretically and in reality are very differant.

    But I have to agree with the power killing.
    Raising the jug about ruined the motor.
    It was hardly rideable.