Tale of 3 engines

Discussion in '2-Stroke Engines' started by Skyliner70cc, May 23, 2009.

  1. Skyliner70cc

    Skyliner70cc Active Member

    I have built many motorized bikes over the past 5 years. I currently have 3 personal builds that ride in my spare time.

    I'm amazed by the differences in the engines and their unique personalities. Currently, I have an engine that is just not developing the same amount of power as my other two. Simply put, its a dog but I do ride it a lot because it also happens to be my smoothest running engine and is comfortable, but not fast, on long rides. Luckily, I have a SBP shifter kit that helps a little in trying to extract the most power out of this engine.

    Here's what i've done to date:
    1. No airleaks in carb, head, head base gasket, or engine cse. I've ruled these out already.
    2. Engine broken in as my others and has at least 300 miles This engine has less power than my others before they were broken in. Engine just recently got a new head and new rings. 100 miles on "re-ringed" engine and it runs the same way..weak. The base gasket under the head was partially blocking the ports but replacing it and ensureing it was correctly trimmed did not help much.
    3. Added a high compression slant head. Unlike other two engines, no noticeable increase in power although compression is noticeably higher because pedal start is much more difficult.
    4. I've done the replacement carb and gasket thing several times. I've got 6 spare carbs from other engines and have tried 3 of them. They all run the same.
    5. Tried new CDI, different plugs, swapped magnetos, done about everything under the sun with no improvement. Swapped 4 different mufflers. The one muffler that actually seemed improve things was one with baffle removed. Maybe its the louder sound that makes it feel stronger :)

    My next step is to add a tuned pipe and see if that helps. This bike is a good ride and with the decreased power, it will be reserved for street use and I'll use the other two for trail rides where I really need the torque to climb hills.

    Anybody else had an engine like this? Maybe this one is sleeper and is taking much longer for rings to seat. I usually see an incredible amount of power different at the 100 mile mark. Maybe this one will come around at 500 miles but not sure if I'll ever get there. I ride mostly on trails cuz cagers scare me.

  2. BSA

    BSA Guest

    I wonder if they cast the ports in the wrong place, maybe only slightly. That might have been a batch of engines were the engine was cast with the intake to high or something similar. I remember hearing that the really early 50cc square head engines had the ports cast in the wrong place, so they offered a 55cc upgrade kit.

    I have two engines, the 60cc is a real screamer, and very smooth. It can really go fast. My 80cc used to run dreadful, until if ported the intake. It's much better now, ive also increased the compression.

  3. spad4me

    spad4me Member

    I decarboned, then ground down he top of the piston to add duration to the intake stroke where the ports are, grind until you get close to the ring lands.
    Removed the botom gasket and notched the piston skirt.
    Added a tuned pipe rejetted the carb addded a lower restrction air filter velocity stack.
    I ported and polished , some smoothing not to major.,the intake and exhaust. Then port match the intake and exhaust ports.
    You will need to adjust your clutch about every week or so
    Too much wear.
    Your brakes will last about a week also.
    You will probably need to upgrade your drive chain I am up to a 420 drive chain.
  4. Skyliner70cc

    Skyliner70cc Active Member

    No significant improvement with the tuned pipe. The performance is similar to an unbaffled stock exhaust and pipe is a little loud for my taste. I'm convinced that i got a bum engine. I'm putting stock exahust back on and this will become my wife's ride becuase it is so smooth and she doesn't care to go fast anyway.
  5. Skyliner70cc

    Skyliner70cc Active Member

    Well, I've given up on the engine and installed the tuned pipe on one of my other bikes that also has a high compression head. I'll still ride the weak engien as my my leisure ride. Its one of the smoothest engines I have ever owned and I don't need to use foam grips like I do on all of my other rides and builds.

    All I can say is WOW on the tuned pipe on the other engine! I had trouble tuning the carb with the WH carb but replacing it with an NT brand HT carb fixed the problem. The HT carb as a finer tapered tip and the WH carb has a very blunt short taper that isn't as conducive to find tuning the high end mixture. I lowered the e-clip one notch and you should now hear this engine sing as it comes on the pipe. The pipe comes on a little later (higher rpm) that I want it to so I'll put in a slightly longer copper fitting to extend the length of the header so the pipe comes up at a slightly lower rpm.

    Anybody who wants ultimate power, a high compression head and a tuned pipe are a must. There are steep hill sections that my engine couldn't pull up before and now it pulls and accelerates with NO problem and makes trail riding here in Colorado easy.

    In fact, I was having so much fun with the tuned pipe on the trails that I ran out of gas and had a fairly long distance to pedal back home.
    Last edited: Jun 13, 2009