uneven power

Discussion in 'Motorized Bicycle General Discussion' started by leo, Aug 31, 2015.

  1. leo

    leo Member

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    (kind of hard to explain in words, and sorry for long winded-ness)

    i believe i am missing some power, mostly on the upper end.
    so far, i have updated the carb, short intake, ngk b5hs, expansion chamber and a 56t. i built it to ride moderately steep trails, (all low end torque) top speed on level ground is 18-22 mph. it has about 500 miles on it.

    every now and then you can feel there is way more power in there. i can hear a difference in the sound when it kicks in on those "cherished" strokes. it's like there is a monster in there, but "he's on a chain most of the time."
    it's not that it cuts out sometimes, it's more like it really kicks in, once in a while...

    at first i thought compression issue. i checked the head gasket and plug area, and they don't seem to be leaking, no oil or discoloration, bolts all tight.

    i have read about people doing "burnouts" on theirs, but mine will not even remotely do that. (should it?)

    don't get me wrong, it runs more than well enough. very much money well spent.
    i just get the feeling it could be much better with what is most likely a minor tweek.

    i'm kind of leaning toward the head needs to be modded or better head gasket to extend the stroke... or maybe a better head all together...

    what do the experts think???

    (the pic is from last week. i changed the front mount and leveled it out a bit since then. and the stubby intake wasn't on then either.)
     

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  2. Slogger

    Slogger Member

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    Lean it out one step. Free the Kraken.
    If your spark plug gap is over .028, tap it on the bench a couple times. They say the CDIs can fire better at high rpm with a narrower gap to jump.
     
    #2 Slogger, Aug 31, 2015
    Last edited: Aug 31, 2015
  3. leo

    leo Member

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    thank you slogger, i will try that later today.
     
  4. crassius

    crassius Well-Known Member

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    I've had motors back a few years ago that took forever to run in & then were still somewhat weak (doing measurements showed that some trans ports were a bit lower than others from same factory), so I don;t use them anymore.

    It does sound like you're too rich on yours tho, so I'd drop the needle right to the bottom and see what happens, then maybe lower the float level a bit also if it gets better. After that, I'd be looking at making or buying a new main jet.
     
  5. leo

    leo Member

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    nope, not much difference.
    can these carbs be adjusted without changing jets?
    would i maybe be better off for now to reinstall the factory one and move the ring down a notch?

    it seems to fire faster with less power at higher rpm's, is that basically what "4 stroking" is?
     
  6. leo

    leo Member

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    sorry. i mean move the c clamp to the top notch, moving the needle down.
     
  7. Agreen

    Agreen Member

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    The coiled up ignition wire is not helping. I won't say that it's the problem, but it should be straight to prevent it from losing power to a magnetic field.

    The standard NT carb can not be adjusted without changing the jets. They are, however, incredibly easy to change out. I had an old broken carb that I stole the jet from and started by soldering the jet shut and drilling it to a smaller size. Turned out, I needed a larger jet because it made far less power. When I went down in jet size, it felt like the throttle was closing some when I opened it past 3/4. It just bogged down and got more quiet. That meant it was lean. I went to a larger jet then, and WOW what a difference it makes! If you go too rich, the engine will also lose power, but it'll braap at you a lot. Instead of it making the normal 2 stroke noise, the frequency gets cut in half, thus the term 4 stroking.

    The needle setting depends on the size of the main jet. Before I changed the jet, it was happy at the 2nd from the top notch. The needle controls everything upnto 3/4 throttle. After I drilled the jet larger, I had great WOT power, but partial throttle was 4 stroking. I raised the clip all the way up (which leaned it out) and the normal power returned. The main jet changes the fuel flow to the throat of the carb, the needle restricts it below 3/4 throttle.

    Having the extra jet made swapping them out way easier. It minimized the amount of time the carb was apart, which helps keep dirt and debris out.

    If you need to go to a smaller jet, soldering the hole shut can be accomplished a couple different ways. I used a plumber's torch and electrical solder. I soldered on the non-threaded side, and just a LITTLE dab is all you need. Otherwise you'll be drilling it out a lot. And only keep the jet in the heat for about 5 seconds. Anything more and you'll melt the jet. Some people use a soldering gun and plumber's solder with brush in flux. It doesn't matter, so long as you solder it right and don't make globs.

    The tiny drills are the hardest part. I was so afraid I'd break one that I drilled for like 30 minutes before I grew a pair and put pressure on it. They do break easily, but they aren't made of glass.
     
    #7 Agreen, Sep 1, 2015
    Last edited: Sep 1, 2015
  8. crassius

    crassius Well-Known Member

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    I'd at least try the old NT to see what happens.

    the two rules are:

    don't get in a hurry
    don't get lazy
     
  9. Davezilla

    Davezilla New Member

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    Good advise so far, but I'll give my 2 cents worth of free power...
    If the engine feels like it's lacking on top you can remove the cylinder and run 2 base gaskets, this will raise the tops of all the ports in relation to the piston, or even more importantly, crank position. This trick will add a few more degrees of transfer and exhaust duration and give a little more power up top.
    Now to get back the power down low you can lap the head while you got it off, just set a sheet of 180 grit sandpaper on a flat surface like a sheet of glass... the surface needs to be perfectly flat. Next just take the head and run it over the sandpaper in circular or figure 8 motion and shave off at least 1mm (2mm if running 2 base gaskets). This will boost your compression and will give an improvement from idle to redline... if you got a compression tester, ideal compression is 135 to 150 psi (it may need 93 octane premium fuel above 15psi tho).

    For the intake, the shorty intakes usually work best for mid range and top end power, but the stock shorties are rather restrictive since their opening is only 13mm, this can be opened up to about 16mm to allow all the flow from the carb, a stock nt carb has a 14mm bore and the nt speed has a 14.5mm bore. If you're running a Dellorto clone, these are usually 14mm, they perform better because they're less restrictive internally and they mix the fuel and the air a little better so they feel and act like a slightly bigger carb comparing to the nt type carbs... anyway, opening up the shorty intake to at least 14mm will unlock some more power from midrange on up.

    For the exhaust, the banana pipe is a poor performer because it's "tuned" to a higher rpm than most of these engines can reach... they're also quite restrictive if you're looking for more rpm or top end power... depending on what kind of tooling you have access to and your skill set, there are other pipe options that aren't too expensive... you would also be surprised how well a straight pipe that's tuned to run best at a certain rpm can do too...
    And pretty much for all pipes you can buy, if you got a dremel and some carbide burrs or grinding stones, the inlet flange on most pipes is a huge restriction since the hole is usually small and round, open this up enough to match your exhaust port outlet and this will free up a few more ponies...

    Just by doing all the free mods mentioned above and in previous posts will give you a very noticeable improvement, some of the stuff I mentioned is free or really inexpensive to do except the exhaust, but doing all the free mods will free up enough to get you a good pipe... there are a few mods you can do to make a banana pipe perform but it takes some cutting and welding, if you got a welder and can weld, I can help ya out there as well as a few other members in here who know how to mod these pipes...
     
  10. Davezilla

    Davezilla New Member

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    I agree on the nt carbs... they are highly underestimated and I got my engine over 10,000 rpm using a plain stock nt carb. On the street that equates to over 40mph with a 44T sprocket.
     
  11. crassius

    crassius Well-Known Member

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    I agree on recovering compression by lapping, except I hate to see the compression ring on the bottom of the head go away - same technique, but applied to top of cylinder instead.
     
  12. leo

    leo Member

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    thanks guys, very much appreciated.

    Leo
     
  13. Davezilla

    Davezilla New Member

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    That's how I set up my own engines by lapping the cylinder, more like cutting off about .040 " from the top of the cylinder on my lathe, then using a .016" thick copper gasket to set the squish clearance down below 0.5mm. I also use copper base gaskets since they don't compress like the paper fiber ones do and there's no issues with stacking 3 or 4 of them if needed. On one engine 1 shaved .044" off the bottom of the jug and .040 " off the top so I could set the port hight as well as get my squish clearance where I wanted it. The real reason for shaving so much off the bottom tho was because it took that much cutting before the mating surface was flat and true. This one I made a .040" thick spacer out of aluminum and used a .016" thick copper gasket on both sides of the spacer, then I ended up cutting .052" off the top of the cylinder to get my compression and tight squish clearance back.... yeah I probably spent way too much time on that engine but I just got a new lathe and what a better way to test it out...
    But that engine was a screamer when I was finally done with it...
     
  14. maniac57

    maniac57 Old, Fat, and still faster than you

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    ALL chinadolls come jetted too rich for good top end. The factories ship them rich to reduce warranty returns.
    Try a smaller main jet and remember to reset your needle full rich (circlip in bottom groove)when you change mains for proper tuning range.
     

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