After exhaust mod...cutting out at top-end.

Discussion in 'Performance Mods' started by mrsaxman99, Feb 22, 2007.

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  1. mrsaxman99

    mrsaxman99 Guest

    This mostly has to do with fuel. This situation has made itself much more evident after opening up the exhaust a little (35% less back pressure or so).

    So consider fuel setting A:

    Engine has plenty of torque down low, midrange is good. Pulls hard till a certain (unknown) rpm, and then it just bogs. If you stay on the throttle, it'll bog till it gets back down to under the bad point, where it kicks back in and pulls strong to the "bog point" again, to repeat indefinitely.

    Fuel setting B, where I've leaned it out a good bit:

    Little less torque in the low range, about the same in the middle. Pulls the same up high, except the "bog point" is all but gone. It pulls under load to a MUCH higher rpm, and pretty much at redline, it finally reaches the similar conditions of the previously mentioned "bog point"

    Here's what I think:

    I think since I opened up the exhaust a bit, the engine is obviously flowing air much better. I think the intake is the limiting factor, and under fuel setup A, the intake simply isn't flowing enough air (really small intake), and it's sucking in way too much fuel, and bogging the engine. I think the richer fuel condition A is really the ideal fuel mixture, but when the intake restricts airflow it just starts sucking in too much fuel.

    I know I know...typically an engine leans out under higher rpms...but then how come the early "bogging" symptoms disappear when I lean out the mixture.

    The surefire way to test my theory would be to open up the intake, which I plan on doing.

    What do you all think about this situation?

  2. gone_fishin

    gone_fishin Guest

    this is what i think...i've taken the end of my muffler off & played with the mixture settings. and i've been trying to find the right words to describe the exact same conditions!!

    i know it's not a leak, because she runs too goldurned consistently, but i can surely feel the difference between under load and when inertia takes over.

    i really look forward to seeing this one get worked out, because i can just feel it trying to go faster...

    i wonder if the directhits thing would do it, i eagerly await feedback on that lil cutie.
  3. chieflets

    chieflets Guest


    when you make the exhuast bigger you must make the intake bigger

    the standard exhuash manifold and intake manifold are great for 48cc motor, but the chinese have used same exhuast and intake for 48cc which don't match up the cylinder ports

    1. you must custom make new exhuast and intake flanges and larger diam pipes

  4. gone_fishin

    gone_fishin Guest

    well, the "simple" part is a given, plenty of fuel and the ability to use it, but...even if i dremel out the intake port and manage to make a bigger intake manifold, won't i still be limited by the carb inlet size?

    so, since i have no fabrication facilities, and certainly can't afford to modify the carburation, i'm still gonna cross my fingers and hope the directhits will burn that extra rich high-end mixture, then i'll just use a bit more oil.

    this is why we need to organize a real motosport, it would encourage and reward innovations that can eventually be used by all.
  5. mrsaxman99

    mrsaxman99 Guest

    I'm just curious why it is acting the way it does. Can the carb/intake not flow enough fuel or not flow enough air?

    I would assume that with lessening the restriction in the exhaust, the engine would flow more air, which would lead me to believe that the carb ISN'T flowing enough air or fuel. But when I modded the exhaust and this problem started, I actually changed the needle to a LEANER setting, and it improved the situation.

    Just seems kind of contradictory, doesn't it?
  6. gone_fishin

    gone_fishin Guest

    whenever you hit a slight incline or face the wind it breaks into the sweetest & strongest "buzz" eh?

    doesn't it feel that once your weight/momentum takes over it's like a retarded timing setting? like, if you could advance the spark a bit it would take off like the proverbial bat?

    problem i have is i keep trying to relate my 4-stroke knowledge to these things & i wind up at impasses like this one.

    is it just that the 70cc needs more everything than the universal carb/intake can give it? what about if we do manage to increase our top-end, won't we just experience the same thing at the new higher speed?

    just for giggles, i'm gonna go try all 4 notches right now...i'll pay close attention and give as full a report as i can.

    EDIT: as soon as i posted this it started raining again, this may take a while...
  7. mrsaxman99

    mrsaxman99 Guest

    yeah lemme know what you find out with all 4 carb settings.

    Do you have stock exhaust / exhaust restriction?
  8. gone_fishin

    gone_fishin Guest

    i took the end clean off...full-flow, at least as much as port/pipe allows.
  9. mrsaxman99

    mrsaxman99 Guest

    hmmm I am starting to wonder if it is more than just intake...more internals/timing
  10. mrsaxman99

    mrsaxman99 Guest

    I am at work and can't get to my bike, so you might try removing your air filter just for a quick spin to see if that helps. I'm kinda curious about that one
  11. gone_fishin

    gone_fishin Guest

    i have that beercan flows really freely, i can put my fingers about an inch away and feel it.
  12. mrsaxman99

    mrsaxman99 Guest

    so I guess it's not a restriction by airfilter issue then... Did you try the leanest setting? I've tried all but that one, and found the leaner I go, the more the issue disappears. I'm just a bit weary of running lean, though by the engine sound, it definitely isn't running lean right now.
  13. gone_fishin

    gone_fishin Guest

    funny you should ask, because i just got back from trying all 4 settings...slot #1 is the most reliable throughout throttle range with a bit of "pop", #2 gives me more "scoot" but i get more "pop" at top-end.

    i'm stumped...and tired of getting nowhere.

    low-octane, 36:1 real oil, new NGK plug, #2 slot, no leaks, great starting, excellent performance...but still i can feel that top-end isn't everything it could be.


    anyone at all have any ideas?
  14. mrsaxman99

    mrsaxman99 Guest

    on the way home just now it was doing it in full force. I did notice if I gave it full throttle going down hill (no I didn't let it over rev, just let it get up to what I am used to it being it's max rpm on flat ground), i could get it to go up to the normal top end rpms...and it didn't cut out at all.

    It was pulling really hard on flat ground, but once you hit that point, the engine bogs, almost like it just instantly stops firing. You can let off the throttle, or keep on it, or go back and forth between full and no throttle, and you just get nothing...this happens for about 3-5 seconds...after that it smoothly kicks back in, sounding like it is running REALLY rich, and then gets back to normal...strangest thing...
  15. chieflets

    chieflets Guest


    take the airfilter box off and just screw back the filter and hard platic to hold it there and see if that improve it also cheap mod cost you nothing there was picture of some doing that here on this forum

    I see if i can dig it up or someone might have it handy

  16. gone_fishin

    gone_fishin Guest

    i never get the feeling it quit firing, i just feel like it's firing too all the combustion is pouring out of the exhaust instead of staying in the chamber.

    i'm gonna sit back and hope someone hops in with a solution.

    (i re-labeled the subject to catch more attention)
  17. mrsaxman99

    mrsaxman99 Guest

    thanks for the relabel...tomorrow I'll try to take out the air filter element. I don't need pictures to do that. I've had this engine completely apart save for the crank. I know how everything confusion lies in WHY things work the way they do. I'm about to just buy a stock exhaust (like mine was before I cut out the small tube) and be done with it. The 1/2 hp increase isn't worth this frustration...
  18. Blaze

    Blaze Guest

    I think you guys are having problems because you changed the back-pressure of your exhaust. 2-strokes don't always get more power just by opening up the exhaust, like a 4-stroke would. 2-strokes actually NEED some back pressure to run their best.

    Ever notice how high performance 2-strokes have tuned pipes with large expansion chambers, but they don't usually have wide-open exhausts? The tuned pipes work by timing the exhaust compression wave to force a little pressure back into the cylinder through the exhaust port just before the cylinder goes back up to compress and fire, thus increasing your effective cylinder compression. The exact timing of the pressure wave is either engineered into the shape and length of the pipe (which is why many tunes pipes are only sold specifically for the engine they were designed for), or by adjusting the length of the pipe between the exhaust port and the expansion chamber.

    We don't exactly have high-tech tuned pipes on our motors, but my guess is that you are experiencing a demonstration of the need for back-pressure to make a 2-stroke run properly. It's possible that removing the end of your muffler may have had a negative effect on the timing of the exhaust compression wave, which has ultimately lowered your cylinder compression.

  19. gone_fishin

    gone_fishin Guest

    that was what i think i didn't want to admit to myself...thanks a lot, blaze :)

    putting the muffler end-cap (with small outlet) back on tonite, tomorrow i'll mess with the mixture one last time 8)
  20. mrsaxman99

    mrsaxman99 Guest

    well it did in fact add a good bit of power to my engine, but yeah I completely agree with what you are saying, and it appears that the backpressure gets too low at the higher rpm's.

    Problem is, the long narrow tube that is attached to that end cap somehow mysteriously got cut off :shock: I replaced the end cap, so I didn't think it would drop the back pressure all THAT much, but I guess it did.

    So now the question is, should I buy a replacement stock exhaust, or look for a tuned pipe that might work.