Engine Trouble High rev problem

Discussion in '2-Stroke Engines' started by Big Red, Jul 28, 2011.

  1. Big Red

    Big Red Active Member

    Well guys, I guess I'm not the MB guru I thought I was. My engine with new top end and an offset intake (otherwise stock) runs fine at low and midrange. As soon as it starts really zinging it sounds like it gets gas starved and dies. I've checked gas flow and float level. and for air leaks. Also has a good spark couse it bit me real good pulling the wire to kill it. (no kill switch yet). Spark plug is new autolite mid range. I dont know if it matters but I have the carb at a slight upwards angle. I've done this before with no problem. Is it possible I'm running the gas out of the bowl becouse it cant fill completely becouse of the angle? I've tried everything I can think of. any ideas?
    Thanks,
    Big Red.
     

  2. GearNut

    GearNut Active Member

    Fuel foaming in the float bowl?
    How bad does the engine vibrate at high RPM's?
     
  3. Big Red

    Big Red Active Member

    Thanks GearNut, That's something I never, ever considered. Perhaps I'll try rubber mounting the carb instead of the solid offset mount I have on it now. I've been tweeking on this thing most of the day, so thats something left for tomorrow. Thanks again,
    Big Red.
     
  4. motorpsycho

    motorpsycho Active Member

    attach a pic of the carb...i want to see the upward angle that it's sitting at.
    it sounds like it;s running out of fuel and the angle could be causing the bowl to empty faster than the float can open enough to keep the bowl full.
    any time you get a high rev and then the engine dies....it normally means that it's running out of gas.
    not out of gas in the gas tank, what i mean is that the float bowl can;t get refilled fast enough to keep the engine running.
     
  5. Lazieboy

    Lazieboy Member

    Air

    Hi, I had an air leak and or a pinched line from carb to motor intake and it gave me a similar issue. muffler was clogged up also.
     
  6. motorpsycho

    motorpsycho Active Member

    an air leak will usually affect the way the engine runs at all rpms.
    a float bowl running out of gas will usually only affect an engine at high rpms.
    i'm sure that he can ride around at slower speeds and the engine is ok.
    this is because the float bowl is able to stay full longer because the engine isn't using a lot of gas.
    at wide open throttle, the engine is using a lot more gas and the float may not be able to keep up.

    A pinched fuel line will also affect the engine at higher rpms too because the fuel flow is restricted to the point to where it can't flow fast enough to re-fill the float bowl before it runs out of gas.

    On this one, i'm leaning more towards the float being at such an angle that it's not letting the bowl get filled back up fast enough. since his carb is mounted at a slight upward angle, the float level may be set perfectly (for a level carb).
    but, with the angle of the carb, the float will also be an an angle in the bowl. since the carb is at an upward angle (assuming that he means the rear of the carb is higher than the front). the needle & seat in these carbs is at the front of the bowl, so that would put the fuel level in the bowl higher in the front than the rear. the float would be at the same angle as the carb (lower in the front and higer in the rear.) once enough gas is burned the float level will drop. fuel starts to flow back into the bowl, putting less fuel in the rear of the bowl than the front. when the float rises, it's on an angle so the needle & seat is closing before the bowl can actually be filled enough to keep it running at w.o.t.
    there will be more fuel in the front of the bowl than the rear and by the time the float drops enough to open the needle & seat, the bowl is almost empty....the flow of fuel from the fuel line can't fill the bowl back up fast enough.
    This is my theory anyway...:)
     
    Last edited: Jul 29, 2011
  7. Big Red

    Big Red Active Member

    I've got the float level set as high as it can go without hitting the primer "thingy". The needle seats well and allows a real good fuel flow when released. I've had the carb set at a higher angle on other bikes with no problem, between 90% and 45%. Not really that much. It does get a LOT of vibration so GearNut may be on to something. I'm going to level out the carb and use gas resistent rubber hose, (like smog hose) as an intake today to see what happens. It may be a combination of a couple of things, angle,vibration, causing my problem. We'll find out todayone way or the other.
    Thanks,
    Big Red.
     
  8. Big Red

    Big Red Active Member

    Hey Lazieboy. No air leak, no pinched line. I do have the tank setting above, but close to the carb. So perhaps there is not enough gravity involved to help keep the bowl filled. combined with the angle and the vibration I might get this figured out today.
    I went to the links you included thinking it had something to do with my carb problem. Imagine my suprise when they turned out to be about getting stopped by the police, And pot. I've only been on this site a few months, but don't those belong in another forum. I have no problem with the cops and I don't smoke weed, (anymore). I think it should be legal but that is for another forum, don't ya think?
    Thanks for the input anyway, ALL suggestions about how to get my engine running right are welcomed.
    Big Red.
    Oh yeah, I forgot to add, Brand new Muffler.
     
    Last edited: Jul 29, 2011
  9. motorpsycho

    motorpsycho Active Member

    I doubt that it's a gravity feed problem. On one of my bikes, the fuel line from the tank to the carb is only 2 inches long. is your tank cap vented correctly and is the vent open and clear?

    I assume that you have had the carb apart to make sure that the jet is actually screwed in and not laying in the bottom of the bowl :).
    It sure sounds to me like it's running out of gas in the bowl.
    But, it seems that you are not willing to take my word for it....so let us know what you find out.
    Think about how much fuel these engines use while riding at w.o.t..
    When you are at w.o.t. there's more gas getting sucked up through the main jet than what is being returned to the bowl. The float bowl should never go empty while riding.The gravity feed is more than enough to re-fill the bowl as soon as the float drops and opens the needle. more fuel will flow out of the tank, than what will be used by the engine at w.o.t. for a long time.
    If your float is at the incorrect angle, no matter how much fuel is flowing into the bowl, if the seat closes before the bowl gets filled, (float at an agle) it will run out of gas.
    While you are riding, the float is constantly going up & down to keep the bowl full. If the float drops too low before the seat opens the fuel going into the engine will be more than what is going back into the carb. and it will run out of gas.
    I had this same problem on my 49 c.c. because the carb was at too much of an angle (rear higher than the front). it ran perfect from idle to 3/4 throttle and then at w.o.t. it would run great until the bowl would run out of gas and then it would rev up and die. The gas flowing in could not keep up with the amount of gas getting burned and the bowl never truly filled up before the needle & seat would close (because the float was at an angle).
    I lowered the rear of the engine and raised the front of the engine to make the carb sit level and i have not had a problem with it since. I can ride at w.o.t. for as long as i want and it never runs out of gas or dies.
    the only thing i changed was the angle of the engine and carb and it fixed the whole problem.
     
  10. Big Red

    Big Red Active Member

    Hey Motorpsycho, It does it with the gas cap off too. And I am taking every bit of advice and gonna go do most all of it right now. Gonna level out the carb and use a gas resistent hose for the intake to avoid vibration. I think yer right about most everything, from the angle of the carb to the vibration causing it to foam. ANY foaming, with the carb not being able to fill all the way, Would cause it to get more air than fuel.
    So, I'm headed out to the shop right now to find out. Wish me luck.
     
  11. goodtime65

    goodtime65 Member

    If you feel that the carb angle is not an issue than I would be more inclined to look into your fuel valve on the gas tank. Iv'e come across this problem a few times and found out there was not enough fuel flow from the valve. To verify this you can replace the valve with a straight fitting and try it out
     
  12. motorpsycho

    motorpsycho Active Member

    Just for the record, it was gearnut who mentioned the gas foaming, not me.
     
  13. motorpsycho

    motorpsycho Active Member

    or a simpler solution is to just pull the fuel line off the carb, and open the valve letting fuel run out of the hose into a gas can. look at the stream of fuel and you'll know if it's flowing well or not.
    This is easier than replacing the valve right from the beginning.
    if the flow is very wek, let the tank drain empty (you'd have to drain it to replace the fuel valve anyway). pull the old valve out of the tank and look at the screen on the end. Take the screen off, and that may solve the fuel flow proble, (if there is one).
     
  14. Big Red

    Big Red Active Member

    Well guys, I've been out in the shop most of the day AGAIN. I've got it reving with no load but bogs out with load. Also, it acts like it's out of time or something couse it's spitting a gas fog out of the carb intake. How can this be? I thought these engines were factory set for timing. I've tried different muffler setups for some back pressure. I think I might still have a fuel flow problem plus a timing or carb problem. What the H E double hockey sticks? I've NEVER had this much trouble with a little china girl in my life. You guys have got me this far and I really apreciate it. Any more ideas out there?
    Thanks,
    Big Red.
    Hey bro's. Right after I posted this I THINK I figured out my carb blow back problem. I think I'm using WAY TOO BIG of a hose for my intake. The engine was not made to take in that much volume. What don't get sucked in has to go somewhere, and thats back out of the carb right on my UPPER leg. A little too close to my n^%,s. Yeah, thats right, I smell like gas and my leg is burning. I'll check back when I get out of the shower.
    Thanks guys,
    Big Red.
     
    Last edited: Jul 29, 2011
  15. Big Red

    Big Red Active Member

    Yer right bro. I did level out the carb as you suggested. I think that was at least 50% of my problem.
     
  16. motorpsycho

    motorpsycho Active Member

    well, the timing on these engines is keyed, it's either right or 180 degrees out. there's no way to adjust the timing unless you cut a new notch in the magneto for the woodruff key.
    gas spitting out of the carb is normal on these engines because there are no reed valves, just ports in the cylinder.
    the size of the intake tube has nothing to do with how much fuel gets taken in by the engine. The amount of fuel that gets taken in by the engine is regulated by the main jet. The amount of air that gets taken in is regulated by the size of the carb throat.
    so even if you had a 3" diamater intake tube the same amount of fuel and air would be sucked into the engine with the stock carb.
    If you put a carb off of a 250 c.c. motorcycle on it, then you would get major blowback out the back of the carb because there would be too much fuel going into the engine for it to burn all at once. the engine probably wouldn;t even run with a carb that big, but i'm just using a 250 cc carb as an example.
    the only thing intake diamater and length affects is the rpm range where the engine makes the most or least amount of power.
     
  17. Big Red

    Big Red Active Member

    Intake

    So I'm guessing that the factory boys already have it all worked out. How long the intake is and the I.D. are at optimum for the best power at most RPM levels. Then what would one do, as far as intake goes, to get best power at higher RPMs?
    This is my first offset carb build and It's not going as planned. I don't give up easy though so I'll be on it again tomorrow.
    Thanks,
    Big Red.
     
  18. cloud_2901

    cloud_2901 Member

    I read somewhere the rule was shorter intake = more top end power, longer intake = more bottom end, think it was jaguar who said it somewhere in these forums. No idea how the diameter would affect ideal flow ratios, some here seem to think it wouldn't matter at all for the flow, but bigger/smaller, one of the two, don't know, could end up with less impedance? thereby not really increasing the flow, but making more of said flow enter the cylinder? Is that possible or am I talking out my arse here?
     
  19. Big Red

    Big Red Active Member

    Intake

    Hey Cloud2901,
    It seems to me that as long as you don't make the intake any larger than 66/80cc capacity then the cylinder should be able to take in the entire fuel discharge from the carb. I think also if the intake is the size of a house, (yeah, stupid I know) There's no way much of the fuel mix is gonna reach the cylinder.
    Anyway, like I said before, The factory boys must have worked this out already so I'm gonna go with tubing as close to the stock size as possible.
    So I'm off to the shop to try it again. Hope I have better luck today.
    Thanks,
    Big Red.
     
  20. Lazieboy

    Lazieboy Member

    Ammemendment rights

    It's not about Illegal substances, but that WE have rights and even a square like you dont have to let your property be searched illegally. Cops are not always honest and will try to Intimidate people to get what they want to build a case. I dont hate em though. O ya bike running ok still in break in. 27 mph. so far.
     
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