Carby Dax RT carb tuning

Discussion in '2-Stroke Engines' started by snarlofdisgust, Nov 16, 2012.

  1. snarlofdisgust

    snarlofdisgust New Member

    So, I finally got my bike started but I need to get it tuned correctly.

    I read the directions for plug chopping but the bike is surging a lot and only seems to run ok at full throttle.

    I have all the different carb jets from dax.

    I'm assuming smaller number (ie 60 65) is a smaller opening letting in less fuel? and hence leaning the fuel/aire mixture?


    I'm at 500ft sealevel and it's around 40-70F most of the time, what would be a good jet to start testing on? Currently I have a 60 in there and when I pulled it there was a lot of fuel residue on the plug which surprised me as I though the mixture was going to be lean with the 60 in there.
     

  2. snarlofdisgust

    snarlofdisgust New Member


    It tends to chug up hills, and when I do a plug chop it's oily and wet. This makes me think it's running rich but I have the leanest jet I can find (60) in there.

    Any advice?
     
  3. jaguar

    jaguar Well-Known Member

    take the air filter off to see if that is choking the flow and causing a rich mixture.
    if the float is not set to cut off the fuel flow into the bowl at the right level then the carb will run richer if the gas level is too high. take off the bowl, turn the carb upside down, and see if the float is horizontal with the carb. if not then bend the tab to adjust the float height so that it will be parallel with the carb.
     
  4. Fabian

    Fabian Well-Known Member

    Are you doing the plug chop correctly, because there are two ways to do a plug chop: the right way and the wrong way!

    Tell me exactly the procedure you use to do a plug chop?
     
  5. jaguar

    jaguar Well-Known Member

    I never ever do a plug chop. I think that idea is just one more half-baked fuddlestick some idiot made up that we two strokers were left thinking is "bible". Well, as in real life I found out all religious books are way not worth believing because they're full of errors and fairy tales etc proving they're a far cry from being the final word, I don't believe in doing "plug chops" either (or the nonsense about there being just one "tuned length" for an expansion chamber whereas in reality there are three). Why? Think about it. Does the combustion process continuously put layer upon layer of new deposits on the spark plug? NO! It's a very slow process. If it weren't then we'd have to totally scrap it clean once a week. Shoot, I clean mine like once every 6 months! If it is running rich at low speed and lean at high speed then it just doesn't change from black to white and back to black every block. forget it. don't happen.
    Believe nothing - test everything
     
    Last edited: Nov 18, 2012
  6. Fabian

    Fabian Well-Known Member


    Interesting that a man of your calibre would say something like that, because if it were a true statement, then everyone who races 2-stroke motorcycle, up to and including Grand Prix 2-stroke motorcycles is nothing more than a half-baked fuddlestick, working with a technique that some idiot made up.

    If it's good enough for Honda to do plug chops on their million dollar 2-stroke 500cc V4 Grand Prix motorcycle (back when they were racing in the 500cc category), and good enough for every other manufacturer competing in the 500cc category to do the same thing, and good enough for everyone competing in the 250cc Grand Prix category and good enough for everyone else competing in the 125cc category, then it's good enough for the rest of us.

    When Honda spends 500 million dollars per year on piston engine development, you would be unwise to argue against their research and development departments, or their analysis techniques; plug chops included.
     
  7. jaguar

    jaguar Well-Known Member

    Completely different engine categories. Maybe there is a very slight difference between the two methods but not enough for me to worry about. It reminds me of an average racer telling me how he wanted everything to be perfect on his ride because that's how the factory race teams do it. I reminded him that he was just a common Joe with average talents and not able to utilize his machine to its utmost potential. If doing a plug chop gives the "perfect" reading then of course the race teams will use that method. But for you and me to be that picky is just ridiculous. A 2% difference is just not enough for me.
     
  8. jaguar

    jaguar Well-Known Member

    Fabian if you don't believe me then test it for yourself. Go riding and do the sacred plug chop at full throttle. Then look at the plug color. Then repeat the same ride but keep the engine running till you come to a stop. Pull the plug and look at the color. You'll see no difference. Maybe there is a 2% difference but anything other than the eyes of an expert race team mechanic won't see it.
    Since color can be affected by the type oil u use and the ratio, I prefer to not pay any attention to the color and just go by how it runs. For main jet I keep increasing size until the top speed decreases slightly. That's the size I keep because it is ever so slightly rich which will help prevent a seizure when I do a mad blast for a few blocks wide open throttle at 8500 rpm. For low speed jetting I go for optimal torque and acceleration without causing irregular running. For the idle jet I set it lean enough that its hard to start it without the choke on and then refine it for optimal idling rpm.
     
  9. Fabian

    Fabian Well-Known Member

    A plug chop tells you exactly what is happening in the combustion chamber and not just the air/fuel ratio, but more importantly, evidence of detonation if you have a carburation problem.

    I have a cylinder head temperature gauge and an exhaust gas temperature gauge on my bike, and i still use plug chops to verify what's going on in the combustion chamber, even if the CHT and EGT are in the right zone, because combustion starts at the spark plug tip; the very place you want to go hunting to find evidence of what's happening in the combustion chamber.
     
  10. jaguar

    jaguar Well-Known Member

    "exactly"?!! not quite. I guarantee you there is no visible difference in color between the perfect size main jet and .02mm plus or minus that size.
     
  11. snarlofdisgust

    snarlofdisgust New Member

    Thanks jag, my carb was at a big of an angle, I heated up the pipe real good and bent it straight then welded up the holes. If it's till running rich I'll check the float but I bet it was the carb angle.
     
  12. Purple Haze

    Purple Haze Active Member

    That's a good point, snarlo, if the carb is at an angle, float level changes. A lot of people miss this, then can't figure out why their carb isn't running right. Thats one reason the off-roaders run spring-loaded needles in their carbs. Now, on to the plug chop debate. I want to add my 2 cents, and some will be shocked, because I usually agree with Fabian on most things. But Jaguar is right on this one, plug chopping is an out-dated tool, since it originated in the days of leaded fuel. Todays fuels don't leave the residues that change the color of the plug, the only color you'll get is the oil, so it might be somewhat useful in checking your mix ratio, but it's just as accurate to pull the plug anytime after a run. If it's brown, you're good, if it's too light or ashy-looking, add more oil to the mix, but it's useless as a way to check mixture richness. Old habits die hard, but leave the plug-reading to Smokey Yunick and the mid 20th century.
     
  13. jaguar

    jaguar Well-Known Member

    cool! I hate being the lone voice of reason. So with synthetic oil is the plug color providing any useful feedback of info?
     
  14. crassius

    crassius Well-Known Member

    Can't leave you hanging on this jaguar. I do a chop every so often to see if I see anything relevant, but never seem to unless the problem is so bad it can be figured out without the chop : (

    Back in the 70s I was considered the local expert on 'cycle carbs, but that was because I had a dyno & could tell a lot about jet performance from its gauges.
     
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