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Thread: Setting carb fuel mixture screws

  1. #1
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    Cool Setting carb fuel mixture screws

    I wrote last fall about how unimpressed I was with the Colortune to set the fuel mixture, and I promised then to report back on how my exhaust gas analyzer compared - well, here it is. Sorry it took so long.

    Most exhaust gas analyzers are quite expensive - $2,000 to $5,000! And my research shows that there are not a bunch of Cheap types, then a bunch of expensive one, they are virtually ALL expensive. So when I found one that I might actually think of buying, the Gunson Gastester, about $230, I had to think long and hard about just how good this thing might be. The few reviews I found on the web were contradictory - some said it worked well, others said it was junk. I took the chance, and I am glad I did.

    Before I go there, however, let's talk about the three ways I know of to approach this task.

    1. First is by ear and a tach. You simply turn out the mixture screw until the engine reaches the highest RPM, then back in just enough so the RPM begins to drop off. I am never comfortable with this method, since it seems so danged imprecise, but when I have used it and then compared it to instrument measurements, I have always gotten it quite close.
    2. Second is to use the Colortune - a device with a glass window in the spark plug so you can see the color of the flame. I only tried mine once, and I was unimpressed by how far you could turn the mixture screw in either direction before seeing ANY change in the color of the flame. That one time, when I got done - just guessing where to leave the screw, it turned out to be almost the exact right setting. I can't imagine it was anything but luck. So I think the Colortune is good only if you have a carb so far out of whack ANYTHING you do might be better.
    3. Real exhaust gas analyzer, where you can actually know how a change affects the combustion.

    The Gunson Gastester only measures CO%. This is not everything that a quality instrument would measure, but CO% is a good indicator of the combustion efficiency. The biggest problem I have with using a CO meter is that I have not been able to find any published spec from Yamaha on the CO setting for this engine, so I just have to use general engine info. Basically, CO% in free air is about 2%, so CO in exhaust at 2% is about as perfect as you can get - less is too lean, more too rich. Generally, an engine running anywhere between 2% and 4% CO is good.

    The Gastester is very cheaply made - seems like flimsy plastic, and the part that actualy reads the CO is just held on with double sided tape that falls off. This does NOT give one confidence. Most important to know, when the instructions tell you that this device cannot be moved after the 10 minute calibration, they ain't kidding! Any movement at all will send the numbers out the window!

    It is slow and tedious, with lots of waiting for calibration and readings to stabilize. But it DOES work. I don't have time to do a proper writeup with pictures right now before my trip, but I will when I return. I found that the setting of the mixture screws is fairly sensitive, with as little as 1/4 turn making a noticeable change in the CO%. This is particularly important when I contrast it to the Colortune, where turning the screw TWO or THREE entire turns did not seem to change the color of the flame at all!

    Hope that info is of value. Ride Safe!
    Goose
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  2. #2
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    Default What He Say?!?!

    Well Goose,
    Thanks for the tip on the Colortune! I was wondering after setting your Carbs. with the exhaust gas analyzer how close were you to the "Tride and True" seat the screw and back off 21/2 turns. Is a poor smoe like me even close using this method? Would you modify the rule to 2 turns...or 3? I would be grateful for any light you could shed on this issue.
    Thanks,
    Earl
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  3. #3
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    Cool Uhhhh, don't use that method on the Venture

    Quote Originally Posted by skydoc_17 View Post
    Well Goose,
    Thanks for the tip on the Colortune! I was wondering after setting your Carbs. with the exhaust gas analyzer how close were you to the "Tride and True" seat the screw and back off 21/2 turns. Is a poor smoe like me even close using this method? Would you modify the rule to 2 turns...or 3? I would be grateful for any light you could shed on this issue.
    Thanks,
    Earl
    The Venture, at least the second gens, seems to be a bit of a strange duck when setting up the carbs. If you check the parts list you will see that three of the four carbs use different jet sizes! I have no idea why. Furthermore, the shop did a professional carb setting on my bike a few years ago with a real shop quality exhaust gas analyzer, and when they got done I checked and wrote down where each screw was set - none were close to the others! And my own testing later with the Gunson Gastester absolutely confirmed the individual settings they left each carb at was correct; they were all running right at 2% CO.

    I'd have to check my notes to see what the "average" setting might be for the mixture screws, but my best advice for someone without testing equipment is to use a tach and find the highest RPM for each carb, then turn that screw in about 1/2 turn. That will get you close on these engines. The RPM change from adjusting only one carb is quite subtle, so you really have to pay attention. And it is possible to do this just by ear on an RSV - I've done it - but that is even harder than using a tach!
    Goose
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    Default

    Thanks for the write up.

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