Carb Adjustment Question - RCCanada - Canada Radio Controlled Hobby Forum
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Old 08-10-2006, 07:39 PM   #1
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Carb Adjustment Question

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This is on a Webra Silverline 40 motor. For the low speed needle (idle mixture) - how many turns out should this be, as a starting point? I goofed and turned out this needle, when the engine was upside down, thinking it was the high speed needle (normally I never touch the low end). I just want to know a good starting point on this needle. Thanks.
Colin Jensen
Regina Windy Flyers
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Old 08-10-2006, 10:17 PM   #2
Cecil Marshall
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I'd try one and one eight open from the closed position for starters..You'll probably have to "fine tune" from there!
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Old 08-11-2006, 12:44 AM   #3
Wayne MIller
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Carb Adj

Hi Cjensen,

If the above does not work, try the following to set the initial starting points for the needles. I posted this someplace here before but can't find it - so here it is again. Hope it helps.


Wayne Miller


If your glow plug is OK, you don't have old fuel, your fuel delivery system is OK, no engine air leaks and the spray bar is positioned correctly (as posted earlier) then you may want to try the following initial set up.

First, try resetting your needle valves to an initial start position. To do this, open carb to about 1/4 throttle setting. Turn high end needle about 5 turns from closed. Open carb to full throttle and use a clean piece of fuel tubing attached to the carb intake nipple and blow gently into it with your mouth - listen for air going into the carb (a hiss) - if you don't hear air, back out the high speed needle valve a little and try again - keep doing this until you hear air. This ensures your high end needle will allow fuel (in this case air) to flow into the carb. If it doesn't, than try another turn out or find out what is the obstruction.

Once you've verified the high end needle valve is working properly and allows air to be blown into the carb at full throttle, you need to set the low speed valve. To start, keep the same high end needle setting at 5-6 turns out and close throttle to about 1/4 throttle setting. Turn low end needle valve so it is completely seated (meaning closed by turning it clockwise, do this gently and don't force anything). Now with the carb opened at 1/4 throttle setting, blow air into the fuel tubing and SLOWLY rotate the low end needle valve counterclockwise (to richen the low end mixture). Stop richening the low end needle valve when you first "feel", or more accurately, hear (a hiss) air leaking into the carb.

Now, return to high end needle valve and close it completely and open it about 2 1/2 turns from closed. Recheck to make sure this 2 1/2 turn setting allows air to flow into carb when you blow air into the fuel intake nipple. Then double check the low end needle setting to see if you can blow air into it with carb at 1/4 throttle setting.

If all checks out, then attached fuel tank line to engine, prime engine manually (a few drops of fuel in the carb opening), return to slightly less than 1/4 throttle, attached glow driver and flip the prop, or start with starter. Most engines start better at 1/4 throttle than full throttle.

For the next step, remember turning the needle valve counter clockwise (out) makes the setting richer, turning the needle valve clock wise (in) makes the setting leaner.

As soon as the engine fires up, don't touch anything and let it run. Leave glow driver on and let engine warm up for about 30 seconds. Now with glow driver still on, slowly advance throttle setting. You should be able to get it to full throttle. If it dies from going from 1/4 to 1/2 throttle and above (with glow attached) it probably means you are too lean on the low end needle valve. Just open it (low end needle valve) up a little bit (about an 1/8 of a turn or less). Repeat the process as necessary until engine can advance above 1/2 throttle without dying. Once it can run at 1/2 and above, remove glow driver and go to full throttle. At full throttle, lean out high end needle valve to max lean rpm and then richen the high end needle mixture by 400 rpm (when richening, the RPM will drop slowly) or so when you pinch, and release, the fuel line briefly at full throttle the engine speeds up a little and then returns to its setting. The pinch test verifies if you are too rich or too lean. If you are too lean, when you pinch and release the fuel tubing the engine will die or not speed up at all. If you pinch and release and engine speeds up a lot, you are too rich, etc..etc.. The right setting is when your engine speeds up a little, about 300-400 rpm (a tach would help with getting your ears tuned in this area)

Have fun and if you use this methodical method to tune your needles, you will get a good "feel" for how to set carbs quickly and easily. Remember, high end needle normally works at 1/2 throttle and above and low end needle affects the idle and transition to full throttle. Also, if you readjust the low end needle valve, you must recheck and readjust high end. The two needles can interact with each other. If you forget which direction to turn the needle, just use a clean piece of fuel tubing attached to the carb and blow into it to determine if your adjustment reduced the flow or increased the flow for whatever needle you were adjusting.

Its really simple once you get the hang of it.
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