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Aug 01, 2003, 02:38 AM
Thread OP

K&B 40 w/ K&B carb...80s model

Hi all,

I have a K&B 40 from presumably the 80s. It seems only mildly used, and the local hobby guys said it had unbelievably good compression. Once installed on my Sig, I had it running well (I thought) till the 2nd day that it all seemed to go wrong. Though I had very, very little smoke (how do I get visible smoke?), the engine flooded and the idle never seemed smooth...lots of "missing" like you'd hear on a 4 cylinder engine with a bad spark plug. It has the stamped "K&B" carb w/ the screw and brass collar on the throttle side, and the "L" shape high idle adj. on the fuel intake side. As a good starting point, from closed, how many turns out should each "screw" be? Also, what glow plug is required?...short, long, hot? Does it really matter? The first flight had about a 75 degree incline from take off (70" wingspan)...lots of power, and no stall. I know folks dispute this engine and carb, but when it ran the first day, it was music. Thank you so much for any help!
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Aug 01, 2003, 03:08 AM
That carb was made before CNC was really popular, which means that the modern "how many turns" thing does not work reliably.

I once asked a fellow how long I should cut the pipe header for an engine of mine. He said 14". That was where his engine ran really great. Like an idiot, it was my first tuned pipe set up, I cut mine to 14". His engine was a Rossi and mine was an OS. Guess what? It not only didn't work, but I had to buy a new header because mine was too short. He warned me, but I didn't listen.

The same is true with carbs and their settings. Forget about what other people's engines are set at. None of that is important. What matters is where your engine is set and how it runs.

Save yourself some grief further down the pike and get really familiar with that engine. Set aside some time just to learn the engine and its setting needs. Do some hands on adjusting and don't be too impatient. You can master it.

Good flying.
Aug 01, 2003, 03:02 PM
God is good
Viper Pilot's Avatar
Have one of your local pilots adjust the low-end setting.

Sounds like it may be too rich at idle.

Aug 01, 2003, 05:21 PM
Registered User
Tony Oliver's Avatar
A quick check for setting the idle:-

Set the high speed needle at full throttle but just backed off from highest speed. Let the motor run at slow speed until as even as it goes, then quickly put the throttle wide open. If it coughs and splutters and either dies slowly or takes time to clear, it is too rich and the needle needs altering 1/8 turn maximum to close the fuel (that is the idle needle, not the main needle). If the engine dies or stops cleanly, there is insufficient fuel at idle and the mixture needs to be richer - again, no more than 1/8 turn maximum at one time.

When you think it's right, try it with the model tilted up at 45 degs. to check it will still run that way under flight conditions.

As it is an older engine, don't expect a linear speed range from that carb..


edit - Use a mid-range long reach plug, an idle bar will work well as they tend to make for more reliable running if the carb's not set accurately. Experiment with others after you have the motor set up correctly.

You can also check to see if there are any perished 'O' rings which may need replacing by now. They are a notorious cause of poor carburation due to air leaks.
Last edited by tonyo; Aug 01, 2003 at 05:27 PM.
Aug 02, 2003, 02:19 PM
Thread OP
Excellent advise, thank you all! It starts right up. I have it running pretty well now. I'm now using a long reach plug. It idles very well (and slow), but I'm still adjusting the dies sometimes when full out. It follows the "too rich" symptoms, but I get no visible smoke. Reason? Is it really too lean, but doesn't act like it according to traditional wisdom? Thank you!
Aug 02, 2003, 05:50 PM
Thread OP
Ok, adjusted and flown. It didn't take long running out across our high school's soccer field before it ran sweet. Good, light smoke, and a somewhat loud but very smooth run. Nice transition, and dead stick to full several times, was perfect. 3 point short field landing, from a medium bank. Next flight I'll dive a steep spiral. Thanks for the advise. I'll stay w/ the vintage motor till my next ARF. Happy trails!

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