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Old Jan 01, 2008, 11:23 AM
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London, UK
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How to make sure your new engine is not running too lean

We have all been told about the dangers/damage of running a new engine too lean.

I have a new K&B 20 engine which is running well at the moment after the break in. After opening the needle valve 3 turns, at full throttle I closed the needle valve and observed the RPMs increase and then I richened it out again a half a turn.

The test I am using (as suggested in the engine tuning sticky) to make sure it is not running too lean is as follows:

At full throttle I pinch the fuel line to the carb and observe RPMs to increase (no need for a tachometer the loud wail was obvious! ). I hope this is sufficient to ensure I am not running it too lean.

Are there any other simple tests/checks you are aware of?
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Old Jan 01, 2008, 02:20 PM
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Wayne V's Avatar
London, Ontario, Canada
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Mostly you can do it by sound, but using a tach and finding peak rpm and then richen it up so you lose 200rpm is common practise.
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Old Jan 01, 2008, 03:17 PM
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Jacksonville Fla.
Joined Mar 2007
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I use the pinch test for low end adjustments only..high end is throttle wide open and slightly rich...lean out the engine, you'll hear the rpm increase and then start to decrease..stop then richen 1/4 to half turn just like you have done...watch for a smoke trail while flying...
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Old Jan 01, 2008, 06:56 PM
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Adelaide, South Australia
Joined Sep 2003
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I know you're still running in your engine but this is what I do to check the mixture at full throttle. If it's running in a full 2 stroke scream I'll immediately open the needle (richen) and listen to the revs. Obviously if it picks up in revs then it would have been way too lean but usually it'll start to slow down a little. I'll continue richening until it starts to 4 stroke and that's my basis for setting the mixture. Usually most engines with a normal size flying prop will begin to 4 stroke around 2000 revs below peak lean. My flying setting is where it's just barely into a clean 2 stroke which is then about 1000 below peak. I'd much rather be a little too rich than a little too lean in the air . Another way to do it is to go to peak revs then richen until there's a quite obvious drop in revs but I never like to peak out an engine even for a short time.
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