User Manual

PAGE 13 OF 22
Tuning Your Gas Engine using the Spark Plug Colour
In general, engine manufacturers set their engines to run when they leave the factory. This does not
mean, they are set at best needle settings, it just means, that the needles are adjusted in a way that the
engine will start and run. Obviously, some needle tweeking is required if you want the engine to show
what it is capable of.
Walbro carbs, as well as other carbs without floats, normally have two needles to set. The high speed
needle labeled "H", and the low speed needle labeled "L". The idle needle always is the one closest to
the engine.
It is important to keep in mind that the low needle is always active, and that the high needle is only active
above 1/4 throttle, when there is sufficient low pressure in the venturi for the H needle to start drawing
fuel.
Here is the drill to start from scratch:
idle mixture
Open both needles L=1.5 to 2.0 turns, H=2 turns. This will ensure a rich setting.
Close the choke valve and turn on ignition.
Start until the engine pops, or runs. It will stop again.
Open the choke valve
Start the engine at slightly high idle rpm, and let the engine warm up a bit.
Adjust the idle needle (closest to the engine, and marked "L") for best rpm, and then open it 1/4 to 1/2
turn. This will be a safe rich setting to start tuning, and from here, the high speed needle can be set.
High speed mixture:
With the idle needle set about right, slowly apply full throttle. If the engine stays too rich, close the high
needle a bit so the rich condition is cured, but no more than that.
This is the basic high speed needle setting, at which the engine will run without damage due to lean
mixtures.
Keep this setting while running in the engine.
If the engine has run about five minutes, the plug can be checked. It should have a desert sand tan, or
slightly darker. Not black, because that is an indication of an overly rich mixture, nor pale-white, because
then the engine is too lean. This causes damage!