PRME Engines User Community

[preivers]

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. See the Walbro manuals
http://www.prme.nl/forum/images/WalbroHDA.pdf
http://www.mvvs.nl/Manuals/WBmanual0.pdf
http://www.mvvs.nl/Manuals/WBManual1.pdf
http://www.mvvs.nl/Manuals/WBManual2.pdf
http://www.mvvs.nl/Manuals/WTmanual.pdf
http://www.mvvs.nl/Manuals/Walbro-maintenance.pdf

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.
Very good, and almost a must reading (many pictures) on understanding the Walbro is this:
http://www.helifreak.com/showthread.php?t=326599

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!
Read this article on plug reading (Gordon Jennings)
http://www.strappe.com/plugs.html

Here are some plugs from my engine. The measured upper Cylinder wall temperatures are noted on the plug in °C

This plug looks about right. On disassembly, notice the black mixture ring in the isolator shoulder.


Plug 1 is slightly lean, as indicated by the head temperature.
plug 2 is slightly richer, the black ring still is not as high as 1/4 isolator length
plug 3 is too rich. The black has crept up to 1/3rd isolator length.
plug 4 is too rich, but heat range is OK (champion DJ7Y)
Plug 5 is too lean (no black ring) It also is too cold (champion DJ6J black nose)
plug 6 is right mixture as shown by mixture ring, but too cold.

The head temperatures are quite high, because measurements were made on a stationary test stand. In a plane, with baffles installed, temperatures drop 20 - 40 degrees.

Also check out
]http://www.dansmc.com/Spark_Plugs/Spark_Plugs_catalog.html
Very clearly documented information from TSR:
http://tsrsoftware.com/read-plugs.htm
http://www.helifreak.com/showthread.php?t=326599[/url]
_________________
PRME - Pe Reivers Model Engines
Dealer for MVVS, Mejzlik, MTW
http://www.mvvs.nl
http://www.prme.nl
http://www.mvvs-nl.com

[preivers]

The next step is to get the midrange just right. This can only be done after the engine is run in. If done before that, settings will change, and the process has to be repeated.

So far, you have been running the engine with a rich idle, and quite rich main needle settings. This causes four-stroking in the mid-range.
Let the engine warm up, and apply full throttle, until the rpm has stabilized.
Now slowly reduce throttle, until the engine starts to four stroke. Cure that condition by leaning the idle needle.
Go back to full throttle, and adjust for max rpm, then open the needle 1/16 turn extra, and throttle back again. This time, the four stroking transition will be at a lower throttle setting.
Repeat the above steps, until the engine runs well at all, but slightly high idle.
check for crisp throttle response. If the engine lags, then open the idle needle until the condition is cured.
Let the engine idle for a prolonged time, and apply throttle. All should be well now, and throttle should be accepted quickly and clean, with maybe a very slight initial fourstroking to clean the lungs.

It is a fact of life, that two stroke engines do not fire every stroke when the rpm approach idle speeds. That means, that below half throttle, the engine may break into an uneven pace, so do not worry too much if the above mentioned method does not provide clean running all the way down to idle.
If after these adjustments the engine becomes harder to start, the idle needle is too lean, and if easy starting is needed, the needle should be opened up again and the high needle readjusted (leaned). Clean midrange will suffer though. Sometime you can, but most of the time you can't have the cake and eat it.

edit:
to recap:
Because at full throttle both needles contribute to the mixture, you can run the engine with lean idle and richer main needle for good midrange, but starting will suffer.
You can also run the engine with rich idle, and leaned out main needle, but midrange will suffer and burble.
A larger prop will run cleaner at midrange.
Too large a prop will make a good tuning very hard to do, and it will be almost impossible to get full bore just right.

On most carbs, a very quick tune goes like this:

at about 1/4 throttle (1/2 peak rpm) tune the engine for best rpm using the idle needle only. Then tune for max full bore rpm using the H needle only.
set idle with the idle stop, and check throttle response. Adjust the idle L needle as needed, but NOT LEANER. After every idle needle change go back to the H needle and adjust for best rpm, just at the verge of loosing rpm rich.
_________________
PRME - Pe Reivers Model Engines
Dealer for MVVS, Mejzlik, MTW
http://www.mvvs.nl
http://www.prme.nl
http://www.mvvs-nl.com