Fuel to air ratio

[D._Frederick]

With the new saws required to have fixed jet carburators, how can you determine if the engine is not running too lean?

[Kevin]

I use a tach.

[D._Frederick]

Kevin,
Since there is nothing to adjust, I don' t understand how the rpm would help you. My understanding on adjusting for the correct mixture is that you adjust for maximum rpm then increase the mixture until the rpm decreases a certain %. Just reading the rpm does not tell how much richer the mixture is. Am I missing something?

[Kevin]

Are you sure that they aren't just caps placed over the screws?
The older manuals describe how to adjust the screws to change the air/fuel ratio, I'm not sure about the last year or so.
What does your manual say?

[TJACK]

My Husky 359 2002 and 372XP 2001 is adjustable

TJACK

[jokers]

The factory max rpm as checked with the tach is determined by the engineer who designs the engine. Optimally the port and ignition timing can be factored with the displacement and target rpm of a given engine and a fuel quantity(fuel/air ratio) is derived. Don`t ask me to do the math for you because it`s likely that I can`t, .

The bottom line is that the tach setting is a generic "one size fits all" way for a potentially unskilled person to adjust the carb for minimal emissions while getting reasonable life and performance from the saw. From my experience the factory spec is usually at least a few hundred rpm above where the engine makes maximum power but to lower the rpm you must enrich the mixture which also increases emissions. Up to 30% of the fuel mixture leaves the engine unburned in a typical two stroke.

Russ