The basic setting for the MS 290 (Walbro HD) carburetor is H=1 and L=1. You may have a version with or without limiter caps, I don`t know. But the basics are the same. The 1 and 1 is the setting without caps. A good tachometer is required to get the right tune.
Make sure to have a proper spark plug and clean air and fuel filters and check the spark arrester that it`s clean. Start engine and warm it up with some cuts. Also make sure to have a properly tensioned chain.
Idle speed: 2800
High speed: 12.500 (is what you should have at the end.)
With the LA screw you turn the speed to 3300 rpm. Now search by turning the L screw (leaner or richer) the highest possible idle speed. If the drop of point is higher as 3700 rpm go back with the LA to 3300 rpm and repeat this step. If you have found the drop off point (highest idle speed) go back with the LA screw to 3300 rpm, after that richen the idle mixture with the L screw back to 2800 rpm and you`re done.
Now the full throttle speed with no load. Give a few seconds full throttle and see where you are (with tach). Turn the H screw (richer or leaner) to get the required 12.500 rpm. I would go 200 or 300 rpm below the 12.500 as safety buffer. (temperature changes, altitude etc.) Done.
If you know from your machine what carburetor you have and what the basic settings are and what the required speed (idle and high) is, you can use this tuning method on nearly every model with diaphragm carburetor.
