Stihl MS 440 top end rebuild

[wildlandfire]

Hey everyone,

                     New to the forum, and looking for some insight!  Thanks in advance for any help.  I just started in on a MS 440 top end rebuild on one of our fire saws.  I replaced the piston rings, and didn't note any damage to the piston itself or the head, so I put that all back together.  Once I began the tuning process to get the saw back into commission, I was unable to get the old carb to work correctly due to years of overzealous tuning; ruining the screws.  So I put a brand spanking new one in, as well as a fuel filter, cleaned the air filter and changed the spark plug.  So with the new carb in place, we adjusted the high end screw completely counterclockwise then 3/4 turn back, and the low end completely counterclockwise then 1/4 turn back in order to get it to base line settings.  Including this setting, and all others possible, we were only able to get the high end rpm's to 10,500, using a stihl brand tachometer.  Additional symptoms include: the motor bogging down slightly before revving to high end upon full throttle input, engine stall after seemingly coming into a correct idle rpm range (1800-2400), and engine stall upon tilting to one side or the other when angle reached 90 degrees or more.  My next thought is to purchase a whole top end kit...hopefully someone out there has experienced this before...thanks again for the help...

[dougand3]

Sounds air leaky.
Impulse line in good shape?
Intake boot and clamp in good shape?
Crank seals in good shape?
What is compression after new ring?

[blueberrymuzik]

Glad to hear that you didn't find any damage to the piston and cylinder when you got into the top end.

With this being a fire saw, most likely it has a lot of hours on it. I would highly recommend that you change the seals, intake manifold, and impulse line since you have got it out of action and on the bench. At a minimum, do a vac/pressure check on it. Usually a stall when turned 90 degrees indicates a seal problem and possibly a worn crank bearing.

Really sorry to hear that you weren't able to get the original carb to adjust. Usually the best place to start on adjusting a carb from scratch is to initially set it to Stihl's standard settings. That calls for turning each screw clockwise until it just seats. Then turn each one counterclockwise one full turn. You can then adjust the low speed screw for best idle/good throttle response. As per the service manual, the high speed screw can then be set with your tachometer. As service manual states, the high speed screw should never be set at less than the standard setting (1 turn out).

Was the carb that you installed an OEM carb or an aftermarket?  I have had problems with most aftermarket carbs not wanting to adjust properly. They will usually start and run, just don't perform as good.

If your cylinder and piston are not scored, a whole top end kit would be unnecessary.

[ZeroJunk]

Not sure about the 1/4 turn out on idle. Maybe I misunderstood. Try tuning the low for max RPM at idle and then open it up another 1/8 to 1/4 turn. May need to adjust the idle speed screw if it wants to cut off. But, the idle needs to be a tad rich to offset the initial lean condition when you throttle it up before the main jet gets fuel moving from the far side of the butterfly.
As far as the 10,500 that may be because it is too lean as well as was previously said.
And, as the first reply indicated, make sure it doesn't have an air leak although that would typically make it over rev not under.

[mike_belben]

Read the tuning section on madsens saw site and try again.   You want the L screw set for highest idle speed and the H screw set to where it 4strokes without load and 2 strokes when you dig in.  There really are no "base settings" on a 2 stroke thats used in every climate. User is responsible for good tune all year long.

[joe_indi]

Wrong thread