MS 440 cold start carb issues - fixed

[Philip Wheelock]

Earlier I posted for help because my newly-acquired 6-yr. old MS440 required 10-11 pulls cold before showing signs of starting. Once warm, it was fine. Finally had the carb rebuilt; turned out the metering diaphragm was stiff, probably from fuel/ethanol damage over the years. Not sure of the mechanics why this would result in cold start issues, but the problem has now been eliminated. Took awhile to replace broken parts and solve idling and starting issues, but the saw runs fine now. Ready for work.

 

[ErikTheRed]

Thanx for the "heads up" and the update.

 

[pioneerguy600]

Earlier I posted for help because my newly-acquired 6-yr. old MS440 required 10-11 pulls cold before showing signs of starting. Once warm, it was fine. Finally had the carb rebuilt; turned out the metering diagram was stiff. Not sure of the mechanics why this would result in cold start issues, but the problem has now been eliminated. Took awhile to replace broken parts and solve idling and starting issues, but the saw runs fine now. Ready for work.

The first couple of pulls creates enough vacuum to pull the metering diaphragm down on the metering lever thus lifting the metering needle off its seat allowing fuel to flow into the carb. If the metering diaphragm is stiff it takes more pulls than usual to pull that stiff diaphragm down.
Really the diaphragm is not pulled down, its pushed down by atmospheric pressure acting on the top of it, when the vacuum is created by the piston moving up and down in the cylinder the diaphragm is forced down upon the metering lever.

 

[Philip Wheelock]

The first couple of pulls creates enough vacuum to pull the metering diaphragm down on the metering lever thus lifting the metering needle off its seat allowing fuel to flow into the carb. If the metering diaphragm is stiff it takes more pulls than usual to pull that stiff diaphragm down.
Really the diaphragm is not pulled down, its pushed down by atmospheric pressure acting on the top of it, when the vacuum is created by the piston moving up and down in the cylinder the diaphragm is forced down upon the metering lever.

Cool. Thanks for the info.

 

[mountainlake]

Makes sence, cold daiphram is stiff, when warm it worked good. Steve

 

[XSKIER]

Just went through the same thing with my 08. Upon carburetor disassembly, there was fuel in the pump chamber but not in the metering chamber. The metering diaphragm was stiff as a board. Not too surprising for 50 years of age.

 

[pioneerguy600]

Makes sence, cold daiphram is stiff, when warm it worked good. Steve

Also Steve, when the carb is warm, has been running , it will already be full of fuel and that makes it much easier to start.