MS391 chainsaw - no start

G

Gumby83

Well-Known Member
See pics - these are the only 2 labels I found on the saw.

It will fire on full choke. It will start and run for 1-2 seconds on partial choke, but as soon as I try to give it throttle, it stalls.

It’s only a couple of years old and has never had trouble starting before.

I’m using 50:1 premix.

Spark is good and fuel is getting into the cylinder but not flooding.

I disassembled & cleaned the carb, and made sure the adjustment screws were in the same positions as they were before cleaning.

All the gaskets were in good condition and re-used.

Same problem.

I’m following the manual step for step in starting and it just won’t start.

Thanks in advance. 😁
 

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Most likely the metering diaphragm has stiffen. IF diaphragm can't move to open the metering valve the engine will not run.

Repair Kit K10-WTE.

If you replace the metering valve and lever they will need to be set using the new W tool [500-528].
 
Thanks for the kit & tool numbers. I’ll have them on Tuesday and will update once the repair is done.
 
UPDATE:

Got the new parts installed today. I was surprised by how stiff the old diaphragm felt compared to the new one.

It’s starting now, and I was able to use it for an hour or so, but I can’t get it to idle for more than about 10 seconds.

Do I need to adjust the mix screw or idle first?
 
you can adjust either. If you turn the metering the wrong direction, it will get worse. 10 seconds should be long enough to improve the mixture. Or you can speed it up a little, tune the idle mixture, slow it down and adjust it some more. Basically walk it down.

Running it dry before putting away will help the diaphragm last longer. I've found they last only maybe 2 seasons when ethanol fuel is left in them.
 
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