MS251 idle stall and hard restart when hot

[Dra2650]

Saw is about 5 yrs old. Has cut a dozen cords in that time. This year the saw started bogging in big wood, requiring a very ligh touch and making cuts last a lot longer. Until recently always easy to start cold and usually 1 pull restarts when hot. I replaced the fuel and air filters, and spark plug (all oem) for the first time ever.
1st use... started very easily, idled perfectly for several mins to warm up, and cut big wood with no big.
When fully hot, it would start to stall at idle, and would take a 6 or more pulls to restart... sometimes I'd have to apply full choke for a pull before starting back up after a a few pulls. I have not try a cold start yet. No tinkering with the carb ever.
Any ideas as to what is going on?

Edit: No ethanol ever. Same gallon mix for both examples above.

Thanks, David

 

[WoodmanSr]

Saw is about 5 yrs old. Has cut a dozen cords in that time. This year the saw started bogging in big wood, requiring a very ligh touch and making cuts last a lot longer. Until recently always easy to start cold and usually 1 pull restarts when hot. I replaced the fuel and air filters, and spark plug (all oem) for the first time ever.
1st use... started very easily, idled perfectly for several mins to warm up, and cut big wood with no big.
When fully hot, it would start to stall at idle, and would take a 6 or more pulls to restart... sometimes I'd have to apply full choke for a pull before starting back up after a a few pulls. I have not try a cold start yet. No tinkering with the carb ever.
Any ideas as to what is going on?

Edit: No ethanol ever. Same gallon mix for both examples above.

Thanks, David

I'd speculate you have 2 different issues. No experience on Stihl ms251 but symptoms are telling.

The stalling when hot idle is possibly a weak ignition coil, or possibly wide air gap. I've experienced those symptoms on Husqvarna/Electrolux coils. Stalling hot & then wanting to be a wee-bit flooded (choke) to restart. I'm betting Ignition Coil. If so will get progressively harder to start cold as well.

Bogging it cut is Lean. Get that fixed ASAP. Possibly may need carb diaphragm kit (even running non-Ethanol). First try opening High screw 1/16-1/8 (CCW) turn at a time. If no improvement after 1/8 turn than might need carb kit. Again, I'm unfamiliar with ms251, assume it's an adjustable carb.

If High screw already full open CCW against OE Limiter Cap stop then prehaps speaks to an air leak performance change. Crankseals, Intake Boot, or Carb gasket. Cheap evaluation to install Carb gasket (if ms251 uses). I fixed an ms210 that very minor Carb Gskt lean conditions. Could richened High to improve but not correct until new Stihl OE carb gasket. Like $2 fix sourced from John Deere shop.


Good Luck.

 

[Dra2650]

I'd speculate you have 2 different issues. No experience on Stihl ms251 but symptoms are telling.

The stalling when hot idle is possibly a weak ignition coil, or possibly wide air gap. I've experienced those symptoms on Husqvarna/Electrolux coils. Stalling hot & then wanting to be a wee-bit flooded (choke) to restart. I'm betting Ignition Coil. If so will get progressively harder to start cold as well.

Bogging it cut is Lean. Get that fixed ASAP. Possibly may need carb diaphragm kit (even running non-Ethanol). First try opening High screw 1/16-1/8 (CCW) turn at a time. If no improvement after 1/8 turn than might need carb kit. Again, I'm unfamiliar with ms251, assume it's an adjustable carb.

If High screw already full open CCW against OE Limiter Cap stop then prehaps speaks to an air leak performance change. Crankseals, Intake Boot, or Carb gasket. Cheap evaluation to install Carb gasket (if ms251 uses). I fixed an ms210 that very minor Carb Gskt lean conditions. Could richened High to improve but not correct until new Stihl OE carb gasket. Like $2 fix sourced from John Deere shop.


Good Luck.

Yes. Carb is adjustable. Again, the bogging problem was fixed after fuel and air filters + sp plug were replaced. I assume a clogged fuel filter. But, the stall at idle after full temp achieved also resulted. It is just odd that the symptoms inverted after the tune-up. I just went out and the saw started super easy and idled fine. Didn't do any cutting to get it hot. May try that tomorrow.
Could the low idle screws need opening?
Would a leaking carb gasket also affect the idle as well? You are referring to the gasket between the carb and block, right? And not to a gasket internal to the carb?

Thanks for help.

 

[WoodmanSr]

Yes. Carb is adjustable. Again, the bogging problem was fixed after fuel and air filters + sp plug were replaced. I assume a clogged fuel filter. But, the stall at idle after full temp achieved also resulted. It is just odd that the symptoms inverted after the tune-up. I just went out and the saw started super easy and idled fine. Didn't do any cutting to get it hot. May try that tomorrow.
Could the low idle screws need opening?
Would a leaking carb gasket also affect the idle as well? You are referring to the gasket between the carb and block, right? And not to a gasket internal to the carb?

Thanks for help.

Yes, gasket between Carb flange & bulkhead. Assuming one present on ms251 as similar to ms210/250.

Edit... Oops kicked myself out on Reply.

Yes, a carb gasket air leak could affect Idle but cause lean condition. Increased idle speed or even crawling chain when brake disengaged.

You described stalling that not time bound or additional symptoms + choke restart (vs fast idle restart). So my suspected weak ignition coil, a guess.

Is possibly that Low speed is too rich. If loads up on idle, speed slows, struggles to say running, then stalls as "crankcase loaded up on excess fuel". Becomes harder to hot restart after rich stall as is a partially flooded condition. Requires restart on Fast Idle of master control switch, or holding trigger down when restarting. Allowing saw to breath more fresh air, say 2 to 4 pulls. If this then close (CW) the Low screw slightly until idles smooth. Set carb screws after saw completely warmed up.

Maybe read here on AS about carb tuning method. Been covered by others.

Also. Fyi, helpful discussion on Zama carb accelerator pump (oring leak, throttle response lean hesitation)

https://www.arboristsite.com/threads/stihl-ms250-zama-carb-info.345449/

https://www.arboristsite.com/threads/accelerator-pump-o-rings-carburetor-lookup.374153/

Common sizes PN.
• 195503010 = 3.0 x 1.0
• 195503510 = 3.5 x 1.0
• 195702308 = 2.3 x 0.8

195503010 (superceded to Z000-955-Z014-A, prehaps typical usage for Zama C1Q carbs... 3.0 x 1.0 mm)

 

[Dra2650]

Yes, gasket between Carb flange & bulkhead. Assuming one present o

Thanks. I'll have a look.

 

[Dra2650]

Thanks. I'll have a look.

Thanks for the heads up about your edited reply.