MS250 help.

[stretch5881]

I'm trying to figure out a friends' bucket saw. It's a MS250. Starts great when it's cold. Shut it off and go to restart it and the rope will break or tear out of the handle. It acts like it is advanced, but only when warm. I have tried 4 different oem coils (2 new and 2 good used), swapped the flywheel, tried 2 other carburetors and changed the vent. Switch off, it pulls over normal. Switch on, it will stop the recoil dead. When it starts there is no smoke and it runs great. Never acts flooded.
Any ideas?

 

[farminkarman]

Sounds like you have ruled out coils & flywheels. Being that you can turn the problem on & off with the ignition switch sounds like it is trying to light off a little early. Do you know if the saw has had this problem ever since it was new? Have you tried a different batch of gas/oil mix?

 

[Squareground3691]

Sounds like an advancement under live fire , sure no keyway issues ?

 

[Wonkydonkey]

Flywheel gap ? I’ve heard a bigger gap retards the ignitiona abit..

just a thought

 

[stretch5881]

Sounds like an advancement under live fire , sure no keyway issues ?

Keyway on the crank is perfect. Keys in both flywheels that I tried look good.

 

[stretch5881]

Flywheel gap ? I’ve heard a bigger gap retards the ignitiona abit..

just a thought

I have tried .008 - .012. No change at all.

 

[stretch5881]

Sounds like you have ruled out coils & flywheels. Being that you can turn the problem on & off with the ignition switch sounds like it is trying to light off a little early. Do you know if the saw has had this problem ever since it was new? Have you tried a different batch of gas/oil mix?

He said it started doing it this year. I have changed fuel.

 

[farminkarman]

He said it started doing it this year. I have changed fuel.

So, being that it hasn't always acted this way means the key seat in the crank probably wasn't cut wrong. Might be worth pulling the cylinder and having a look at the combustion chamber & piston crown.

 

[stretch5881]

So, being that it hasn't always acted this way means the key seat in the crank probably wasn't cut wrong. Might be worth pulling the cylinder and having a look at the combustion chamber & piston crown.

I had looked inside with a scope. There doesn't appear to be anything unusual looking there. No carbon buildup.

 

[farminkarman]

Can you start the saw warm using the technique that @Stump Shot shows in this video?

 

[Pincher]

I found on my 025 and 250's there is some play between the crank and flywheel key when assembling. With the flywheel loose, install a piston stop or starter rope on top of piston to prevent rotation and make sure flywheel is fully clockwise prior to tightening the flywheel nut. That should be fully retarded (timing wise). If it still kicks too hard, file the key slightly and try again.

On of my 250s kicks pretty good but not enough to break the rope or starter.

 

[Stump Shot]

I'm trying to figure out a friends' bucket saw. It's a MS250. Starts great when it's cold. Shut it off and go to restart it and the rope will break or tear out of the handle. It acts like it is advanced, but only when warm. I have tried 4 different oem coils (2 new and 2 good used), swapped the flywheel, tried 2 other carburetors and changed the vent. Switch off, it pulls over normal. Switch on, it will stop the recoil dead. When it starts there is no smoke and it runs great. Never acts flooded.
Any ideas?

Sounds to me like the problem might actually in the muffler. As the motor is being pulled over on the down stroke exposing the exhaust port where a hot ember of carbon resides, igniting the fuel charge and giving it a boost in downward direction.
This is why it only does so hot. If you clean the exhaust port and muffler can, should alleviate the issue I believe. If this comes to fruition then tuning, etc. should be performed so as to eliminate from further issue. If Stihl Ultra is being used it also should be eliminated as well, as nothing carbons up like that junk.

Can you start the saw warm using the technique that @Stump Shot shows in this video?

I learned it from @tree monkey

 

[stretch5881]

He did say that he uses the newer stihl oil. I'll pull the muffler tomorrow.

 

[stretch5881]

No go. Complete teardown and clean. Got every trace of carbon cleaned out. Muffler is clean also. No change in starting.

 

[farminkarman]

No go. Complete teardown and clean. Got every trace of carbon cleaned out. Muffler is clean also. No change in starting.

Well, unfortunately it's not fixed, but you ruled out a few possibilities.

 

[Agent Smith]

Might sound dumb but does it have a standard or resistor spark plug in it? If it's got a standard, try a ngk bpmr7a resistor plug. If a resistor plug is already in then I got nothing. I only suggest because some say resistor plugs retard/ delay timing a bit and that saw calls for a resistor plug. Sometimes anything is worth a try.

 

[stretch5881]

The only thing I can think of, is maybe the crank somehow got a slight twist??? Is that unheard of?

 

[stretch5881]

Might sound dumb but does it have a standard or resistor spark plug in it? If it's got a standard, try a ngk bpmr7a resistor plug. If a resistor plug is already in then I got nothing. I only suggest because some say resistor plugs retard/ delay timing a bit and that saw calls for a resistor plug. Sometimes anything is worth a try.

Nothing sounds dumb anymore. I replaced the Bosch with an NGK resistor. Same thing.

 

[Stump Shot]

The only thing I can think of, is maybe the crank somehow got a slight twist??? Is that unheard of?

Then why start okay cold? Which is what keeps this thing going around in dead end circles.

Normally the crank is only pressed in at the connecting rod, I'm not sure how this crank is made if the shafts were indeed pressed in it would totally be a possibility.

Could you get a timing light on it? You would have to degree wheel it to TDC and mark it on the flywheel and case for a reference when using the light, then mark how far off you are on the case and see how many degrees that is with the wheel.

 

[Pincher]

Did you try retarding the flywheel? Is the plastic chassis holding the engine in place?