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MS250 Doesn't Like to Rev if Cold


adamelder
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After a long while I've finally replaced all the gaskets in the carb, cleaned it out, replaced fuel line.

 

The problem still persists. Saw won't rev. It'll start fine and tick over (just) but as soon as I touch the trigger it dies.

 

Anyone got any thoughts what to try next?

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After a long while I've finally replaced all the gaskets in the carb, cleaned it out, replaced fuel line.

 

The problem still persists. Saw won't rev. It'll start fine and tick over (just) but as soon as I touch the trigger it dies.

 

Anyone got any thoughts what to try next?

 

Have you touched the low and high speed mixture screws at all?

 

Take a note of screw positions before you start.

 

I'd turn the low speed screw out a 1/4 turn anti-clockwise (= richer) and see what that does. You might get away with less. It might also need the engine speed screw winding in slightly (to increase speed) at the same time just to keep the speed the same.

 

When its up to temperature do the low speed carb adjustment properly. And high speed if you know what you are doing.

 

bmp01

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By "cleaning it out"...you have removed the H & L screws, got some carb cleaner down them and then wound them all the way in and back out 1 - 1 1/4 turns?

 

I didn't remove the H&L screws, no. Every other screw, needle though. Sprayed the whole lot with carb cleaner and let sit, twice.

 

Looks like I'll be removing the H&L screws then!

 

Cheers Spud!

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Have you touched the low and high speed mixture screws at all?

 

Take a note of screw positions before you start.

 

I'd turn the low speed screw out a 1/4 turn anti-clockwise (= richer) and see what that does. You might get away with less. It might also need the engine speed screw winding in slightly (to increase speed) at the same time just to keep the speed the same.

 

When its up to temperature do the low speed carb adjustment properly. And high speed if you know what you are doing.

 

bmp01

 

No I didn't touch the screws at all. The saw had been running fine and then started not wanting to rev from cold. Then that problem got worse, difficult to start past an initial cough. That's when I asked for thoughts at the start of the post and took the advice to change fuel line and service the carb. But as Spud noted, I didn't unscrew the H&L screws and clean inside these, so that's next.

 

The gaskets and membranes inside the carb looked fine though. However the gasket between the carb and the saw body looked rough so I changed that.

 

I'll see what happens with a more thorough clean of the carb.

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It sounds like an issue with the high speed carb circuit of just lack of fuel delivery. There is enough fuel to idle but not enough to allow the saw to rev up.

 

I would think cleaning the holes the H&L screws sit in may well do it as may just turning the H screw 1/4 turn out.

 

Have you checked the gauze strainer is clean? You need to do it by taking it out and inspecting it with an eyeglass as they can get a bit scummed up and easy to miss!

 

Other than that - make sure the fuel line and fuel filter are in good shape.

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No I didn't touch the screws at all. The saw had been running fine and then started not wanting to rev from cold. Then that problem got worse, difficult to start past an initial cough. That's when I asked for thoughts at the start of the post and took the advice to change fuel line and service the carb. But as Spud noted, I didn't unscrew the H&L screws and clean inside these, so that's next.

 

The gaskets and membranes inside the carb looked fine though. However the gasket between the carb and the saw body looked rough so I changed that.

 

I'll see what happens with a more thorough clean of the carb.

 

Spuds the man. I hadnt even considered you might have left the screws in place !

 

Worth gently winding the screws in first and noting number of turns it takes, sometimes gives a clue when you know where you are relative to the factory start point, (the 1 - 1 1/4 turns out).

Just winding screws in and back out is sometimes enough to reset the clearances but if you have the time do it right.

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It sounds like an issue with the high speed carb circuit of just lack of fuel delivery. There is enough fuel to idle but not enough to allow the saw to rev up.

 

I would think cleaning the holes the H&L screws sit in may well do it as may just turning the H screw 1/4 turn out.

 

Have you checked the gauze strainer is clean? You need to do it by taking it out and inspecting it with an eyeglass as they can get a bit scummed up and easy to miss!

 

Other than that - make sure the fuel line and fuel filter are in good shape.

 

When I opened the carb up there wasn't a fuel screen in it so I can only assume someone has been there before me. I put a new one in.

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When I opened the carb up there wasn't a fuel screen in it so I can only assume someone has been there before me. I put a new one in.

 

Thats never a good sign - fuel screen missing.

When you spray through the low speed adjuster hole you should see the spray coming out of the tiny jet holes in the carb port - 2 or 3 jets up stream of the throttle plate. Make sure each hole is flowing.

Spraying through the high speed adjuster hole, spray will come out of the main jet (brass cylinder protruding into the carb post).

 

Some folk like to use compressed air to blow through the passageways but Id be cautious of that. Sometimes the main jet has a rubber disc in it which needs to stay put, fuel screens can be dislodged, even the welch plugs ....

 

HTH.

 

bmp01

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