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Ms 250 erratic idle issue


Statichum
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Hi guys, I have a still 046 that goes well, I know how to tune it, never an issue.

but this MS250 that I bought second hand on impulse just wont idle...

 

it starts easily, hasn’t been tested but feels like it has good compression.

whats happening is that when it idles I can’t tune it to run at a low enough idle that doesn’t drive the chain.

 

I can get it to idle stable-ish (mostly) without it driving the chain (still with the issue below though) by using a significant amount of the idle screw (not the low jet) wound in. which is not right but it’s the only way I’ve found to make it idle sort-of ok.

 

So the weird thing that it’s doing which I’ve never come across before is that when the saw is moved around it revs and drives the chain fairly fast (not cutting fast but unsafe) no touch if the throttle at all, simply move the saw around, especially tipping it side to side and it will rev. it doesn’t matter how I adjust the saw, it does this. 
If I adjust the low jet to about where i think it should be (saw starts, idles a bit but I have to keep a finger on the trigger to throttle every now and then to keep it running), I can actually keep it running by moving around, it revs fairly high though. Stop and it will stall out after 10 seconds or so.

 

if I start the saw, have it sitting on the ground, as above it’s a fine line but I can get it to run and idle ok-ish without it driving the chain as below but as much as simply tipping the saw to one side can make it rev like crazy (for what should be idling anyway). 

 

I’ve fitted a new carb rebuild kit and cleaned it out - made no perceivable difference.

 

has a new spark plug (didn’t suspect it was part of the issue but had one hanging around)

 

Fuel filter and fuel line are new.

I haven’t replaced the impulse line but it looks good to my eye. Certainly no signs of cracking or perishing.

 

When running it revs up high nicely and cuts well, it just won’t idle as it should.

 

What could be causing it to rev when moving it around?

 

 

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If moving a saw around makes the saw rev then it is most likely a split inlet manifold where the split is opening up when the saw is moved letting in more air.

The carb is most likely set where you have shut off its low speed fuelling and are using the high speed circuit and the air leak to somehow compensate.....badly.

Simple test is to get the saw idling as stable as you can then move the engine around whilst holding the fuel tank flat to the ground. If the revs significantly increase, expect a split manifold or possibly an impulse line.

The saw is a bit of a mare to work on so good luck with that.

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That was my thinking too Spud. 

 

Another handy tip (for those that didn't know) is to have a can of maintenance spray at hand, and spray down on those parts while running on idle, you'll hear it change or even stop running.  It's a quick and dirty way to find air leaks.

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15 hours ago, spudulike said:

If moving a saw around makes the saw rev then it is most likely a split inlet manifold where the split is opening up when the saw is moved letting in more air.

The carb is most likely set where you have shut off its low speed fuelling and are using the high speed circuit and the air leak to somehow compensate.....badly.

Simple test is to get the saw idling as stable as you can then move the engine around whilst holding the fuel tank flat to the ground. If the revs significantly increase, expect a split manifold or possibly an impulse line.

The saw is a bit of a mare to work on so good luck with that.

Definitely a pain to work on, you're not wrong there! 

Originally I held both handles and gave the anti-vibe a good wiggle around while holding the saw stationary on the ground and it didn't make a difference so ruled out the rubber manifold but after this suggestion I tried again, this time with the cover off, holding the saw stable and giving the carb a good wrench around with my hand, I could induce the high idle sooo it seems that's the cause. Because it's such PITA to get to, i've ordered a replacement (as well as an impulse line just incase although I inspected that and it looked good). Will pull it apart again once that arrives.. hopefully that'll be problem solved!

 

Thanks for the suggestions all, hopefully it'll be sorted shortly!

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On ‎29‎/‎09‎/‎2019 at 19:00, spudulike said:

Trouble is that I have done that on well running machines and it still stops them as the spray lowers the temperature in the manifold and affects the vapour causing the engine to stall. Not been a good test for me.

Yes I guess it would if you go a bit mad with the spray, it only needs a sniff.  Does work though, on some things, like crank seals, base gasket leaks etc, and on the manifold if used sparingly.  Alternately use something like duck oil, or oil from a squirty oil can.

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