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Vacuum leak


MATTMOSS
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Im trying to sort out a 2004 ms 260.

At first wouldnt run or or idle at all so i stripped, cleaned and rebuilt carb.

Changed fuel filter, tested pipe and tank vent.

Now idles ok but wont rev high it just coughs and splutters and burbles from mid to high revs almost like an electronic limiter noise.

Pressuming it isnt the carb i have done a pressure test which is fine but when i did a vac test i have found its got a quite fast vacuum leak.

Its easy to spot a leak under pressure but a bit more difficult to pin pont a vac leak im thinking possibly a crank seal but which one? or maybe even the decomp button but how would i know for sure?

Any suggestions from you lot while i think about it. :001_smile:

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I think you can spray a bit of soapy water onto the crankshaft to see if it's blowing, assuming you can get the damn thing running. Mine was a right sod today (MS181) and don't know whether it's an air leak or blocked jet in the carb. I'll see if I can dismantle it a bit tomorrow as got a lot of logs to cut up!

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I think you can spray a bit of soapy water onto the crankshaft to see if it's blowing, assuming you can get the damn thing running. Mine was a right sod today (MS181) and don't know whether it's an air leak or blocked jet in the carb. I'll see if I can dismantle it a bit tomorrow as got a lot of logs to cut up!

 

Yes doing that would certainly find your pressure leak but not your vacuum leak.

Because its and old saw im going to replace the impulse line and the inlet boot anyway so ill do that and vac test again, so that hopefully eliminates them.

Then i can find a way to block the de comp button to iliminate that, then im just down to the seals.

Not the end of the world if replace it all, it keeps me out of trouble and then the saw should be good.

Still like to know how to find the place the vacuum leak is tho....if it can be found accurately.

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If it sounds like an intermittent rev limiter then check the leads that run around the top AV mount just in front of the air box, they often chafe!

 

An air leak will tend to make the saw rev to high, hold on to descending revs and cause uneven or fast idle. A BIG airleak will cause a real boggy saw and will possibly give the symptoms you have.

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