MS240 carb air leak?

[Olddevonstihls]

11 hours ago, spudulike said:

I think the H screw at 3/4 is probably a bit lean, 1 to 1 & 1/4 is probably more like it but can't be 100% sure without adjusting it myself by ear or with a tach. The L screw is probably pretty much OK.

You normally don't do a leak down test by sealing the back of the carb for the reasons you have found...you get spurious leaks from the carb. I usually use a soft rubber blanking piece using the carb to clamp it to the manifold and the carb to seal the impulse as you have. 

Thank you.  Even quite big differences (and settings in between) on both H and L) don’t seem to overcome the fundamental problem. Carb stripped down again and inlet gauze/screen replaced (again) as there was some residue on it. Another new set of diaphragms and gaskets on the carb.  New fuel line still ok, and tank/vent still passes vac and pressure test.  But the half choke start remains and symptoms still suggesting air leak but where is it?

[Muddy42]

6 hours ago, Olddevonstihls said:

Thank you.  Even quite big differences (and settings in between) on both H and L) don’t seem to overcome the fundamental problem. Carb stripped down again and inlet gauze/screen replaced (again) as there was some residue on it. Another new set of diaphragms and gaskets on the carb.  New fuel line still ok, and tank/vent still passes vac and pressure test.  But the half choke start remains and symptoms still suggesting air leak but where is it?

 

Have you tried a pressure test with the saw under water in a large sink? Best to apply pressure through the impulse line.  When the pressure is there, rotate the crank, wiggle the impulse line and poke the intake book.  In my experience, water is the best way to find stubborn crank case leaks. Then dry the saw off and try the same for vacuum (best to not do this under water).

 

As described if both pass, the carburetor isn't providing enough fuel so clean the carb fuel gauze, clean below and check the adjustment of the needle, spray cleaner down the H and L pipes.  Soaking is better for the carb than compressed air, which can damage the carb.  

 

 

 

[Olddevonstihls]

I’m pretty sure the crank seals are ok.

 

i’ve now got it running reasonably smoothly but it needs the half choke setting to start.  When I look back at the original ms240 manuals, what I’m describing as half choke is what stihl describe as the warm start position, and the manual says to use this position for a warm start and, from cold, to use the full choke position to get the blip, and then move to the warm start position.  I’m pretty sure that until I started getting problems I could start the saw by going from full choke to run, and to warm start directly from the run position without using the warm start/half choke position in either case.  I’ve read recently that ms240s/024s are difficult to start.  Is what I am describing normal?  If so perhaps I’m worrying unnecessarily.

[spudulike]

Your saw has settings for "full choke" where the choke valve is completely closed and the throttle is partially opened," Fast idle" where the choke valve is slightly closed and the throttle is held slightly open and "Run" where both the choke valve and the throttle opener are not used.

Most saws bar the Stihl Mtronic ones usually warm start on the "Fast idle" setting which is normal.

[Olddevonstihls]

On 14/02/2024 at 19:44, spudulike said:

Your saw has settings for "full choke" where the choke valve is completely closed and the throttle is partially opened," Fast idle" where the choke valve is slightly closed and the throttle is held slightly open and "Run" where both the choke valve and the throttle opener are not used.

Most saws bar the Stihl Mtronic ones usually warm start on the "Fast idle" setting which is normal.

That confirms the manual and also what I’m experiencing now (though not for the many years of using the saw before the problems began).  Seems to be running hot though - smoke coming off the cylinder when running for a few minutes.

[Welshfred]

I haven't read the old thread so this may have been covered but have you checked the manifold? I have a hot running 260 with a manifold split. Waiting on the part but hoping it's the root of the problem. 

[spudulike]

The guy pressure checked the engine with the manifold in place so it should be good.

[Botty Cough]

I think I would just shelf it and buy a new saw 

[Vedhoggar]

On 14/02/2024 at 17:22, Olddevonstihls said:

I’m pretty sure the crank seals are ok.

 

i’ve now got it running reasonably smoothly but it needs the half choke setting to start.  When I look back at the original ms240 manuals, what I’m describing as half choke is what stihl describe as the warm start position, and the manual says to use this position for a warm start and, from cold, to use the full choke position to get the blip, and then move to the warm start position.  I’m pretty sure that until I started getting problems I could start the saw by going from full choke to run, and to warm start directly from the run position without using the warm start/half choke position in either case.  I’ve read recently that ms240s/024s are difficult to start.  Is what I am describing normal?  If so perhaps I’m worrying unnecessarily.

On comment you've read MS 240s are difficult to start that is not my experience, quite the opposite in fact, I use 4 Stihl  saws one of which is a MS240 purchased new in May 2006 (£315) so almost 18 years old, it gets quite a lot of use (Stihl logo and MS240 marking completely warn off now) and is my go to saw for felling smaller dia timber with 13" (32cm) bar is easy on the back and great for sending. In 18 years apartment from guide bars, drive components, chains, plugs other replacement parts one air filter has been replaced, maybe two fuel filters (never replaced oil filter yet), fuel pipe (split), and I shock absorbers might have also been replaced.  I used to run it on regular petrol but switched a few years back to an ethanol free pre-mixed fuel.  When cold the saw usually fires straight away in the cold start position, when warmed up in usually on first pull. Great saw and engine still seems to run as good as when new, I  just wished that it had a heated handle at times. May be some operators reporting that MS240s are difficult to start have't been maintaining the machine properly, aren't paying attention to the 4 positions on the starting control lever or maybe at times have winter/summer shutter i wrong position, or have adjusted carburettor setting incorrectly (in 18 years I've never touched the carburettor setting with exception of idle speed screw a couple of times and a Stihl dealer may have tweaked carb settings years ago once.