MS240 carb air leak?

[Olddevonstihls]

Hi, having a lot of trouble with MS240.  Seized up with damaged piston last year.  Just before it finally went bang it would only start on half choke.  From what I’ve now learnt this suggests either an air leak or saw otherwise running very lean.  When rebuilding, replaced seals and fuel lines etc.  It now runs, but seems low on top end power, and will only start reliably on half choke still.  Idle is around 2800-3000 and wto 12500 rpm. On vac and pressure tests with inlet blanked off BEHIND carb it’s tight as a drum.  When I do the same tests with the front of the carb blanked off it doesn't hold pressure at all.  Bubbles come from around throttle linkage.  A bit of grease on the linkage improves things so that it will hold pressure which slowly subsides.  I’ve tried a cheap Chinese carb replacement but that also shows leakage around the throttle and won’t tune properly.  The original carb is the genuine stihl/walbro item that came with the saw (15 years old) and has had two rebuild kits.  From what I’ve read on other threads here throttle linkage leaks are common and should not generally cause these sorts of problems.  Can anyone suggest where else I might be looking (I’ve read the long thread about an 024 that had a mysterious air leak).

 

thanks.

[pleasant]

Try a genuine carb 

[Stere]

£150!?

 

WT-426B

 

 

 

[Olddevonstihls]

That is the problem.  If I could be sure it was a defective carb then buying an original new one might be worthwhile if it gets the saw going.  But £150 and it is still not working ….

 

are there any sources of after market carbs that are better than others?  

[pleasant]

Why don't you take it to a dealer to do the diagnosis work, then you won't be buying parts that may not cure the problem? We offer an estimating service which cost upfront £35 which allows us to accurately diagnose what the issue is and then quote for the repair. If the customer wishes to progress, then the £35 estimate fee is deducted from the final invoice costs, or if they don't want to go ahead or wish to repair it themselves they can have the machine back and we keep the £35 for the time we spent of it so far. Sometimes it takes us a matter of minutes, sometimes it's a mornings work to diagnose correctly, but its still the same £35 so some we win and some we lose. If a customer wants us to go ahead with the quoted repair and we then fit a new carb for instance, and it doesn't cure the issues, then the cost of that is borne by us, as it is then us who have made an error (doesn't happen often!)

 

It's a win win for the customer.

[spudulike]

Was the replacement cylinder kit OEM or aftermarket?

[Olddevonstihls]

10 hours ago, spudulike said:

Was the replacement cylinder kit OEM or aftermarket?

Aftermarket from L&S Engineers.  I did an 038 at the same time with the same kit from L&S and that’s fine.  The reason I’m circling around the carb is that the symptoms of needing half choke to start were the same before the failure as now.  

[spudulike]

No one does a vac/pressure check by sealing the back end of the carb. The engine is always tested by sealing the exhaust port, impulse, plug hole, decomp and the inlet with the manifold fitted, not the carb unless using it to clamp a sealing plate/rubber. You need to input the pressure/vac via a fitment to one of these orifices. 

The leak around the mechanism will always happen and should only stop an engine idling if there is excessive wear. 

If you needed to half choke the saw and it has been tested as sealed then your carb isn't providing enough fuel so perhaps the gauze strainer, diaphragms, fuel filter, holed fuel line, tank breather or the carb adjustment has just been too lean.

[Olddevonstihls]

11 hours ago, spudulike said:

No one does a vac/pressure check by sealing the back end of the carb. The engine is always tested by sealing the exhaust port, impulse, plug hole, decomp and the inlet with the manifold fitted, not the carb unless using it to clamp a sealing plate/rubber. You need to input the pressure/vac via a fitment to one of these orifices. 

The leak around the mechanism will always happen and should only stop an engine idling if there is excessive wear. 

If you needed to half choke the saw and it has been tested as sealed then your carb isn't providing enough fuel so perhaps the gauze strainer, diaphragms, fuel filter, holed fuel line, tank breather or the carb adjustment has just been too lean.

Thanks.  Very helpful.   I probably wasn’t being clear about the tests, sorry.  Normal vac/pressure was with rubber seal behind carb (ie next to cylinder inlet), impulse sealed by the connection to the carb, exhaust sealed similarly behind muffler, and test hose connected to modified spark plug.  Both vac and pressure holding solidly at 0.5 bar.    Carb itself holding pressure separately.  The other pressure test, when I sealed the air filter side with the carb attached was to establish if there were any leaks on the carb side, including at the gasket between carb and cylinder: there were not, other than at the throttle linkage.

 

I look again at the other things you suggest, but there is new fuel line (a split in the original lead to starting difficulties and fuel starvation and so was replaced) and filter.  Carb settings at the specified starting positions of L 1 turn/H 3/4 turns plus a small amount and giving the right engine characteristics but perhaps they need richening up more boldly.

[spudulike]

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.