MS390 won't idle

[Crazy_Bull]

Evening Gents,

 

I have a MS390 that over the weekend decided to go tempramental on me , racing then stalling then simply not starting.

 

Tonight gave the fuel tank a good rinse out (little bit of crud in there) cleaned the air filter and fuel filter.

 

Saw now starts fine on half choke and runs fine on a bit of throttle, however it simply dies when the throttle is released.

 

I have tried adjusting the idler screw however what ever position i have it in does not seem to make a blind bit of difference.

 

Any of you knowledgeable folks have any suggestions or should i just admit defeat and get it down the menders?

 

Cheers

 

C B

[spudulike]

These carb sort of problems are hard to diagnose without having the saw in front of you.

 

A common problem is crud in the gauze filter in the pumoing section of the carb directly under the single screw cover on the carb.

 

Fuel lines can go very soft and cause issues.

 

Check the breather is clear - just spent longer than I should on a Jonsered with a plugged breather:thumbdown:

A quick check on this is to release the fuel cap and try running the saw with it canted over - otherwise all the fuel will run out:001_rolleyes:

 

Other than that, an air leak can cause a racing idle and bogging in the mid range.

 

You aren't a million miles from me, can't do anything until Sunday if you are stuck.

[Crazy_Bull]

Cheers Spudulike,

 

It's not an urgent sort i will have a further investigation on the points you have kindly raised if i haven't sorted it by the weekend I might send you a PM about getting you to have a look see.

 

Thanks

[spudulike]

No Worries - I would steer clear of adjusting the H screw (closest to the air filter) as if you lean the saw out too far, it may seize. The settings that the saw ran on before the problem should allow the saw to run when the fault is found.

 

The idle can be adjusted with no problem but again - once the problem is found, the saw should run fine on the original settings.

[GardenKit]

Spot on

Too many people try to tune carburettors rather than cure the fault which make the saw seem out of tune.

Carbs seldom go off tune on their own, and if they do they do it very slowly.

Contamination problems can happen quickly, as can fuel line deterioration.