stihl ms021 help please

[Matthew Burton]

A friend of mine asked me to have a look at an 021 of his that was cutting out as soon as you opened the throttle. On first inspection the air filter was missing and there was fuel leaking from a series of splits in the pipe between the fuel tank and the carb. Quick trip to my local dealer later and I replace the pipe, treat it to a new carb kit and find out the air filter I have been supplied doesn't quite fit.

 

Reset the high and low running screws to 1 turn which the label of the cowling suggests thinking this might be a good starting point. The saw runs on full choke and half choke but rapidly dies on normal running with or without throttle. Spent a couple of mins tweaking the low running screw once i'd warmed it up a bit but with mo real improvement in performance. Will the missing air filter be causing a lean mix? can I tune to compensate for this until i can get the correct filter? is this wise?

 

Any suggestions/tips very welcome!

 

Thanks in advance, Matt.

[WorcsWuss]

Should run without the filter... Has the carb kit been fitted right and everything oriented correctly? Check and double check. If it's running choked it could be fuel starvation. Check spud's favourite, the fuel breather, although I'd be surprised if in this instance it was the culprit. Does it run ok with the fuel cap slackened?

Has the compression been checked? Could be air leakage into the cylinder. You're into leak down tests there for diagnosis....

[Rich2484]

I wouldn't run it without an air filter..

 

Sounds like either not enough fuel is getting through to cylinder or too much air.

 

Could be the metering valve isn't opening enough to allow fuel through. Did you put the diaghram in the right way up?

 

Other than a pressure test to check the seals on saw body. I would look into the metering valve.

[Stephen Blair]

I wouldn't run it without a filter mate.

Don't know why,but I just wouldnt:biggrin:

[Ray1]

If the fuel pipe has deteriorated, chances are that the impulse line and possible the inlet manifold could do with looking at aswell. Or if you have replaced the fuel pipe, did you completely remove the orange handle to get at it or replaced the fuel pipe with the handle in place. Could have disturbed the impulse line where it mounts onto the handle.

Edited January 30, 2013 by Ray1

[WorcsWuss]

Could be the metering valve isn't opening enough to allow fuel through. Did you put the diaghram in the right way up?

 

 

That's where'd go first, either diaphragm upside down or diaphragm and gasket the wrong way round.....

[Matthew Burton]

Thanks for the replies guys!

 

I did double check i've put the diaphragm and gasket back correctly but I can always check again.

 

I wasn't going to use it until i have the filter, just try and get it running again a bit better.

 

I did give the impulse line and inlet manifold the once over and they seemed to be fine and I made sure they went back into place correctly.

 

I'll give the diaphragm and gaskets another look and try to find out if the breather is blocked. If that doesn't help i suspect the diagnosis/solution is beyond me.

 

Matt

[spudulike]

Well it sounds like lack of fuel as using the choke a bit will allow fuel to be dragged up the high speed jet that is probably working OK and is making up for the low speed part of the carb that probably isn't working correctly.

 

A holed impulse line will stop the carb pulling enough fuel through so is worth looking at as are carb inlet manifold boot leaks - if it is bad enough it will let loads of air through and stop any sort of idle.

 

A blocked tank breather will normally cause this sort of issue after a minute or two running but can occur immediately - loosening the fuel cap and trying this is a classic test to do.

 

Metering arm IME - if they are set wrong, you will either get a very rich running or it won't run at all - look at where the diaphragm gasket sits - it should be BETWEEN the carb body and the diaphragm:blushing:

 

Have you tried 1 1/2 turns out on the L screw?

 

One possibillity is that the low speed part of the carb is either blocked, got resin in it etc - a good carb cleaner would help, ultrasonically cleaning the carb would also help and if all this fails - it sounds like the MS230 I have just fixed - had to remove the welch plug on that one!!

[Matthew Burton]

Thanks for that spud!

 

Opened up the low running screw to about 1 and half turns now and its idling and running well. The only slight problem now is if you throttle hard from tickover its very slightly sluggish picking up the revs and i do mean very slight. I'm more than happy with how its running now and have had a little play with the low running screw to try and prevent this but don't seem to be getting anywhere. Any suggestions welcome...

 

I do appreciate everyone taking the time to read this and reply, its a good learning experience, always picking up little tips!

 

Matt

[spudulike]

Thanks for that spud!

 

Opened up the low running screw to about 1 and half turns now and its idling and running well. The only slight problem now is if you throttle hard from tickover its very slightly sluggish picking up the revs and i do mean very slight. I'm more than happy with how its running now and have had a little play with the low running screw to try and prevent this but don't seem to be getting anywhere. Any suggestions welcome...

 

I do appreciate everyone taking the time to read this and reply, its a good learning experience, always picking up little tips!

 

Matt

 

That sluggishness can be the L screw rich or lean - if it does it a little when the saw is stone cold then it can be normal if it clears in 15 secs of running - you could try turning he screw in 1/4 turn and see if it improves or out 1/4 turn and try again but an L screw on 1 3/4 turns is not normal IMO on this saw. The MS230 I have done was around 1 1/2 turns out to get it right.

 

Give it a bit of a run and see what colour the plug is after half an hour - it wil give you an idea of what it is doing - it is possible the H screw needs turning in slightly to lean it up but can't be sure without hearing it or using a tach.

 

Glad it is useable now:thumbup: