stihl loss of power at low revs

[Aicchalmers]

So I thought I had learned the golden rule: never lend your chainsaw to anybody!

But I was involved in a project where we were doing some planking and thought 'well if it's all bolted into an alaskan mill theres not anything they can really **** up is there'

WRONG!

Got my saw back a couple days later 'uh it won't start' .. somebody had poured straight petrol into it :facepalm:..

But anyway, I took it all apart, unseized the piston and it was scored to all buggery, got a new piston and sanded off (!) the bits of aluminium stuck onto the cylinder wall. Got it all together again and it actually isn't too terrible compression on it, I don't have a tester at the moment but it has a fair bit of rebound.

Since then I've used it a fair bit and it's had just about the same power as it did before but when accelerating from idle in the mid-low rpm range it really struggles even with no load, once it's up to speed it's fine and it idles fine. I've checked the rpm with a laser tach and it seems in spec.

I'm thinking it's probably a crankcase air leak or could it just be really bad compression? Anybody have any tips on using liquid gasket properly?

N.B it's a stihl MS171; was just making some wee planks with it.

[doobin]

I'd say it's more the fact that you put an MS171 on an Alaskan that's the problem!

[Aicchalmers]

I'd say it's more the fact that you put an MS171 on an Alaskan that's the problem!

 

Was just making some 4" wide planks out of softwood

[Stubby]

I'd say it's more the fact that you put an MS171 on an Alaskan that's the problem!

 

Not the saw of choice I admit but neat petrol cant be a cool trick either ?

[spudulike]

Your method of cleaning off the transfer is harsh - cylinders don't like rubbing hard in one area as it will cause a cavity and cause leakage round the rings.

 

You probably have low compression - this may cause loss of power, especially in the cut.

 

Best measure it - 150psi is normal.

[Aicchalmers]

Your method of cleaning off the transfer is harsh - cylinders don't like rubbing hard in one area as it will cause a cavity and cause leakage round the rings.

 

You probably have low compression - this may cause loss of power, especially in the cut.

 

Best measure it - 150psi is normal.

 

What would you recommend instead?

I might as well splash out on a cheap compression tester- sure it'll come in useful .. but it cuts with plenty of power and idles fine it's just the mid range that it struggles with.

[spudulike]

What would you recommend instead?

I might as well splash out on a cheap compression tester- sure it'll come in useful .. but it cuts with plenty of power and idles fine it's just the mid range that it struggles with.

 

The aluminium should have been removed with acid or alkalie.

 

If the saw has full power but bad pickup, it sounds like the carb is a little lean on the L screw or is it one with only one screw!

[Aicchalmers]

Right will give it a shot when I next can ..

Is there any decent threads here explaining how to tune carburettors? I can basically understand them but still a bit unsure sometimes, with this I just put it to standard settings and it was within spec so I was happy..

It has the L/LA/H screws.

[wisecobandit]

Being a 171 I would expect lower compression. Out the factory new they only tend to be around 120-125 psi if I remember correctly which doesn't leave a great deal of tolerance.

 

I would fire it up and with it running just twiddle the low screw a tadge one way and the other to see if it improves your problem. Factory settings tend to be a bit rich for safety reasons.

If you find it improves it by altering it slightly find the best point. You may have to reset the idle slightly afterwards.

If the H is set as per factory setting and the higher side of the power is fine then leave as it is. A rich saw is much better to have than a lean one.

[morten]

Right will give it a shot when I next can ..

Is there any decent threads here explaining how to tune carburettors? I can basically understand them but still a bit unsure sometimes, with this I just put it to standard settings and it was within spec so I was happy..

It has the L/LA/H screws.

 

I have sent you a PM with a link to the STIHL MS 171 Service Manual. See section 14.5 regarding carb adjustment.

 

This link is also good for info about adjusting the carburetor http://www.madsens1.com/saw_carb_tune.htm

Edited January 25, 2014 by morten