Friday, 8 May 2015

Chainsaw Chain Keeps Moving at Idle

Chainsaw Chain Keeps Moving at Idle - Intro 

I have an 10 year old husqvarna chainsaw with a chain that keeps moving at idle, I used to apply the chain brake to stop the chain when not using the saw, but the problem has got steadily worse and now the clutch overheats so I had to try and fix it.

Trying to fix this chain moving at idle problem has lead me on a journey of discovery which I will now share with you in the hope it may help you fix the same sort of problem.

Chainsaw Chain Keeps Moving at Idle - 2 Causes

I may be wrong but there are 2 basic reasons why your chain might move at idle:

  1. A faulty clucth (or something else) is engaged and is causing the chain to spin.
  2. The clutch is OK but the saw is revving too high and so the clutch engages (as it should) at higher revs.

"Clutch" Based Reasons (1)

Chainsaw Chain Keeps Moving at Idle - FIX A - Faulty Clutch

The spring on the centrafugal clutch may have become weak over time, and now when the RPM is correct (sat around 2800 RPM), the clutch engaes and the the chain moves.

Either replace you clutch, clutch spring, or tighten the spring.

There could be lots of sawdust or wood chips stuck in the clutch, if so get rid of this.

Further reading.

Chainsaw Chain Keeps Moving at Idle - FIX B - Seized Sprocket Bearings

While you are inspecting the clutch springs it is worth taking a look at the sprocket bearings, they may have overheated and seized. The clutch drum (and sprocket) should rotate freely on the crankshaft. 

Further reading

"Rev" Based Reasons (2)

I am no expert on carburettors please read this guide before following any of the below suggestions.

Chainsaw Chain Keeps Moving at Idle - FIX C - Adjust Idle Speed

on the carburettor there are 3 screws a low jet screw, high jet screws and a "T" screw. This T screw is for setting the idle. Turn it out (anticlockwise) to lower the idle speed, until the chain stops spinning. If the saw stalls before the chain stops spinning or this does not lower the revs enough to stop the chain moving, then it is likely you will have to adjust the carburettor.  

Chainsaw Chain Keeps Moving at Idle - FIX D - Adjust Low Jet

Turn out the low jet screw, this will richen the mixture and make the chain speed slow down. I think that adjusting this screw affects the setting of the high jet. But if you are just adjusting a small amount, perhaps leave the high jet screw alone, if this is set over lean it can damage the engine. Make adjustments very small and take not of which way you are adjusting them, so you can put them back how they were is the adjustments do not improve things.

Chainsaw Chain Keeps Moving at Idle - FIX E - Your May have an Air Leak

If the saw is running at high revs with the idle screw out and the low mixture set over rich then it is likley you have an air leak.

Put simply the engine requires air and fuel to run, the mix is controlled by the carburettor. If extra air gets in from else where then this will cuase an over lean mixture and the saw to run at higher revs.

Air leaks can come from

  1. Seals - Port Seal, Impulse Seal, Cranks Seals
  2. Cracks - In Cylinder Block
  3. Loose Nuts - on carburettor mounting etc.
I had no special machinery so I improvised in the following manner.

You will need a bicycle inner tube, and a bicycle pump.

Cut some piece of inner tube to fit behind the carburettor and the carburettor mount. Fit these is place to seal the port.

Refit the spark plug.

Cut the valve from the inner tube together with a patch big enough to fit behind the exhaust port. As pictured below: fit behind the muffler and re-attach.

Very carefully now use a pump to very gently blow a littl e air in the the seal chamber. Ensure that the piston head is down low, or up high ( you need to try both up and down I think) and not blocking the ports.

Listen for air hissing out, with a wet hand you may be able to feel the air escaping from behind a seal. Spray the outside of the sealed area with lashings on bubbly liquid, this could be kids bubble bath, or washing up liquid.

The picture above shows some bubble coming out of the crank seal. The man cuase of the air leak on my saw was a completely trashed impulse seal. Pictured below.

Further viewing (video below).

Chainsaw Chain Keeps Moving at Idle - Conclusions

So I hope you can see from the above that there are many reasons why your saws chain may be moving at idle. And hopefully using the baove information you can fix your saw, and get back out in the woods.


I am not an expert and this is my first foray in to the world of small engines so please read this guide at your own risk. My saw is worth about £50 / $90, and worth nothing broken, so I had nothing to loose here. If your saw is worth a lot of money then please read up on the each fix before attempting it. Chainsaws are really really dangerous obviously, and despite trying to fix this saw I have in the mean time bought a new one, because I value my legs and other limbs, and want a saw that is safe to use because they are inherently dangerous anyway. Perhaps consider doing the same your self.

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