Stihl engine knocking when hot.

[milkloss]

Evening All.

Soon be time to wake up the strimmer/brush cutter. When I put it away last year I was having trouble getting it to idle.

I can get it lovely when it's doing light work but when it's hot it sort of revs up a little with a knocking kind of noise. I initially put it down to something to do with the clutch as it starts to engage when it does it. Someone that heard it said it was 'knocking'.

Any ideas as to what the issue is and a solution?

thanks.

 

[C.J]

Is it a 4-mix engine - they don't like cheap 2 stroke oil

 

[milkloss]

Is it a 4-mix engine - they don't like cheap 2 stroke oil

no, it's a proper 2 stroke and mostly use stihl red 2stroke oil.

 

[Lincs Lass]

Try Husky XP 2 stroke oil ,,its a much cleaner burn ,if its high idling the mixture screws could need tweeking or the idler screw just half a turn out

 

[milkloss]

Try Husky XP 2 stroke oil ,,its a much cleaner burn ,if its high idling the mixture screws could need tweeking or the idler screw just half a turn out

thanks. I can't adjust it out. If I try then it will die when i off the throttle after hard work. If I adjust it for good idle when warm (not hot) then after hard work it idles for half a minute then starts to knock and high idle on its own.

edit. I'm fairly sure it's not clutch noise.

 

[Lincs Lass]

Not coming from the starter cord ,could be the reel tapping or the spring catches onto the flywheel

 

[milkloss]

Not coming from the starter cord ,could be the reel tapping or the spring catches onto the flywheel

wouldn't cause the idle speed to change wh i is the problem I'm having though would it?

 

[Nithsdale]

Runs on when hot - it's losing pressure somewhere... crank bearings and seals need checked.

Don't use it any more as it is, it'll overheat and melt the piston

 

[Lincs Lass]

wouldn't cause the idle speed to change wh i is the problem I'm having though would it?

No ,not that then ,,check any gators or seals from the air filter ,,it could be drawing excess air in from behind the filter and not through it

 

[milkloss]

Runs on when hot - it's losing pressure somewhere... crank bearings and seals need checked.

Don't use it any more as it is, it'll overheat and melt the piston

No ,not that then ,,check any gators or seals from the air filter ,,it could be drawing excess air in from behind the filter and not through it

Both could be right, just having a google, looks like I'll have to investigate now I know where to start.

 

[Lincs Lass]

I had a husky 560 chainsaw from new ,,it ran perfect for 6 months ,first it lost its computer memory ,then it played up again and wouldnt run smoothly till it eventually wouldnt idle and the revs were getting faster ,,crank seals were deformed

 

[Nithsdale]

Both could be right, just having a google, looks like I'll have to investigate now I know where to start.

2 strokes are very simple.


The knocking is normally known on a big end, on a car... the symptoms are transferable to the 2 stroke crank bearings.

If they've failed or are on their way out, the seal will be gone, too. The lack of bottom end 'back pressure' means the piston accelerates faster and faster as the air becomes hotter, meaning it passes through the leak easier/quicker. This is noticeable as the engine revs faster on idle, becomes difficult to set the carburettor and runs very lean at top end revs (which causes the cylinder and piston to get very hot. The piston ring usually fails first, this failure causes scoring to the cylinder causing the engine to jam/cease!)

 

[startinghandle]

How clean are the fins around cylinder?
Are you using Tesco fuel?
Is spark plug gap correct?
Is spark plug correct type
Is air filter clean

 

[milkloss]

How clean are the fins around cylinder?
Are you using Tesco fuel?
Is spark plug gap correct?
Is spark plug correct type
Is air filter clean

yep! Ran through all the basics last year when it started playing up but never got round to getting any further

 

[Lincs Lass]

Take the plug out and see if theres a build up of carbon top of the piston and take the exhaust of and see if theres any minor scoring

 

[Smith31]

I had a husky 560 chainsaw from new ,,it ran perfect for 6 months ,first it lost its computer memory ,then it played up again and wouldnt run smoothly till it eventually wouldnt idle and the revs were getting faster ,,crank seals were deformed

Computer memory? Sorry do chainsaws have computers now? I have an old stihl so nice and simple

 

[forblue]

what model is it

 

[milkloss]

what model is it

FS160, a fair age but 'one of the good ones'.

 

[8100]

Sounds like pre ignition .As above may need a piston crown de coke as when its hot the muck on top of piston will glow and cause pre ignition

 

[Lincs Lass]

Computer memory? Sorry do chainsaws have computers now? I have an old stihl so nice and simple

Yes the auto tune saws have a tunning chip built in ,,it controls the emissions, 2 stroke engines run differently at higher atmospheres and colder temps and on a standard carb engine ,you alter the fuel mix to compensate , auto tune does it for you ,when you use an auto tune from new ,you have to work.it hard for the tunning chip to memorize the fuel mix and emissions, it then keeps those settings ,,Stihl have the M Tronic models ,same system.
Thus is why you cant buy a new saw over 75cc now as the main brands are not allowed to sell them.in the UK so the big saws over 75cc have vanished from the market