Husqvarna 435 cylinder/ piston fault or something else?

[DaveyCrockett]

I have been trying to diagnose a fault on a Husqvarna 435.

 

The saw was initially running poorly and hot.  It was taken for a serviced by a local company after which the saw wouldn't start.  It was taken back.  The company swapped the carb which didn't fix it then they said it might need a new piston and quoted a high price..  The owner brought the saw to me.

 

The saw will fire but won't run for more than 1/2 second, even with direct priming with carb cleaner.

 

The plug has been changed and it produces a healthy spark which jumps a gap.

 

Compression was measured at 120 psi.  The piston ring is free and piston skirt un-scored.  There are some mystery scratch marks on the back of the cylinder opposite the exhaust post and  on the front of the piston below the ring but they seem quite superficial and not enough to cause a problem. 

 

On removing cylinder, there were no foreign objects, nothing stuck in the transfer port, manifold and cylinder seemed in reasonable condition.  There were dark marks on the piston skirts so I think there is some blow by past the ring.

 

Just in case, I stripped the carb to look for obvious faults but all looked clean and in good condition.  I have also tried every tuning point of H and L including the standard tune in am effort to get this saw to run at any RPM.

 

Has anyone any suggestion where I should go from here?  My feeling is that it needs a new piston or piston and cylinder but I am not at all sure.

 

Thanks for any help

[Khriss]

Check exhaust gasket, that would lean it, often forgot.  K

[DaveyCrockett]

32 minutes ago, Khriss said:

Check exhaust gasket, that would lean it, often forgot.  K

Hi Khriss,  Actually the exhaust gasket was missing, but I didn't think that would effect how the saw ran.

[Khriss]

Oh dear!!  Cos two strokes use the piston to 'breathe' in and out, its fuel air mix, they  are needed so it all goes where it should. Its a cheap part so buy two - one fr yer next overhaul. Then see if it helps.  K

[adw]

Ignition unit every time, there are different units for different years and they are no interchangeable, my guess is the unit was bad and the dealer has now fitted with the wrong unit.

[Paddy1000111]

120psi isn't anywhere near enough compression in my mind? 

[DaveyCrockett]

2 minutes ago, Khriss said:

Oh dear!!  Cos two strokes use the piston to 'breathe' in and out, its fuel air mix, they  are needed so it all goes where it should. Its a cheap part so buy two - one fr yer next overhaul. Then see if it helps.  K

It still had the alu heat shield to seal it so any leak would be very minor.  Most of the tuning effect from the muffler would still be there.  This saw hasn't got a flat spot in the power band -  It won't run at all.

[DaveyCrockett]

7 hours ago, adw said:

Ignition unit every time, there are different units for different years and they are no interchangeable, my guess is the unit was bad and the dealer has now fitted with the wrong unit.

 

I have considered ignition module, but most I have seen failed are either totally caput or fail when hot.  The wrong module hadn't crossed my mind.

Edited December 30, 2020 by DaveyCrockett
typo

[DaveyCrockett]

5 minutes ago, Paddy1000111 said:

120psi isn't anywhere near enough compression in my mind? 

I was guided by the book by Paul Dempsey which quotes 90 psi as a working limit, but I think you could be right.  There is no figure quoted by husqvarna.

[adw]

Remove the flywheel, on the back will be the part number of the flywheel, you then match the unit to the parts list that shows that flywheel.