Husqvarna 540xp Mk1 cuts out on idle

[Mark_Skyland]

16 minutes ago, spudulike said:

Try putting the ring in the bore to make sure it fits OK and the gap being around 7-12 thou. 

Not being a dick but you are aware that there is a ring location peg in the groove?

You can measure the thickness of both rings with verniers but, unless you have the wrong ring, it should fit and as Stubby said, clean the groove with a piece of busted ring as carbon can build up in the groove.

Have you tried pushing the ring edge in to the groove, not as it would normally be fitted but the outer edge of the ring in to the piston ring groove?

I was just about to say the same thing. As spud says if it fits in the bore it should fit the piston.

 

[Jon IOW]

The gap on the old ring is approx 0.7mm which according to google is approx 28 thou. Quite a lot!

 

I checked the thickness of the old and new rings and they are actually the same, must have been a trick on the eye!

 

The old ring is 1.5mm. The new ring is 1.7mm. The 0.2mm difference means the new ring needs to sit deeper inside the groove on the piston and seems to be this that's causing a tight fit. 

 

Cleaned out the gap with the old ring, got some carbon out but still the new ring is a tight fit. The new ring just won't go deep enough into the groove. 

 

And yes I have got the ring in the correct orientation etc relative to the little alignment pin thing. Even a blind man could spot that. 

[spudulike]

You are using a ring compressor to get it in to the cylinder bore aren't you? 

If all else fails, remove the piston and try to insert it, with the ring fitted, in to the cylinder and see if that does it. 

It will get a tight fit, that is how it works. Use a bit of oil to ease it....you could measure the piston ring groove depth so you know it will work.

Have you carried out many rebuilds?

[adw]

Sounds like you have a mark 11 ring, you are not going to get that ring into that piston sir.

[spudulike]

21 hours ago, adw said:

Sounds like you have a mark 11 ring, you are not going to get that ring into that piston sir.

Well old boy, on July 20th the OP stated the machine was a MK2  so please keep up and stop drifting in to that boxed set of Downton Abbey again, I know it is difficult🙂

 

[adw]

Oh Pooh, I thought we had not established it was a mark 1 or mark 11, took my eye of the ball there, I bought a box set of Doris Day films, living the life now.

[adw]

[Jon IOW]

Good news and long story short - saw is now running like a sewing machine!!

 

The supplier informed me there are actually two rings for the 540XP Mk2 depending on serial number.  Unfortunately both products were labelled on their website as being compatible for the Mk2, but did not specify the serials in the description. I only noticed the one ring, not the second.  The ring which I purchased was the wrong one according my serial number, I needed the later one. 

 

In lieu of this they agreed to send me a whole new piston assembly minus the cost of the original ring.  New piston now fitted and straight away the saw fired into life and idled nicely for 5 mins, and revs up nice.

 

Tbh I don't think a ring compressor is necessary for this dinky toy stuff, mine slipped in with a little bit of lube and finger tip encouragement no problem (no pun intended).  The rings got naff all resistance and the head is rounded off a little at the end to help ease it in so no drama. I didn't bother to measure the new ring gap, but it was a noticeably closer fit compared to the old worn out one! 

 

Lesson learned and case closed. Many thanks for all the advice etc!

[Jon IOW]

FYI - as for the mystery adjustment screw on the carb.  I set mine to 3 turns out from closed (as a semi-intelligent guess). No idea if this is as per factory, but saw runs great so I guess it doesn't matter.