Husqvarna 540xp Mk1 cuts out on idle

[Mark_Skyland]

Did you check the piston ring end gap? They can wear fairly quickly which affects tick over. If you strip a machine down to the piston and cylinder its nearly always worth replacing the piston rings as they are fairly cheap.

[adw]

So if it’s a mark 11 it will say it on the ser no. plater in Roman numerals, completely agree with replacing the ring, unfortunately that screw is set by the carb manufacturer so no basic setting, bit late to say but you should not have removed it, with any auto tune saw idling is the first thing to loose if there is any drop in compression.

[spudulike]

The issue with measuring the compression is the small plug adaptor which can lose around 30 psi on the large plug type and also using a car type compression gauge which will top out on a small engine at 75psi. In short, use a Gunson HiGauge or something similar to get a correct compression reading. 150psi and higher is decent on a cold engine after 5-7 pulls.

As Mark and ADW have said, these machines need a piston ring in decent order and if worn, the idle suffers.

If you hadn't said the impulse line was connected, on Mk1 machines, this was one of the most common issues and have seen them come off or even the brass insert on the cylinder come out, the symptoms are stalling.

The carb - worth checking the gauze strainer is clean. The pump and metering diaphragm, especially the metering diaphragm,  can get a bit baggy causing running issues.

On the CST test your dealer did, if you have the printout, the carb H&L settings can often be revealing.

 

[Jon IOW]

Thanks guys for all the advice!

 

I checked, it is a Mk2.

 

Ordered carb rebuild kit and piston ring this evening. These parts are fairly cheap so worth a try. Will fit those all see what that yields!

 

Unfortunately dealer didn't give me print out of the autotune stuff....seems the norm now everyone wants your money and you get less and less in return!

[Doug Tait]

Don't think they give a printout of the stats as standard because they're of little use to most saw owners.

 

Last time a saw was at my dealers just out of interest I asked for a printout and got it no problem. Was interesting seeing the rev hrs on it and remembering where it was when the high/low operating temps were recorded (Brittany/South Scotland), but not much practical use, carb settings etc mean little to me.

[adw]

The figures can tell you an awful lot, the L setting is very important, the figure runs from 5 to 125, the lower the figure the richer the mixture, auto tune will always try to keep the engine running at full power, a typical figure of 10 or lower will most likely be an engine air leak, the fault codes given are taken from these readings.

[Jon IOW]

Hi guys,

 

Parts arrived today.

I replaced all the bits in the carb, all good.

 

I am having some problem with the piston ring though. On opening the new one I noticed it is very slightly thicker and wider than the original. Obviously I put this down to wear on the old one.

But I am struggling to fit the new ring. It wants to go in, but I cant squeeze it together and get the gap to close up. I checked the piston, its clean and no obstruction in the gap.  The ring does fit in the gap but only like half way inside before it starts to bind.  As far as I know the ring should have some small freedom of movement once installed on the piston to prevent damage to the bore.

 

Any ideas?

 

Cheers,

Jon

[Jon IOW]

By the way I bought the new ring from L&S engineers:

Piston Ring for Husqvarna T540 XP, T540 XPII Chainsaw - 577 96 05 01 | L&S Engineers

WWW.LSENGINEERS.CO.UK

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Piston Ring Husqvarna T540 XP, T540 XPII Chainsaw - 577 96 05 01 Genuine Husqvarna Part. OEM No. 577 96 05-01

 

[Stubby]

1 hour ago, Jon IOW said:

Hi guys,

 

Parts arrived today.

I replaced all the bits in the carb, all good.

 

I am having some problem with the piston ring though. On opening the new one I noticed it is very slightly thicker and wider than the original. Obviously I put this down to wear on the old one.

But I am struggling to fit the new ring. It wants to go in, but I cant squeeze it together and get the gap to close up. I checked the piston, its clean and no obstruction in the gap.  The ring does fit in the gap but only like half way inside before it starts to bind.  As far as I know the ring should have some small freedom of movement once installed on the piston to prevent damage to the bore.

 

Any ideas?

 

Cheers,

Jon

 I know you say the groove is clean  but break off a bit of the old ring and use it as a scraper and clean the groove out .

[spudulike]

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?