T540 XP I & II Compression Issues

[TheShopForestry]

Hello all! I'm hoping someone with good experience with MKI and MKII T540 XP Autotune Husqvarna's could possibly give me some advice. I'm working with two T540 XP saws one is a MkI other is a MkII. The Mk I would run fairly normally without load and for the first 5 minutes it would run in the wood well also but after it got to operating temp and you dogged it in it would bog down and die and take forever to restart. The Mk II was worse it would bog whenever given any throttle and could barely be coaxed up to working rpm. Both saws had low compression at the exact same numbers 94-96 psi hot 103 psi cold.They were both tested with a genuine husqvarna 2stroke comp gauge. The Mk I after disassembly was found to have blowby but zero wear to the jug so we replaced just the piston assy. This repair didn't really change the compression number still only hit 96 hot and 103 cold, but it runs much better and without any bogging issues when given the adequate amount of warm up time and even from a cold start the acceleration is much better. So that one was a success obviously but the Mk II not so much. We found minor lean scoring in the exhaust port side so we replaced the whole cylinder/piston assy. Once again with a different saw only hit 94-96 psi warm and 100-103 psi cold, but this time the saw barely ran and was able to barely make it through a tuning cut. Then after talking to tech support at husqvarna they said reset the carb settings and then the saw wouldn't start at all. Both saws were rebuilt to husqvarna specs following the workshop manual. Both saws also got run time after the rebuild and were broken in with incremental increases in load over multiple cutting sessions equaling 1.5-2 tanks of fuel for each saw. I have also pressure/vacuum tested the crankcase, fuel lines, and impulse hose and they all check out good/airtight, and were tested with a mityvac. They both also have strong spark with the plug in the hole and Husqvarna's three point tester at the 6th hash mark. At this point after replacing the cylinder and piston I don't want to load the parts cannon again without a second opinion(s). Anyone that has a theory let me know any input would be appreciated.

Regards The Shop Forestry & Supply

 

[cscltd]

Ya, I was hoping these saws would be better than they turned out to be. they are great when working and guys love them, but when they give problems, they give problems. Hence why they are on the mark 3 version. if you look at the bulletins, they have a piston cylinder issue from the start. Mk2 was suppose to be better but I have a couple sitting with a couple mk1 than are also sitting as they have poor Idle and off idle rev. There is a bleed screw on carb face by the choke butterfly, can try turning that a bit to help idle.
new cylinder kit should have corrected you mk2.
do you have the common service tool to update software?

 

[TheShopForestry]

Ya, I was hoping these saws would be better than they turned out to be. they are great when working and guys love them, but when they give problems, they give problems. Hence why they are on the mark 3 version. if you look at the bulletins, they have a piston cylinder issue from the start. Mk2 was suppose to be better but I have a couple sitting with a couple mk1 than are also sitting as they have poor Idle and off idle rev. There is a bleed screw on carb face by the choke butterfly, can try turning that a bit to help idle.
new cylinder kit should have corrected you mk2.
do you have the common service tool to update

I have both cst and service hub, cst is by far superior. Both saws firmware is up to date. I'm gonna try and find that screw you're talking about. And I can't find anything mentioning a bulletin about the cylinders on the power site. Is there a different site I might find those at?

 

[hotshot]

Don’t know much about Husqvarnas’ saws, but do know that a 2 stroke saw engine will not cut well hot with that 95 psig low compression. Factory new they should test hot above 140 psig.

Are you sure the mufflers’ spark arrestor screens are not fully plugged up?

Did you take piston ring’s end gap & piston squish readings using soft solder to see what ring & piston clearances are on the rebuilds?

https://************/threads/t540xp-woes.24813/
Edit- What’s up with this? You have to replace the asterisks in the link above with ‘o* *p *e forum*’, but no asterisks or spaces.

 

[av8or3]

I’ve had to fix two of those saws and a new cylinder kit fixed them both. I don’t know what’s up with them. I dicked around with the first one for about two weeks taking it apart and putting it back together looking for a reason…. I finally changed out the p&c and it ran like new again.. I didn’t waste a minute on the second one, just went straight for the p&c and it worked too. Same tree service. If another one comes along that won’t run for no apparent reason, it’s getting a new p&c kit too!

 

[HumBurner]

I’ve had to fix two of those saws and a new cylinder kit fixed them both. I don’t know what’s up with them. I dicked around with the first one for about two weeks taking it apart and putting it back together looking for a reason…. I finally changed out the p&c and it ran like new again.. I didn’t waste a minute on the second one, just went straight for the p&c and it worked too. Same tree service. If another one comes along that won’t run for no apparent reason, it’s getting a new p&c kit too!

Transfer covers leaking....?

 

[av8or3]

No, it was pressure checked under water even. We could get it (the first one) to blow 120psi but it would not start and run. I can’t remember if it would hit at all. “We” was me and my friend Daniel who owns Dover & Tatum , Husqvarna dealership. I enlisted his help and he couldn’t find it either. I took it home defeated. I found a cylinder kit and ordered it for no good reason. After I installed it I was surprised when it started. I kicked it back into play and when the fellow brought me another that wouldn’t start for no apparent reason I ordered another cylinder kit and sat it in the floor until the kit arrived. Replaced it and it started and ran like new. It’s real simple for me now with the 540T (or T540). If it has compression, fire and fuel but won’t start (and there’s nothing obvious (impulse line bad to come loose on this saw, for example) it gets another cylinder.
This saw is such a PIA to take apart and put together I was certain it would soon fall by the wayside. But it looks now like folks like it and will be fixing them up. I hope I don’t get any more, that’s for sure.

 

[TheShopForestry]

Transfer covers leaking....?

Saw crankcase is air tight

 

[Tobystihl]

Just finished rebuilding five of these mk1s, out of a big box of bits! (Over the last years, I have rebuilt a fair few!! ) I have found any running problems tend to be down to be a worn piston ring or stiff carb diaphragms. Pretty good saws and I don't mind admitting that I prefer them to the 201tc