Stihl MS391 suddenly lost power.

[Paddy1000111]

1 minute ago, Paul Tomo said:

What would he be talking about then?

Maybe the "gasket" on the decompression valve? They have a crush washer? 

[Moose McAlpine]

4 minutes ago, Paul Tomo said:

What would he be talking about then?

Not sure mate, check the things listed in here and report back. Most things can be checked with minimal disassembly.

[Stubby]

2 hours ago, Paul Tomo said:

What would he be talking about then?

Cylinder base gasket , or , maybe just bollox ! 😁

[Silverhooker]

I would agree with the bollocks suggestion! Did he try to sell you a 500i replacement yet ? 

[spudulike]

WTF...either you have completely misunderstood him or he was talking gibberish in a twatish dialect!

If the base gasket fails, the saw will still make pretty much normal secondary compression, you will have a big air leak, you may feel the cylinder is moving on its screws and it may not run at all. 

If you pick the saw up with the starter handle, does the saw fall to full extension of the recoil rope immediately, does it take 2-3 seconds or 8-15? 

Have you taken the muffler off and looked at the piston? You can take the spark plug out and shine a torch down the bore to look for streaking/scoring on the bore surface around the port.

Not being funny but....your saw is 65cc and if you pull it over hard with the decomp out, you will feel it, it is your saw, you know what it normally does. You initially said there was no tension in the starter chord.....in my world that means the engine spins over like the plug is out and that means a bad failure.

I can usually tell within seconds what has probably happened and verify the issue for fact in a minute or two. I am NOT bragging, anyone who does this sort of work should be able to do this as many of my customers have witnessed when they hand their saw over and I pull it over and wince at the lack of compression or dodgy noises coming from the saws engine!

Checking the compression, checking for a seize takes a few minutes by a tech. 

Just follow the advice from us and lets follow this through and get the thing going again!!

 

 

[bmp01]

Could be wrong, MS391 = clam shell engine? Doesn't have cylinder to crankcase gasket. But on the plus side, a pain in the backside to strip down   Which makes it a priority to diagnose the problem with certainty before launching into it.

 

 

 

 

[Stubby]

Just now, bmp01 said:

Could be wrong, MS391 = clam shell engine? Doesn't have cylinder to crankcase gasket. But on the plus side, a pain in the backside to strip down   Which makes it a priority to diagnose the problem with certainty before launching into it.

 

 

 

 

Yep  Home owner clamshell dsign . Will have gasket between 2 halves of crankcase though ?

[Paul Tomo]

I’ve found what the problem is and those that mentioned about air leakage you are all correct on that part of the problem. I may have misled you when I said the chainsaw suddenly died when what I should have said it seamed has though the saw was gasping for air as it came to a stop. I thought at the time that no sudden stop would mean that not a lot of damage as been done but maybe a blockage in the fuel or air pipes or something to do with the compression valve.

 

The fuel was new and the correct oil added and I had just fuelled and oiled up and sharpened the chainsaw as I was using it the day before.

 

On taking your advice I took the saw into my workshop prepared the tools that would be needed turned the heater on and started to take the top cover off. First job I did was to clean the air filter, this was pretty clean so on to the next job of dismantling the saw.

Has soon as I touched the spark plug cover I could see that I had found the problem and this is where you are correct in saying that there could be an air leak. What a relief I felt, straight away I thought there is nothing else wrong with the saw and can only be a loose spark plug. I took the chainsaw outside and within three pulls on the cord my Ms391 fired up and running perfect. 
 

Thanks to you all and thanks to myself for taking your advice and working this problem out My chainsaw is now up and running again although it just being a loose spark plug. Thank you very much for your help...Paul Tomo 👍

[Rich Rule]

I think you owe Spud a beer.  Sounds like he diagnosed exactly what you experienced.

[Doug Tait]

21 hours ago, Paul Tomo said:

I’ve took the saw down to a local Stihl dealers and said there’s still compression and the problem maybe the gasket that’s at the top of the piston ? He said the card that’s at the top might be to flexible and gone. Stripping it down will give me a better idea, looks like I’ll be doing this at the weekend. 
 

 

 

Doesn't give much confidence in the local dealer. My local husky dealer didn't know what a T540 was and said they'd have to try ordering in an air filter.

Anyway, glad you're up and running. Back to cutting logs 👍