Stihl MS660 Won't Start - Can't Work Out Why!

[atomicduck123]

Hi All,

 

   I recently bought a secondhand Stihl MS660, fired it up, ran it through a couple of logs and as I finished going through the last one, the idle dropped and shortly stopped all together and was unable to get it started again. I went through a number of checks, cleaned the carb (ended up replacing it altogether), checked the fuel line was clear including the fuel filter, air filter looks brand new and clean, spark plug is firing nicely, spark plug gap is spot on, tried a new spark plug altogether as well. I also checked the flywheel key which is perfectly intact, the exhaust/muffler is clean, set the carb to recommended settings - all of which made no difference, so I decided to take the head off.

 

   Upon doing so, the cylinder looked a little scored and the piston was mostly black so I ordered an aftermarket replacement kit. I installed it today, popped the choke on and after 2/3 tugs it fired up for a few seconds. I then went to try and start it again, this time it wouldn't start. If I pull relatively softly, it will pull as it normally should and I can feel the piston moving. If I give it a harder pull, i.e enough to actually start the engine, the decompression valve pops and as soon as it does, the pulling comes to an abrupt stop, almost as though the piston is hitting something. As soon as I press the decompression valve down again, the piston can freely move, until I pull hard again. Does anyone have any ideas at why it is doing this? I took the head off again just to check I had installed everything correctly and to check nothing in fact had broken off inside the engine and it's all fine, piston rings are intact and in position, the little clips holding the piston in place are also intact and in position, the piston arrow is facing the exhaust port, presuming that is correct and nothing looks scored, really bizarre. I really can't figure out what the problem is though and have been trying to get it running for the last few weeks without much luck and wondered if anyone here had experienced a similar thing and new a remedy.

 

   Many thanks in advance,

 

Joe

 

 

[Muddy42]

Are you sure the 'aftermarket' piston and cylinder are exactly the same as the original (many aren't built with the same tolerances)?

 

Do you get the same abrupt halt with the old p&c re-installed, its all about eliminating variables?  You say the cylinder was lightly scored, it might be repairable?

 

Could there be an issue with the decompression value - making it turn off too quickly.?

 

Have you been checking compression, pressure and vacuum before and after?

[atomicduck123]

Yeah that's what I wondered, it's almost like the decompression valve closes before the engine has had a chance to fire up. I've ordered a replacement so I'll see how that fairs but doesn't appear to be any visible defects

 

The old cylinder head and piston didn't have this issue of the abrupt stopping/decompression valve.  Perhaps, as you say, the make up of the aftermarket head isn't quite fit for purpose? Would it be possible for an aftermarket part to create too much compression, hence the decompression valve closing too early? I did try honing the original cylinder, placed it back on but had no luck there. 

 

I tried to do a compression test and discovered my tester is faulty, but a new one is in the post so will give that a try as well. I've never done a pressure and vacuum test so will look into that to and let you know how it goes 

[ChrisNewport]

Does it pull over a full stroke with the spark plug out? if it does it appears like its just got very good compression,

[spudulike]

As Chris said, the only issue you have is a weak decompression valve popping out when you don't want it to. The resistance you are feeling is compression and probably isn't being helped if you lined the bore with oil from the rebuild.

Personally........

1) I would have tried to save the cylinder after ensuring it is the original part and not a £25 ebay POS. It is very possible the seller has swapped out the top end with a low cost option which if the case, is a real shame. Take a pic of the top and sides of the original and we will be able to tell. 

2) I would have pressure and vac tested the saw after rebuilding it.

3) I would have serviced the carb...I am assuming the new one is a £25 special off eBay....I have found these to be a bit unstable on the idle, OK flat out but very sensitive on adjusting the H & L settings.

4) Finally I would have tach tuned the saw for around 12500rpm...........1000rpm below manufacturers. maximum. 

My skeptical mind is saying that the seller purchased a non runner, stuck a cheap top end on, did none of the good stuff listed 1-4 so missed the original fault that caused the issue and you purchased it and it went pop.

You could get the saw going but the only aftermarket kits I would trust on a saw of this type is Meteor or Hyway. Anything else is a poor compromise unless OEM.

A cheap top end will show it's heritage as soon as a 36" bar is stuck on this 660!

METEOR Piston & Rings Fits STIHL 066, MS660 Chainsaw 54mm 1122 030 2005

WWW.EBAY.CO.UK

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Find many great new & used options and get the best deals for METEOR Piston & Rings Fits STIHL 066, MS660 Chainsaw 54mm 1122 030 2005 at the best online prices at eBay! Free delivery...

 

Edited January 9, 2023 by spudulike

[pleasant]

As Chris Newport states above- does it still do it with the plug removed? If yes, then you don't need to wait for the new decompressor valve to arrive, as you have another issue somewhere else. One step at a time to eliminate and start with the cheapest and easiest, then work up.

 

I will just add further....have you removed the bar and chain, then tried pulling it over? A couple of cuts would be enough time for the bar and chain to heat up, and if you have a chain with burred drivers where it may have flown off previous then this can jam in the bar and stall the saw. Admittedly once cooled it would rotate within the groove of the bar again so maybe not your primary issue. I cannot suggest incorrect width of drive link as the 660 takes 1.6mm which (aside from .404 or harvester) is the largest....unless you have the wrong bar with a narrow groove? But like I say, if it still does it when cold then discount that suggestion....I would still pull it over without the plug in and with bar and chain removed. Try and remove external resistance first, then delve in deeper.

Edited January 9, 2023 by pleasant

[atomicduck123]

Hi all,

 

   Thank you all so much for taking the time to reply. I've just pulled out the spark plug, given the cord a good pull and the engine is easily able to complete a stroke without any abrupt stopping.

 

   In regards to what you said Spudulike, I did line the bore with oil before installing. I took some photos whilst I was there of the original cylinder, looks like a genuine Stihl head. I've added a couple of pics of the scoring and the condition of the piston. On closer inspection, those piston rings look like they've become one with the piston itself. Would that be a result of heat? The fuel was mixed 50:1, it wasn't a particularly large log and the chain had just been sharpened. Do you think it would be at all possible, in its current state, to save the original cylinder head? 

 

Thanks,

 

Joe

[pleasant]

Blimey that piston is scrap..the cylinder is bad but may be salvagable. Although i am no expert in resurrecting these things unlike others on here. As a main dealer effectively 'making good' a repair when the correct process as far as stihl are concerned would be to replace with new oem.is a no no.

 

We had a 661 in last year...similar story to yours, although wasnt a recent purchase from the company owner that brought it in. I normally charge an upfront estaimate fee, but on this occasion it was never taken...presumably because the assumption was it had been run on a weak mix. So a piston and barrel were the obvious issue as far as my receptionist thought.

 

In a cock up of all proportions, my more 'junior mechanic assumed the same, so he fitted new genuine piston, cylinder and base gasket at £170. Labour was around £50 all plus vat.  All parts now fitted, mechanic pulled it over with fresh correct mix in the system. Fired once or twice then tried to fire a few times after than but wouldn't start and sounded pretty bad when pulled over. Further tests showed up a collapsed crank bearing on the recoil side so was leaning the mix off hence the scoring. I told guys to stop pulling it over....rung customer to explain. Not happy as bill was goinmg to be huge. Told us he wanted it back as it was when he brought it in as wasnt paying anything. So another hour or so labour later we re fitted his knackered piston and cylinder and he collected it making sure he told us we were a bunch of idiots...which on this occasion we were and i made the workshop know about how unhappy he and i was. Thinking i could salvage some loss by holding on to the fitted but never run piston and cylinder for another job somewhere down the line i asked for it back from the workshop. Upon inspection those two or three times our guys pulled it over an small part of the broken beearing had made its way into the combustion chamber an put two or three 'finger nail' indents in the crown of the piston and corresponding surface of the inside of the combustion chamber......so now I had a pot and piston I really couldn"t pass off as new, fitted but not run' on the 'bay' or fit as new to a future customers saw. Consequently i still have the piston and cylinder and out of pocket for that and at least two house labour....a very expensive lesson there.

 

I would be more than happy to fit the piston and cylinder to my own saw, but not a customers even though the indents to the crown and cylinder top are miniscule.

 

Anyone want to make me an offer?

 

Sorry slightly off topic, but COULD be a similar issue, so be careful fitting new cylinders until you have checked the actual cause first.

Edited January 9, 2023 by pleasant

[spudulike]

Yup, fortunately the top end is 100% Stihl and looks to be salvageable to me - the piston is toast but does look pretty clean - perhaps the seller changed it....possible.

There is another recent thread on this site about a MS880 and we have discussed all about cleaning up the cylinder. What happens is the saw can overheat through old fuel/straight fuel, air leak, bad carb setting etc, the exhaust side always is the hottest side, the piston alloy melts and smears itself on the bore  (known as "transfer") sealing the piston rings in to the piston thus losing compression.

In your case, I would :-

1) Clean the bore, lightly hone or roughen using emery paper.

2) Fit a new piston from Meteor, Hyway or OEM. (eBay link below) 

3) Pressure and vac test (if possible), if not, inspect the impulse line as it is the most likely culprit.

4) Service the carb and then tach the saw as I previously mentioned. The limit cap on the H speed may need pulling but the saw needs to run 12500rpm with your normal bar on the saw.

If you do this, the saw will be good and have done this process many times before.

You can stick with your aftermarket kit - choice is yours, was it a decent brand?

Do you have any idea how the saw seized? Was the fuel mix fresh (mixed under 10 days before use) with a major brand quality oil. 

Whatever you do, do make sure it isn't over revving once done.

[spudulike]

8 minutes ago, pleasant said:

 

 

Anyone want to make me an offer?

 

Sorry slightly off topic, but COULD be a similar issue, so be careful fitting new cylinders until you have checked the actual cause first.

Similarly, I once got a 346XP in with......3 top ends, the owner had just kept fitting new top ends not knowing the big end cage had failed. Often difficult to see a cracked cage (unless you have done a few).

That 661 is a good deal for someone - I wouldn't rule out eBay as long as it is a good honest description....if I had a 661 that needed work, I would consider it....obviously passing the saving on to the owner.

Usually when the cage fails, you get tell tail signs of vertical gouging marks in the bore, dings in the top end and/or port ceilings.

Fortunately I have repaired many saws in the way I have described. Most dealers will have more constraints than my business had but have had huge success with most saws - I have found the 660 top end is one of the most resilient for salvaging.