MS390 decompressor failure

[windfall]

Hi All,

 

My mate came round this afternoon to borrow my saw to disc up sycamore he dumped here on saturday, i leant him my trusty MS390.

 

He had done half a tank when he came up to the house saying it just stopped and now would not turn over. As he is a bellend i suggested the chain brake but he came back in the kitchen saying no its still not working.

 

So after tea i whipped the plug out and it was squashed and bent. Arse.

 

Saw is now in a million peices. During the strip down i thought i had lost a bit of the decompressor and figured it would turn up.

 

It did, in the bottom of the crank case!!

 

So the barrel has a fair few dents as does the piston, in general though the piston and barrell are is very good condition as is the small and big end bearings.

 

Questions are as follows:

 

Should i replace the piston small end and bore or just clean it up the burrs with the dremmel? Just a bit concerned its all had a good shock coming to a sudden stop. Thing is i dont think i have much to loose other than time replacing the decompressor etc and seeing how she fairs.

 

Is this a common failure? seems bonkers to me that this could happen!

 

Any tips or advice most welcome

 

ps if i ever see anyone from stihl id like to insert this decompressor sitting on my keyboard into their bum hole.

[Mike Hill]

Take the "Sissy Valve"out of your Saws,replace with a Bolt and learn to start them properly.LOL.

[windfall]

Good plan, i never use the "Sissy Valve" anyway as I am no Sissy. lol

 

will save a few quid too! and every little helps.

[spudulike]

Very difficult to judge the damage - put some decent pictures up on this thread.

 

I would think that the con rod may well be bent - the way I would check this is to pop off the piston rings (with the piston still on the rod) insert the piston in to the cylinder and bolt it down so it is held in position but you can still lift it 5mm upwards, spin the motor over with the pull start and watch out for the cylinder rocking from side to side - apply the lightest of pressure whilst doing this - you should be able to spin the motor over slowly or with around 1/2 starting speed without the head moving or rocking.

 

Areas of other damage to look out for are around the transfers, inlet and outlet port, gouges in the plating and debris in the crank bearings. Make sure the piston crown isn't cracked!

[Forest2Furniture]

I had 2 of those break on my 660 within 2 months. After the first one I had to replace piston and pot, when the second one went I knew what the problem was and stripped down the saw straight away.

I spoke to a Stihl service engineer who told me the decomp valves are a pain so I fitted a blanking plug. Just as easy to start as before and no problems, the engineer also suggested I wait until my new 880 runs out of warrenty then I should change that valve for a plug as well, that'll be interesting to start!

[windfall]

Hi Spud,

 

Yeah checked both of these, conrod and ports both seem fine.

 

My 660 will be stripped and the sissy valve will be removed as soon as the 390 is up and running again!

[spudulike]

I had 2 of those break on my 660 within 2 months. After the first one I had to replace piston and pot, when the second one went I knew what the problem was and stripped down the saw straight away.

I spoke to a Stihl service engineer who told me the decomp valves are a pain so I fitted a blanking plug. Just as easy to start as before and no problems, the engineer also suggested I wait until my new 880 runs out of warrenty then I should change that valve for a plug as well, that'll be interesting to start!

 

The 880 will be a pussycat to start compared to my 298XP - no decomp and a small pulley - thought it was seized when it came in:thumbup:

The art is in making sure the saw is in fine tune and will start on the two choke pulls and one fast idle otherwise hope you are a big bloke - being 6'2" and 15 stone helps:thumbup:

[windfall]

I went to my dealer this morning, got a new exhaust bolt to replace the one that sheared during strip down and a blanking plug for the decompressor.

 

Just got to put it back together now!

 

Any tips Spud?

[spudulike]

I went to my dealer this morning, got a new exhaust bolt to replace the one that sheared during strip down and a blanking plug for the decompressor.

 

Just got to put it back together now!

 

Any tips Spud?

 

Yes - send it to me:thumbup:

 

If the piston has been removed, make sure you get the direction correct and the circlips in correctly - it is very easy to get this wrong, removing the handle gives you extra space to work in, oil the bore before reassembly, make sure th eopen ends of th erings are central to the piston lacation pins, make sure no chips or crud has fallen in to the crank, lube the big end, small end and crank bearings, ease the cylinder over the piston, make sure the closed part is located in the cylinder held at a slight angle to the piston, ease the ends of the ring in to the bore carefully - leave the mallet in the box:lol:

 

When fitted, do up the bolts in a diagonal cross pattern and at even pressure same as any cylinder head, turn the engine over slowly when thehead is slightly loose and check for catching and then crank it down.

 

Make sure all boots and carb mating surfaces are holding presure - I always pressure check and compression check at this stage as it is important for me to know the health of he rebuilt top end but many are less festidious.

 

I than tach the rebuilt engine but hope that you will get away without doing it.

Hope it all goes well - be careful not to break a ring - I never have but.....

[windfall]

I am struggling to get the piston into the bore due to the shape of the cylinder, tried a lovely jubbly clip and all sorts. Next step was to visit my neighbour to see if he has a ring compressor.

 

If its possible without one then ill have another go!

 

How much do you charge for a port and exhaust open out and tune spud?