Stihl FS-55 locked at bottom of stroke

[Jimbo]

Hi folks,

I replaced the perished fuel hoses on my FS-55 brush cutter at the weekend. On pulling the cord to start there was a metalling slapping noise and the engine locked immediately. It never actually started

Stripping away the housing, I can get a socket on the pull start mechanism and with the plug removed, I can see that the piston gets stuck at the bottom of the stroke.

I don't think it's seized as it ran okay before, and it locked immediately, so no chance to run, get hot and seize.

Will strip down over the next couple of days, but wondered whether anybody else had seen this problem and could guess at the root cause?

Thanks
J

 

[Burnerman]

Ye gods that does sound terminal.....wondering if the engine was full of fuel when you tried to start it?
This is called a hydraulic lock and can bend con rods and fracture pistons.
Please post back with what you find!
John

 

[Jimbo]

Ok so I have a banana-shaped conrod.

Replaceable but I can't understand why the cylinder would have been full of fuel unless the carb is faulty, and the carb was relatively recently replaced with a genuine Stihl part.

I don't want to rebuild only for this to happen again - any clues why a cylinder would fill up with fuel?

 

[Burnerman]

Blimey, what next? Talk about bad luck!
There's really only a couple of ways that the engine can fill with fuel......being stored upside down maybe, or even the tank can be pressurised by sunlight / heat if the carb needle lets a tiny amount of fuel through.
I can't say for the FS 55 but pattern cranks / pistons / barrels are available for the popular Stihl machines....genuine parts are eye wateringly expensive.
John

 

[Burnerman]

Actually, the genuine item doesn't seem to be too bad, price wise at around £70.
I think the part number is 4144 030 0400 but do please check.
Firms such as L&S Engineers are top dog for info like this.
John

 

[Jimbo]

Actually, the genuine item doesn't seem to be too bad, price wise at around £70.
I think the part number is 4144 030 0400 but do please check.
Firms such as L&S Engineers are top dog for info like this.
John

Yep. I already have that part in my basket. Trying to find advice on how to remove the crank bearing without damaging it, I don't have a bearing puller small enough but if it's quite robust, a pair of screwdrivers might do the trick...

Update: bearing puller fit after all! Onwards and upwards!

 

[Burnerman]

It could be worth replacing the crankshaft oil seals at the same time.....if air gets in, the carburation all goes to hell.
A gasket set may also prevent misery!
John

 

[JohnD]

presumably this is a 4-stroke engine?

 

[Keithmac]

If you give it a couple of gentle rotations before starting you could tell if it's hydrolocked before giving it the beans.

 

[Burnerman]

I'm sure the 55 is a two stroke motor.
Stihl have produced a four stroke engine as a wet sump, but that didn't last long and now they use their 4-mix engine - which is a four stroke motor with total loss lubrication i.e it's run on a 50:1 petrol / oil mixture.
John

 

[Jimbo]

It could be worth replacing the crankshaft oil seals at the same time.....if air gets in, the carburation all goes to hell.
A gasket set may also prevent misery!
John

The oil seals are in superb condition, as is the head gasket (the only gasket affected). The piston and barrel are very clean.

That said, I'm quite tempted to replace the piston and barrel anyway, in case there's been some deformation that isn't obvious. In which case for a few more pennies I'll add gasket and seals!

 

[Burnerman]

If the piston is scored, the barrel will be too - if not, and the rings are free then back on it goes - or it would be if it was mine! With these machines, a peek down the exhaust port reveals all nasties.
For sure, the cylinder base gasket nearly always splits.
Good luck with the rebuild - the cylinder base is tapered to accept the piston, just keep the ring gaps over the pegs. The piston will have an arrow pointing to the front.
John

 

[Jimbo]

So quick update: Been getting on with other things the last couple of weeks. Just went to rebuild the brushcutter and discovered that the crankshaft I've been sent is the wrong one. It's the part I ordered, but the website simply lists the wrong part number for my machine. The conrod is too short; at the bottom of the stroke, the counterweights will hit the bottom of the piston.

I contacted the dealer, DIY Spare Parts, and they've basically refused to take any responsibility, even though it was their website that indicated this part number. Unfortunately it's after the 14-day return period and the part isn't faulty so they're saying it's my problem. So point 1 - never use DIY Spare Parts, their customer service is awful and they'll avoid taking responsibility for their mistakes if they can.

The part they now tell me I need has a different part number to the one that came out of the unserviceable machine, so I'm not sure they're giving me the right info even now. I've emailed Stihl for help.

Stihl appear to offer a part number lookup service to their registered dealers, but it's not available to the public. Given the sheer number of different part numbers for different variants of machines this is staggering. I wonder how many people end up buying Stihl parts in good faith, only to find that the part isn't right.