One of these valved engine Stihl strimmers

[Baldbloke]

I’ve had one for a few years (decade) and the things steadily lost compression over the last couple of years.

I see that the barrel and piston kits for these things are about double the cost of doing a more conventional 2 stroke repair. Over £100. I presume these engines never caught on and I should probably scrap it as an uneconomic repair?

[arbwork]

check valve clearance, 1st may be easy fix!

[bmp01]

1 hour ago, Baldbloke said:

 

I’ve had one for a few years (decade) and the things steadily lost compression over the last couple of years.

......don't they have some decompression mechanism thingee on the valve train, for easy starting. ....? That's from memory, I could have imagined it , but if true how would you know if the compression was low? Can't see why they would wear out the cylinder or rings unless mega mileage,  no ports to worry about. 

The one i looked at (a non starter) - one of the push rods had popped out of the tappet due to clearance issue. Put it back together correctly, started like there was never anything wrong with it. Don't give in yet 

 

 

[Richard 1234]

check valve clearance, 1st may be easy fix!

This.
They don’t like starting hot at all either when valve clearance is wrong

[spudulike]

The compression usually goes up if the valve clearance is too great. There is a reason for this but I won't bore you with it!

[Baldbloke]

All I have done so far is to remove the tappet cover to check that there is some free play/clearance. Also can confirm push rods appear to be correctly in place

[lurkalot]

Yep, make sure tappets are set correctly, very important on the 4Mix engines.  You'll need a special feeler gauge with a narrow end on it to set these properly.  You can obtain these from your Stihl dealer.

 

As for compression, it shouldn't have much at all if the tappets are set correctly, and the decompressor is working properly.  Though they do have a habit of destroying the decompressors.  They part company from the plastic camshaft.  Not one of my favourite engines if I'm honest.