Km dropped valve rod

[Sam white]

Hi I've been buying used and non running  stihl chainsaws ,strimmers blowers etc for the last year and servicing them getting them running well and selling for abit of spare cash .

Just picked up a km unit not quite sure what model as the sticker with the serial number is missing.  I paid £25 for it . It's not seized as the flywheel turns then stops before the piston gets to the top. I thought for £25 id buy it as I was interested in diagnosing the problem and seeing if I can fix it. After taking the valve cap off it became clear one of the valve rods had dropped and was jamming the piston.  Only the 2nd time I've worked on a 4-mix engine. The 1st was a br600 I picked up for £80 . All what I did with that was sort the valve clearances and got her running great. My question is . Has anyone had this problem and did you get it repaired? If so how much did it cost? did you repair it yourself and is it worth the time to repair it. I'll repost with my progress  . Also if anyone can identify what model this is . It would save me abit of time researching it.

Many thanks 

Sam

 

[ChrisNewport]

Happens all the time if you dont keep up on checking the valve clearences every 20 hours use or so. It will be pretty much a write off, valve guide will be knackered, valve seat will be dented so a new valve wont seal and as these are parts of the cylinder it makes a uneconomical repair, piston will probable have half the valve stuck in it, con rod will possibly be slightly bent. 38mm dia piston is a km90, 40mm is a km100 and 43mm diameter is km130. 

Edited July 3, 2022 by ChrisNewport

[Sam white]

Cheers Chris.  I took a better look at it and the piston and rings looked almost new  but inside the pot is some nasty scoring.  I'll use it for parts I think . Thanks for the info on the size I'll go and check now. 

[Dan Maynard]

4 hours ago, ChrisNewport said:

Happens all the time if you dont keep up on checking the valve clearences every 20 hours use or so. It will be pretty much a write off, valve guide will be knackered, valve seat will be dented so a new valve wont seal and as these are parts of the cylinder it makes a uneconomical repair, piston will probable have half the valve stuck in it, con rod will possibly be slightly bent. 38mm dia piston is a km90, 40mm is a km100 and 43mm diameter is km130. 

Yikes I'd better check my KM90!

[spudulike]

I have seen the top collar break up on these engines. The valve clearances do need regular adjustment.

If the engine feels difficult to pull over, the valves need adjustment. The large valve gap knocks the auto decompressor out so you feel the full engine compression if the gap is too large and is an easy way of telling it is out.

The OPs machine.....strip it down, see what parts are needed and if uneconomical, sell it on or strip for parts to sell on. It is a bit catastrophic when a valve drops but you may get away with a small repair bill if very lucky.

Good learning curve and make sure you set the correct valve timing when rebuilding.

[gand]

You might well be able to find the serial number stamped on the engine. It's on the underside of the engine usually visible through a cut out in the plastic casing.