KM100 Problems

[gardenmad]

Hi all.

I have a stihl KM100 Combi motor. One of the valve retainers broke the valve dropped and bent and put a dent in top of piston. I replaced the obvious but now I can't get it to fire up. The coil is fine, carb is fine. I put some petrol down the head and it reluctantly fired up and stopped. Does anyone have any ideas? it seems like there is a fuel problem it's not the carb as swapped it for one I know is ok....Any help appreciated. Thanks

[pleasant]

Common problem on the 4-mix engine, and Stihl know of it. I wrote off one of my customers engine because of the same fault and cost of repair (very badly damaged) and he complained to Stihl even though it was out of warranty and they collected it and repaired it free of charge for him.

 

...anyway- have you done a compression test?

[gardenmad]

Hi I haven't done a test on it It did seem the faster I tried to turn it over the closer it was to firing . I am thinking valve problem? (Shot in the dark) I have read another post where the valve clearance is essential? unless I have not done something right on rebuild. It seemed straight foreword just didn't want the expense of taking into shop

[pleasant]

Do a compression test first.

[gardenmad]

OK Never done one before is there a certain reading it should be?

[pleasant]

Anything below 100psi is junk. 100-130psi it should start, and ideally you want 150psi+

[gardenmad]

Well I guess it's junk lol it done 60! so I take it the barrel is knackered as well? something else for the spares bin!! Thanks for your help

[pleasant]

Well, assuming the valve(s) are seating correctly, then at only 60psi I would certainly start inspecting the internals (piston/ring/barrel etc)

[GardenKit]

This engine has a decompressor which reduces compression at cranking speed. This will affect the normal compression test readings.

 

The decompressor lever can also stick in the cam gear and not close.

 

This may be worth a look before pulling down the cylinder.

[gardenmad]

Hi thanks to both of you. I have fitted new piston and rings. I looked at the barrel and there are no marks at all that's why I can't understand it as it was running fine. Gardenkit is the decompression thing behind the cam wheel? Thanks for all your help