stihl fc 75 trimmer runs loud like a piston knock

[dswiss2424]

I was wondering if this is normal or not i have just purchased two older used stihl trimmers one is a fc75 and when running it sounds really loud like a piston knock it runs and trims fine it just has a reallhy loud knock or pinging noise has anyone else encountered this? any input would be appreciated thanks

[topsites]

Does it turn over (without starting) fine?

You'll have to get it in a quiet spot and see if you can hear or feel anything as you turn it over,
with the ignition off see if it does anything out of order, like a scratching or dragging feel or a noise.

But no, and not to break bad news but it doesn't sound good.

[rotti1968]

One thing you could be hearing is the clutch drum and clutch making noise. If you take the recoil off the motor and turn the hub by hand or kind of turn it back and forth do you hear it ? also while your there see if you have excessive end play or up and down movement. If you have end play or up and down movement then you have bad crank bearings. But I have noticed over the years of working on older Stihl equipment they tend to have noise from the clutch and drum and that is just from wear and use .