FS85 won't start when hot

[Treewolf]

We have a well-used Stihl FS85 which starts easily when cold and runs well. When it's hot though it is almost impossible to start.

 

The spark seems OK and there is fuel getting through (and it doesn't seem to flood) - it just won't fire when hot. It feels to me as though there is not much compression especially when hot, so I am thinking maybe worn piston and bore.

 

Before I rip it apart doers anyone have any suggestions or come across anything like this before? I vaguely recall reading about some exhaust fault that can make these difficult when hot.

 

Thanks!

[Mortimer Firewood]

Coil?

[Treewolf]

Coil?

 

Could be I suppose, but if you take the plug out, reconnect it and lay it on the head, there's a respectable spark when the starter is pulled.

 

Think I'd better get the spanners out.

[Rich2484]

Do you have a comp tester? If so plug it on when it's hot to check.

[GardenKit]

It feels to me as though there is not much compression especially when hot, so I am thinking maybe worn piston and bore.

 

Thanks!

 

I reckon you are on the right lines there, so best check the compression.

 

Worn (wide) plug gaps can cause this, as can too rich a mix. Fuel vapour locks can also cause it, especially if using stale fuel.

 

New plug, new fuel, quick tune, then if still no good do a comp test. Oh, and clean the air filter.

[spudulike]

Sounds like worn rings/piston, engines loose compression when hot so it may have 135psi when cold and will run but when hot it will fall to 115-120psi and cause starting/running issues - worth fitting a new piston and rings if they are worn.

 

The ring ends will be thinner than the centre of the ring when laid flat on a table viewed from the top - I don't mean the thickness of the ring in the up and down direction -hope that makes sense:confused1:

[leedeerman]

Sounds like worn rings/piston, engines loose compression when hot so it may have 135psi when cold and will run but when hot it will fall to 115-120psi and cause starting/running issues - worth fitting a new piston and rings if they are worn.

 

The ring ends will be thinner than the centre of the ring when laid flat on a table viewed from the top - I don't mean the thickness of the ring in the up and down direction -hope that makes sense:confused1:

 

Not likely to be piston rings IMO and experience, more like a crankcase leak with those syptoms.

[WoodED]

my fs85 used to die when hot.. but it was the coil..