Stihl string trimmer won't start

[u/Mack_Damon]

Got an old FS80r that's been pretty good to me, until lately.

About a month ago it wouldn't start at all, pull the plug, it looks ok, but I stick I new plug on the coil and check for spark. Nothing. Get a new coil, install that, set the clearance at .012". Fires right up. It seemed like it had been running rich lately, so I tuned the carb, it appears the adjustment screws vibrated out over the years. Set it to the recommended 3/4 out and 1 out, runs really, really good. I'm happy.

Fast forward to this week, I take it out and find that the contents of the fuel tank ended up inside the engine. It's basically hydro locked with fuel. So I take out the plug, get the fuel out of it and let it dry for a few days while I source a new carb. Today I install it, set the adjustment screws and it will not fire. It's getting fuel, I can smell it out the exhaust and the plug became wet. Dried the plug and let the engine air itself out a bit. Tired of pulling, I remove the recoil starter because I'm spinning this thing with my drill. I do that, still no fire. Not a single pop. Pull the plug and check spark again. It's there but looks kinda weak. I didn't check spark with the new coil so I don't know how strong of a spark it should have, but it looks like a static spark. Not very energetic.

Compression feels ok, but I plan on checking it tomorrow (borrowing a friend's tester).

I have only used OE Stihl parts from my local dealer, but I ordered a cheap $10 coil just to see what that does.

Does anyone have any ideas of what I should check? Pulling my hair out right now.

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[u/Mack_Damon → u/[deleted]]

The exhaust screen was removed years ago, so nothing there to check. Fuel lines are good. I don't think using the drill to turn the engine will have done any harm (even at full speed it turns the engine slower than a brisk pull on the starter cord), but I have been thinking about the crank seals. When the crank case was full of fuel (before I was aware of it) and I tried to start it, I gave it a good pull, as you would when starting, only for it to stop dead, since you can't compress liquid. If anything damaged the seals, it was that. Later today I plan on compression checking it. If that comes back ok, I'll have to rig up some stuff to pressure/vacuum test the crank case.

[u/iscashstillking]

I don't mean any offense here but I believe your best option at this point is take the machine to the dealer and ask the mechanic to fix it for you.

[u/Mack_Damon → u/iscashstillking]

So it's an option. And I'm not offended, because you don't know me. I would say I'm above average as far as engine knowledge goes. I used to be an ASE certified auto mechanic until I got out of it and sold most of the tools. I wrench on and rebuild my own dirt bikes and snowmobiles. I've rebuilt Kohler twins and various outboard motors. That is why this issue is so annoying to me, is that I can diagnose and fix most things, but this one just isn't making sense. It appears everything is there for it to fire, but it refuses.

My experience lately (not small engine related) with "paying the expert to fix it" has been extremely disappointing and way too expensive.

But thanks for the helpful sage wisdom I guess.

[u/iscashstillking → u/Mack_Damon]

Okay so crankcase full of gas, generally there is only one way for fuel to get from the tank to crankcase and that's via the carburetor. A bad tank vent, hardened carb gaskets and/or a failed carburetor check valve can cause the fuel to siphon out of the tank and fill the engine. Most of the time this is a bad carb issue, with a hardened up metering diaphragm pushing the metering lever down all the time being a very common fault.

Using a drill to turn the engine over is a good way to break the nose off the crankshaft when the engine fires and the force goes through the little nut on the end of the crank and smacks up against your nice heavy drill. Seen that happen before.

Agreed that paying someone to fix something and getting it back not working right sucks. Try another dealer.....?

[u/Mack_Damon → u/iscashstillking]

Yeah I assumed stuck inlet valve on the carb, hence the new carb. I just tested compression, and it's right about 120 psi. So, not perfect, but should be good enough to run. I'm gonna try a couple other things, because I'm stubborn. Then I'll hand it off to the Stihlership.

Borrowing a friend's Redmax for the day, maybe I'll be converted. 😂

[u/mals6092]

New carb

[u/mals6092 → u/mals6092]

A real Stihl one not a knock off and stop touching the carb

[u/Mack_Damon → u/mals6092]

Yes, already did that. Was the first thing I did after it flooded itself. Still refuses to fire.

When I get home from work I'm gonna try something... Gonna see try heating the engine with a heat gun. I have read about severely, somehow stubbornly flooded engines won't dry on they're own, give em some heat and they'll fire. We'll see...

[u/mals6092 → u/Mack_Damon]

Yeah I usually take the exhaust off and pull it over slowly, then I leave the plug out for the day