# Stihl FS86 trouble: Rings, gaskets, carbon deposits, and ring gap



## ralphbsz (Aug 4, 2014)

Warning: Amateur ahead! I have two Stihl machines that are giving me engine trouble: An FS86 trimmer, and a BR400 blower (I also have a host of other 2-stroke machines, some of which get even more use, and all of them are fine). They are the same age, about 16 years, and both get serious amateur use, about 100-200 hours per year. This being California, they are usually used in very dusty conditions. Neither have had serious maintenance, other than replacing spark plugs regularly, and in the case of the BR400, a new ignition module a few years back. Carburators are clean, and are adjusted as per manual. Exhausts are OK, with the screens in good shape (not plugged up). I have multiple air filters, and try to keep them clean, but on the FS86, that's hard, and it may have eaten some dust (the flat air filter doesn't always fit the filter housing perfectly).

Both have compression loss, with the FS86 at about 120 psi, and the BR400 at 110psi cold, and 80 psi (!) hot. Fuel is automotive premium (typically 76 brand), with Stihl 2-stroke oil mixed 50:1. 

Interestingly, the symptoms are very similar. They are both hard to start. Once they start, they run for maybe 5 or 10 minutes, and when they warm up, they don't want to run any longer. What happens is that the RPM under load cycles up and down every few seconds, and when the RPM goes down, they often stall. By playing with the throttle, I can sort of keep them running. On the trimmer, applying the choke half-way makes it run a little better; on the blower, that completely kills the engine.

Given the low compression, I've decided that they need a top-end rebuild. Even if there is another problem that is responsible for their behavior, the low compression needs to be fixed first. So I pulled the cylinders from both of them. The news on the BR400 is pretty good: All gaskets are in good shape, and there is absolutely no scoring or wear on piston and cylinder. The inside of the crank case looks clean. There is a thin layer of carbon deposits on the piston, and on the inside of the cylinder (the combustion volume), and in the exhaust port, more about that below. So I'll just order replacement rings (either from the local dealer, or from some online place like DLA), and hopefully that will fix all problems. Do you guys suggest some other parts to replace? For another $80, I could just replace cylinder, piston, and all gaskets with a kit; is that worth it?

The FS86 is worse. None of the gaskets on the cylinder survived; it looks like it overheated at some point in its life. There are black/brown deposits in the crank case. There is very light abrasion on the side of the piston (strangely, on the intake side), none on the cylinder. Very heavy layer of carbon deposits on the piston; on the cylinder the deposits are thick enough that they are flaking off. I think I could get compression back to normal just with a set of rings, which are also available from places like DLA.

But there is one big problem: I can't find replacement gaskets for the FS86. The local Stihl dealer says that they are no longer orderable from Stihl. I can't find any online store that sells them (or perhaps a complete cylinder/piston kit that includes the gaskets). I might be able to piece all the necessary gaskets together by getting them from ebay or smaller dealers. Anyone have a good idea what to do? Other than buying a new trimmer?

Next question: What to do about the carbon deposits? I'm tempted to just leave them, or try to remove them with Seafoam or similar. In the exhaust port, I might scrape them, but not on the piston or in the cylinder. Advice please?

And the last question: After I get new rings, does the ring end gap need to be adjusted by filing? On 2-stroke motorcycles, this is necessary, but I don't know whether hand tools (with their two rings and more standardized piston sizes) also need adjustment.


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## backhoelover (Jun 6, 2015)

check tank vent with vacuum 
stihl decarbonizer for carbon or you can clean it out with something plastic your muffler heat up with a torch till it red hot then blow out with compressior when it is still hot 
the long part at the end of the ring is the bottom 
if old gasket is still in tach go to auto parts store get a roll of gasket material 
keep me up to date 
there is no 86 manual


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