Hello,

I have an old Stihl FS96 trimmer that I picked up for close to nothing last year. My original plan was to remove the handles and put them on my much newer machine (an FS120). However once I got it home I began to notice that it was in very good condition considering it's age (The model debuted in 1987). I could see through the exhaust port that the piston was scored most likely from the machine being run on straight gasoline. After some time and ebay hunting I managed to locate a new old stock piston and head for a good price. I installed them last week and we got the machine running! Awesome!





Now here is where i'm having trouble....it looked like I had a fuel leak between the carburetor and the head so I ordered a replacement gasket and installed that but i'm still leaking fuel.

The gasket that I replaced is part# 3 on page 8 of this link.

http://www.motoculture-jean.fr/upload/pdf/FS96.pdf

I should note that the new gasket that came in was a simple flat "paper" gasket whereas the old gasket had bumps and groves embedded in it. Is there something that i'm missing here or do I need to get some sort of gasket material to get this sealed?

The arrow points to where I "SUSPECT" the leak is coming from.


Here is the old gasket.

__________________
1982 300GD Carmine Red (DB3535) Cabriolet Parting Out
1990 300SEL Smoke Silver (Parting out)
1991 350SDL Blackberry Metallic (481)

"The thing is Bob, its not that I'm lazy...its that I just don't care."

The bumps and grooves would be formed by the gasket being compressed at installation, when new the gasket would be flat.

My WAG internet diagnoses, I'd suspect that the leak you describe is the actual cause of the scored piston. It may leak fuel when off, but when the engine is running and that area is under vacuum, it'll be pulling in air from that same area and leaning out the air/fuel ratio. You can test this by spraying the area with carb cleaner or WD40 or even water while running. If the engine changes tone or idle raises or lowers, that's strongly suspect. Make sure you're not getting any carb cleaner or WD40 in the intake of the engine while this is happening, as that'll fowl the results. The gasket you removed looks fine on the visible side. I'd closely inspect the mating surfaces on the carb and the engine, make sure they're very very clean, and then double check that the carb is mounting securely and compressing the new gasket. If there's still a fuel leak, I'd check the carb gaskets and diaphragms and such. Complete carb rebuild kits are available on Amazon for $8 or so.

__________________
617 swapped Toyota Pickup, 22-24 MPG, 50k miles on swap

Well your WAG turned out to be 100% correct! I reopened the unit, removed the gasket that I installed, opened the fuel cock and just watched. Sure enough fuel started leaking from the gasket on the bottom of the carb (Which is what made it look like it was coming from the gasket that I installed)

I'm going to spring for a full rebuild kit for the carb to settle any other problems that it might have. The tricky part now will be finding the correct kit...as best I can tell it's a model 38B but I need to triple check this (Right now I have to run back to work)

__________________
1982 300GD Carmine Red (DB3535) Cabriolet Parting Out
1990 300SEL Smoke Silver (Parting out)
1991 350SDL Blackberry Metallic (481)

"The thing is Bob, its not that I'm lazy...its that I just don't care."

Good news! My old machine is now running wonderfully! Details below!


OM617YOTAs WAG turned out to be 90% correct. While the carburetor was indeed leaking from where turned out not to be the gaskets. I ordered the rebuild kit and redid most of the internals (The only thing I didn't replace were the welch plugs which were still in great shape) In the end this turned out to be a thing as there was quite a lot of debris which needed to be cleaned out anyways so i'm glad that I got the kit.






The real kicker is that the leak was actually coming from the end of the throttle shaft which was VERY difficult to see. I removed said shaft and unfortunately there are no seals which could be replaced so the shaft must have simply worn down over time. I was afraid that I would have to buy a whole new carburetor but a bit of barn yard engineering presented a solution. The side of the shaft which was leaking has the throttle spring pushing against the body of the carb. I thought that if I managed to get an o-ring in between the two that it just might seal. A rubber o-ring was too thick but amazingly the carb rebuild kit came with a paper gasket which did and much to my surprise it worked! I'm not sure how long this setup will last but for now all is well!

__________________
1982 300GD Carmine Red (DB3535) Cabriolet Parting Out
1990 300SEL Smoke Silver (Parting out)
1991 350SDL Blackberry Metallic (481)

"The thing is Bob, its not that I'm lazy...its that I just don't care."