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Old 09-26-2010, 08:35 PM
Roger Roger is online now
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Join Date: Dec 1999
Location: McMurray, PA
Posts: 5,805
Stihl FS90R carb repair - replace gasket/diaphragms (pictorial)

I have used Stihl handhelds for many years. I used an FS85 string trimmer for many, many years, and replaced it about four years ago with an FS90R (4-Mix). I replaced gaskets and diaphragms in the FS85 after a few years of use. Likewise, I did the same task with an FC85 edger.

I started having some unusual operational patterns with my FS90R a couple of weeks ago. It was demonstrating slight rpm variations when trimming, but typically only on the first job of the day. Also, sometimes it would choke out, that is act like it was depraved of fuel. The choking pattern was erratic. Some days it would never happen, other days it would happen a couple of times. It often happened when I was switching from right hand to left hand use.

The trimmer is about four years old. I work solo, mowing/trimming about 1,100-1,200 yards per season. Typical use on one yard is 10-15 minutes of use. It has had other concentrated use (trimming down a bank, along a creek, etc.), but most has been routine turf trimming.

The pattern was strange, not very well defined. I did a quick valve adjustment check, and found no change was needed. I checked the fuel pickup and found the filter to be clear of any debris. I checked fuel lines, and did not find any kinks, or other changes that might be cutting off fuel.

Having run out of ideas for further diagnosis, I concluded the carb diaphragms were getting weak, and the carb was not pumping fuel as it should. I got the information off the carb, Zama C1Q-S110A. This was a number different than I found in charts on Internet (this is critical information). I ordered a kit from Power Equipment Warehouse. Lisa helped me get the right parts for my carb.

I took the carb off and replaced the gaskets and diaphragms. What follows is a description of that task, and pics to support the replacement. If you follow the pics and descriptions, perhaps you will learn how to do the task yourself, or if you wish to farm out the work to others. Also, you will learn what I found, and the problem. Follow along ...

The carb is removed from the machine. Remove the air filter cover, the air filter. Inside the air filter housing are two nights that hold a sandwich of the housing and the carb. Two posts come from the engine, and the nuts are at the end of the posts.

The housing comes off, and the carb comes off over the two posts. Three hoses attach to the carb. Two are smaller lines going to/from the fuel tank. The third hose is a larger hose that runs from under the engine. I am supposing this to be a vacuum hose that drives the diaphragms to make the pumping operation work (somebody more skilled can confirm or deny my assumption).
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