View Full Version : Stihl FS75 carb cleaning

09-19-2009, 10:07 AM
I have a FS75 Stihl weedwhip that will idle all day, but has no acceleration. When the throttle is wide open, it just bogs. If I feather the choke and the throttle at the same time, I can get it to spin up, and it will stay that way as long as the choke is nearly all the way closed. When I remove the choke, it will go back to idle.

Based on this, I have deduced that the main nozzle is plugged. I just bought this used with this problem. I have opened the carb and found deposits at the welch plugs, so I pretty much know that bad-gas/varnish is the problem.

The carb that I have is the Zama C1Q-S45 and this does not have H or L screws...just an idle screw. The Zama TechGuide_2007.pdf states that the mixture screw must be removed before removal of the nozzle. The nozzle is a screw in type (if I recall correctly).

So, my question is, can I just remove the nozzle, clean it up and replace it with no problem? Or, what is the procedure for this? Or, how can I get this back up and running?


09-19-2009, 02:12 PM
That is not the nozzle. What is screwed in is the jet. The nozzle is pressed in and has a check valve in it.(which is fragile). Nozzle is non serviceable for all intents. Stale fuel kills them (the check valves). I'm sure some "experts" (one hammerhead in particular) will chime in to debunk, but, be very careful around the check valves. Soak the carb in fresh fuel over night and try a kit for about $12.00. I think it's a RB-69. Don't be surprised if it doesn't work. New carb is about $65.00.

09-19-2009, 06:02 PM
From what I can tell, the kits do not have the nozzle, check valve or jet. I've read on another forum that idling with 25% Chevron Techron (I used Yamaha Ring Free, which is the same thing) & 75% 50:1 for 20-30 minutes might do it. I tried it and junk (carbon?) was dripping from the muffler, but it didn't free up the High speed path...presumably since it was just idling...

I've also read that dechlorinated brake cleaner is safe on these carbs, but compressed air is a no-no...unless at 20psi and not directly into the orifices.

I'd really like to get this back on-line without springing for a new carb. If I were to unscrew the jet, would that allow me better access to the obstruction?

09-19-2009, 07:07 PM
No, the kits don't. Parts are available but take forever to get. Brake cleaner is best and NO compressed air. Problem is, IF the check valve is stiff or sticking; spraying cleaner directly into the jet will lift it off it's seat and everything looks fine. But when running, and you go from idle to high, it doesn't react to the pressure change to allow fuel into the venturi. Problem is compounded by it being a fixed jet design. And yes, if you can remove the jet. It will allow you to see the check valve. Again, sometimes soaking them in fresh fuel will work if they are just gummy. Carb cleaning dips might do more damage than good. BTW. I have tried replacing the nozzle. It is a PITA at best, and if it is not exactly the correct height (or depth), the engine will run poorly. Isn't Techron a carbon cleaner? Stihl does not recommend adding anything besides oil to fuel in their products. You have no idea what it will do to fuel system parts or any other rubberized parts. Try a soak and cleaning along with a kit. It's the cheapest and you don't know if it will/will not work till you do it. I have had carbs at work that were fairly clean and not work, and have had carb that were gooey yet cleaned up fine.

09-20-2009, 05:16 PM
I removed the jet and I don't think there was any obstruction through that...just a bit of gas. I blew through it and saw a nice clear shot of daylight.

Next, I sealed my lips around the nozzle part of the carb and blew through the nozzle and with an exhale could blow air though and with an inhale, the check valve would prevent airflow. Good, the check valve is there and functioning in some capacity. However, I noticed that I had to use a bit of pressure to open the check valve after an inhale seated the valve. So, I think there is a bit of junk in the check valve/seat that is preventing the check valve from opening with the vacuum/pressure found within the carb. I've got the carb soaking in fresh gas right now. If that doesn't work, I'll try some brake cleaner at the check valve.

Do you think my symptoms are indicative of no gas going through the nozzle/check valve? This is where the high speed fuel comes from, right? Remember, it will idle all day and will throttle up with the choke nearly all the way closed...allowing for the add'l fuel to come from the idle circuit, I suspect

09-20-2009, 09:59 PM
Correct, and there is nothing more you can do. And it is a symptom I have seen all too often with primer equipped carbs (because of the check valves).

09-22-2009, 01:56 AM
I left it soak in fresh gas for a day and did the breathing through the nozzle trick and it seemed better...less sticky on the exhale after the inhale...maybe... Then, with a few more breaths, the check valve didn't work (as if something was obstructing it)...I could inhale and get air. I put it together, started it and it would spin up, but wouldn't idle. I let it sit for an hour and tried it again and this time it would idle and would spin up, but only with a bit of help from a temporarily closed choke...it is better, but still not 100%. I took it apart and the check valve was working again. So, I put it back in for another soak. We'll see tomorrow...and, I'm going to pick up some brake cleaner for some higher intensity cleaning.

My question is: What does bad gas do to the check valves? Does it distort them and ruin them forever or just coat them with crud that has to be cleaned away? Is dechlorinated brake cleaner the toughest stuff I can use to clean the crud away?

09-22-2009, 09:05 PM
Stale fuel does both. As for them working, that's the problem, sometimes they work, sometimes they don't. I will tell you they HAVE to be wet with fuel to work at all. Strange things happen to fuel when it gets old. I've seen fuel lines that you could smear like grease of being exposed to stale fuel. Brake cleaner is pretty much harmless to rubberized parts. Sitting with the fresh fuel might do the trick. Not to bum you out, but there is a check valve in the idle circuit also, but that didn't come up because at first it was working. If anything, you will learn never to leave fuel in equipment. One thing, the inlet needle lever must be level with the body of the carb. If too low, wont accelerate clean.

09-22-2009, 10:04 PM
I have to post here...even though I have bad news. Today, I had a correspondence with Zama customer service. He mentioned that he would first try methanol, then dechlorinated brake cleaner. I got both...a bottle of HEAT (methyl alcohol variety...not the isopropyl version) and the brake cleaner. So, I got home to the second day of soaking, it still acted the same. I didn't have a good method to apply the methanol, so I first tried the brake cleaner. After some directed squirt, it seemed to be working...maybe the fuel soaks loosened up the stuff. I tried the inhale/exhale test and the check valve seemed to open OK. I was going to put it together and did just a bit more. I put the straw of the brake cleaner into the nozzle and pushed it tightly to make a good seal and added a few more squirts...then POP. I knew instantly what happened. I popped off the check valve...uugh! The Zama guy gave me a part number for the nozzle. I might try to press in a new one...we have a press at work...else I'll watch eBay. Drat!

Indeed, I have learned to not leave fuel in the machines. What is the best method to rid the machine of the fuel? Drain it, then run it until it won't run? I've heard of fogging...what's that procedure?

10-04-2009, 11:41 PM
For $23 shipped on eBay, I found a used carb. Of course, when I got it, it didn't quite work right. I took it apart and found that a welch plug was out of it's spot, so I pressed it back in and gave it a once over cleaning and it fires up and runs like a champ.

Lesson learned. Yesterday, I fired up all my two-cycle engines, then drained the gas and ran them till dry.

I have a friend that has a Stihl leaf blower that wasn't running quite right. He added sea-foam to one tank and now it runs like a champ. Does anyone have an opinion on sea-foam as a preventative measure for 2-cycle engines (carb cleanout)?

Turf Dawg
10-05-2009, 01:23 AM
If it just needs a light cleaning Sea Foam is great. This is the only "snake oil" that I use. Well I have been know to use some ATF in the spark plug hole every now and then. I know some people will disagree with me, but on these new carbs I think it is best to go get a new one and stick it on there.