FS45 won't start - need some ideas!

[buening82]

I have an FS45 that is about 10yrs old and haven't had any issues with it until this spring. This spring I added new gas mix (fresh gas and new Stihl mix per directions) and it ran fine the first use. The last (2nd) time I went to run it, whenever I gave it a lot of throttle (past 3/4) it tried to die, but letting off the throttle completely it also attempted to die. In the revving up condition is when it'd run, so I was blipping the throttle constantly until it got up to operating temp and then it worked fine with good throttle response. I went to use it again (3rd time) and now it won't even fire. Pull and pull and pull and no attempt to even fire, choke or no choke. I first pulled the spark plug and I can see spark when pulling the cord. Plug was wet from my start attempt. I pulled the in-tank fuel fillter and it looked perfectly clean, but also installed a new one. I pulled the carb and completely took it apart and cleaned good with carb cleaner and reassembled. The carb bulb primes fine, and I tested the prime function when I had it pulled off and it gave a good squeak. I then put a spoonful of gas into the cylinder to bypass any potential fuel delivery issues and still nothing. Air cleaner is new. I have air, fuel, and spark so the next thing I checked was compression and I got 105psi. I'm at a loss. Any ideas?

Can the ignition modules slowly go out, i.e. there is spark but has weak spark? I could see spark but it wasn't crazy bright. Does anyone have instructions on testing the coil? I hate throwing parts at something until I figure out the cause, especially a $50 coil on a $150 trimmer.

 

[Fish]

You could just have it still flooded. Dump out all of the fuel and remove the plug, and pull the rope 20 times or more.
Reinstall the plug and pull the rope, trying to get it to start. It should eventually start to run briefly, then die a few seconds later when the fuel is burned off.
Then refill with fresh fuel and try to start.

 

[bertsmobile1]

Firstly the spark on trimers & chain saws is always feeble & small at cranking speeds because they are designed to be strong at running speeds up to 12,000 rpm.
They are very hard to see which is why we use spark testers and in particular the ones with an orang/red globe.
They are not servicable so they either work or don't work and the only real test is to remove the kill wire to see if there is a problem with the kill circuit and to take a resistance reading from the plug to the coil laminations to see if there is a break in the HT lead .
The condition you described originally were consistent with a partially blocked carb.
Using it like that has now made it a fully blocked carb.
It could be something as simple as a blocked internal filter or as complicated as a blocked fuel passage inside the carb.
Cube carbs are notorious for being next to impossible to clean
If it has never had ab overhaul the metering diaphragm will be hard so making the throttle response sluggish and he pump diaphragm will also be worn & stiff.
So the carb will be in need of a rebuild and as the carb itself is now suspect the easiest thing for you to do is replace it.
At 12 years old the fuel lines have probably shrunk a bit & are not making a pressure tight seal in the tank and the tank filter is way past replacement time.

 

[buening82]

Thanks for the info guys! I should add that the teaspoon of fuel into the cylinder was done on a different day than when I tried to start it, and left the spark plug out of the head overnight so it shouldn't have been flooded the 2nd time. I've got a new carb on the way, so hopefully thats the easy fix! Good to know the coils are spark or no spark, so its definitely not the issue then.

 

[StarTech]

Never had a problem cleaning cubes; but of course, I have right cleaning equipment. My biggest problem is those none replaceable welch plugs that some have that are not in the kit or available separately. I had to replace few Zama carburetor because of this.

 

[Fish]

so it shouldn't have been flooded

Well, I have heard "shouldn't" a lot.

 

[Fish]

Plug was wet from my start attempt

Flooded.

 

[bertsmobile1]

Never had a problem cleaning cubes; but of course, I have right cleaning equipment. My biggest problem is those none replaceable welch plugs that some have that are not in the kit or available separately. I had to replace few Zama carburetor because of this.

Well I have definately had some that I could not clean & I have ultrasonic cleaners.
In particular I find a lot of the check valves stuck closed and on most carbs they are not replaceable .
However that is you & me and not buening82 who does not have all the gear you & I have .
I doubt he is willing to buy a $ 200 ultrasonic cleaner rather than a $ 20 carb
The homelite trimmer I have just sent back needed over 3 hours of ultrasound before the carb came good.
Most of the rotary valve carbs I get is are a lost cause due to damage to the check valves

 

[buening82]

Just to follow up, a new carb fixed the issue. I replaced the fuel filter, fuel line, air filter, and others while I was in there.