fs45 wont start

[nhyrum]

ok guys, this one has me stumped.

my fs45 was running great one day, went to use it a few days later(we've been having hot and wet days so the grass is growing crazy fast) and I couldn't pull the rope. it would hardly budge. after a few minutes attempting to get it to crank over, I finally got a full rotation, when suddenly fuel came gushing out of the muffler, which then continued for the next 5 minutes or so till a beach ball sized pool of fuel was on the pavement.

took the carb apart, all looks fine. still couldn't get it to start(even after multiple days of attempts, with the plug out and covered so the cylinder could vent and not get dirt in it) bought a new carb from the local dealer(I did just put a new fuel mix in it, amsoil 100:1, as they recommend that for 50:1 equipment, and I'm a huge believer in amsoil) so figured maybe the richer fuel(also, I use 110 race gas as i already use it for a lot of other equipment, and its the only place i can get ethanol free gas. DOWN WITH ETHANOL!!!) needed an adjustment range beyond what the factory carb could do with the plastic lock tabs. after about 10 pulls when I pull the plug it is dripping with fuel.

today is day 9 of no start. I decided to completely gut the thing to see if there was any noticeable damage, or any reason for the machine not starting, and I couldn't notice anything. everything is actually very clean, and no signs of excessive wear. I was given it by my grandfather who is just not wanting to pull start anything anymore, and was used very seldom. piston looked good, rings looked good, weren't cracked or anything, and they moved in and out freely. I am pretty familiar with stihl products, just not "factory trained" but i have taken a few courses.

I do get spark, and my kill switch is functioning.
I have 5 foreseeable courses of action (COA from here on out).

COA 1: I see a pulled short block(no carb or muffler) on ebay for about 35 bucks. take the gamble in it actually functioning? It claims to have been pulled from a working unit.

COA 2: replace the coil pack. possible weak spark? gamble on that not being the fix.

COA 3: Replace piston and cylinder. Maybe I'm getting low compression? I don't have a gauge, but it feels good pulling on the rope. but that doesn't tell much. Now, I may get a little weird with this. Are there any larger bore pistons and cylinders that would "fit"? If im gonna be replacing it, may as well do it Tim Taylor style, right?

COA 4: both 2 and 3, possible that I have both problems compounding on each other?

COA 5: take it to the dealer and let them fiddle with it (had they been open Saturday afternoon, they would have it now, but, that didn't happen) I'd prefer not to take it in. Not because I don't trust them, but because well, I'm slightly embarrassed about it. I took out the spark arrestor and drilled out some of the baffles to get a bit better flow, which will result in needing a richer mixture, and probably less efficient running, as more fuel will be escaping out of the now free-er flowing exhaust system.

me personally, I'm leaning towards 4, because... why not?

your opinions?

 

[nhyrum]

Update 1.

Ordered an ignition coil.

I got it to a point now where it will backfire out of the muffler once about every pull. I'm going to make sure the timing is *generally*correct and not 180 out or something

If the coil doesn't do it, cylinder is next. After that, I give up and I'm taking it to the dealer.

Edit: timing seems OK. The poles on TV flywheel and coil line up about 30 degrees before TDC and IIRC spark occurs when the magnets break, which is just a tad shy of TDC. Gap is set to .30mm(don't have a standard feeler gauge set, don't ask why) which is about .012 inch, and spark plug gap set to .50mm which is roughly .020

 

[bertsmobile1]

Carb is not OK if you get fuel in the muffler.
What has happened is the carb has leaked fuel into the sump because the needle was not able to hold the fuel back. hen when you started to pull the rope liquid fuel went into the the cylinder which was a greater volume than the compressed combustion volume , thus the hydraulic lock.
The some of the liquid got forced past the piston ring (2) think the FS 45 has 2 so the piston could go to exhaust and force the liquid out into the exhaust.
SO now you have a doggy carb & potentially bent con - rod &/or broken rings depending upon how much grunt you applied.
Good chance the tank vent is also blocked so when the trimmer got hot the air pressure inside the fuel tank went above the 7-10 PSI needed to lift the needle valve so that will need to be looked at as well.

The cost of a rebuild kit is only marginally less than the cost of a replacement carb so I have given up trying to clean them out and just toss them into a box for "latter on batch cleaning" & fit new ones.

 

[nhyrum]

Carb is not OK if you get fuel in the muffler.
What has happened is the carb has leaked fuel into the sump because the needle was not able to hold the fuel back. hen when you started to pull the rope liquid fuel went into the the cylinder which was a greater volume than the compressed combustion volume , thus the hydraulic lock.
The some of the liquid got forced past the piston ring (2) think the FS 45 has 2 so the piston could go to exhaust and force the liquid out into the exhaust.
SO now you have a doggy carb & potentially bent con - rod &/or broken rings depending upon how much grunt you applied.
Good chance the tank vent is also blocked so when the trimmer got hot the air pressure inside the fuel tank went above the 7-10 PSI needed to lift the needle valve so that will need to be looked at as well.

The cost of a rebuild kit is only marginally less than the cost of a replacement carb so I have given up trying to clean them out and just toss them into a box for "latter on batch cleaning" & fit new ones.

I already put a replacement carb on, I haven't thought of the tank vent, that's in the cap, correct?

I didn't apply too much grunt to try to turn it over, Just kinda worked it a few times till it purged the fuel. The machine turns over fine, and seems to have good compression. I pulled the piston, and it and the rod looked OK, but I wasn't checking for a bent rod.

I have a coil coming soon, if that doesn't fix it I think I'll just upgrade.

 

[bertsmobile1]

The pop off pressure of the needle is between 5 & 10 psi which is very small as it is a very small fuel pump on these carbs.
Hand helds are a regular PIA.
I test the carb by removing it then blowing low pressure air through the verturi.
You should see the fuel being blown out the engine end.
With the carb connected it should start with a little starting fluid and you should be able to keep it running with small & I mean really small shots of starter fluid down the carb just remember the engine is very small so they will flood real easy.
A red neon spark tester on the HT lead allows you to see if it is sparking.
If not then take off the kill lead from the coil and try again.
If it starts you can whip off the plug tester to stop it.