Stihl FS46 won't run

[oldsaw]

Okay, it isn't a chainsaw, but this is stumping this chump. I've always been able to get things running, and keep them that way. That's old stuff, this thing is only a year and a half old with very few hours on it.

It will start, run okay, but then starts loading up, and eventually dies. I've checked everything. Had the carb apart twice, it's spotless, with everything looking like new. Pulled the screen, that was perfect. I've replaced the fuel with new, twice (both fresh from two different stations two weeks apart). Not that the fuel was "bad", my saws and lawnmower have worked perfectly on the same mix. This thing never saw the can of bad gas I had early in the summer. And it will run, which means I have spark, at least sometimes.

Piston and cylinder are perfect, but it seems that there is a lot of fuel bubbling past the rings on the exhaust side, compression seems okay though, similar to my old Homie 150 with only slightly larger displacement.

Like I said, this chump is stumped.

Mark

 

[046]

check to make sure exhaust is not plugged up...

put small amount of fuel inside intake or use small amount of starter fluid to eliminate possiblity of not getting fuel.

rebuild carb making sure ALL passages/jets are clear by using a visual indicator. I spray chemtool w/thin nozzle down each and every passage and see spray come out other end. set float level to spec's.

I've rebuilt hundreds of carbs this way. has never failed me yet.

do a spark plug test (use clean/new plug) to make sure you are getting fat spark. nice if you have a plug tester under pressure dome.

next up is to do vacum/pressure crank test. but only 1.5 years old, seals are probably still good.

most of the time it's fuel/carb related...:greenchainsaw:

 

[oldsaw]

Nope, exhaust screen was perfect. Checked before I posted.

mark

 

[Lakeside53]

Definitely make sure the exhaust isn't blocked. Unscrew the fire screen assembly (either 15 or 16mm) to check it.

The Zama carb is a real pain as there are parts not accessible (like the low speed check valve) or available (outlet valve). However, you say it starts and run fine, then bogs down and dies... so it's likely a mixture issue (way too rich) or you have a leaky needle valve. Just guessing though, as I've been caught by these before.

Don't use carb cleaner on these carbs as it will ruin the check valves.

If it dies or runs like crap when hot, the ignition is always suspect on these units.


First thing is to make sure it is in fact "flooding" or starving for fuel. Check the plug after it dies.

 

[Stihldoc]

OK- FS 46's are pretty dependable units. 2 problems I'v repeatedly seen: 1. the fuel hose- if it is green its the good one. If its black replace it with p/n 4140-358-7702. Black hoses crack & suck air. Second issue: Zama carb. check valve problems. There are 3 nescessary tests for the C1U/C1Q series purge-type Zama carbs: you need a pressure & vacuum pump to test it. Step 1: attach pressure pump to fuel inlet & pump up to 6psi. If it holds pressure, the inlet needle is good, if not, needle or seat is leaking. Step 2: attach pressure pump to fuel purge outlet (line that dumps fuel back into the tank). Push the purge bulb a few times and see that it develops & holds a few psi pressure.Use the purge bulb, not the pressure pump to develop pressure. This tests the purge body & its check valves. Step 3: Attach your vacuum pump to the purge outlet and pull a vacuum- 7'' or 0.5mb. If it holds, all is well. If the vacuum gauge drops in less than 15-20 seconds, the main (nozzle) check valve is bad. Steps 1&2 can be fixed, failure of step 3 cannot- its new carb time. The ignition module is a possible problem, but on this series of trimmers, module problems are USUALLY confinded to "loss of spark when hot" issues. Good luck.

 

[oldsaw]

Okay, she's running lean, not rich. I can pump the primer and get all kinds of revs, stop and it will just die.

It's got the green fuel hoses, still nice and pliable.

Well, it runs like crap when cold, never gets hot. All this was "catastrophic", it ran great, then just stopped.

Bad carb would suck, there's only about 15-20 hours on this dang thing.

Mark

 

[Lakeside53]

Exactly what carb do you have? (numbers on sides).

Check your mounting block screws. If you have any leaks, it won't suck gas.
Pressure test you fuel lines and tank anyhow, and, check that you tank vents (take of cap while testing).


It's still under warrenty... 2 years to homeowners... Stihl has been honoring warranty on blocked carb outlet and L check valves, on a case by case basis.