Another Stihl 025 Mystery

[Wood Doctor]

These Stihl 025 engines really stump me. I imagine the MS 250's do the same thing now and then. Here's the problem and associated observations:

(1) The engine has great compression -- over 145 psi. Rings are fine.
(2) The engine has a good spark but will not start and not even pop.
(3) Four carbs have been tried, including one that has no hours on it at all.
(4) The spark plug is getting fuel -- always damp when removed after several starting attempts.
(5) The carb gasket is new, fuel line is new, fuel filter is new.

What do you suggest?

 

[kyle.kipple]

Just because it has spark...doesn't mean it's sparking when it's suppose to. Check the flywheel key. Impulse hose on these?

 

[bigbadbob]

different plug.

 

[Wood Doctor]

different plug.

I tried three different plugs. No change. No pop.

 

[kyle.kipple]

You need spark, compression and fuel. You say you have great compression with a wet plug. How are you checking for spark? If the flywheel key checks out good, then I'd be looking for a donor coil to test on it.

 

[Wood Doctor]

Simple, traditional check for spark: Jumper wire from plug threads to cylinder with plug inserted in boot. Good spark, plainly visible. It's there, believe me. Flywheel key is also intact and nothing is loose. This saw ran a whole year and then stopped suddenly during a buck cut, dead as a mackerel.

Frankly, I have never seen any other Stihl saw do this. Give them enough compression, a spark, a good carb, good pickup body, and a good fuel line, and they all run. Even with a lousy impulse hose or bad carb gasket, they will at least start. This one does not.

 

[big t double]

Vac/pressure test performed?

 

[Chainsaw Jim]

The metal clip in the spark plug boot might have broke. When broken it'll lose contact when it has tension on it while attached to the seated plug. Then you check for spark after removing the plug which relieves that tension and it connects for a good spark to fool you.

 

[DELETED (Struggle)]

If it was vac tested someone forgot to remove seal between muffler and head

Provided timing is correct and a different spark plug has been tried

 

[huskihl]

Spark arrestor screen plugged up?

 

[brushwacker]

+1 timing is way off or when you reconnect to the spark plug some thing could be pulling apart the circuit or could you be pinching a wire somewhere ? Surely the switch is on ? No 2 bare wires being pushed together by a cover being reinstalled ? Probably not as complicated as it seems.

 

[czar800]

Make sure it's not completely flooded

 

[Chainganger]

Spark arrester plugged burn it with a torch

Could be flooded, I have 2 of these and they flood super easy and even with the plug out will not dry up for hours

 

[kyle.kipple]

You've eliminated compression being an issue so I would focus on spark and fuel delivery. youve tried several different carbs so I would like to think its not that. But for sh*ts and gigs, try to un hook the fuel line from the carb and block the end off. Squirt a drink down the throat and try to start it. That will rule out the carb feeding to much and flooding if it doesn't pop.

 

[jake wells]

try a different coil

 

[blueberrymuzik]

Sounds as if it is badly flooded. Take the carburetor off. Crank the engine until the piston is at or near the top of its stroke. Tip the saw up and see if fuel runs out of the intake port. Then take a blow gun and blow air through the intake for about 60 seconds. Put a dry spark plug in the cylinder and try to start. If it was badly flooded there will be enough residual fuel to get a pop. You should be able to put the carb back on and go from there.

 

[Onan18]

Simple, traditional check for spark: Jumper wire from plug threads to cylinder with plug inserted in boot. Good spark, plainly visible. It's there, believe me. Flywheel key is also intact and nothing is loose. This saw ran a whole year and then stopped suddenly during a buck cut, dead as a mackerel.

Frankly, I have never seen any other Stihl saw do this. Give them enough compression, a spark, a good carb, good pickup body, and a good fuel line, and they all run. Even with a lousy impulse hose or bad carb gasket, they will at least start. This one does not.

It takes four times more energy to fire a plug under compression than atmosphere. You need an adjustable air gap spark tester, with the plug in the cylinder and the tester in line it should be able to jump a 6mm gap with a good blue spark.

Joe

 

[angelo c]

Edwin,
Take a look behind the carb mount and confirm the impulse line is connected. Damn things act funny if they are a little loose or leaking. Beyond that if you wanna play "endless swap" I have a couple good running 250's just sitting that you can mix and match parts with till your heart finds the true meaning of love.
lemme know where to send one ifn ya want.

Im betting on flooding though. this series is a known "NO BURP" saw.

 

[Fish]

The025/MS250 had 2 different ignitions, you have to make sure that you have the correct flywheel.

 

[jmester]

As mentioned above. I would look into the ignition module. Have seen those modules come loose and rub the flywheel causing them to run funny or not run at all. But still show spark. I have also noticed on that series that the shim behind the carb will deform if the carb is over tightened and cause an air leak. Hope it helps