Hard Pull and Combustion Jerk Back on a 2005 Stihl MS280

[russcron]

Hello Out There. I have an MS280 that failed recently. One day it was running reasonably well (fast idle was manageable) during a two hour cutting session. The next day it will not start and the T handle is jerked out of your hand when the first spark fires. It feels like very high compression.

I think it is due to excess fuel in the cylinder. To that end I put a new Walbro carb on it. My hope was that the fuel metering and/or pump diaphragm were somehow failed. That didn't fix it but at least I have a nice new clean carb.

I noticed that there was significant fuel pressure when I removed the fuel line from the old carb. I had to plug the hose. I wonder if this is normal.

Any tips or suggestions are gratefully appreciated.

Russ in West Virginia

 

[coffeebrk]

Pull muffler and look at piston. Maybe take a pic to show.

 

[slimbo3006]

If you removed the fuel line with the engine hot or even warm, without opening the fuel cap first, this is normal for there to be fuel pressure on it. Basically the fuel (more-so the fuel vapors) expands when heated up in the tank and creates pressure. Just pop the fuel cap off first before working on any carbs or fuel lines. this will relieve the built-up pressure.

If you believe that it is flooded to the point of hydro-locking (fuel puddling in cylinder) you can just pull the spark plug and drain the cylinder. pull it over a few times without the plug i it and it will shoot any excess fuel out of it. Then just leave it to sit for a bit to dry up.

Also, I agree with coffe--take the muffler off and take a look at the condition of the piston/Cylinder.

 

[russcron]

Yes, there must have been some tank pressure from expansion. It had been outside before spending the the night in the heated workshop.

I pulled the plug and could not drain or pump any fuel from the cylinder.

Photos are attached. The piston looks good to my eyes as does the wall.

Thinking about the kickback, it seems like the plug is firing well before TDC and causing counter rotation (??)

Thanks guys.

Russ

 

[Farmer_Nate]

Last saw I had to show strange backfiring and kickback needed a coil.

My saw was weird. It would not work with a resistor spark plug, only a non-resistor plug.

Some have said on other threads this may happen if the flywheel somehow rotated to where it isn't supposed to, such as when a key shears? This causes timing to be off.

 

[russcron]

Makes sense, thanks. I'll dig into the flywheel - ignition module area tomorrow. Hope you don't need a special puller. I guess the timing is fixed static and defined by the crank angle and shaft keyway.

 

[heimannm]

The basic timing is set by the position of the flywheel on the crankshaft. On most modern electronic coils there is some advance built into the electronics so the advance increases a few degrees as the engine speeds up. In the starting mode, the spark will occur around 25 degrees before top dead center.

Most Stihl saws require a certain cranking speed before they will generate a spark to avoid the conditions you are describing. I would start looking for a replacement coil.

Mark

 

[stihltech]

Same as above. Have seen coils stuck in advance. Tough on the starter hand.

 

[Glenn50]

Say heimannm, does the distance from the coil to the magnets have any effect on timing? Say from .010 to .015?

 

[heimannm]

The distance between the coil and the flywheel can affect the strength of the spark, but should not have any measurable impact on the timing.

Mark

 

[russcron]

Good question Glenn. Thanks Mark.

I may have had a weaker than normal spark because the gap was around .04. There is a YT video of an MS280 coil replacement where the mechanic uses a common business card to gap the coil and flywheel. That’s about .015. So closer equals more magnetic flux equals more energy created equals more juice to the coil. But I think this is a CD coil so it may be academic if the electronics compensate. My saw did seem to flood easily however.

My failed coil was an aftermarket Proline. It was installed about 7 years ago. Given the 19 year age of the saw, that’s pretty good compared to the OEM since the cost is about 1/2 of the OEM. I think I would base my purchase decision simply on availability if you are in a hurry to return the saw to service.

Russ

 

[cookies]

Earlier today I finished repairing a Stihl 051 ignition system, it fired and ran well on the first fire up attempt. The second time I tried to start it the sucker painfully snatched the handle through my fingers and refused to let me pull it over with the coil on. I rechecked the coil gap and slid the coil in the retard direction approx .03 and found the coil was cocked a hair leaving one side just a touch further from the flywheel magnets. I readjusted it and that joker starts smooth and easy every time now. Tats what I would check before spending money.

 

[Farmer_Nate]

One of my saws did that until I went to a non-resistor spark plug. Weak coil I guess. Seemed to "flood" a lot before the non-resistor plug made her run like new. I will probably need a coil in the future, but for now I'll milk more use out of the saw.

 

[thenne1713]

Say heimannm, does the distance from the coil to the magnets have any effect on timing? Say from .010 to .015?

Guessing from my experience, without looking at Spec before posting, that 010 within spec, but 015 maybe out of spec?

 

[stihltech]

Look at he primary wiring carefully. I have found wear through in one of the twists and turns the wiring makes.

 

[thenne1713]

Hello Out There. I have an MS280 that failed recently. One day it was running reasonably well (fast idle was manageable) during a two hour cutting session. The next day it will not start and the T handle is jerked out of your hand when the first spark fires. It feels like very high compression.

I think it is due to excess fuel in the cylinder. To that end I put a new Walbro carb on it. My hope was that the fuel metering and/or pump diaphragm were somehow failed. That didn't fix it but at least I have a nice new clean carb.

I noticed that there was significant fuel pressure when I removed the fuel line from the old carb. I had to plug the hose. I wonder if this is normal.

Any tips or suggestions are gratefully appreciated.

Russ in West Virginia

PULL FLYWHEEL AND INSPECT KEYWAY. I did not think that was issue on a saw I was given, but it was. = installed wrong, not on key, but previous owner/ tech, who also messed up OIL Pump button linkage.

 

[scruffy808]

Hello Out There. I have an MS280 that failed recently. One day it was running reasonably well (fast idle was manageable) during a two hour cutting session. The next day it will not start and the T handle is jerked out of your hand when the first spark fires. It feels like very high compression.

I think it is due to excess fuel in the cylinder. To that end I put a new Walbro carb on it. My hope was that the fuel metering and/or pump diaphragm were somehow failed. That didn't fix it but at least I have a nice new clean carb.

I noticed that there was significant fuel pressure when I removed the fuel line from the old carb. I had to plug the hose. I wonder if this is normal.

Any tips or suggestions are gratefully appreciated.

Russ in West Virginia

Russ Take a look at my post "Stihl saws kickback on starting. Posted ways to check ignition timing and IF its too advanced how to retard timing.

 

[russcron]

Thanks for the lead scruffy. And to all who responded.

Problem solved. I learned that the coil gap should be .008 which is easy to achieve with 4 thicknesses of common printer paper (the thin cheap stuff).

Fired on first pull and runs great now. No jerk back at all.

The factory coil was about $70 US with tax. It is visibly better made than the Proline. Still, I’d get the one that restored operation fastest.

Now back to the beaver trees…