Stihl HS56 Problem

[KE4AVB]

Let's see anyone can guess what was wrong with this HS56 since the dealer couldn't find anything wrong with it. Actually still under warranty.

Initial problem:
After it warms up it will accelerate and then dies.

After getting it from the dealer still accelerating way pass specs. And also surging. My Tiny Tach indicated no spark during these extreme speed episodes.

BGS I figure you probably seen something like this.

Now i have found and fix the problems that the dealer could not even find.

[KE4AVB]

I would add that this thing would hit on choke with the spark plug lead disconnected every once a while.

ADDED info...
Plus I even a 4 cycle several that would not stop running after it warmed up even if you pulled the spark plug lead off the plug. Of the cause there was slightly different as I had induced that problem during a temporary repair.

[bgsengine]

well , have a few thoughts,but ,never having seen a Stihl HS56 or how they are put together,I couldn't even really hazard a guess.

[KE4AVB]

Well it is a 2 cycle engine and it is a Hedger if that helps and it was less 6 months old when it came into the shop. Basically it is the same engine as the HS46 4242 power plant.

I even see one case with a 4 cycle Briggs that similar though the root cause was introduced by me during repairs. See my added note in post 2.

BGS don't be afraid to hazard on this one. This one could get anyone of us in trouble. The lack of tach readings during these overspeed episodes is what lead me to the solution. It actually was two things that was wrong with it.

[bgsengine]

Well, never seen a dieseling 2-stroke , but I Have seen one unit with a broken coil ground wire that could back-feed a spark through the cylinder jug but it wouldnt start and run,it'd just hit every so often.. since yours would only do it warmed up, I'd have to guess at a carbon hot spot. though I don't 100% trust an inductive tach of any kind - if they act weird its usually EMF interference. or its just not connected in a good spot. Only other thought would be low octane fuel (high volatility)

[KE4AVB]

Well don't feel bad I have never had either until this one.

Well it was dieseling at full throttle but once it return to idle the dieseling would stop. This is because of rpm limited coil that was completely shutting down after it surpassed 9500 rpms which explains the tach not registering any spark. The surging was also caused by coil trying to limit the rpms.

Now the spark plug is one those NGK CMR6H's that I been seeing weird failures with in the last three years so I keep a box or two of 10 on hand. It was failing. A secondary problem that showed up is more common and was probably was caused by the failing plug was a plugged spark arrester.

It was strange that the engine ran fine until warm as it also ran fine during below 40F air temps even when it was fully warmed up. The strange thing that I seen and this is because the dealer had left the plug wire off was that it would hit once fully choked even without a spark. Kinda strange that it would only diesel at full throttle, once the rpm dropped below the limiter it return to normal every time.

After I seen the strange tach shutdown reading I changed the plug then it would only idle. This I have seen with especially Stihls where the spark arrester is clogged so pulled cleaned the clogged arrester.

[Luffydog]

I would have to guess someone installed the wrong coil. Just a guess though