Stihl MS291 Strange Problem

[Wood Doctor]

I've run into a Stihl MS291 that has good compression and spark but makes a whap whap sound when cranked and will not start. My first guess is that the crankshaft bearings and seals are shot, but there is no play or wobble on the flywheel or clutch. Everything seems tight. I have removed the muffler and checked the rings and they look OK. Frankly, I have never heard a whap whap sound like this before. I suppose there could be something banging against the crank shaft, but whatever it is, the saw refuses to fire and run. Anyone ever run into this? Any ideas what gave up or what is causing this noise?

 

[frank_]

bent rod maybe ?

 

[Wood Doctor]

bent rod maybe ?

I hope not, but that's certainly a possibility. This saw's compression has been really high in the past and the engine has had remarkable power to the point where a decomp valve would have made a nice addition. I've never started a 56 cc saw that was this tough to pull. Regardless, it looks like a repair is going to take a top-end rebuild.

 

[HarleyT]

These have side port covers that like to come loose.

 

[HarleyT]

Here is a view of a side.

 

[BuckthornBonnie]

I've run into a Stihl MS291 that has good compression and spark but makes a whap whap sound when cranked and will not start. My first guess is that the crankshaft bearings and seals are shot, but there is no play or wobble on the flywheel or clutch. Everything seems tight. I have removed the muffler and checked the rings and they look OK. Frankly, I have never heard a whap whap sound like this before. I suppose there could be something banging against the crank shaft, but whatever it is, the saw refuses to fire and run. Anyone ever run into this? Any ideas what gave up or what is causing this noise?

If not the cover, it could be the crankshaft itself. We had one go in an 1127 series and it made the same sound you’re describing. Main bearings were tight.

 

[cscltd]

2nd the transfer port covers, usually flywheel side- there is a reason husky glues and screws theirs on

 

[Wood Doctor]

Great replies, guys. Looks like it's going to require a take down of the top end, regardless. The extra high compression that I noticed could have eventually broken loose these side transfer port covers or bent the crankshaft as mentioned in Post #6. It's somewhat surprising that most of the compression is still there, but whatever is causing the whap whap is also blocking the ability of the engine to run at all. It sounds a bit like a cement mixer.

 

[Maintenance supervisor]

Coil bolts loose? Magnets sucking it in on rotation?

 

[Wood Doctor]

Coil bolts loose? Magnets sucking it in on rotation?

I checked that and reset the gap which, in fact, was a tad too small (about .005"). Coil bolts were tight. Spark is good but no change. Still getting the whap whap and no start. Frankly, I was hoping you were right.

 

[Maintenance supervisor]

I often hope I'm right also, lol!

 

[PV Hiker]

What does it sound like without the spark plug in? Is the noise a effect of under compression?

 

[Fatherwheels]

I checked that and reset the gap which, in fact, was a tad too small (about .005"). Coil bolts were tight. Spark is good but no change. Still getting the whap whap and no start. Frankly, I was hoping you were right.

Split / cracked piston, cylinder or transfer leak / crack.

 

[Tobystihl]

Worn small end bearing resulting in the piston skirt whacking the crankshaft perhaps?

 

[HarleyT]

Take off the top cover and peek.

 

[Wood Doctor]

What does it sound like without the spark plug in? Is the noise a effect of under compression?

The whap whap noise effect disappears without the spark plug in. If under compression, the noise returns immediately as the starter cord is pulled.

 

[Wood Doctor]

Take off the top cover and peek.

Harley, what "top cover" and "peek" at what are you referring to? The cylinder I assume? The shroud? If the cylinder, that means almost a complete engine take down. As you know, this rebuild is never that easy with a residential Stihl saw like a 290 or 291.

 

[HarleyT]

Here, I believe you can get a look of them with the top cover off.

 

[HarleyT]

I am waiting for windstream to fix my internet, before I can load the part 2.

 

[Fatherwheels]

The whap whap noise effect disappears without the spark plug in. If under compression, the noise returns immediately as the starter cord is pulled.

Is the plug wet when its in, is there a flood of fuel going into the cylinder by any chance.