MS 650 Now Has Zero Compression?

[Wood Doctor]

... Thats right, zero compression, none at all. The engine was running beautifully, made it to full throttle, and just plain stopped suddenly as the chain entered a big log. If you turn the flywheel, it's as if the spark plug has been removed, even though it's tight. I checked first to see if the piston was moving at all, and it is. You can stop the crankshaft rotation with a screwdriver inserted inside the spark plug opening. And, nothing is rattling or seems loose.

What on earth happened and what is the most likely fix? I'm stumped at this point. Any ideas?
:msp_confused:

 

[deye223]

pull the muffler and have a look and see if the piston is scored

 

[Wood Doctor]

Scored Piston and Zero Compression?

pull the muffler and have a look and see if the piston is scored

OK, I'll start there, but frankly, I have never heard of a scored piston dropping compression all the way to zero. You can lose a lot that way, but to lose all of it and so suddenly seems difficult to believe.

 

[watsonr]

broke a cylinder screw or a ring?

 

[zogger]

Rings stuck into the piston? Hole in the crankcase some place? Cylinder gone loose and massive leak in the gasket area?

 

[sachsmo]

Even with stuck rings you will get a reading.

Sounds like you have a hole on the piston crown.(broken piston) Pull the jug and you will see the problem.

 

[morgaj1]

Broken ring gouged the piston?

 

[morgaj1]

Edit to above: broken ring gouged the cylinder?

 

[rupedoggy]

Even with stuck rings you will get a reading.

Sounds like you have a hole on the piston crown.(broken piston) Pull the jug and you will see the problem.

Listen to this man.

 

[angelo c]

Grab the T27 and see if the head bolts are still present and accounted for....if so then pressure test it for leaks.....many leaks....

good luck doc...post up some piston/cyl pics too

 

[wigglesworth]

Sounds like u sheared the flywheel key to me...

 

[FATGUY]

Sounds like u sheared the flywheel key to me...

+1

 

[Wood Doctor]

Sounds like u sheared the flywheel key to me...

Well, it seems to me that if the flywheel key was sheared, the piston would not be moving when in fact, it is. Please correct me if wrong.

On the other hand, a hole in the piston crown makes the most sense because then air is leaking right through it. A broken ring is possible but even then you might get some compression reading. Believe me there is none. I'll see if I can take some Pics as I go along, but what I am trying to do is avoid replacing as many parts as possible. This engine has hundreds of hours on it in over five years or professional logging use, so that may be difficult.

At this point, I may be looking at a new piston, ring clamp set, and new gaskets. Thanks for the feedback so far. My sleeves are rolled up.

 

[Wood Doctor]

Found Hole in Piston Crown

Yep, thar she blows. I found a hole in the crown of the piston, right next to the outer rim, about 1/4" across, just visible from the spark plug opening. So, at best case, only a piston assembly is required to get this saw going again.

However, judging from 500+ hours on this engine, does the forum recommend a new cylinder as well with new gaskets, etc.? That adds another C note to the repair cost. We would then be looking at about $150 for the parts alone, perhaps more.

What also worries me is that most of these aftermarket cylinder kits (such as NWP) do not support the compression release button. Anybody have a reading on that?

 

[wigglesworth]

Well, it seems to me that if the flywheel key was sheared, the piston would not be moving when in fact, it is. Please correct me if wrong.

On the other hand, a hole in the piston crown makes the most sense because then air is leaking right through it. A broken ring is possible but even then you might get some compression reading. Believe me there is none. I'll see if I can take some Pics as I go along, but what I am trying to do is avoid replacing as many parts as possible. This engine has hundreds of hours on it in over five years or professional logging use, so that may be difficult.

At this point, I may be looking at a new piston, ring clamp set, and new gaskets. Thanks for the feedback so far. My sleeves are rolled up.

Just from reading ur first post it sounded to me like everything looked good, but when u pulled the rope, it had no comp. I've seen the flywheel key shear, and exhibit the same symptoms.

Glad u found it. Don't waste ur time with an aftermarket top. That's a $1000 saw. Why risk it again.

Fwiw, a hole burnt in the piston top is usually caused by bad fuel, or too much ignition timing...

 

[sefh3]

Post a picture of the spark plug. Was it brown or tanish color? How old was the fuel that you were running in it?

 

[ZeroJunk]

Might as well put a MS660 top end on it. Make sure there are no fragments running around in the crankcase. They should all have compression release unless you block it off, I think you are looking at something wrong.

 

[barneyrb]

I recently went through a MS-650 and even though there is no aftermarket piston listed in Meteor or Espian catalthe 064 is an exact replacement

 

[Rounder]

If the cylinder looks fine, don't worry about it. I've had saws with a lot more than 500 hours on them.

 

[kr5258]

My experience with old two stroke dirt bikes points to preignition burning a hole in the piston. It's usually caused by burning carbon deposits or the hot tip of a spark plug (heat range too hot).

Detonation causes erosion around the perimeter of the piston. The engine can run like this a long time.
Preignition burns holes through pistons in a matter of seconds. The charge ignites near BDC, the heat cannot be pulled away from the piston into the cylinder walls before the piston crown softens and a hole results.