The MS650 Project

[Bobby Kirbos]

So, the FedEx man dropped off a lovely box from Hungary yesterday (thanks @David Major ).

I'm looking at an MS650 with:

• Some scoring on the exhaust side of the piston and rings.
• The intake side of the cylinder looks good.
• Pulls 120psi compression on my gauge.
• A muffler that still has the baffle in it.
• The kill switch doesn't work.
• A few missing screws.

Nothing that I can't handle.

Here's where it gets interesting... being that the kill switch doesn't work, I still needed to shut down the saw. I choked it and it died. Then it was flooded and a ***** to start. The next time, I pressed in the decomp and the saw died. Not being flooded, it restarted easily. Should I be able to press in the decomp valve easily while the saw is running? Could a leaky decomp valve (over time) lead to the scoring that is on the piston, speciffically on the exhaust side?

Other than those minor issues, it's a runner. Not 6 cube, but bigger than my rebuilt Poulan 245, and I won't need to be pushing the 245 to its limits with the longer bar.

 

[cus_deluxe]

Should I be able to press in the decomp valve easily while the saw is running?

yes. you have to hold it if u wanna kill the saw that way, usually theyll pop right back out.

 

[rupedoggy]

120#?

 

[Bobby Kirbos]

120#?

Yes, 120 #.

 

[Bobby Kirbos]

yes. you have to hold it if u wanna kill the saw that way, usually theyll pop right back out.

It pops back out if I don't hold it. If I hold it, it does not push back. So, the fact that it doesn't push back... does that tell me that the piston and rings are too far gone and should be replaced? ...or that the decomp is shot, and should be replaced?

 

[Bobby Kirbos]

Sorry for the questions. This is the 1st saw (engine) I've had with a decomp and don't know the finer points surrounding it. I want to either confirm it as or rule it out as a problem.
Also, I'm not familiar with Stihl saws. I know 120psi is low, but is it too low to the point that I should put a new top end on it?

 

[cus_deluxe]

yes 120 is low, you said you have visible scoring, so at least a new piston, very few cylinders ive seen havent been able to be cleaned up. dont worry about the decomp. they can leak and cause a hotspot, but not super likely to cause major scoring.

 

[Bobby Kirbos]

Since I need a new piston (and possibly a new jug) -- yup, I'm going to do a 660 top end and call it a 650 MAGNUM.

 

[farmer steve]

Text me a pic of the switch Bobby so I can see what ya got.

 

[Bobby Kirbos]

I haven't torn into that yet. I'll so that this evening.

 

[Bobby Kirbos]

The clip on the right side of the "switch" mechanism had fallen off and was floating around inside the air box. I reattached it and it now works correctly.

I think the decomp is shot. I took the valve out and there is almost no "snap" to it. After replacing that, I will see what kind of pressure it pulls. I expect the numbers will improve.

 

[Bobby Kirbos]

New decomp came in yesterday. It only improved thr compression gauge by 5psi, so we're up to 125#.

660 top end parts are inbound.