Mculloch PM 610 odd exhaust port?

[purdyite]

I stripped down my PM 610 for a modest muffler mod and found this notch on the exhaust port. Is that factory? What's it for?

View attachment 196932View attachment 196933

Should I match the muffler opening to the notch? It doesn't have a matching notch at the seal.

 

[Eccentric]

I stripped down my PM 610 for a modest muffler mod and found this notch on the exhaust port. Is that factory? What's it for?

View attachment 196932View attachment 196933

Should I match the muffler opening to the notch? It doesn't have a matching notch at the seal.

It looks somewhat like the "Q Port" on a McCulloch PM800. It is a way of bleeding off compression at cranking speeds, eliminating the need for a decomp valve. Doesn't affect compression at running speeds supposedly. A PM800 is a STRONG saw, so there must be something to it. I haven't had the muffler off of a PM610 in a LONG time, but I don't remember seeing that notch. McCulloch was famous for making running changes during a saws production run. Could well be factory...

 

[Andyshine77]

I've seen that on all the 610 I've seen. I think it also helps to give you a few more top end rpm's without hurting compression and torque, it's an old school trick.

 

[trappermike]

It's actually a decompression device/trick,I've seen other large 2-strokes with the same thing,it's so people can more easily pull start a large cc motor. Toro had a H.D. lawnmower with a big 2-stroke motor and it had exactly the same thing,so Joe Home-owner could easily pull start it.

Not a performance thing at all. If you raised the whole top edge of the exaust port to the top of that notch you would get a huge power gain(From having more exaust duration,like a performance engine),IF it had any compression left after raising the port.(over 100 or more psi).
Do a compression test,see what it has now,hold the throttle open but don't pull the motor over too fast so the decompression notch is doing it's job. If the motor has a removeble head,or you did some other minor machining,you could regain or increase compression if raising the port lowered the comp. too much.If it's easy to still raise the comp. after raising the port then you could do the mod if you wanted to.
If you don't want to experiment on it then better leave it alone.

 

[Andyshine77]

It's actually a decompression device/trick,I've seen other large 2-strokes with the same thing,it's so people can more easily pull start a large cc motor. Toro had a H.D. lawnmower with a big 2-stroke motor and it had exactly the same thing,so Joe Home-owner could easily pull start it.

Not a performance thing at all. If you raised the whole top edge of the exaust port to the top of that notch you would get a huge power gain(From having more exaust duration,like a performance engine),IF it had any compression left after raising the port.(over 100 or more psi).
Do a compression test,see what it has now,hold the throttle open but don't pull the motor over too fast so the decompression notch is doing it's job. If the motor has a removeble head,or you did some other minor machining,you could regain or increase compression if raising the port lowered the comp. too much.If it's easy to still raise the comp. after raising the port then you could do the mod if you wanted to.
If you don't want to experiment on it then better leave it alone.

It does both, many saw builders did this back in the day, I've seen the work with my own two eyes. I've ported and machined a few saws cylinders in may day, simply raising the exhaust doesn't necessarily increase power, especially if you don't know what you're doing.:msp_wink:

 

[Warped5]

It looks somewhat like the "Q Port" on a McCulloch PM800. It is a way of bleeding off compression at cranking speeds, eliminating the need for a decomp valve. Doesn't affect compression at running speeds supposedly. A PM800 is a STRONG saw, so there must be something to it. I haven't had the muffler off of a PM610 in a LONG time, but I don't remember seeing that notch. McCulloch was famous for making running changes during a saws production run. Could well be factory...

I had the muffler off my former PM 610 a couple weeks ago. Did not see that notch. But look at the non-quality of that notch; defintely not factory issue.

 

[trappermike]

It looks factory to me,no signs of filing or grinding,plus there are cast lugs hanging down into the port from the inverted "V" notch,no one can do that....
Like I said,I've seen other brand factory motors with the same notch,in fact I'll bet the Toro motor I saw is about 200cc at least,it would be very hard to pull start without some form of decompressor,in fact that mower had a sheared flywheel key from striking a rock(Which shifted the flywheel out of time-advanced) and it was literally ripping my hand off trying to start it until I replaced the broken key. It's a decompressor notch,but I guess not all Macs had it.
I've also seen large 2-stroke "Jumping Jacks" (Compactors) that use the same notch too,and also a large older Skidoo single cyl. motor.

 

[Roanoker494]

Here is a auction for a NOS 610 cylinder and it has that V in the port.

oem McCulloch 94536 94304 Cylinder PM610 PM655 EB3.7 | eBay

 

[purdyite]

Thanks for the replies! I got the saw a few years ago with a load of other stuff; it was neglected, so I'm sure the former owner didn't try any modifications. I just left it alone, put the saw back together and it runs very well. I suspected it was intentional, figured it had to do with easy starting. Still haven't got that compression tester yet.

 

[struggle]

This week I had torn mine down for cleaning and resealing the oil tank and mine was just as you pictured.

 

[Eccentric]

It looks factory to me,no signs of filing or grinding,plus there are cast lugs hanging down into the port from the inverted "V" notch,no one can do that....
Like I said,I've seen other brand factory motors with the same notch,in fact I'll bet the Toro motor I saw is about 200cc at least,it would be very hard to pull start without some form of decompressor,in fact that mower had a sheared flywheel key from striking a rock(Which shifted the flywheel out of time-advanced) and it was literally ripping my hand off trying to start it until I replaced the broken key. It's a decompressor notch,but I guess not all Macs had it.
I've also seen large 2-stroke "Jumping Jacks" (Compactors) that use the same notch too,and also a large older Skidoo single cyl. motor.

I replaced the flywheel key on one of those Toro 2-strokes 16 or so years ago. Thankfully, the crank didn't bend when they hit the rock or whatever it was. Wasn't any damage to the blade or blade adaptor either. The owner (a friend's mom) didn't even recall hitting anything more than a clump of weeds. The key was small, so I suspect that it was a bit 'weaker' than needed. Flywheel spun about 30deg outa time IIRC. About pulled my fingers off when it kicked back. That was NOT an easy flyhweel to get off. The key meat was 'smeared' around the crank...

 

[Pendagator]

I've got a 610 parts saw that I haven't tried to crank. I checked compression and it was about 100 psi, but the piston and rings look good. It has two small rings, I thought maybe one looked stuck. The cylinder also has the v- notch. Since it acts as a decompression valve when cranking, could this cause a low compression test?

 

[Eccentric]

I've got a 610 parts saw that I haven't tried to crank. I checked compression and it was about 100 psi, but the piston and rings look good. It has two small rings, I thought maybe one looked stuck. The cylinder also has the v- notch. Since it acts as a decompression valve when cranking, could this cause a low compression test?

Yes, the 'Q-Port' equipped McCullochs will give a lower compression reading.