What makes a 797 Super?

[cpr]

Super 250s are factory hotrods, but what about the 797s? Eh, not so much...

I bought this one a while ago and it has always been easy to start, ran well enough, but I never could register more than 120psi compression which is not where it should be. I raced it against my MC-91 and it got thumped and, in hindsight, I had put this saw at a disadvantage I either didn't realize or had forgotten. It had a 7 tooth sprocket on it and I could have sworn there was an 8 in there. So, I tore it down to get to the bottom of the lack of compression, but first the differences.

1. 797s had flatbacks, most Supers didn't. They had an adapter to fit the Walbro SDCs. This is an improvement.

2. Thick ring full-circle piston. I don't know how much difference ring thickness makes, but the thick ones are harder to find now. This is no change, but the full-circle part is not an improvement.

3. The piston is full circle because there's no boost ports (although I have heard that some early ones did. Neither of mine do). This is not an improvement.

4. The porting is different. They're all lowered. Takes away the high-rev, hot rod feel the 797s are known for, but adds in the long-bar torque the 125s wound up with. Depends on what you want, whether it's good or bad. I have a 50" bar it could wear so...
797 map courtesy brian13.

Super 797 map.

I believe that McCulloch put this saw out as a bridge between the 797 and the CP-125. It was a hard-tail and used up that production, but introduced a block design and power curves that the 125s would have. I have 101 maps, but it would be cool to see CP-125 and an SP-125 to see all the changes.

Now, on to the compression problem...

 

[Ambull]

I thought that all Supers had SDC carbs.

I know that a 797 Super will thump a SP125. Uncle Lee told me so.

 

[cpr]

I can still see hone marks on the cylinder and spot-checking the piston clearance at various places on the piston and in the cylinder comes out at .005, right where it should be (bore is stock). The ring pins are .072 and while I couldn't get an accurate end gap measurement (didn't feel like disassembling my gauge set) it is less than .080. The spark plug threads aren't the greatest, but I don't think they were leaking, but the decomp sure was.

This saw was owned, at one time, by a village highway dept. I don't think they ever closed it. The top corner is worn off from vibrating against the muffler.

The socket is wallowed out on the shoulder...

And the tip is all peened over. Bad one on the left.

Hard to say whether the seat is damaged or not, it might be...

After specing everything out and replacing the decomp valve, I tacked it back together enough to check compression and found I still had none. I'm kinda stumped here. Not sure the valve isn't still leaking and that the rings are just tired, but it seems hard to believe that even with new ones I won't find much more than a 10-20psi improvement. I have half a mind to just plug the decomp all together...

I have time to clean it up and think while I hunt for a set of rings and wait for the exhaust gasket and crank seals to come.

I welcome any thoughts on what I may have missed or where I should go from here.

 

[RandyMac]

Neither of my 797s had decomps, one was plugged, the other hadn't been drilled.

 

[ozflea]

Like you i have found a few tied Macs down on compression i usually rering the power unit change the seals and all is sweat just check piston to wall clearance old engines may have a high rate of wear between the too a piston port engine is not as tolerant as a reed valve when it comes to wear

Hated the style of decompressor the 797 had the SP125c decompressor was miles better

McBob.

 

[67L36Driver]

On the decomp problem.

Ran into something similar on a Poulan 202. Pumped 110psi and then I put in new notched rings with .005" endgap vrs. .075" before and got the same 110psi after.

Decomp is leaking but how do you 'fix' it unless you just drop in a lead shot and follow it with a socket set screw.

 

[WidowMaker1]

I don't mind what makes a super 797, i just be wanting a S797 ....hard to find though i think, well untill Mcbo0b sends me one to play with anyways

 

[ozflea]

My SP125C'S will keep me happy bugger the 797's

McBob.

 

[WidowMaker1]

I will happily take either!

 

[CM76]

I thought that all Supers had SDC carbs.

I know that a 797 Super will thump a SP125. Uncle Lee told me so.

Early Supers had a Tillotson HL carb.

Chris.

 

[Bill G]

My SP125C'S will keep me happy bugger the 797's

McBob.

Sounds good,

I will send you 125's and you send me 797's. There will be a 797 Super listed on a sire Sunday evening. I expect you will not bid ????????

 

[LegDeLimber]

based on the way pic number 4 is showing here:
the bottom of the hole looks (to my eyes, at least) like it's got some damage
from the 9:30 to midnight range, then 12:00 to 3:00 looks questionable.

Do those areas line up with the damage on the plug/plunger?

wondering if you could carefully stuff a crayon in the hole, without any twisting
and get an impression of what the bottom of decomp port looks like?

Could a new or undamaged plunger be used to lap in the seat area?

 

[leeha]

I often wondered why Mac called the Super 797's supers when
they really weren't. Besides what has already been mentioned
i don't know of any other changes from a 797 that makes then
supers. The early Super had the HL103 carb while the laters had
the SDC set up. And why go to a thick ring when you would think
the thin ring in the regular 797 would make more power. The CP
125's came both ways. Either thin or think. I'm not sure if the lack
of a boost port in the supers was a decrease in power as i have a
few very strong runners as well as boost ported ones. Some early
supers had boost ports.

Charles did you try lapping the new decomp valve to the block with
valve grinding compound. If it's leaking you should hear it in the exhaust
just by turning the motor by hand. The rings could also be a problem.
They may not have the tension as they use to.



Lee

 

[cpr]

The seat damage is 90deg. to the pin being peened over.

I was thinking of plugging the decomp with a bolt and shellac. Never thought about lead in there. Not sure it would stay?

 

[Modifiedmark]

The seat damage is 90deg. to the pin being peened over.

I was thinking of plugging the decomp with a bolt and shellac. Never thought about lead in there. Not sure it would stay?

I had a bad decomp like that on a early Poulan with that same basic decomp setup and ended up plugging it off and I'm thinking that is your only option as well judging by the pictures you posted.

Right, wrong or otherwise, I used a cut off wheel to cut the decomp plunger off and used it to actually fit down in the hole to use for the plug itself. I then tapped the decomp hole in the cyl and used a allan head set screw to hold the cut off plunger tight in the cyl.

You want to cut the plunger so it sets to where the set screw is just below flush on the cyl as I took a center punch and actually peened the cyl so it staked the set screw in. I also did the same on the hole going into the exahust port but I'm not so sure you really have to do that.

If you try this, just make sure to snug the setscrew and plunger when you tighten it, over torqueing it will dent the cyl.

I thought I had a picture of that one that I did but sure cannot find it now.

 

[Modifiedmark]

No wonder I couldnt find the picture, I didnt take it. Wildman took it and posted it after I did that saw for him.

This method worked like a charm.

 

[cpr]

Quick update. Still waiting on seals to arrive and hunting for rod bolts. I may just make some to work. In the mean time, I found rings and a couple sets wandered in here today. Sure enough, after 44 years inside the bore the old ones were tired.

View attachment 267168

 

[leeha]

Yep, I've seen a few pretty worn rings come out of some 797's also.
They had a hard life if they came from the PNW where most mine
came from.
Have you tried Terry Ives in Ca. for rod bolts? and do you want the
Mac spline head bolt. Or the earlier socket head bolt.
Also, you could try vintagevixon on ebay. He's selling off lots of Mac kart stuff.




Lee

 

[cpr]

It had early socket head in it. That's fine by me.

 

[Jacob J.]

Al Smith was making his own rod bolts for the big Macs using grade 10 fasteners he got from McMaster-Carr and then turning the heads down on his lathe to clear the stuffer.