Exhaust Delayed Scavenging

[bikemike]

I personally would still cut those skirts up to the machined ridge inside the skirt. That where I went with mine. I don't have the squish reduction or much done with the jug ported exhaust out and did the same work to the intake manifold like your saw

 

[bikemike]

Well, I lucked out - there is no pin in the port, or ring end either! I'm betting it's really close, but it should be OK.

View attachment 487702

Here it is in all its glory. I had to use a mismatched clutch cover until I fix the brake on the green one. The muffler doesn't really look that rusty, it's a flash thing.
View attachment 487703

Now I can keep experimenting with this project.

Oh yeah did you close off the back exhaust ports near the jug. My guess is you did

 

[Chris-PA]

I personally would still cut those skirts up to the machined ridge inside the skirt. That where I went with mine. I don't have the squish reduction or much done with the jug ported exhaust out and did the same work to the intake manifold like your saw

LOL, I knew you were gonna say that - I could hear it coming while I was grinding! I'm usually quite conservative with port widths and muffler mods and such, and find good gains anyway. I was concerned about the strength of the piston skirt, given the large intake notch I have there too. I can always make them larger, but suspect I won't have to. Keep in mind it's only needed right at BDC - a few degrees on either side and it has plenty of clearance, especially with the bevel on the outer edge of the counterweight.

Oh yeah did you close off the back exhaust ports near the jug. My guess is you did

Actualy, I have them pried open further. It's an older muffler I did as I'm out of the better deflectors for the way I like to do them now. It's the one in the middle here:

 

[Chris-PA]

I was so nice today I took the afternoon off. I worked on that small hickory and cleared a bunch of brush to get at it. I took a couple of videos, but the first was running an 18" bar and Oregon 91PX, and I had a heck of a vibration problem. The wood is pretty hard, but it still seemed extreme (more about that later), so I changed to a 16" bar and Carlton N1. That was a big improvement.

The saw ran quite nicely - still not super exciting, but certainly competent. I think adding the transfer access notches in the piston skirt made a noticeable improvement.

Here's the video - I think the tuning is a bit fat, but not too bad. It's mostly running around 10,500rpm, sometimes over 11,000:




The no load rpm was around 14,200rpm.

Later I looked at the 18" bar. It is a PowerCare bar, and while it's hard to see it turns out the bar rails are not high enough relative to the bottom of the trough. The bottom of the drive links have been peening the laminate center insert in the middle of the bar, and the drivers have been flopping to the sides and wedging open the rails. Both sides look the same. No wonder it vibrated some!

 

[bikemike]

Sounded good. When they run like that makes like the small cheep saws. Is the the crapsman with the Poulan block? And cut skirts. Did you notice any gains with skirts cut?

 

[Chris-PA]

Sounded good. When they run like that makes like the small cheep saws. Is the the crapsman with the Poulan block? And cut skirts. Did you notice any gains with skirts cut?

Yes, I put the Poulan engine I've been working on in this thread in the Craftsman chassis. Actually the engine originally came out of a Jonsered 2137, but it is the same non-plated engine as were in the Poulans.

I do think the skirt notches helped, but it was not as radical as I had hoped.

 

[bikemike]

Cool so it wasn't a bad thing for the saw. I may thunk about a squish reduction or popup for 2 of my poulans. I have 1 crapsman 36cc strato that runs really good never tore apart the engine or advanced timing just a loud muffler mod and bored the carb out a bit but it's sounds nice and the now 11 yr old loves it. Especially when he revs it to the max.
Got 1 strato woodshark 42cc and I hate it. May have to do a major muffler mod on it cause the baffle is removed and it is a turn that is just an inconsistent runner.

 

[LegDeLimber]

Has anyone tried .325 chain on these 42cc units?
If so, what clutch shell did it take?
The craftsman that I have has a 6 tooth spur shell.
It would be nice to have a rim drive setup for it.
Not sure if you could even get a small enough (7?pin at the least) 325 rim to fit it though.
I suspect you'd be looking at a limbing only saw, with the 7 pin.
And I believe that an 8 pin would be quiet overly optimistic on the little saw.

and this could just be my nausea & fever talking.

 

[bikemike]

I want to find a oregon rim clutch for my 46cc pro so I can try a 7 pin chain speed is slow on a 6 tooth 325

 

[Chris-PA]

Has anyone tried .325 chain on these 42cc units?
If so, what clutch shell did it take?
The craftsman that I have has a 6 tooth spur shell.
It would be nice to have a rim drive setup for it.
Not sure if you could even get a small enough (7?pin at the least) 325 rim to fit it though.
I suspect you'd be looking at a limbing only saw, with the 7 pin.
And I believe that an 8 pin would be quiet overly optimistic on the little saw.

and this could just be my nausea & fever talking.

I am not aware if a rim drive for these, but then I have not looked as I've never had an issue with spurs. The 7 pin 0.325 drive sprocket (530069342) from the various 46cc units (like my 2775 or the PP295 and 4620) will fit fine. I prefer 3/8 lo pro, and am considering swapping my 2775 out to that.

Hope you feel better!

 

[bikemike]

Never mind my poulan is running a 7

 

[Chris-PA]

I ran the saw again today in some white ash. This time I used a 17" bar and TriLink 3/8 lo pro safety chain. It still runs around 10,500rpm - the saw doesn't seem to want to rev over that under load, but it holds it well.



I'll probably keep this bar on it. It's really an A095 60DL bar for some cheap McCulloch, but I modified it so it would oil and have a couple of chains for it. It's just been hanging on the wall in the barn and since this saws is a collection of misfit parts, it seems appropriate!

 

[bikemike]

For aged ash it's cutting smooth and holds nice rpm

 

[Chris-PA]

For aged ash it's cutting smooth and holds nice rpm

I'm not sure which is harder, green pignut hickory or old white ash. Probably about the same. The TriLink is safety chain with large ramps like Carlton - it is smooth and also good for clearing brush.

Likely it is the larger combustion chamber/reduced compression ratio and the more restrictive transfers that keep it from revving higher. Maybe a shorter intake duration and longer case/primary compression would have helped, but I'm skeptical of that. Still, it's a nice usable saw and I'm happy with how it runs.

 

[bikemike]

Well either way it's holding it's own for a 42cc

 

[Chris-PA]

So having a bit more time on this saw I'm quite pleased with how it runs. Clearly it's no 11k screamer, but it's extremely useful - it'll pull in the 10k range in some decently sized and hard wood, and it doesn't bog easily.

That got me thinking about the original configuration, which I do think was set up for exhaust delayed scavenging, and that the higher transfers with lower volume made a good starting point for increasing performance.

But what about that larger combustion chamber? That has me considering combustion chamber size in general. Obviously larger combustion chamber volumes negatively impact compression ratio, but do they also trap a larger volume of fuel/air mix? At the point of ignition, is there more fuel trapped in the chamber to burn with a larger combustion chamber?

Clearly chamber volume can't be zero or way too big, but what is optimal? Last, does a larger chamber help with loop scavenging and preventing loss out the exhaust?

 

[bikemike]

So having a bit more time on this saw I'm quite pleased with how it runs. Clearly it's no 11k screamer, but it's extremely useful - it'll pull in the 10k range in some decently sized and hard wood, and it doesn't bog easily.

That got me thinking about the original configuration, which I do think was set up for exhaust delayed scavenging, and that the higher transfers with lower volume made a good starting point for increasing performance.

But what about that larger combustion chamber? That has me considering combustion chamber size in general. Obviously larger combustion chamber volumes negatively impact compression ratio, but do they also trap a larger volume of fuel/air mix? At the point of ignition, is there more fuel trapped in the chamber to burn with a larger combustion chamber?

Clearly chamber volume can't be zero or way too big, but what is optimal? Last, does a larger chamber help with loop scavenging and preventing loss out the exhaust?

Do you have timing advanced? Did you say you had a bigger carb installed? You did reduce squish and opened up ports, and cut skirts. Did you also make a cut or 2 with the top cover off? Maybe it might be starving for a bit of air flow

 

[Chris-PA]

Do you have timing advanced? Did you say you had a bigger carb installed? You did reduce squish and opened up ports, and cut skirts. Did you also make a cut or 2 with the top cover off? Maybe it might be starving for a bit of air flow

Yes, squish reduced, ports widened and timing changed, spark timing advanced, larger carb, muffler mod - I'm happy with the saw. I think mid 10k is fine for 42cc cutting hard wood with semi chisel chain, given the humble engine architecture (dual open transfers). Especially since it holds that steady and does not struggle.

I'm more interested in discussing the effect of larger combustion chambers, and what size is best given that I've found it does not seem to be much of a handicap on this saw.

 

[bikemike]

Yes, squish reduced, ports widened and timing changed, spark timing advanced, larger carb, muffler mod - I'm happy with the saw. I think mid 10k is fine for 42cc cutting hard wood with semi chisel chain, given the humble engine architecture (dual open transfers). Especially since it holds that steady and does not struggle.

I'm more interested in discussing the effect of larger combustion chambers, and what size is best given that I've found it does not seem to be much of a handicap on this saw.

Well for chits and giggles of it maybe pull a old piston weld a pop up and shape it to the combustion chamber and se if it improves performance. If so than they probably made a big combustion chamber for easier starting for the dumbazz home owners than need electric start for there mowers and snowblower.

 

[bikemike]

Either way your saw is running good according to the vids. I'm not sure about the new frame work on ur crapsman saw chassis. But the non strato wildthingys have 1 small air inlet near the flywheel. If the crapsman is the same way then open it up a bit to compensate for the bigger draw from the carb and porting. Maybe make a louver to catch more air to act more like the jred turbo