40cc Ported Stratos

[Chris-PA]

This is interesting - if you listen to the sound in this video you can hear it surging. That is another thing I did not like about how it was performing:



This is the same 16" bar and TriLink lo pro, but this time it's full bar. This is very hard and dead ash, and it makes the cut in about 23s, and throws nice chips. That's not too bad. When I looked at the spectrum plot from the cut, it turn out to be averaging 12krpm through the cut, sometimes dropping to 10.7krpm, but also going up to 14k. here is the plot - the width of that spike is the range of rpms it ran during the whole cut, centered on 12krpm (200Hz on the graph):


I have seen clear signs of rev limiting on these saws at 14.5k, but I'm not sure exactly where it kicks in. If that happened under load it would really knock it back, and I'm wondering if that is what is causing the surging during the cut. So I guess a 40cc saw running those rpms at full bar under load in hard wood is not too shabby.

 

[svk]

That pulls very nice rpm's considering what it's doing.

 

[scottr]

I think that saw is running very good. I heard A change in sound when you rocked the saw for a new bite. Wonder if a smooth felling plate would help .

 

[Chris-PA]

I think that saw is running very good. I heard A change in sound when you rocked the saw for a new bite. Wonder if a smooth felling plate would help .

I've made a couple of those, including one on the same chassis as this saw. I like them a lot actually.


It's too bad I didn't have anything large or better secured to cut on, as having the log shift around didn't help me in making consistent cuts.

 

[scottr]

A folding sawbuck with a moveable center cross member helps. It uses a 1.25" diameter 40" long dowel. A 15" rubber bunge keeps the log steady.

 

[Chris-PA]

I was looking at some plots from a portion of a video I did not upload where I was tuning the saw, and it would appear that the rev limiter kicks in at 15krpm. The saw did not rev that high on the full-bar video, so I think my speculation that it was hitting the rev limiter is wrong.

That means I still don't know why it was surging, but I probably need to look at enlarging the fuel outlet in the carb. Maybe it was maxing out the fuel flow on the carb and leaning at high rpm? It seems like the most likely explanation. If so then it will probably reduce the rpm a little bit but allow it to hold steadier.

 

[Chris-PA]

I tried drilling out the H needle seat to allow more fuel flow, but I went too far. It was at 0.040" and I drilled it to 0.046", but it turns out the taper on the needle does not get quite wide enough and so I could not lean it far enough. So I put the original carb back on and it worked fine. The surging it was doing last time is gone, and it's running around 12krpm through both cuts:



I'm happy with it now and I think I'll leave it alone. It seems to run about as well as the Ryobi, but I should to a comparison with the same bar and chain again.

 

[scottr]

Hey Chris, that's running great. Was that the ramped drive link tri link chain?

 

[Chris-PA]

Hey Chris, that's running great. Was that the ramped drive link tri link chain?

Thanks! Yes it is, the standard TriLink lo pro sold in box stores. I think Oregon 91VXL and 91PX are faster, as well as Carlton N1, but not by much. Actually I think 91PX is the fastest of those when properly filed, but the cutters are too short for it to be a good value. So I've pretty much switched to TriLink for common lo pro chain loops as I'm just not going to put it on a credit card and order online. It holds up well and has long cutters.

 

[scottr]

Thanks! Yes it is, the standard TriLink lo pro sold in box stores. I think Oregon 91VXL and 91PX are faster, as well as Carlton N1, but not by much. Actually I think 91PX is the fastest of those when properly filed, but the cutters are too short for it to be a good value. So I've pretty much switched to TriLink for common lo pro chain loops as I'm just not going to put it on a credit card and order online. It holds up well and has long cutters.

 

[scottr]

When I see full bar cuts like that I wonder about the cutters lifting up to take the next bite while going around the low side of the nose sprocket. Got any 12" thawed oak ?

 

[Chris-PA]

When I see full bar cuts like that I wonder about the cutters lifting up to take the next bite while going around the low side of the nose sprocket. Got any 12" thawed oak ?

I looked for some today but didn't come across anything suitable in the little time I had. I think maybe the larger diameter nose sprockets might help a little in that regard.

 

[Chris-PA]

OK, I lied - I didn't leave it alone. I ran the saw more last weekend and it worked well enough, but it really wanted to rev to the sky. When I tuned it so that it ran well under load it wanted to take off with lighter load. When I tuned it so that the max revs were more reasonable it was pig rich under load.

Then I remembered that when I first ported this saw I increased the strato (air-only) intake duration, but did not increase the fuel/air intake duration. The air only intake was at 163deg and the fuel/air intake was at 144deg duration. At the time I wondered if this might delay the arrival of fuel into the cylinder too long, such that it would lean out at high revs. That did not happen at the time.

Now however, I've increased the exhaust duration and moved the rev band higher, and it would seem like it is exactly what is happening - at least it seems to describe the symptoms.

So I went back in and increased the fuel/air intake duration to 152deg. Now the timing looks like this:


Becasue there is an impulse port right under the intake I decided to notch the piston:






For reference, one these saws moving this edge is how you increase the strato air only intake duration:


Then, just on a whim I bored out the factory carb a little bit. It's a 24/64" stock, but I took it to 26/64". I didn't go any further in part because I did not want to reduce the fuel signal too far, and because the WT750 has a smaller throttle plate than the 28/64" WT391 I had played with. Not much to see:


It started and ran, and I gave it a preliminary tune. It felt responsive. Hopefully I'll get to try it tomorrow.

 

[Chris-PA]

I did a little cutting on a different ash today, using an Oregon 91PX chain. The saw behaved much more consistently, running at a little lower rpm as I expected.



The first cut averaged 11krpm through the cut. The second cut I pushed it harder and the rpms dropped a bit and varied more, but it cut just over 1s faster. NOW I'm done with it! At least with the engine anyway - the air filter still sucks.

 

[scottr]

I did a little cutting on a different ash today, using an Oregon 91PX chain. The saw behaved much more consistently, running at a little lower rpm as I expected.



The first cut averaged 11krpm through the cut. The second cut I pushed it harder and the rpms dropped a bit and varied more, but it cut just over 1s faster. NOW I'm done with it! At least with the engine anyway - the air filter still sucks.

Hey Chris, here's a old thread that member Moddoo started that might help 'Prefilter material to help keep saw air filters clean' my PP255 came with a petrified air filter that I used this on and it works well.

 

[Chris-PA]

Hey Chris, here's a old thread that member Moddoo started that might help 'Prefilter material to help keep saw air filters clean' my PP255 came with a petrified air filter that I used this on and it works well.

Thanks Scott - that looks interesting and I may have to try it. This filter seems to be leaking at the interface where it connects to the carb and the air valve. Or maybe where the two halves join.

 

[Chris-PA]

I learned a few things modding the McCulloch saw to try to match the Ryobi, primarily because the two engines are mostly identical.

First, by increasing the air-only intake duration you can delay the arrival of the fuel mixture into the cylinder too long, such that the transfers close before it is all in. This causes it to lean out at high rpm. The strato system creates this delay by filling the transfers with air, and the fuel can't get into the cylinder until all the air is moved through the transfers first. If the delay were in terms of degrees of crank rotation, then it would not matter. But in a strato the direction of flow in the transfers reverses - first it pulls air down the transfers to the case, and then pumps it back up. So there is going to be some charge inertia, and this will make the delay more of a fixed amount of time. That means that at some rpm there just isn't enough time to get it done and the fresh fuel charge gets starved out.

That did not happen the first time I ported it, but once I increased the exhaust duration (and operating rpm) it did. Increasing the fuel/air intake duration a little fixed it by changing the balance toward more fuel/air instead of air-only.

Second, I believe the reason the Mculloch does not run as strong is because the longer intake duration reduced the case compression angle too much. If you look at the port timing diagram above, the case compression is 39.5deg, which I thought should have worked well as the case has a small volume. But it's over on the other side of the stroke that the problem lies. A strato has a lot of intake area, but the air-only intake path must go through the air valve, across the piston, then into and down the transfers. It's a bit contorted. Plus, it has to pull air through the small carb at the same time. Both the small carb and the strato intake are restrictive, and my guess is that just having a lot of intake duration is not good enough, rather you need to have a good pressure differential to move the air. A longer angle between the time the transfers close and the intake open allows for a lower pressure to build up before the transfers open, and I suspect this is more beneficial than intake duration.

 

[Chris-PA]

I was using this saw today, and realized that I never posted the end of the story for these obscure old license built GZ4000-based saws. In short, the factory modified air valve design doesn't work after a little run time, and the design can't work either. The Zenoah air valve is pretty crappy too: It has no shaft seal on one side, and a rubber lip seal on the other - but the nylon shaft doesn't fit well when new, and after a little sawdust and wear it moves enough that the lip seal also doesn't work. However, the Zenoah air valve passages seal after/below the shaft, so the crappy shaft seals don't matter.

The Jenn Feng air valve moved the main seal to above/before the shaft, so the air leaks around the shaft matter - it cannot possibly hold an idle setting. The solution was to make a cap to block off the end where the rubber lip seal is. Since the shaft doesn't stick out past the lip seal it never made any sense anyway. It will leak on the other side anyway, but I had replaced that end of the housing with a brass bushing that fit better already.



Probably no one will ever mess with one of these, but if you do just throw out the air valve and find a Zenoah GZ400/GZ4000 or Ryobi air valve - it isn't worth the time to bother with. At least the Jenn Feng saws have Walbro carbs rather than ZAMAs.