40cc Ported Stratos

[Chris-PA]

Sometimes things don't go like you want them to. I had intended this to show that you can get good power gains while porting a strato without gutting it, and while I'm happy with one of them the other is not there yet.

I have a couple 40cc saws based on the GZ4000 design. One Craftsman yellow saw that is really a Zenoah GZ4000. One Ryobi 10532 that is really a GZ400. And one weird "McCulloch" saw made by Jenn Feng in 2005 just as Zenoah was releasing the GZ4000 - it is clearly a licensed design, with the same engine and chassis design, but different castings, different plastic parts, and different air valve and carb.

They're all the same engine design. I ported the McCulloch MS4018 a few years ago and did a thread on it. I increased the exhaust duration from 132deg to 142, raised the transfers to maintain the blowdown angle, increased the strato port duration, removed the base gasket and muffler modded it. It ran well.

Then I put the Ryobi together from a box of used parts, removed the base gasket and matched the lower transfers to the case. It ran well but the McCulloch beat it handily. So a few weeks ago I was bored and decided to port the Ryobi. I increased the exhaust duration to 150deg and raised the transfers to match. I left the intakes alone:



It smoked the McCulloch rather badly. So naturally I had to mod Jenn Feng engine more!

I figure the difference was likely to be the increased exhaust duration, so I just raised the exhaust and transfers, leaving the intake where they were. I ended up with a n exhaust at 148deg, just a little shy of the Ryobi, but with more intake:


I thought it would run really well, as I had been very happy with the Ryobi, and I took it up to the woods on Sunday to work on some dead ash. I took the video camera and set up a log, and even used the same bar and chain so I could compare.

First, the wood was so hard it looked like I had the chain on backwards. Yes, it's an 18" bar on a 40cc saw, but they sure took forever to cut through. It's actually quite sharp Carlton N1, but while I like semi chisel I've found with light saws and very hard wood sometimes it just won't bite. Next, I was disappointed in the McCulloch. With the same wood, same bar and chain the Ryobi is a good second or more faster:


So now I'm trying to figure out why the McCulloch is behind. It could be the different port timing, but I'm not convinced. Jenn Feng made their own air valve, and I suspect it may be more restrictive than the Zenoah design, so I'm going to look into that. The Jenn Feng valve is on the right, and it will be hard to increase the opening as there is not much surface for the filter to seal to ( the Jenn Feng filters kinda suck):


Next, the muffler on it was one of the first of this type I did, so I need to see how that compares to the one on the Ryobi.

The Ryobi is no slouch though - here it's running at 11.5k to 12k in the same tree, but lower where it was a little wetter and not so hard:


And here at 11.5k but running TriLink:

 

[merc_man]

Ryobi looks like a good runner.

Sent from my SM-J320W8 using Tapatalk

 

[GeorgiaVol]

Have you ever ported one of the cheap 33cc Homelite rangers? I have 2 that I could try for my first port job that wouldn't kill me if I messed them up.

 

[Chris-PA]

Have you ever ported one of the cheap 33cc Homelite rangers? I have 2 that I could try for my first port job that wouldn't kill me if I messed them up.

No, I'm afraid I have never messed with any of the Homelite clamshells - I would of course, but I've never run across one.

 

[svk]

I'm sorry if I missed this from a previous thread. Have you gutted the strato on these models previously so you could compare?

 

[Chris-PA]

Some pictures of the modified transfers. The outlets were raised with a hand file:




The cover had to be thinned further so they would not block the top of the outlets:



I took a good look at the Jenn Feng air valve and it appears to be much more restrictive than the Zenoah design. The thing that irritates me is that I'm sure I have a spare Zenoah valve but I cannot find it!

 

[KingDavey]

I would like to see the ported Ryobi and a stock ms241c go head to head

 

[Chris-PA]

I would like to see the ported Ryobi and a stock ms241c go head to head

I don't think the stock MS241 would stand much chance.

 

[Chris-PA]

I looked at the muffler, and I don't think that is slowing the Jenn Feng/McCulloch down. But the air valve probably is. I tried to get a picture of the problem, but it's a bit blurry:


The valve is like a poppet valve, and it has a projection that extends down into the passages. Because the stroke is not very long they never really clear the openings. If you compare to the Zenoah valve picture up thread it is quite restrictive. The gray part here is the original, and you can see the restrictive projections:



I could just buy a Zenoah valve, but I have several of these and I figured it was an excuse to play with the 3D printer! An additional problem is that the valve sits down in a well, and under the air filter lip, so I decided to make an extension to raise it up further into the filter for less restriction. First I printed a housing extension:


And then the extended valve shown above. I put the parts in the air valve I had repaired ( http://www.arboristsite.com/community/threads/fixing-a-gz4000-air-valve.305727/ ), and use JB Weld to attach the housing extension:


This the shaft and crank that extends the valve:


The opening stroke is still short, and I doubt it will flow like the Zenoah valve, but it's much improved and maybe enough:


Here is how it sits in the filter base now:


I hope to get it back on the saw and try it out soon - I'm betting it will make a difference.

 

[Chris-PA]

I'm sorry if I missed this from a previous thread. Have you gutted the strato on these models previously so you could compare?

No, I have not. I still cannot see any advantage to introducing fuel into the strato air path as a goal in itself - rather I think any benefit is simply in reducing intake restriction, with the merging of the two flow paths as a consequence. That's not to say some of the gutted stratos don't run very well, as of course they do. I think these particular engines can be made to breathe well enough with the strato intact.

 

[Chris-PA]

I got the saw back together, and everything fit and worked. I started it but had to move snow so I did not have time to use it. Plus it's cold and blowing and much nicer here by the stove!

Here is a air valve installed:


And with the lower half of the filter:


The real test is if it will hold more rpms in the cut or not.

 

[GeorgiaVol]

Have you messed with a 4218 poulan strato?

 

[Chris-PA]

Have you messed with a 4218 poulan strato?

No I haven't. The closest I did was a Poulan Pro pole saw/trimmer with a similar strato engine design.

 

[Chris-PA]

I wanted to give the saw with the modified air valve a try today even though everything was covered with snow, but I ended up having an air leak between the 3D printed housing extension and the air valve housing. I guess the expoxy did not fill every gap. So I sealed it with a bead of Yamabond4, but it did not set up in time to run it.

In the mean time I decided to put together a larger carb. The WT750 on the Jenn Feng/McCulloch saw only has a 24/64" venturi (the GZ400/4000 has a similar sized ZAMA), since much of the air does not pass through the carb. Given the reduced air flow you don't want too big a venturi or it won't meter well. However, given that the saw is ported and that it is not performing as well as the Ryobi, if the air valve mods don't help I may try a bigger 28/64" carb. I combined the shafts from the WT750 and a WT391 from a Poulan - I had to file the notch in the throttle shaft for the larger diameter throttle plate, and I used a spring from out of my box of carb parts for the choke shaft (the hole in the body was in a different spot). The only down side is that the WT391 has no accelerator pump.

I have not tried it, and I might not depending on how it runs.

 

[Chris-PA]

I finally got run the saw today, and the results were mixed thanks partly to an overly aggressive chain that was a bit too much for the saw, at least on an 18" bar in hard wood. The modified air valve worked well once the sealant set:


However, when I first tried it the idle and tuning were inconsistent, and it was not too impressive. So I decided to try the larger carb I put together. It was a little more lively, but no more consistent in idle or tuning.

I was concerned about an air leak so I pulled the cylinder. I did not see anything obviously wrong with the base sealant or the transfer covers. I cleaned up the edges of the transfers a bit, and noticed that the cylinder walls were pretty polished, probably from the lousy air filter seal this thing had and ingesting sawdust. I went over the cylinder with 400 grit by hand, and got a nice scuff on all of it. Then I polished the rings with 800 grit and put it back together. Now the idle is steady and it holds a tune nicely.

However, I should have changed the chain as this one is too much for the saw. It is Oregon 91PX but it has a lot of hook and the depth gauges are a bit too low - it is grabby and rather uncomfortable, and it keeps pulling the saw revs down. For all that it didn't really cut too bad - I will try it again with a more appropriate chain.

This is with the 3D printed air valve mod and the 28/64" carb:

 

[rmh3481]

Quite impressive Chris, I think its cool that you work on these. Thanks for the graphs.
The Ryobi is holding its rev's real well. Was it Zenoah that originally patented the strato scavenging or was it RedMax?
How about heat, have you noticed if its getting hot? Sometimes youll hear the metal ticking.

 

[TBS]

Redmax is zenoahs us market name.

 

[Chris-PA]

Quite impressive Chris, I think its cool that you work on these. Thanks for the graphs.
The Ryobi is holding its rev's real well. Was it Zenoah that originally patented the strato scavenging or was it RedMax?
How about heat, have you noticed if its getting hot? Sometimes youll hear the metal ticking.

I'm not sure who had the patent as I believe Zenoah and Husqvarna were both working on it. I'm pretty sure the GZ4000 was the first to market with it though, and of course Husqvarna owns it now.

 

[TBS]

Zenoah had the patent in 98 introduced the first stratocharged trimmer in 03 and first strato saw in 04.

 

[Chris-PA]

I had to do some work on my car first, but I did manage to get up in the woods and try a 16" bar with some TriLink lo pro. It's the same wood. The saw was averaging 12krpm through both cuts:


Somehow I enjoy running the Ryobi better. Partly I think it's that the Ryobi has better power at lower rpm, but still spins well. Partly though I think the McCulloch seems to make a nasty sounding racket, and I don't know why! It's the same basic design and the muffler mod is similar, so I don't know why it would sound different.