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041AV rebuild/porting question/build

DucTruckin379

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I've owned this saw since I was 18. The last time I loaned it to a friend, it came back only half running and not idling (this was about 15yrs ago). It would start right up cold, and run well until it got good and warm, and then it lost tons of power, and wouldn't idle at all. I assumed they'd run straight gas in it, and at that point in my life it went in a box, without the slightest troubleshooting (I'd already had trouble getting parts in the past, and I wasn't online then).
Fast forward to now, and I decided to tear into it and see what might be the issue...
Turns out, the coil wire had gotten displaced and was getting into the flywheel. Evidently it just needed that little bit of heat to make it really short out.
Anyways, I've decided to go through it anyway, and as of today all the parts I need to bring it back to top notch are in the mail on the way. Currently it's got a Mahle P+C. I don't know if that was factory back then or not, but the casting/ports look way better than what I've seen on current Sthil stuff. Both P+C are still in really good shape, and I plan to just lightly hone it and install a new set of rings, which leads me to my dilemma...
It should have room to do a base gasket delete (will be measured when I put the piston back on the rod, as the pin is a heat/press fit).

Assuming I can get at least .020 of squish without the base gasket, will this be something worth doing? This thing won't be anything other than a play saw, but I'd like to get whatever I can out of it without going crazy.
The next question would be, if I do a base gasket delete, do I bring the exhaust port up to match the original timing? I won't be able to bring the transfers up with the exhaust, as I don't have anything that I can get into the cylinder with. This would leave me with a decent amount of extra blow down, but on this saw I don't know what that alone might do to the powerband. It's been years since I've run this thing, but I remember loving how much low grunt it had, and if I can get a little more while I'm in there great, but I definitely don't want to do anything to hurt the grunt...

For background, I've done a bunch of port work on 2stroke bikes and 4 wheelers years ago, just not on saws, so I'm confident in my ability to do the layout/grinding work, but I don't have a 90 head small enough to do the transfers and I don't want to ruin this cylinder because the alternatives are either crappy or very expensive.

I know the safe thing is to leave it alone... But I don't want to leave any easy power on the table if it's there for the taking either...

So... What would y'all do?
 

MemphisMechanic

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You’ll need to use a timing wheel ($30 worth of stuff on Amazon) to measure where you’re at.

I did a base gasket delete on a 1989 Stihl 044 and only lowered the intake to 78 and exhaust to around 102 if I recall, and left the transfers alone, as it timed out to around 25 degrees of blowdown and my dremel wasn’t going to reach.

It turned out to be a very strong saw for such a mild build, and pulls a 32” lightweight bar and full comp chain all day long in professional service.
 

DucTruckin379

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Been tied up all weekend, gonna try to run the numbers tomorrow evening or Tues evening. Should've come prepared... I'll report back when I've got timing. @MemphisMechanic , sounds like exactly what I was hoping for, thanks!
 

DucTruckin379

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@Redfin
So here's where it's at without a base gasket.

The squish is even more than I thought it would be.
The PTO side transfer opens closer to 120.5...

I typed out a whole plan, then realized that it would be pushing it closer to newer saws, and might kill the grunt. The more I think about it, the more I think I see what they were trying to accomplish with the numbers as is. That said, I've got zero experience with anything without a pipe, and zero experience in the last 15yrs so... How can I keep the grunt, but maybe move it up in the RPM range just a little?

Now that I'm re-reading @MemphisMechanic 's post, it's sounding real close to what I was thinking of doing.
 
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cus_deluxe

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Without machine work i doubt theres a ton to be gained. Pics of inside of cylinder?
 

MemphisMechanic

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Personally if you want to keep the grunt, I’d do a conservative job on the exhaust, perhaps 103ish. Widen it a little if the saw will accept it, but be conservative there.

I am sure you already know, but higher exhaust means more top end power and higher RPM, at the sacrifice of some bottom end grunt. My 044 was really good about slinging a 28” chain back into motion when slightly pinched in the kerf. Great balance of torque and top end - no one who ran it would ever want to return to a factory saw.

Bringing the intake floor down to 76-78 is going to be what really transforms your 041, I did something similar on the 044. Do your checks to make sure you won’t free-port or spit back through the carb before grinding, because that’ll probably be a big 1/16”-1/8” chunk of aluminum…

If you want to play it safe, cut it to something like 104 and 74, put it together, and see how she runs. Leaving the transfers alone should be fine; just open up the lowers, then taper/blend them into the uppers.
 

DucTruckin379

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When you say "check that you won't be free porting", in this case, without machine work to the base of the cylinder, that would imply cutting the skirt to get the intake numbers, correct? I've actually marked this off already @75°, and still have plenty of room. My logic for piston vs. intake floor was to maintain velocity (by not increasing runner volume). Sound plan, or no?
(Oh, and yes, it's a big chunk, but I didn't measure it)
 

MemphisMechanic

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I lopped a huge hunk off the floor of the cylinder on the 044 rather than cut the piston, since it was a rather small port. Runs great. Might work just as well with a trimmed pistion but I haven’t tried.

What I mean is that you need to verify that after you cut either the intake port floor or the piston skirt, ensure the intake will still fully close when it’s supposed to.

If it’s opened up too far, your intake won’t be closed when the piston’s stroke is beginning to pressurize the crankcase, and the saw will spit back through the carb.

The 200T, the gold standard of top handles, made such good power partially because it already had agressive intake timing for a factory saw. It’s already close.
 

DucTruckin379

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What I mean is that you need to verify that after you cut either the intake port floor or the piston skirt, ensure the intake will still fully close when it’s supposed to.

If it’s opened up too far, your intake won’t be closed when the piston’s stroke is beginning to pressurize the crankcase, and the saw will spit back through the carb.

Ok, yeah, I was getting too far ahead of the discussion with my thinking I guess. I thought that since, by my measurements (that you obviously didn't know about) there wasn't a way to get it to free-port without cutting the cylinder base, that you were implying cutting the piston. I've got a bad habit of thinking all of these things are the same, and that you guys can see everything I'm seeing. My apologies...

So, yes, even with the piston trimmed to where I have it marked (75deg), there's still plenty of skirt to fully close the intake port. It'll obviously be closing later in the cycle, but it will fully close at any timing number that I might consider. If I still seem confused about what you're saying, please let me know.

I was actually wondering if the stock "early" closing was contributing to a lot of low rpm crankcase pressure and transfer flow. I'm sure it is to a point, but I agree that it closes pretty early, and opens pretty late compared to the numbers you've provided and that I've been able to find.

Anybody else have a pro or con to doing the port floor vs the piston? I won't/don't want to do any carb work, which was part of the logic for piston. I didn't want a big high volume/area behind a smaller volume/area further up the intake runner/carb, as it'll take a lot of material removal from the port floor to get to the mid upper 70's timing numbers.

If I'm thinking about any of this in the wrong way, please point it out!
And, @MemphisMechanic , Thank You for the discussion so far!
 

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Max rpm for that saw is 11,000 according to one specification website. I might have an aftermarket piston you could try porting the piston first to see what numbers you want. I like the idea of 103EX and 74IN or somewhere around there. Lower on the intake if the carb and intake boot looks unusually small for that size of engine. The 59cc Echo 590/620 also has a long stroke small bore and has great torque still with a 103 exhaust, and I take the intake down on the lower side since the 590 has a small carb and intake tract.

I just checked and my piston is for the 48mm super.
 

DucTruckin379

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Ok, based on y'all's responses, and more measuring, I think I've got a plan. I've said this before though, and I'll say it again, if at any point you're reading this and thinking "no dipstick, that's not how this works", then for my sake please say something!

For the intake:
Below is the piston with three lines drawn on it. Top line is the bottom of the exhaust port at TDC, the middle line is what I'd have to take off the floor or skirt to get 78deg, and bottom is 75deg. You can see that if I took material off the floor, it would be a whole lot. Memphis Mechanic got it right, it's a little over 1/8" to get to 78. (Ignore the "scuffing", it's almost all graphite from the pencil and me turning it over a gazillion times)

IMG_20220824_190201.jpg

This is how much extra volume the intake port already has over the carb/adapter block (spaghetti fills the intake track/carb bore)... This was the only way I could get a picture to show up, but it's the same in the port.

IMG_20220824_184549.jpg

So... I'm really leaning towards getting the intake timing with the piston skirt, even if I had plenty of room to get it on the floor. The way the boots/adapter block etc are aimed, it would make the weird kink that's already in the flow pointing to the top of the port worse, and it would add nearly 30% or so to the already large area in comparison to the carb, and I'd think that it might cause the charge to slow enough to drop fuel out of suspension. (yes, no?) I think this is what you were talking about @Nutball . Not getting too crazy with intake timing if you had a smaller carb? You were talking about making the timing number numerically lower and not about lowering the floor right?

For the exhaust:
These lines are drawn off the floor of the exhaust, just to show the vertical distance between 108 (where it's at now) and 105. This is just my rough way of seeing how much I'd need to raise the exhaust to get to 105.

IMG_20220824_190530.jpg
It looks like there's plenty of room there, and I could go a little further later if I decided to. Because of the way these ports are stacked, the intake track and the muffler flare away from each other and put elbows in exactly the wrong direction for each of the ports...

So, finally, the Plan:

Raise exhaust to 105
Get 75 on the intake by porting the piston skirt. The 75 would also allow me to later take a little off the cylinder base to reduce the squish some, and I'd still only be around 76 or so, and I wouldn't be adding to an already huge cavern behind the bend in the intake track that's pointing at the top of the port.

I re-measured all the numbers before doing all this tonight, and I think my wheel or pointer slipped a little between doing the int/exh numbers and looking at the transfers. The int/exh are right, but the transfers are actually at 120.
This would leave me with 15deg of blow down vs the stock 12. Raising the exhaust to 105 would only put me about 1.5deg less power stroke than it had stock with the gasket too.

Tear it up, or prop it up, what do y'all think?
 

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I meant a physically lower intake port: more duration to compensate for a small intake bore. I judge small by comparing to other saws, not just comparing the port area to the rest of the intake area. For example, a 064 is known to run very well, and it has a larger bore carb than a 395, 9010, and chinese 660. But I've heard that the venturi size doesn't really affect much, just the overall diameter of the rest of the carb and intake tract. I don't know, but I do know that saws like the 9010 and 590/620 have overall small intake tracts.

I think what you are doing and the numbers so far looks good. Don't forget to mark the port widths on the piston, so you know how far it is safe to widen the ports. I would only grind on the piston as wide as the port will be, so there's still some original skirt length for giving the piston stability. Doing so might allow you to shape the piston such that the ports open very fast or square without risk of hanging the piston. I did this to a few saws on the exhaust port, it allows for a safe curve for the rings while letting the port open to full width a few degrees sooner than it would.


DSC04772 (1280x960).jpg
 

DucTruckin379

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Ok, I see about the intake... At some point opening earlier and closing later starts to kill crankcase pressure to drive the transfers though right? Mainly the closing later part. I guess this is where being able to test comes in...

I was actually considering doing the above for about 1/2 the needed exhaust modification, it was a common thing on the stuff I worked on years ago and did work well... It would leave me plenty of area above the top ring, and open the port faster, and be easier on the ring as you say...
 

DucTruckin379

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75 to 120 is still good at 45degrees of case compression. I'm sure someone else knows better, but over 40 is good.

I've not run across a good number for case compression, thanks!
 

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Nutball is giving good advice and has built far more saws than I have.

Biggest pro to trimming the piston skirt for your intake timing: if it sucks and you want to try again, pistons are cheaper than cylinders.

Biggest con: You lose some skirt height and stability in the bore.

I’d trim the piston for 75-76 and grind the exhaustxroof to 103. Advance the flywheel 5 to 8 degrees. Open the muffler up. And go run it.
 

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Chances are an aftermarket replacement piston, if that's all that's available, is going to be heavier the OEM, so hopefully the piston won't need replacing. I think 103 on the exhaust would be good, no higher. I'd stay conservative on the intake at first just to make sure it doesn't loose too much low end. I wonder how the small bore affects the response to intake timing? A 660's bore/stroke is 54mm/40mm while this is 44/40. A good intake number for a sporty but still torqy 660 is 78. With the exhaust at 103 the 660 has gobs of torque, and so does the 45mm/37.5mm Echo 590.
 

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I wonder how the small bore affects the response to intake timing?
This is what was driving my lean toward conservative here. I'd think that with the reduced displacement in the same stroke, that it might want a little less timing to maintain transfer volume/velocity, especially considering they ran the super P+C on this same crankcase, but again I'm rusty and out of my element so...

Between y'all's responses, it sounds like the exhaust is safe, and could even go higher than I had planned without fear, and maybe I'll stick a little shy of or right at 75 on the intake, knowing it'll be easy to get more, but it's never easy to put it back...
I do already have the ignition timing at 5deg advanced from where it was when I pulled it down, but I still need to confirm where it was and is now etc. I am curious how that is going to turn out at idle/starting, as it looks like this thing is just flat in it's timing curve (it's points with a simple coil). That should be easy enough to change (go further or back off, depending on behavior).

You guys are awesome, Thank You.
 

Redfin

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I like a few degrees less intake duration then ex. If you are set on grinding piston to set timing you can always just grind notch to match port wall, that will keep piston stability in bore.
 
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