getting some revs out of the 041

[Brmorgan]

Glad you're OK. There's nothing like a good close call though to improve future safety though! Damn trees not going where they're supposed to... I tell ya.

You're lightyears ahead of me as far as the port work goes though, so I'm afraid I can't be of much help with a lot of your more specific questions. I've never bothered to measure anything more than squish and the max width of the exhaust as compared to the cyl. diameter & piston skirts. But I'm just the type who would rather get in and start doing something than play with numbers and spend more time thinking and preparing than actually doing the work. Just not my way of doing things, though admittedly probably to my detriment if anything. Maybe in time. I guess I haven't felt the need for all the analyzing because I'm not out to push a saw right to its maximum potential, or to get the absolute fastest cut times or anything. I just aim for a decent, noticeable gain with minimal risk to durability; on newer models that pretty much just means modifying them back to how they SHOULD be if emissions regs were thrown out the window.

Further, I really don't understand all the theory behind ideal port durations as expressed in degrees yet. I mean, I understand 2-stroke operation, how the ports work etc. and (largely) how the durations can factor in and everything; I just don't know how to come up with the ideal numbers or the reasoning behind them yet. But I'm learning and every saw I've worked on gets easier and seems more routine.

 

[Brmorgan]

thanks for the tips. i built honda and acura race engines for a number of years so that stuff is all laying around the shop.

LOL we mod Hondas in Canada too...:

<object width="425" height="344"><param name="movie" value="http://www.youtube.com/v/TM48RnRQQ9g&hl=en_US&fs=1&"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/TM48RnRQQ9g&hl=en_US&fs=1&" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="425" height="344"></embed></object>

 

[jsmith40004]

Careful on the intake

Three or four years ago I ported a 041 Super and it had so much reversion that it would spray back 2" above the air filter base. This made it very picky to tune. At part throttle it ran lean. At full throttle it would richen itself up as the mist would hit the plastic part of the air filter, directly above the throttle bore, and charge back into the cylinder. A porter that use to be here, made a 5/16 spacer plate to put between the cylinder and plastic carb adapter and it helped some to increase the powerband but was still a tuning nightmare.
If you need another factory carb adapter PM me as I have a few laying around.

James

 

[mcb]

im really glad you mentioned that james.. gives me something to ponder. lately i find myself wondering how important crankcase volume is.

brad- im not ignoring your videos, i just cant view them at work. gray boxes. probably youtube or something?

 

[mcb]

Three or four years ago I ported a 041 Super and it had so much reversion that it would spray back 2" above the air filter base.

did you cut the skirt or maybe lower the intake floor appreciably?

 

[jsmith40004]

041 intake

I lowered the intake and bell shaped the port. I was told to do this before I started to avoid reversion. I couldn't imagine what it would have been like with a straight tapered port. The builder said that this was a problem on all saws with this style piston port (041, 051, 076) because of the short and straight intake manifold.

James

 

[Brmorgan]

Yeah but how much did you lower the intake by? I never lowered the intake on my 041 Super and while I can see a very tiny bit of fuel spray out the intake for an instant when I open the throttle, it doesn't continue once up to RPM. Though I'm not exactly trying to push it to max RPM, so my mods aren't all that extreme.

 

[hotshot]

Transfer timing

MCB,
I've been following this thread due to porting an 041 also.

Did you ever get a chance to measure your 041 transfer port pair timing while you still have the degree wheel on? They look almost level at the tops with the exhaust port, but getting a piece of feeler guage to stay there while checking is a real problem.

Have you thought about milling an inverted V or U in the skirt (on the top side only) to increase CC fill intake duration? I did an 8~9 mm V on an 041 Super skirt and it seemed to increase flow without spitting. Unless the intake manifold port is modded too much up & down, the back spitting is normally due to a worn skirt & I have seen 045/056 series saws spit when they get worn out.

 

[mcb]

got some time to work on this again tonight. sanded, primed and painted all the white parts except for the case halves which are still in the vice. finally got a camera, will upload pics soon.

using a .004" feeler gauge to ensure a consistent "valve lift" amount, i rechecked my numbers and found i was still pretty darn close.

timed the transfer ports at 117* ATDC open and 117* BTDC close for a duration of 126*

this puts the blowdown at only 9* of crank rotation in a basegasket-removed application. in my opinion, the exhaust open point is about 18% too late. i think holding the cylinder pressure past 90* ATDC (max leverage to rotate the crank) is simply a waste of power. and only 9* of blowdown isnt enough time for the pressure above the cylinder to be reduced below the pressure of that in the crankcase.

 

[mcb]

im starting to think the carb spitting is more a factor of inadequate blowdown and restrictive exhaust than intake port timing. with only 9* of blowdown and a high exhaust port backpressure, there is still going to be a high exhaust pressure above the piston when the transfers initially open, which will pressurize the crankcase. when the intake opens with an above atmospheric pressure, its going to push itself out the carb.

what do you guys think of that theory?

 

[Brmorgan]

Well a lot of that is ΩΦπΔΨ to me lol... I'd like to see a picture of those V grooves in the piston skirts if anyone has some though.

 

[AUSSIE1]

Mcb, you should have at least twice that in blowdown.
Widening the exhaust will increase blowdown and revs.

 

[mcb]

im gonna stick the gasket back in to see what .016" cylinder height does to timing, but ill likely be running it without a base gasket and looking for .025-.030 in blowdown after the compression bump unless i find a compelling reason otherwise in the meantime.

 

[mcb]

i got some of my pics uploaded, photobucket being fussy.

im using a .004" feeler guage to stop the piston consistently.

got some paint and primer on, gonna need to spray some clearcoat to make it durable enough.

here is the muffler that im not too thrilled about. between the turn and having only two slits, i think its pretty restrictive. most of the other 041s ive seen had 4 slits on the muffler. not sure what im gonna do about it just yet.

lot of squish in the chamber, you can see the line from my solder checking

port width vs piston skirt contact patch

fairly pleased with the casting quality, ive certainly seen much worse. more pics to come later

 

[Brmorgan]

Regarding the mufflers, most non-AV 041 mufflers have four slits and the AV ones have two. Likewise the non-AV mufflers don't have the second bolt eye welded to the bottom side towards the rear of the saw. Newer 041AV mufflers have a larger port located towards the top of the muffler, with a flange directing the exhaust downwards. These also have a spark screen; I have one of them downstairs, I'll try to take a pic of all three types sometime.

 

[mcb]

thanks for the info, much appreciated!

 

[mcb]

i just degreed the valve events one more time with the .016" base gasket in place. results was a pretty consistent 2 degree advance or retard of every event.

basegasket or not, the puny blowdown is still a major issue holding these saws back in terms of RPM, or atleast this one. keeping the exhaust valve closed well past 90* ATDC is silly in my opinion. the cylinder pressure has major mechanical advantage against the crank pin because its horizontal at 90*ATDC so thats where the 041 gets its mega torque, but the cylinder is running down the bore and reducing cylinder pressure/temp when the exhaust valve opens, so the blowdown pressure is very low. between the blowdown gasses moving slow from lack of pressure, AND the transfer port opening only 9* later (fresh charge starts coming in) this saw has way too little time to exhaust itself. its dumping fresh fuel into a chamber full of spent gasses and really killing its power potential in my opinion.

if the exhaust valve were to open sooner when the cylinder pressure/temp were higher, the exhaust would exit the port at much higher velocity and be more likely to scavenge itself (vaccuum out the chamber of spent gasses with the initial puff of high pressure at valve opening) .. hence having a higher concentration of fresh fuel mix on the next cycle. so says the theory atleast.

 

[Brmorgan]

Well... Sounds like you oughta know! I guess that's just how far we've come in cylinder design in the 45-odd years since that one was designed. I don't care how much of a dysfunctional ba****d it is though, I love mine just the same!

FYI, did you ever see the cylinder pics in my non-AV 041 build thread a few weeks back? I used an AV cylinder on it, and I had three to choose from initially. Two were the same, but one was different.

The first two looked like this one:

But the third looked like this:

Notice how much farther down the transfer is cast into the cylinder wall on that one? It's a much nicer casting in other regards, as well. On the other hand, notice how much more restricted the transfer passage is. I actually never noticed this until looking at these pictures after getting it all assembled and running. I never did any port work to it at all (no need for a non-AV 041 to shake any more than it already does!), but I wonder if just opening up those transfer passages might give it a bit more grunt without really jacking the RPMs.

It's kinda hard to tell for sure, but your cylinder looks more like the top one in the pics.

 

[mcb]

meant to put this up the other day.. heres how i got the transfer port timing down, since i couldnt see over the piston dome at all.

measure the height from the cylinder base to the top of the transfer with a caliper...

then scribe an appropriate line in one of the ports and mark the wheel whenever the compression height edge of the piston lines up with the scribe line.

 

[mcb]

we must have been typing at the same time brad. i think id prefer the first cylinder. easy enough to radius the base of the transfer but not easy at all to get in there to enlarge the conduit.

my jonsered looks to have removeable transfer port covers for access.. pretty excited to open that little guy up one day. think the 385xp will come right after :greenchainsaw: