Tuning question on Husq. 359

[Hawkster]

I did the muff mod on my 359 and had it tuned fairly well. It would 4 stroke at WOT when not in a cut. Then when I put it to work in a cut at WOT it wouldn't 4 stroke. Worked well for a while. Last weekend it was the warmest out that I've used it (70 deg.) and even in the cut it would sometimes 4 stroke. I suspect I need to turn the screw in just a bit? I figure the warm air is the culprit. Boy she sure runs much cooler without the "Enviro Nazi Muffler"

 

[blsnelling]

When tuned properly, it won't 4-stroke continually at WOT. It'll kind of bounce back and forth. A little more back than forth if you want to play it safe. But it should bounce to 2-stroke some.

 

[timberwolf]

In the cut it should not be four stroking much, if at all, just NO load then it should be four stroking a little.

Warm air is less dense, so it could be a bit rich if tuned in cold dry high pressure air then run next in warm humid low pressure air.

Humidity and barometric pressure are minor factors, but if they all work together then it can throw the tuning off quite a bit.

 

[djauto]

Yes, when I had my 359 it did the same thing after the muffler mod on my initial richened guestimate on the H screw. By leaning just slightly is smoothed out in the cut just fine.

Later I did borrow a tach and set the WOT rpm to 13,250 or so. I found that by ear I was tuning to about 12,500. That being said, 13,500 rpms is screaming and without a lot of practice hard to set by ear. I would recommend a tach.

The 359 really cut well after the mod and carb adjustment. I don't have it anymore but that is another story......

Enjoy!

 

[Hawkster]

Thanks for the info everyone. I'll try a small adjustment in on the screw. I have been looking at tachs. Just don't want to spend another 60-100 dollars on an item I may not use much once my saws are tuned. That of course is the logical side of my brain, which also happens to be the side that said I only needed one saw. Obviously I found this place and have purchased 4 saws in approx. 3-4 months.
I did sell one though!!! Does that mean I'm recovering from CAD???

 

[blsnelling]

Lean it out slowly. When you're real close, like I assume you are, it may only take 1/16 of a turn to make a big difference.

 

[FSTS]

noob question

In the cut it should not be four stroking much, if at all, just NO load then it should be four stroking a little.

Warm air is less dense, so it could be a bit rich if tuned in cold dry high pressure air then run next in warm humid low pressure air.

Humidity and barometric pressure are minor factors, but if they all work together then it can throw the tuning off quite a bit.

At the risk of sounding like a noob, and I am a noob, this may sound will like a stupid question. What do you mean by a saw will four stroke and two stroke. Is four stroking more low end and two stroking when the saw is wide open? Thanks, Kevin.

 

[Hawkster]

Well since I started this post I will respond at the risk of using poor terminology or really knowing anything at all. (which I will never admit to). When you have the high speed screw set too rich the saw will not have the smooth two stroke sound that it should at WOT. It will rev up and then when it gets near or at full throttle it will kind of burble or sound like it's not running as smooth. I hope I explained correctly. If not I hope one of the more experienced people will chime in.

 

[blsnelling]

A two-stroke will not rev smoothly and will smoke a lot if running too rich at WOT. When it's doing this it's "4-stroking". If you lean it out until it "peaks out" and revs much faster and smoother, that's called "2-stroking". When a chainsaw is tuned properly, it will straddle the line between the two. Almost peaked out but not quite. Take it too far though and it will build enough extra heat to seize the piston in the cylinder. As a rule of thumb, the leaner the meaner. Just don't melt your piston.

 

[outdoortype]

What about the spark plug color?

Some guys use the tach as way of knowing where they are in relation to max rpms and then check the plug to make sure they aren't lean.
Follow Timberwolf's advice-he has ton's of experience with the 359. I've put my saws on the tach and temp. and barometer have a significant impact on some saws.
FWIW: Buy a tach! You own enough saws to justify spending $100. Especially if your modifying them.

 

[Mr.]

The four-stroking they are talking about is a slight "blurbling" in the sound at Wide-Open-Throttle.

When you turn the high speed screw to where the saw reaches peak RPM, then you richen it up until you hear the slight "blurbling" sound.

It really is a misnomer, but don't tell anyone.

Fred

 

[timberwolf]

I agree, it is a misnomer.

I read it explained like this:

As Fuel air mixture becomes richer, flame speed drops. so there is then a point where increasing fuel will slow the flame speed below the piston speed which peeks around 90 deg ATDC and about the time the exhaust port would be opening give or take 10 deg. So if the piston is moving away from the flame front faster than the flame front is moving there is no "POP" when the exhaust port opens and it sounds a bit like a miss.

Because there is no pop, scavenging is compromised and more exhaust is left in the cylinder, then with only a partial charge on the next stroke the mixture is a bit leaner and the engine slightly slows, so the engine again picks up to fully firing. If the saw is loaded in the cut it can go into two stroking from breaking between 2 and 4 stroking because the piston is then slowed and pushing back more on the charge due to the load on the engine and as a result it stops out-running the expanding gas arround the time of exhaust port opening.

This also helps to explain why as the exhaust port is raised in a modified saw the RPM where the saw break between four stroking and 2 stroking also increases. The sooner the exhaust port opens the higher the pressure inside the cylinder still is and the faster the flame front is moving, so the piston is unable to out run the flame until higher RPM is reached. Besides RPM and stroke length, rod geometry also plays a significant factor in piston speed and where it peeks and at what rpm 2/4 stroking will occure.