Tune Your Saw with Contrarianism

Do you have an Echo CS-590 chainsaw you modified? Did you advance the timing and open up the muffler? Do you have a carb from a more-powerful saw on it?

Are you totally unable to start and tune it now that you’ve worked on it?

Take heart, friend. I may have the answer.

I put a new key on the shaft of my saw’s flywheel a while back, advancing the timing, and it seemed to run considerably stronger. I stuck a new, more open deflector on the muffler, too. Everything was fine. But my motto is, “If it ain’t broke, fix it,” so I got back to work.

I removed my muffler’s deflector and drilled 6 3/16″ holes in the muffler, around the factory opening that shoots gas out of the saw. This reduced back pressure and should end up giving me considerably more power.

The carb I have on it comes from a CS-620P, which is a professional-grade saw. It’s more powerful than a stock CS-590, so the carb has a slightly higher capacity. Unfortunately, it comes with a main jet/check valve that has a little hole in it that always lets some fuel go through, even when you close the H and L screws.

This hole is there to save Echo aggravation. It’s just like the little plastic things my saw used to have, to prevent people from turning the screws too far. The purpose of both is to keep people from leaning their saws out until they blow up. Echo knows people will do this and then expect warranty repairs.

The main jet has a check valve to keep air from being sucked backward into the system.

If you modify your saw, you will need to readjust your carb, so the limiter caps on the screws have to go, along with the hole in the check valve. If you have a hole in your valve, you may not be able to lean your saw out enough to make it run well after modifications. A saw the factory wants to max out at 12,000 RPM may run well at 15,000 after modifications, requiring a different mix, so you don’t want a check valve ruining your day.

I installed a new check valve with no hole, and then I tried to tune the saw. It drove me crazy for several days.

It wouldn’t start. Then it would start, but it wouldn’t idle. Then it would idle, but it died when I goosed the throttle.

A guy who sells modification parts did a video, and he said that if you change your check valve, you should open up the H screw to richen the mixture. Other people on the web seemed to agree that saws with muffler mods needed more fuel. Believing this tripe, I tried starting my saw with the screws out a little past the OEM settings, and I tried the OEM settings. Finally, I tried starting the L screw at one turn out from zero, and the saw ran.

So if you have modified your CS-590, and you’re losing your mind trying to make it work, try leaning out the L feed.

I should add that sometimes tightening a screw will actually make the mixture richer at wide-open throttle, but let’s not go there. I don’t think it applies to the L screw. All I know is, I needed to tighten mine.

I got the saw to run smoothly last night, but I ran out of blood for the mosquitoes, so I didn’t finish the job.

Before fooling with the muffler and valve, the saw was doing something like 13,500, I think. Maybe it was 13,300. This is wide open, with no load. Last night, all I could get was 12,500, which is within wimpy factory specs. Disgraceful.

Today I set the idle at 3,000, pretty much in the middle of the range. I tuned the L screw by ear. Then I opened up the H screw, and BANG, I was at 13,300 with an occasional burp.

It was like, “WAAAAAAAAAAAAA bip WAAAAAAAAAAA bip WAAAAAAAAAAA bip.”

In case you want to know how to tune a saw’s H needle, I have found out, so I’ll tell you.

It’s the last thing you adjust.

You want it to “four-stroke.” This is a misnomer used by chainsaw dudes. A chainsaw has a 2-stroke motor, and it can’t do what a 4-stroke does. It can SOUND a little like a 4-stroke, however. It sounds that way because it’s missing.

You want to make the H feed so rich, the saw misses a little when wide open with no load. Just a little. This means it’s getting more fuel than it can burn. When you put it in the wood, that fuel will be burned to provide more power, and the saw will run smoothly.

My saw has a limited ignition coil. The limit is 13,500.

People say you can’t tune a saw with a limited coil, because when it hits the limit, if starts missing, and it sounds like the saw is tuned correctly. I don’t think I’m having that problem, because I’m not hitting 13,500 and the saw is missing, but I will keep testing it.

The big take-away here is this: if you have been modifying your CS-590, and you’re pulling your hair out because it won’t let you tune it, and you think you broke it, set the L screw at one turn out, or whatever is 1/4 turn in from the OEM setting for your carb. It may be the answer.

Now I have 4 pretty decent gas saws for wood clearing. I have a homeowner-grade 40cc rebadged Husqvarna 435, a modified CS-590 which is maybe 90% professional-grade, an Echo CS-510P, which is a 50-cc pro saw, and a Husqvarna 562XP, which is a 4.9-horsepower 60-cc saw with a 24″ bar.

I only need one big saw and one small saw to work, so with two in each size, it’s pretty likely I’ll always have a set of two gas saws that function. And I have a cordless Makita that will save me if both of my small saws die on me.

I have no idea how much power the CS-590 makes now, but it should be significantly more than the 4 horsepower it was born with.

I’m keeping my chains sharp, so that also helps. Sharpening your chain is like adding one or more horsepower. I also use grown-up chains, not the safety chains lawyers put on saws places like Home Depot sells. Those safety chains are amazing. You buy a 4-horsepower saw, to pick a number, and the chain, sharpened to its peak, makes it cut like 3 horsepower.