Cs-590/620 muffler mod

[Breckinridge Elkins]

Once I did the muffler mod, I wasn't able to get the saw to lean out of four-stroking on the high side; even with the screw lightly bottomed out the saw is running too rich with a dark arrester screen. I thought I had read somewhere that the saw would need to be richened up on the high side after the muffler mod, but I guess I remembered wrong?

All along I thought opening the muffler tended to lean out the saw. Have I had it backwards this whole time?

 

[Johnwell]

All along I thought opening the muffler tended to lean out the saw. Have I had it backwards this whole time?

I always thought it would lean out a saw, but when my cs-590 had a completely gutted muffler it would four stroke super low even with the H screw all the way in. With the muffler slightly restricted it tunes normal. I guess it depends.

 

[NSEric]

All along I thought opening the muffler tended to lean out the saw. Have I had it backwards this whole time?

You have it right.
It's the fixed/governed high jet messing things up. With the increased airflow from mods the governor/fixed jet has more vacuum pulling fuel through it and kicks in earlier.
I dont have the part number for the normal jet without the bypass hole. I used jb weld to plug the extra hole. The saw tunes just like any other saw other than being about 2 turns out on the screws once you plug the fixed main jet.

 

[Woodtroll]

Thanks for the explanation, @NSEric, the "rich after muffler work" situation had me confused, too.

As far as the part number, the 86-578-1 part number is shown at 5:17 in this video. I guess it's the right one?: