All along I thought opening the muffler tended to lean out the saw. Have I had it backwards this whole time?
Cs-590/620 muffler mod
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Johnwell
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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.
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.
I can confirm that the 86-578-1 part number is the correct replacement for the factory part in the CS-590. Received one today, swapped it in, and the saw started and ran with the L screw in the same place I had it before and the H screw two turns out. I ended up having to richen up both screws slightly from there to get within specs, but the saw responds to tuning now as it should. It would not lean out enough after the muffler mod with the factory nozzle/check valve in place.
Is the T27 the correct size for the muffler screws?
Yes sir, that's correct, but you'll need something long and thin to get back in to the two that actually bolt the muffler to the cylinder. A normal T27 socket doesn't have enough reach down into the deep hole before it gets too big at the socket. I ended up buying something like this:
Some of the Stihl screnches have T27 ends too, I believe, if you happen to have one of those. But if you have one you probably already know it's a T27.
No offense intended.What is the cross pattern on the top of the solid jet for?
I don’t know. My new solid jet was completely flat and smooth on top.
On my saw, the factory jet with the hole bored through the top also had a slot in the top as well as the hole, sort of like an offset screwdriver slot. I assume that’s an indicator for lining up the crossbore hole in the jet with the hole in the carburetor bore wall so the two holes align to pass fuel. The new solid jet has an annular ring all the way around it mid-body, so the clocking or indexing of the new jet doesn’t matter. Fuel can be introduced from any point around the jet and it will still get into the nozzle.
On my saw, the factory jet with the hole bored through the top also had a slot in the top as well as the hole, sort of like an offset screwdriver slot. I assume that’s an indicator for lining up the crossbore hole in the jet with the hole in the carburetor bore wall so the two holes align to pass fuel. The new solid jet has an annular ring all the way around it mid-body, so the clocking or indexing of the new jet doesn’t matter. Fuel can be introduced from any point around the jet and it will still get into the nozzle.
Lost the front bolt plate bolt immediately. I’m sure it will turn up after replacement has been gotten. Previous owner started the muffler mod with the two large holes; I added two more and did the jet swap last night. Hopefully I can get the idle and low jets tuned in tonight. Going to start the high jet at 1 turn out and see where it is.
