A few weeks back there was a discussion about the high speed jet, called the fixed jet in a Stihl IPL, and its role in tuning the carburetor.
A little research shows that the US bound MS660s are equipped with a Walbro WJ-69 carb. The IPLs that I have show a .64 jet as standard, with a .62 jet optional for high altitude operation. After porting my 660 I found that the H needle needed to be 1.25 turns out to get the saw to four-stroke. I figured a new jet was needed. I ordered the 1122 121 5607 .74 fixed jet from the dealer. This jet is found in the WJ-76 carbs found on saws not headed for North America.
When I disassemled the carb I found a somewhat unpleasant surprise. The saw had been shipped with a .62 jet. EPA strikes again.
I thought I would share a quick how-to for replacing this jet.
1. Remove the carb and orient the top toward you.
2. Remove the top cover. This will expose the fixed jet. It is the slotted brass part below the lever.
3. Remove the fixed jet. Be sure to use the correct size screwdriver as brass is soft and unforgiving.
4. This is the new jet to install. The jet number is stamped on top.
5. The new jet in place. Reassemble the saw then tune as usual.
After the .74 jet was installed my 660 now four-strokes with the H needle 7/8 of a turn out. That is how it should be.
Thanks for looking!