Originally Posted by Jeff in AL
I'm not sure why the double on exhaust, I saw it on Youtube...
Actually, the real reason is that there was an issue with excessive carbon build up on the exhaust valve years ago. Then they changed the exhaust valve to a "bi-metal" valve. This helped with the excessive carbon build up for the most part. Even with the changes, there is still a unit from time to time with carbon deposit issues. However, today it is mostly due to their fuel or running with a broken tube and not getting the RPM's/engine temp up to burn off the deposits.
The thought process or reason to double up on the exhaust valve on the BR units is to allow a longer seat time to help keep the valve cooler. Even though it is only nano-seconds that it is closed longer, it has made a big difference, combined with other changes to help minimize carbon deposits forming on the valve face and seat. There has been no issue with the trade off of doing so which will cause a slightly lower lift height to the valve when open!
Good to know on the valve adj!
Yeah, it's been awhile since I worked for a Stihl dealer, so have not heard much in the way of tech info. The last 4-mix info I got concerned the valve change,( Sept. 09 or so) but no mention of the clearance increase. Thanks again for sharin'.