Fishy - have you ever tried to jet up when it's doing this? The fuel mix could be going lean and causing detonation - which loses combustion control and releases oxygen in the exhaust and shows it as lean. Just curious...
I haven't tried jetting it up when it acts up because on cool days it never lays down. Have held it to 120 and it was still pulling hard. When I put the carb on I started out with 75's and 82's and it was real rich. Down in the high 10's if I remember right. That's why I have it jetted way down where it's at now. I've heard it spark knock when I had inferior gas in it but I never hear it ping even when it lays down unless it's a really hot day. Although at the speed I'm moving when it lays down things are pretty loud so it's possible I'm just not hearing it. That's an interesting thought you have though. Never considered that possibility. Right now I'm in the middle of installing the new Holley HP150 electric pump so when I have it back on the road (was hoping to have it done today but it didn't work out) hopefully it'll be solved. If not I'll try going back up with the jets and see what happens. Thanks
