[dwaite72lnghrn]

If you are cruising/vacuum high you will be running the leanest the carb can run. I am not sure on the thunderer how to adjust this but on the 1406/1411 you get a tuning kit that has the meter rods and jets. You start by looking in the tunning manual and pick the next step of richness and add the meter rod in the list. This normally is 4% which would probably fix it. The springs are for how fast you go into rich mode which is not a problem at cruise only at lower vacuum. With the new Distrib you might be running more effeciently and burning everything which could cause a lean condition. I think the tunning kit for the 1411 was about $45. The Thunderer I think adjust the same way. you could also get just the rod you need seperately.
By stepping on it or dropping down a gear and the problem goes away I would say it isn't advance You would be reving higher and more advance the problem would get worse. By stepping on the gas and the problem goes away I would say its lean and getting more gas as the pedal gets pressed down.