vac advance is hooked up to the ported source on the carb but there should be no vacuum advance at WOT. I'll disconnect it and see if that changes anything.
The linkage nut is not the problem. It was so loose it backed itself completely off. I found it before it fell off, the only thing keeping it from falling off was that U-shaped rod from the stock linkage but the carb was still sticking. Only happens when it's good and hot. Once it cools off a little the problem goes away.
79coyotefrg - I see you're leaner than I am right now. Interesting. I'll try changing the fuel jets to match yours and see what happens.
If the exhaust has a lot of restriction that should show up when I hook up the pressure gauge. It does have 2-1/4" exhaust with a magnaflow muffler. It also has a high flow cat but the old engine was burning a quart of oil every 500 miles or less so maybe it's plugged. Again, the gauge will show if I have backpressure problems.
What do you think of the Thorley Tri-Y header?
As for gas, I don't know if ethanol free is available around here anywhere. If it is, it would only be at a very few stations which isn't really practical.
