This problem is getting more and more vexing!!! I think "As I see it" may have a point ... but I really am at a loss to know for sure.
Here's what I (and my group members) have found, and done, but to no avail.
Sticking valve? .... had one before and the symptoms don't match.
Symptom. Runs OK to full power on static, but breaks down as T/O speed is approached. Leaning the mixture by the cockpit control helps, but leaning the mix via the mixture screw does not.
Induction tubes leaking? ... Nope, all OK.
Timing? ... OK
Mag problems? ... what both, together??? don't think so?
A fuel line was badly perished and actually leaking fuel! .... problem solved! ....... errr no
So, off with the carb to check the float (seems ok, but hard to tell for certain I guess, without replacing) Jets, airways ect all blown through and a new gasket .... but the problem is still there.
I/we have thought of all sorts? Weak valve spring and a bouncing valve? .... only ever had that in a two stroke? obstructed air feed? could that do it?
The Marvel carb has a venturi and bleed holes to create a vacuum and suck fuel. The mixture screw simply blocks one of these holes to a greater or lesser degree to alter the vacuum. Adjusting this makes no great difference. However, I think the cockpit control actually restricts the amount of fuel allowed and therefore leans in a different way. Whether this is significant to the problem or solving it, beats me?
Any more idea's? Please help .... I'll be bald soon at this rate
SS