Rough running at high RPM

Are you using avgas or 91 octane car gas? If car gas, your carbs are likely slightly fouled with dried gas resin. 

Carburettors appear clean, but we will have a good look at the jets as I do have to use car gasoline.

Problem seems in main jet.

You can clean it or change it.

The main jet affects fuel flow to the engine the most at full throttle but has some affect at part throttle. The closer the throttle is to full, the more influence the main jet has on fuel flow. A larger size increases flow. A smaller size decreases flow.

Check the correct size .

The higher the rpm during a mag drop check will result in higher numbers for the drop.This might be electrical, but should be ruled in or out. What was the mag drop at 4K rpm?  Depending on your answer it may be a plug cap. You can unscrew each plug cap, trim back the plug wire about 3/8", place a small amount of dielectric grease on the tip of the wire and screw it ALL the way back on.

If it isn't electrical then;

Since you're using auto fuel and it sat for a long time I would pull both carbs and clean them well.  When you remove the carbs dissasemble them and spray out ALL openings with some carb cleaner like Brake Kleen carb cleaner in the spary can. Use the little red tube extension as it will help get into the tight areas and into the small openings. Then after flushing all orifices use some high pressure air to flush all the same orifices. Remove the mixture screw main jet and idle jet before the cleaning. The mixture screw is turned out 1.5 turns when you replace it. Take a piece of 16 gauge wire and strip the insulation back about 4". Then untwist 1 strand. Then use that one strand to clean out the idle jet center hole. Using this piece of strand wire will not damage the idle jet. Put the wire through the center and slide the jet back and forth while rotating it around. This will clean out any dried fuel residue. When you hold the jet up to light there should be a very mall, but clean hole down through the center of the idle jet. Then flush the idle jet out with the carb spray and follow it with air. When reassmbling the jets they have "O" rings and since they were rebuilt 2 years ago I doubt they "O" rings need replacement, but some people will always replace them. I would however apply a thin film of white lithium grease to these "O" rings so when you screw them back in place you don't scarpe or damage them. The grease will help them slide into place easier.  When the carbs are back on do a carb sync. I think this will solve your problem. 

p.s.

Make sure the carb vent tubes are connected correctly and didn't come off the carb.

Thank you xing and Roger.

I trimmed the leads when I fitted the new caps which are a NGK 5K ohm type, but made of hard phenolic resin rather than the soft rubber ones. I did think that I had found the problem as one cap had no continuity at all and two others had corrosion around the screw, but the problem remained. I haven't used dielectric grease but I shall apply some.

The longest that it went without flying was just over two months , but the modern car fuel is so different that the blocked jet sounds hopeful.

Unwisely , about a month ago I did actually fly it for half an hour with this problem . After extensive ground runs I did a high power climb and cruised about the local area without any problems. Rate of climb was as before and all the temps and pressures normal. It was only when I throttled back to land that a little "thump" of a missfire every few seconds could be felt. After landing I ran the engine up and the high "mag drop" of 800 to 1000 rpm was still present. I did have a 10,000 ft runway of a former NATO base in front of me, but still silly.

Thanks again

Carb vent pipes in the correct place, floats look good and the bowls had clean fuel in them.