Carb problem with a model 50

After rebuilding the radiator and installing new manifolds and muffler I've finally got my old model 50 back together again. No overheating problems but I've got a serious lack of power and generally poor running. I suspect I've done something to the carb that might cause this as I installed a rebuild kit while doing everything else.

I had not changed the timing. Got nice blue sparks on both cylinders. Both cylinders seem to be moving lots of air (have not tried a compression test yet), but it acts like its running on one cylinder once revs are increased. I can get it to idle okay, responds to both idle mixture screws but looses power when a load is applied. Acts like its not getting sufficient fuel to one cylinder. No black smoke or "rich" smell in the exhaust. Pulling in choke has little or no effect. Needs choke to start though so I know the butterflys are working. Double checked gas flow. Plenty of gas in the carb bowl. As a matter of fact it continues to run for about 20 or 30 seconds once the gas is turned off so there must be plenty in the bowl. The load screw does not seem to be having any effect. Like I said idle is okay but as I add throttle I suddenly hear the exhaust note change to a puffy flat tone instead of the usual powerfull note. I'm guessing a clogged passage in the carb but it ran great before I rebuilt it. I suppose I could have loosened some crud during the rebuild that got stuck somewhere.

I double checked the main jet (all little openings are clear).

Question: What is the initial opening for the load screw?

Any ideas are as always appreciated.

John

Looking forward to your thoughts.

Thanks,
John