31' scarab twin 502 Carb motors, motors have low hours, ran good previously with no changes to setup, one motor now won't turn over 3500 rpm under load but runs fine otherwise, will rev higher out of gear, seems almost like Carb secondaries aren't opening but they are, hard to tell what's happening because it's loud (dry exhaust) and other motor kind of compensates and it only does it on plane around 50 mph, has msd distributors with no boxes and locked timing, any ideas on where to start?

swap carb's

Check fuel pressure and than compression and leak down test.

Thanks for replies, it runs fine getting up on plane where it's under load but rpms are lower which makes me think no internal damage to motor

Ran good previously. How long ago was previously and how was it stored?

Previously was earlier this summer

I wouldn't rule out an ignition issue.

ck timing make sure it has not slipped

i did verify distributor was not loose but I will check with light

Sock on the bottom of pickup tube in the tank.