Have a pair of 425 Mercruisers in a Cafe Racer in the UK.
Having a problem with one engine, it will not run without having to keep pumping the throttle lever and will only reach about 3000rpm under load. Have changed the filter and fitted new lift pumps last season, pressure guage runs at 6psi. Feels like fuel starvation and "pops back" through air filter.
Any thing I should check in the carb??. Ran fine last year.

Check the timing...you may be surprised. Second...look for ignition problems (cap, rotor, wires...the obvious stuff). What carbs are we talking about ?

carbs should be 800 DP HOLLEYS if its 454420 set up if 502s weber 750. carbs I would think happened to a friend.

Holley, I think 4150. I have not checked the filters on the feed into the carb itself - do you think this could be a possible ?

carbs or spark plugs?

Sure...could be the cause. ALWAYS check the simple, cheapest stuff first sir.

Had a similar problem a couple weeks ago. I launched at the ramp and idled out a bit and starboard side died, pumped it and it would not fire so pumped it some more, then after a lot of pumping I was sure it to be flooded. pulled the air cleaner and opened the throttle and noticed some beaded drops dancing inside the intake. Long story is I got it to idle and it took a long time to work it out. It would rev up fine in neutral but would die as power was applied.
The cure was to slowly bring the motor up to the good running one, raise the rpm and then give it a bit more throttle, and after a half hour + It snapped out of it..WATER,, I also cleared the filter several times.

pop the bowl off it its a holley and clean the main jets. check the accelerator pump for squirt. check anti-siphon ball in the squirt tower. rest float level if adjustable.

Nigel - sorry to hear you are having troubles. Is this the first run of the season? Sounds like Carb problems. If it is the first run of the season did you fog it before you stored it?

Eric

P.S. - Going to be in the UK in mid-September. Theale. If things aren't too busy and you are close maybe we'll meet for a drink and some boat talk!!!!