I need the experts on this one

Motors are twin 300hp carbed 350's model year 2006. Started with difficult ignition problem and that problem is now resolved. We have a new problem after 30 hours of pro mechanic work we haven't fixed it. The symptoms are as follows...
Start the boat leave the dock, get on plane, everything works great for as long as you want to run. Both engines tach up fine, all is well with the world. Once you come off of plane, and travel at 800 rpms in gear for 10 minutes, the starboard engine will not tach up past 2500, coughs occassionally out of the carb. If you keep running it, it will eventually tach up very slowly with some surging. After 10 minute run on plane it will get past 3400 eventually, but will not keep up with the port motor. Floscans increase in gph with more throttle, but no rpm increase. If you come off of plane and back to idle speed, it will eventually stall out, but start back up easily.
Things that have been done so far... New plugs twice, swapped carbs, new fuel pump, new fuel filter, compression check good, new plug wires, new coil, new module in mallory distributor, new ignition key switch. We have not yet put psi check on fuel delivery or bypassed tach- that is next. Also - new fuel, both engines routed to same fresh tank of gas to eliminate bad gas. Any suggestions?

You're going the right way. Fuel pressure is a good indicator and eliminating the tach is also a good idea. I would go one step further, if possible, swap the engine harnesses in the bilge. You may have a bad wire in your harness that could be heating up. If you swap the harnesses and the problem moves to the other motor, you have at least isolated the problem. Keep us posted.

Concur with Rick.

Engines are fairly new and so is the fuel....how old is the boat (tank)?

I asked the smartest guy I know what his opinion was:

only 2 reasons to cough thur carb that I know of are intake valve leaking or a lean condition

both will put flames into intake and out of carb

taking it from there, tight guide, bad hydraulic lifter etc for first one

second one, vapor lock, valve sticking in fuel pump. etc

installing fuel pres gage at carb will indicate low psi if it's the second one

carbs need 2 psi min under full throttle

look for hot fuel lines and insulate them



Jeff

update for the experts, it appears that it was the tach wire. When we disconnected it from the coil, the problem went away. I guess it was grounding out somewhere and grounding out the coil.

Now reconect it at the engine and disconnect it at the tach. That will tell you if the problem is in the wiring or the gauge.