2006 90hp TLR runs ruff above 3000 rpm

[CW]

I have twin 90's that both started to running strange at the same time. They run great below 3000 rpm, when I try to go faster they both will not maintain rpm but will oscillate rpm from 2500 to 5000. I changed all the plugs and drained all the carps and the fuel was clean. I have and electric automotive fuel pump that feeds both engines from a single large tank. The hand pump bulbs say hard at all times. If I shut off one engine the other one still runs the same. I only use alcohol free fuel. I always stabilize and ring free every drop of fuel that goes in the engines. They are both low time engines that I have owned since new.

[99yam40]

Why are using the electric pump?
the motors have mechanical pumps to pump the fuel.
If you develop a fuel leak with that pressurized system , you will have a very bad situation.

Being the one tank feeds both motors I would think that is the only common thing to both motors, and it does sound fuel related.
I would monitor the fuel pressures and inspect the mechanical fuel pumps for bad diaphragms to see if they have torn due to the pressure from electric pump, or maybe the electric pump is not keeping up (pick up screen plugging)or sucking air.

[bumperfood]

You may be able to isolate the problem by a process of elimination. First by disconnecting the fuel line at the engines, and running the engine(s) off an auxiliary (portable) tank instead of the main tank. At this point make sure that there is no power to your electric fuel pump, by removing the fuse. Use a different primer bulb then the one you are now using in order to eliminate the old one from the equation. Clean or replace any engine mounted fuel filters under the cowlings. If the engines run fine now, the problem would be from your fuel connections at the engines back to the fuel tank. This could include the fuel line and its connections, the primer bulb, any inline fuel filters, your electric fuel pump and your fuel tank. You could now connect the auxiliary tank to your fuel line on the engine side of your electric pump, should they continue to run fine, you have eliminated the fuel lines, primer bulb and any inline fuel filters. Now bypass the electric fuel pump (make sure you have no power to the fuel pump when running the engines or you will burn the pump out), and run off your main tank. If it continues to perform well, then you have eliminated the fuel tank as the problem, but not the electric fuel pump. Of course if at any time during these procedures should the engine(s) run rough, you would know where the problem lies. I know that this will take some time and effort, but don't do it in a different order, or try a short cut. On a note of safety, remember you are working on a fuel system with a very explosive potential. Any fuel spilled must be wiped up, and fumes eliminated. Have another person stand by with an adequate fire extinguisher whenever you are working on the fuel system.