twin application

port engine feels like it is surging.

Originally on did this in the upper rpm range.
Now does it at 3k and above.

New distributer, rotor, plugs. Used existing ing sensor, coil, and wires. Both sets of plugs looked identical.

while crusing or wot engine builds power then looses it, like its a fuel issue or ignition issue. Tach shows blips in RPM, mph goes up and down as engine surges. Fuel filters new 10 hrs ago. This issue is getting worse, used to be only at high rpm now is above 3k or more.

When I am at wot the tach will bounce up to 5500 down to 4900.

My question-what to troulbe shoot next.

What does the ignition sensor do, just count RPM or what?
Could this be the knock sensor retarding timing?
Could this be the ignition module?

I can advance throttle on one engine and prove that it is the port. The starbord winds up and makes power.

Common fuel tank, not a fuel quality issue.

Before boat would hit 68 to 70 mph

Now 62 light load.....

Before starting engine fuel pump runs and stops. How can I check under load?

No audible alarms.

Thanks,

Ok,

In my opinion if the RPMs are dropping that much and "bouncing" its not fuel. You are loosing ignition and / or power supply. This will not through an alarm usually, are you hearing the beep that you hear when you turn it on? if so that is you engine going on and off.

There was another thread a month ago, someone said that they have seen a wire on the solenoid come loose. I would with the engine running either in the water or on a hose push/ pull every wire and see if the engine dies while doing this. Is this the same engine that you changed the IAC on?
Most of the sensors IAC MAP crank trigger with sound an alarm over 4gs if bad. You are not getting that as I understand



I just reread your post the tach is jumping up to 5500? what is your wide open rpm 5100 5200?

Also is this the same motor that was hard starting a few months ago?

Nope. IAC was on the starboard. Rock solid after that.

WOT is 5100 to 5200. before this issue. this weak motor is limiting the good motor to 4800 to 4900.

I am thinking ignition sensor...but who am I to guess.

LC

ps no beep like when I turn the key to the on position.

why new distributor. could try switching ign modules to see if trouble moves. is there thermal transfer paste behind ign modules.

Is the "surge" gradual or like you shut it off with the key?

If its gradual lets get a fuel pressure reading while running,, you will need a long hose so you can close the hatch and a buddy to watch it. Its just a standard GM schrader valve adapter. I doubt it but lets rule it out. You might want to take it to someone who can see exactly what it is doing on a laptop with Mercury sofware running. I doubt its the distributor, but check the number 7 injector plug (wiggle) while running.

Did not replace whole distributer just cap as pm measure.

Not a gradual decline. Fairly quick and more muffled/ bogged down sounding.

I am going to replace the ing sensor inside the distibutor, mine looks a lot like the ones merc had a service bulletin on and replace the coil. If this doesn't help I am disconnecting the new gaff rig tach, if still no help I guess I will switch ecms and or other various parts and try and get the problem isolated this way.

No, you wouldn't notice it that drastically.