88 454 Merc Momentary Loss of RPM *paging Charlie*

Does it have a moving Advance Plate in the Distributor? Probably does not, but in automotive I've seen where the wire to the points worries into a break after years of advance/retard action. Again, that usually goes with a Vacuum Advance, and boats generally don't have one.
 
I think I'd check the inside contacts dist. cap 1st including the center contact to the rotor.

A quick momentary hesitation like that is usually electrical , so check all ign switch, coil, dist, plug cable and harness plug connections. How old are the plugs?

If the dist cap is still in excellent shape , try the Wiggle Test of all the above-mentioned to see if you can get the hesitation to manifest.

As j-d suggests , besides a loose or corroded connection there could be a chafed wire grounding out.

Is everything original? I.e., same '88 distributor and mechanical fuel pump or have they been upgraded?
 
plugs are replaced every two years(second season now) Cap is 5 years old. Good thought on electrical, as i think of it in each instance i was adjusting the Trim tabs and they share the battery with that engine. Maybe shorting out.....
 
When was the last time you change fuel filters?
How much fuel is in your fuel tank? Sounds to me it is a fuel problem on top of your fuel tank you have a fitting see if you can disconnect fuel line and clean fitting out.Also if you can switch switch fuel tank and let both engines run off the port tank.
 
Interrupting fuel flow at the tank will still have fuel in the supply line as well as the filter. The fuel pump will happily pull fuel for more than a few seconds without interrupting the engine.

There is a vague possibility of fuel flow being interrupted for 2 seconds in the carb but I kinda fu(king doubt it.

Most likely it's an electrical problem. Sounds like a short or bad connection/corrosion. May be anywhere in the primary ignition system...coil/distributor and supply wires. The only component I've run across that can do this on a somewhat regular basis as it deteriorates to non functioning. That's the capacitor/condensor.
 
Rochester quadrajet? If so, it could be occurring when the secondaries are opening. There's a damper you can adjust to combat the fluttering of the secondary air flaps. There are some good vids on YouTube "un-bog your quadrajet" by username "whosaidtyler".

Swapping carbs is an easy task and a good way to confirm or rule out carb issues.
 
Doesn't sound like your problem,but...

If the secondaries are adjusted too loose, There'll be a loud sucking sound from the secondaries and it can bog and start to backfire out the carb. It'll run fine if too tight.

To test, while running with someone at the helm, slowly close the secondary 'trap doors' with your finger. If it's running too lean from being too loose, closing the trap doors will bring the fuel/air ratio back to where it should be. Also, if the cause is too loose secondary trap doors, closing them with your finger will suddenly make the boat surge ahead. Hold onto something when you close the trap doors.

To adjust, there's a tiny spring wound around the secondary shaft on the starboard side. Loosen the little screw and turn to tighten the tension.
 
Just asking the simple stuff, have you actually confirmed it is loosing rpm? Or is this just what the tach says? Can you actually hear the engine slow down? I have seen tachs do this before is why I am asking. Check the simple first.........D..........
 
Check, and clean all your grounds, & electrical connections, and lube them up. Because you mentioned when using trim you had a momentary hiccup, this is usually the classic case of a momentary loss of electrical contact.
 
remove the ignition trigger in the distributor and visually inspect. Could be a failing capacitor. You can see the damage if so.
 
Our later model GM engine has a Delco Distributor. On two occasions now the spring behind the center contact in the cap has rusted, leading to a miss. The miss was violent, though, not just an RPM loss. Detailed cap and rotor inspection doesn't cost anything and is easy to do.
 
Finally got down to the boat last night, Replaced the fuel filter(fuel was clean),dist. cap, rotor, plugs and wires. Wires were 5 years old and pretty gummy. Cap looked ok but the arm on the Rotor was quite worn. The odd thing is this only happens when the engine is Hot. The last time it happened I drove about 45 min at 3300 rpm to my fishing spot no issues. 3 hours later start up and head home as soon as I crossed 3K the RPMs started to drop return drop return. I will get out this weekend and try it out. I do not think it is the head gasket as temps are fine and antifreeze oil remain at level. Thanks for all the feedback. Will update.
 
Did you watch for the drop when trimming again? The tabs can be powered off the same battery as the ignition but should be on a completely different independent circuit.

Did you try the thorough "Wiggle Test" of engine harnesses and dash ignition and gauge wiring ( & trim)?
Check connections at the coil?

If you have an electric fuel pump it may still be fuel-pressure-drop related, but it sure still sounds like an electrical issue (momentary short or open.) You could rule out fuel pressure with a fuel pressure tester gauge, but if it is a carbed engine you'll need adapters for the pressure side of the pump. Definitely "Wiggle " before doing that.

When it happens, does the voltmeter flicker?
 
Thunderbolt ignition. Quadrojet Carb. i was able to rule out Tabs as the last time it happened I was not using them. The Voltmeter does not flicker. This engine is on the house battery bank.
 
Thunderbolt ignition runs straight off the battery. The coil requires a full 12 volts. Make sure you don't have a resistor coil.

Try running without anything else running and draining the circuit.
 
I have twin 454 1988 engines and have experienced similar condition on 2 or 3 occasions. Have not been able to find the cause. Sorry I can't be more help.
 
Check the bottom of your fuel tank for water and nasties. Your pickup is an inch off the bottom of the tank and if the crap is high enough it can momentarialy block the intake.
 
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