Hi all!

how do I go about troubleshooting my starboard engine? 5xx hours factory sealed Merc 454 Mag Efi bone stock. The port engine runs like a top!

It starts up rough and I need to throttle up the idle when it’s cold. Once it’s warm it idles fine but if I rev the engine in neutral quickly it will stumble and almost sound like it wants to backfire.

I assume I don’t notice it in gear because I would never punch the throttle that quickly so I don’t blow out the drives.

Here’s the work I’ve done so far:

new spark plugs gapped to .035” (old plugs looked fine just really old and slightly oily)
new engine oil and filter
new fuel filter
new riser gaskets (as preventative maintenance)
Compression test (cold) results shown below

I should have done a leak down test but I was so fed up with squeezing between the two engines,I called it done after the compression test.


All the plugs looked like this on both engines.


Cylinder 1 150

3 150

5 140

7 120

2 130

4 120

6 100

8 145
I’m hoping to at least get a season or two out of the boat before I have to refresh an engine.

I would start with a distributor cap, rotor and plug wires. Next would be pull the injectors and have them checked.

I would start with injectors. In my 30 years with my existing Merc any running issues have always been fuel related and not plug/wire/rotor etc.

Thank you for the suggestions.

its interesting that if I ease into the throttle the engine will rev up just fine. Its only an issue when you want to snap the throttle in neutral.

cap and rotor is cheap enough for try out.

I’ll have to do some research about the injectors! Gotta find a place that can clean them near me (so cal)

Where in So Cal....LA area or San Diego? RC Injection in Torrance does injector cleaning and flow testing. There are also places in San Diego

Those old plugs look horrible.
At least remove and inspect the dist caps.

I’m closer to LA!

Torrence is close enough. Thanks!

Could also be fuel pump...

Was thinking fuel issue myself. The reason is that you can ease it up and it won't stumble is the fuel demand is less than quick pull up to RPM

Pull the IAC and power cycle the key to the run position and see if its moving, Id hook a scanner to it and see whats going on