Pat McPherson

You did a compression test and it looks good.
Doing a basic tune up and checking the timing would be the next thing I'd do.

ENFORCER24

No way , i have a 23p mirage on my 24 foot , with less horsepower , and i see 60-63 all the time
i would have to guess that the timing is off , or an injector or 2is not firing , or maybe a week fuel pump not supplying e-nuff go juice
just my .02

dcg9381

I think if my fuel pump was lagging or if I had a fuel problem, I'd hear the motor lean out and misfire. I can't hear a hint of misfiring on this motor. I've tuned a few stand alone EFI cars - nothing with this many cylinders, but when the mixture gets off, usually the engines aren't happy and let you know.
I might be losing a cylider and not know it... My ear isn't V8 specific enough.

Timing - maybe... I think I configure that jumper, then set base timing. I'll check the manual to see what the base timing for this motor should be.

the duke

Thats why I said "could be" the wrong prop. There are a ton of things involved in the prop, and unless we are talking the exact same boat/engine, its just a guess as I said.

dcg9381

Well, I replaced me cap/rotor/plugs/wires.
Cap was pretty crapped up, wires looked OK - one rub spot.
Plugs OK.
No change, other than I needed to do the cap.
I had one plug that was a little darker than the others.
I set the tach to 8 cylinder.. so that's working.

I suspect I've got a weak/dead cylinder - not sure.
I didn't check timing tonight, ran out of time.
More later.. as I chase this one..

ENFORCER24

i know you did a compression test with great results , and lean on the top end does give a tell tale sound , probably the same as 4 cyl honda's do , so timing is the last weak link.
i'm curious to see what finally fixes this problem , keep up the input

dcg9381

I took her into a local Marine shop, I asked if they had some EFI experience as well as the EFI diagnostic tool - they said they did. The mechanic I talked to was a bit stumped also, but we'll see if they pull any DTC codes or if they can otherwise work something out.

I did see one plug that was a little wetter than the other 7, unfortunately I didn't mark the cylinder.. It wasn't so bad that I could peg that cylinder as a cause.

I also felt the manifold runners after running the boat.. Old motorcycle trick.. Thing is that the manifold has water in it and the more central cylinders seem to stay colder... More to come (if it gets resolved).

Pat McPherson

My bet is timing.
Either initial is off or it's going into knock mode.

dcg9381

OK guys, quit diverting on torque/hp.. Maybe a new thread with some dyno postings would be great?


Timing: Fair guess. So the timing was off slightly (I didn't ask how much) - probably from me replacing the cap and rotor, but not retiming it afterwards.

The deal was that she's got a bad injector. I asked how they diagnosed it and they said that their diagnostic tool allows them to fire each injector individually and they watch for a fuel pressure drop. Apparently the latest Mercruiser ECMs will self-diagnose this, dropping one cylinder at a time and record the RPM drop. No RPM drop = bad injector.
My MPI motor doesn't have the self diagnostic.

I could have taken a mildly educated guess at that from the one plug that didn't look the same as the others, but it would have been a long shot for me.

Dang Mercruiser injector is $175 (for one). I asked for an auto equavalent - they said there isn't one, which is probably not true - but they don't know the specs on the injector.

We'll see how she runs with a new injector.

Pat McPherson

I've got 2 sets of the stock injectors and 1 set of 30#.
I'd sell them for cheep, (less than you paid for 1) they are just setting in my basement.

It's great you found the problem...