Low RPM at WOT

Did you try running on one engine only and see what RPM it could achieve and then do the other engine and compare.

A bad prop or engine can make the other engine work harder and have a lower WOT.

I presume you already did a complete tune-up. Rotor and cap can build up a carbon like deposit and lower perfomance.

How about a vacuum check. Find someone that knows how to read a vacuum gauge. Look for a valve problem. OR do a mechanical check of the valves...pop the valve cover and put you finger on the end of the valve as it's idling. You can feel a loose valve, It will like a sharp bang or knock.

Did your mechanic read your plugs? To see if you're getting the correct fuel/air mixture. Also feel coil, too hot can mean it's breaking down.
 
Update: went to change the spark plug wires today and lo and behold the wire to the number 7 cylinder was not attached. My current thinking is that I had two problems - one was the paint chips in the fuel cooler/regulator and the other was the wire not fully seated. Earlier in the season I was getting higher rpms, and then it declined to about 3200-3400. Not entirely sure about timing, but I think that was after the tune-up. After the new fuel cooler and injectors I was up 4000 rpm, but still missing about 600-700 rpm. The unplugged wire obviously might explain why I went from 3400 to 4000 rpm with new fuel cooler and injectors, but still was missing the last 600-700 rpm. Unfortunately the boat is on land so I will not test it out until spring.

One thing I am curious about, the mechanic had it hooked up to a computer. Would that have shown that cylinder 7 was not getting any spark? Or, since the spark was getting at least to the wire, would it not be readily apparent?
 
That's great you finally discovered a definite culprit.
My guess would be if it could attain 3400 on 7 cyls., the fully-active 8 cylinders would bump that up to roughly 4600 +/- RPM. (Since V-8: 1/8th of your estimated 4700 is 587.5)

The mechanics scan device may only have clearly indicated #7 was not combusting properly if he had performed a power- balance test . The scan device /computer just electronically shuts down either spark or injection to each successive cylinder while the tech watches the RPM to see if any cylinder being shut down does NOT cause an rpm drop. I would think with your problems that would have been performed as a basic, quick, simple test, and certainly would have saved you time, angst & $.

But ...if... the, say, 600 rpm drop occurred right after the tune-up that should have been a pretty good clue as to what things to check.
 
A vacuum test would have picked up a dead cylinder.

A 'dead' cylinder would not reduce power by 1/8th. It would be a little more because cylinders run in their firing order and a cylinder taken out of the sequence reduces it's potential.

An example would be a 7 cylinder engine of comparable displacement would reach it's designated WOT.

another example more easily seen mathematically is 1/8 of 4400 RPM is 550 RPM, not the 1200 RPM reduction.
 
Had a wire off ours a few years ago. The RPM loss was less than I would have expected. I think on our 5.7/350 it happened to be #5. Single engine 22-ft boat. It'd get up on plane and I think the max RPM was around 4000. That engine only topped around 4600 running its best.

That SBC firing order seems to minimize the roughness when 5 or 7 isn't firing. Right next to each other and right together in that firing order 1-8-4-3-6-5-7-2. I call it the Chevrolet Prayer, to be uttered before turning the Key...

I'm glad you found it, and that it wasn't major!
 
Update to close this out:
Launched this weekend and did a test run. It ran great. Long story short, I had a paint chip issue with my Mercruiser fuel cooler but the first mechanic did a tune up before diagnosing it. When they did the tune-up they did not get one of the wires all the way on. Next, they diagnosed the paint chip issue and fixed it (new fuel cooler and injectors)but the wire was still off, so even thought the boat ran better it was missing 600-700 rpms. Thankfully I decided to change the spark plugs wires last fall, just to see what would happen and discovered the disconnected wire. Pretty much lost my summer due to this. Need a better mechanic.
 
Glad you found it.

My rule when looking for problems has always been to first reinspect all the last work done.
 
Yes you need a better mechanic. Spark plug wires is a basic starting point with gas motors. He should have looked at that at the start.
 
quote:
Originally posted by Robyns Nest

Yes you need a better mechanic. Spark plug wires is a basic starting point with gas motors. He should have looked at that at the start.

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The problem wasn't so much that he didn't look at it, as that he caused it! When he did the tune up he didn't get it all the way back on, so instead of one problem I now had two.
 
Aggravating as it was, glad to hear it's resolved. SHAME on that Mechanic! Always go back and check!

I think it's good that you didn't run the boat through the season dragging one cylinder. The injectors would still have fueled the cylinder, washing the lubrication off the bore walls. Even two-stroke outboards, with oil in their gas or injected into the fuel stream, can damage cylinders that aren't firing, because the gas that doesn't get burned washes the oil away.
 
another rule I always applied on the boat was to remove all old parts and tools from the work area then retighten everything I had touched.
 
quote:
Originally posted by j-d

I think it's good that you didn't run the boat through the season dragging one cylinder. The injectors would still have fueled the cylinder, washing the lubrication off the bore walls.

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Yeah, I am glad too! Kept it at the dock other than to do test runs.
 
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