Here's a stumper: 1998 (by serial number) Mercruiser 5.0 EFI that
runs like the proverbial swiss watch. About 150 hours total. Starts
easily, runs smoothly and strongly. Propped well. 4600 RPM/51 MPH in
2130 Chaparral. Used for family recreation, towing watersports.
Problem begins somewhere after 2-5 hours on a new cap and rotor.
Typical scenario is, while advancing the throttle to 2200-2400 RPM, I
begin to see the tach jump around like it's lost its ability to count
the firing cycles, or is maybe counting extra random firings. Tach and
engine come up smoothly to this range, then tach swings randomly up to
higher ranges, needle swinging back and forth. Once past this RPM
range, tach again comes into line and behaves. Engine is running
smoothly at this point and may continue to do so for a few more hours.
With continued running, however, engine begins to miss and backfire
when accelerating through this RPM range. Concurrently, tach is
having its problem and, as problem progresses, can't rev past problem
area. Inspecting cap and rotor shows excessive corrosion. Replacing
them right there on the water, engine still hot, solves the problem.
Again runs like a top...for a few hours. Only a new cap/rotor solves
problem.

Have inspected everything I can think of. New plugs, wires,
fuel/water filters, fuel lines, fuel pressure tested, all wiring
connectors, ignition timing (with correct timing tool). Even replaced
distributor complete. No water dripping onto distributor.
Disconnected tach to see if it is electronic feedback into the EFI/ECM
circuits. No luck. I even did 'sham' cap and rotor replacements,
where I opened the engine compartment, disassembled distributor, let
engine cool, then put old pieces back on...same problem. Can't figure
out why cap and rotor are getting destroyed.

Doubt serious mechanical engine trouble, as it runs too darned good
until this occurs, then runs great again immediately after cap and
rotor are replaced. Couldn't run this stronly with bad valves, cam,
compression, etc. Have not replaced the coil as it seems unlikely it
could run so well if that were malfuncioning...could a bad coil send
out multiple or random sparks? If so, why does a new cap and rotor
fix the problem? Are the new pieces just able to mask the multiple
sparks occurring?

Seems like corrosion is causing some sort of misfire, and/or
crossfiring (which perhaps could be what the tach is counting making
the needle swing, but the tach should get its signal from the coil or
at least the distributor shutter, not from what's going on in the
secondary wiring, right?). But I can't find the reason why the cap
and rotor get corroded so quickly, and their lifespan is decreasing.

One idea has to do with the advance mechanism on these electronic
motors. The distributor is fixed, unlike the old weight and spring
models that advanced the rotor and breaker cam simultaneously. On my
motor, the advance is electronic and the spark firing occurs earlier
and earlier in the rotation of engine AND distributor which means
that, at full advance, the spark could be trying to jump from rotor to
cap when it is still some distance from the cap terminal. Could this
be causing the terminal to 'burn' and/or the spark jump to another
terminal? If so, why doesn't this occur on all such engines. Why
does it occur on mine with both the stock disributor and the MSD I
replaced it with?

I bought this boat used with 40 hours on it in excellent condition.
I'm not suspicious that the previous owner had this problem and
decided to sell it (but that could be so) because when I first got the
boat a cap and rotor would last all season. Changing it the next year
didn't seem like a big deal. Well now I've had the boat for 4 years
and I'm changing the cap and rotor every time we go out. Any ideas?
I'm about ready to offer a reward to the person who can solve this
problem! Thanks in advance.