5.0 carbureted mercriuser eratic tach with engine miss

[shortbus]

My buddies sea ray 230 runs almost ok.
It sounds like a cylinder or two missing.
Top speed 37. Top rpm maybe 3700.

You can put boat in neutral and get a smooth tach reading over 4k.
Engine sounds ok.

Run the boat under load and you get an erratic tach.
The tach is varying 1500 or so rpm while a very faint motor miss happens at the same time.
Tach was disconnected when he got the boat. I dont think it affects engine.

You mentioned a distributor/coil combo retrofit.
http://www.summitracing.com/parts/dui-m12720bl/applications/?prefilter=0
Ya think ignition problem?

Have not checked plugs, neutral switch, crank sensor.
Peeked inside dist cap and seated good looking wires.
Fuel filter replaced.

 

[rod_stewart]

If it runs OK without the tach, I would start by replacing the tach with a known good one.
You should be able to run 4800 rpm or so at WOT with a good tach. 3700 is way too low and sounds like the tach is shot.
You should NOT be revving it to 4 grand in neutral.

Rod

 

[makomark]

A bad tachometer will indeed keep the ignition system from properly functioning...like rod said, disconnect it and see if the miss goes away...

 

[shortbus]

Do you guys respond before you read the post? Thanks for taking the time to respond, but jeez.

 

[guyjg]

The tach is varying 1500 or so rpm while a very faint motor miss happens at the same time. Tach was disconnected when he got the boat. I dont think it affects engine.

Yes it will. That is probably why it was disconnected. Get the tach out of the equation as suggested above. Do the basics...new plugs, careful ignition parts inspection and spark test at each plug.

You mentioned a distributor/coil combo retrofit.

Who did?