Have a 85 Thunderbird Formula with twin 350's. Last weekend change Cap an Rotor. Noticed rust on sensor so lightly sanded clean. assembled. Did timing, good to go. The port cap was torqued to the max. This weekend ended up getting cap off, did the same as last weekend, assembled. Wont start. WTF . thought it would be a nice day on the water. Boat still in driveway. Took cap off, took sensor out, cleaned well. Pulled center wire off, check for spark. Nothing. Bought a new coil. Installed. Turned ignition key, checked voltage on positive side of coil, 12.4v. Pulled spark plug wire off, installed plug and turned motor over with plug near ground, no spark. Anybody no what I did wrong to ruin my nice weekend? Boat ran fine Tuesday and Thurday. Any advise Please

See if you still have voltage when you are cranking at those terminals. Try and test with a meter if you can and see if the voltage is dropping off to less then say 11 volts. This could be as easy as a bad battery beleive it or not. Not saying it is but that is were I would start.

Jon

After reading your post I would start looking for a grounded wire from the coil to the dist. the sensor may have gone bad after you cleaned it and is grounding out on the dist. case...........Been there done that
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Capt. Steve

check the carbon ball in the top center of cap make sure its not rolling around loosely had a similar problem that was my end result what really got me was it was a brand new cap and it was bad go figure

Figured it out sunday, the steel square in the sensor was fliipped 180 from sanding. started right up. BUT, after setting timing in driving and launching boat. Motor had no power when hit throttle, just bogged down. So drove to shallows on 1 motor, anchored and checked timing. got it set at 8 degrees BTC, starts and idles, sounds like its missing. Hand feels like major arthiritis from getting shocked thru distributor while setting timing. New wires on the way. Ill change plugs and wires thursday and see what happens.

Check the wire harness going to the ignition amplifier, on the earlier model of the 454 the electrical connections get corroded, interupting the circuit. Causing inconsistant ignition. Also check the two leads feeding the distributor occasionally they become loose and also will give you a similiar problem. How old is the fuel??
Have you inspected the water seperators contents in a clear glass container. Often a little water in the fuel will cause loss of power.
Good luck post your findings!

Speed sensor in the distributor. Replaced all wires, plugs an sensors. Put 250 miles on yesterday n today. Purrs nice. Got r up to 54 at 3500rpm. What's a good rpm for cruising? I stayed between 2800 and 3500.

Good to hear your back up and running.
If you keep the rpms just under 3000 the secondaries will stay closed and you will use less fuel.
I'm not sure what your lowest speed is in your boat, run the boat with tabs almost level or just down a touch drives kicked out somewhat so the boat does not porpoise, and listen to the motors, and how the boat is riding.
Of course from time to time you will need trim it to the max, kick the plates up and go by the other guy, just to keep your mind clear.