Merc 900, 1978 90 Hp. Cuts out without warning.

[weeksender]

Hope someone can figure out what could be causing my engine to suddenly cutout. I have been working on this engine all summer, and I still haven't had a boat ride lasting longer than 15 minutes. Summer is over, I'm nearly broke and I'm tired of paddling back to the boat launch.

Engine starts OK. However, after it warms up or runs for a few minutes it suddenly cuts out (no sputtering, smoking, nothing). It's like someone turns the key off. This may occur at full throttle or at idle. When this first happens, the motor restarts with no trouble. However, it then gets progressively worse over a course of a few minutes (where it runs shorter intervals before cutting out, and gets harder to restart).

So far, I've cleaned and rebuilt the carburetors (with new floats, gaskets and needles). I've rebuilt the fuel pump. Replaced the water pump. I've replaced ALL the hoses. Replaced the external and internal wiring harnesses. Replaced the ignition switch. Replaced the gas tank with all a new hose and fittings. Changed the gas in the tank. Although, overall, the motor runs smoother, it continues to cutout as before.

Any thoughts??? I don't know where to go from here, except to the boat dealer to trade "the Money Pit" in this spring.

Thanks for any help,
weeksender

 

[emckelvy]

Re: Merc 900, 1978 90 Hp. Cuts out without warning.

Does your motor have a belt-driven distributor, (1) ign coil and (1) switchbox mounted in front (CDI), or does is have (2) switchboxes mounted on a plate on the Stbd side of the powerhead, with (6) ign coils (ADI)?

Need to know this basic info before attempting to theorize what your motor's problem is............ed

 

[turbinedoctor]

Re: Merc 900, 1978 90 Hp. Cuts out without warning.

I would say it's in the Stator or the switch boxes. Try reading out the stator first.

Turbo

 

[weeksender]

Re: Merc 900, 1978 90 Hp. Cuts out without warning.

ed:

It's a (1) ign coil and (1) switchbox mounted in front (CDI).

 

[weeksender]

Re: Merc 900, 1978 90 Hp. Cuts out without warning.

Thanks Turbo for your suggestion. I'll see what I can do with that. I've never messed with the stator, so I'll have to do some research.

Thanks
weeksender

 

[emckelvy]

Re: Merc 900, 1978 90 Hp. Cuts out without warning.

Then you have a distributor model and the stator has nothing to do with how well the ign works (except for charging the battery).

The switchbox requires a constant +12V on the Red terminal (on the Stbd side of the switchbox), and a switched source of +12V to the white terminal on the same side of the switchbox (from the ign switch).

It'd be interesting to see if, when the motor dies, you're losing voltage at either of these terminals. You could hook up a meter or test light to see what happens.

Also, you've done a lot of work on carbs etc but no mention of looking at anything ignition. How 'bout the distributor cap, have you pulled it down for inspection? "Trash" in the cap can lead to all sorts of strange ign problems. Flush it out with carb cleaner if it looks bad inside. Check the spring-loaded carbon center contact, if it's bound up you won't get good spark to the ign wires.

It's also possible you may have a heat-related problem with either the distributor trigger, coil, or switchbox. Hard to diagnose but you might try a can of that electronics "Freez" stuff which will cool down parts very quickly. You may be able to isolate the cranky component by giving a good shot of the Cold Stuff when your motor starts acting up.

The following test is intended for isolating distributor trigger problems, and may help you in your diagnosis (to either confirm or rule out a trigger issue):

"MERCURY BATTERY-POWERED CDI TEST FOR DISTRIBUTOR MODELS

This test is for the 332-2986 switchbox used from 1967-1978 on all the inlines.

This test assumes your coil is good (problems with CDI coils are rare).

DISCONNECT BATTERY

1. Turn off ignition;

2. Disconnect all 3 distributor wires on the Port side of the switchbox (and the ?mercury switch? if present);

3. Remove the HV lead from the ign coil to the center of the dist. cap (remember it unplugs from the coil and unscrews from the cap);

4. Reconnect the HV lead to the COIL only;

5. Position the free end of the HV lead approx. 3/8" from ground (block, shrouds etc), and find a way to hold it there;

6. Jumper the brown and white terminals on the dist. side of the switchbox to each other.

RECONNECT BATTERY

7. Check that you have +12V at the red terminal (even with the ign off);

8. Turn on ignition and verify +12V at the white terminal (same side as the red terminal);

9. Ground the black terminal on the distributor side of the switchbox - this should cause a spark each time you touch ground.

If you get spark with the distributor bypassed, and it won't fire with the distributor connected, the trigger is bad and the entire distributor housing assy must be replaced.

If you get no spark using the test, the switchbox is probably bad. In that case, be sure to check for correct power on the switchbox, check all connections, and check the coil's resistance to make sure it's OK."

That oughta keep you busy for a while!! HTH...........ed

 

[weeksender]

Re: Merc 900, 1978 90 Hp. Cuts out without warning.

Thanks, emckelvy.

I'll have to give that a try this weekend. I think this may turn into a winter project from the look of things.

Eric