1984 Merc 150 will only fast idle and has constant-intermittent horn

[DB150]

The short story: The motor will start and fast idle, but when I slow down to shift into gear, the motor stops. It emits a constant-intermittent alarm (i.e., it beeps about every ½ second continuously) whether running or not. More details below. Thanks in advance diagnosing this.

More detail: 1984, 150 ELPTO model: 1150724
Previous trip, the motor ran great; 3 weeks later we drove 300 miles, launched, and it would not run. If I push the release to advance the throttle without being in gear, it will start and fast idle, but when I slow down to shift into gear, the motor stops. Was able to get it into gear and mess around near the landing. After a while it started emitting a constant-intermittent alarm (i.e., it beeps about every ½ second continuously). Now it beeps like that all the time whether running or not. I installed a fuel pump kit, and cleaned the carburetors with new gasket kits. The problem persists. Still has the oil delivery system with oil pumping to the top reservoir and at the bleed screw on the oil pump. Ran some 50:1 pre-mix through it and there was no change. Not opposed to having a merc shop work on it, but local shops aren’t “taking new clients” which is code for everybody is short technicians. Consequently, I am DIY motor mechanic. Any help will be greatly appreciated, or if you know someone who knows what they are doing, I am willing to drive it 200 miles (from Lake Geneva, WI) to get it worked on. Last thing: I have a Seloc manual because apparently the Mercury website does not offer one for a 37-yr-old motor; information on where I can get an OEM manual is appreciated. Thanks.
Dan

[rock]

Welcome to the site. Start from the beginning and do a compression test. Look at plugs as you remove them looking for any differences then noting which hole it came from.

Rock

[DB150]

Thank you Rock. Compression test was the next thing on my list. I have inspected the plugs. They all looked similar, no sign of pitting, ash, deposits, or uneven discoloration. Just a thin film of oil.
D

[rgsauger]

Hi DB. Welcome to S&F. As Rock said, need to run a compression test to get a baseline on the mechanics of your motor. How long since carb rebuild? Stock electronics? Is it new to you or old faithful that is starting to let you down?

[DB150]

rgsauger,
Thank you for the help. It's been a busy week, but I'm hoping to get the the compression test tomorrow. I rebuilt the carbs in September. As far as I know it is stock electronics. My Dad was the second owner in 1993. I inherited it in 2004. The motor has been reliable until now. We use it a couple times a year to fish in WI so it has not been driven hard or long. As I noted, the second-to-last-time out it ran great, then the next time it would not start normally. I'll post the compression results when I get them and go over the electrical system for loose wires, etc.
Dan

[FMP]

Check all wiring from under flywheel down.....

[rgsauger]

I’m running a 1990 Merc 200 so yours is just a few years older. Mine was recently acting weird as you described but without the beeps. In my case, I had a stator go out so I replaced it. Then, one of my switchboxes was dying. You should replace in pairs so I did. Now I have a good stator and new boxes and she barks right off and runs GREAT.

Check the compression. Then let’s look at maybe bringing some of the electrics current. Are you willing to ditch the oil injection and run premix?

[DB150]

rgsauger,
The way it stopped running, like flipping a switch, makes me suspect electrical, but I get that compression is a necessary first step. The more I learn, the more the oil injection system scares me. Premix is easy, and no risk of catastrophic failure. Thank you for the help.
Dan

[rgsauger]

Ditch the oil injection and the black boxes. Simplify it as much as possible. Then new electrics and she should be good to go. Run good oil like Penzoil XLF as yours is a chrome bore. Report back on your compression. It's probably electrical. Triggers usually last. Flywheels do too. Stator and a pair of switch boxes will probably have her all back up to snuff.

[OnPad]

Intermittent alarm is usually for oil injection. Check your cap on tank. I personally ditch the system.
Not running at idle could be a few different things, like linkage adjustment, blocked fuel system, timing. Likely low speed windings on stator. You will need a dva tester for electrical tests.
Ebay has lots of service manuals. Sometimes you will get lucky and find one for free download if you search around.

[rgsauger]

Intermittent alarm is usually for oil injection. Check your cap on tank. I personally ditch the system.
Not running at idle could be a few different things, like linkage adjustment, blocked fuel system, timing. Likely low speed windings on stator. You will need a dva tester for electrical tests.
Ebay has lots of service manuals. Sometimes you will get lucky and find one for free download if you search around.

Welcome to S&F, OnPad! Good input.

[OnPad]

You shouldn't run engine with alarm sounding. Unless you premixed fuel.
A inline spark tester would be handy for initial diagnostics.

[DB150]

Hi all,
Made some progress over the weekend. Here is what I found.

Compression results: I started the motor and let it warm up, then moved into the garage and checked compression with all of the plugs removed.
It’s a V-6 with cylinders and compression as shown below if you are standing behind the boat and face the motor:
Left Right
cylinder/psi cylinder/psi
2/109 1/112
4/117 3/117
6/117 5/117

Since all the plugs were out I used a spark tester to check each plug. Found NO SPARK on cylinders 1, 3, and 5. Spark present on cylinders 2, 4, and 6. Jiggled wires with the spark tester on cylinder 1 and nothing changed - no spark. (I can't believe that the thing will run on 3 cylinders, but I swear it started and I warmed it up.)

There are two yellow wires that go from the stator to a square (1-1/2x1-1/2" ) potted circuit board (rectifier?) that is just above the switch boxes. The insulation on these wires is cracked and separating from the wire for about 2" near the rectifier (very bad condition).

What’s my next move? Thanks in advance.
Dan

[OnPad]

You have a bad switchbox.

[OnPad]

Clean up your wiring. Replace bad switch box.
Most people prefer to do both boxes at the same time. You can check resistance between bias wire (white wire) and ground (switch box frame). You will need to unhook white wire and test individual boxes. They should test very close to each other, or you will have problems.

[rgsauger]

Yep, you got a bad switchbox. I totally agree with Onpad to replace both of them at same time. Sierra ones made in GERMANY. They are $300/pair. I just bought a pair from Amazon a few weeks ago. Also, you need to check your trigger and also your stator. A bad stator can take out one or more switch boxes. That was what happened on mine.