Mercruiser 3.0 diagnosis / serial number decode

[Tube1]

You guys are spot on. The distributor internals have obviously been replaced, but the points were corroded and not making any electrical contact. I hit them with sandpaper and will verify spark, then fine tune the dwell.

 

[Eric 3.7LX Alpha 1]

Each spring time when I'm ready to fire it up, one of the first things I do is clean the point contact surface. That brass surface on your rotor button (the part that contacts each post on the cap) looks clean as a whip, you can check the brass contacts on the underside of the cap also for wear, when they get old you will see pitting and wear marks.

 

[Tube1]

Another thing that was an issue is the neutral cutout switch, mounted right on top of my valve cover. It was stuck closed no matter what gear position. I removed it and got it to free up, but obviously needs cleaned.

I don’t really understand what this switch is doing. It’s obviously shorting something out when the button is depressed but I’m not sure exactly what. Would this also hinder spark from firing if the button was depressed?

 

[Eric 3.7LX Alpha 1]

I think your referring to the shift interrupter switch near the valve cover. See this write up:

HOW TO: How the Shift Interrupt system works​

HOW TO: How the Shift Interrupt system works

Usually in the spring and summer we get a lot of posts regarding the shift system for the MC-1 (falsely referred to as Pre-alpha drives) to the Alpha 1 drives. There are a lot of wrong ideas on how and why the system works and why it's there to begin with. Maybe this will help explain how it...

forums.iboats.com

 

[dubs283]

Another thing that was an issue is the neutral cutout switch, mounted right on top of my valve cover. It was stuck closed no matter what gear position. I removed it and got it to free up, but obviously needs cleaned.

I don’t really understand what this switch is doing. It’s obviously shorting something out when the button is depressed but I’m not sure exactly what. Would this also hinder spark from firing if the button was depressed?

You are referring to the shift interrupt switch. It grounds the ignition system momentarily so the drive can be shifted out of gear into neutral. Yes, if the switch is activated the ignition system will not work

 

[ScottinAZ]

the neutral cutout grounds the coil when switching gears to allow the gears to unload the shifting dog clutches. If this were inoperable during shifting, it would be much harder to shift out of gear. If this is not working properly, disconnect it (troubleshooting only) and verify that the engine runs.

 

[ScottinAZ]

geez, all three of us were johnny on the spot with that one !!

 

[achris]

OK, really briefly. (Ignitions 101)

Consider it as 2 circuits, the low tension (12v) side, and the high tension side (20,000v+).

The LT side consists of (in order) the battery, the ignition switch, a ballast resistor or resistive wire, the coil, the points and condenser. (The condenser just suppresses the small arc generated across the points as they open, reducing points burn).

When the points are closed, current flows in the LT circuit and 'charges up' the primary winding in the coil (actually creates a magnetic field inside the coil). When the points open that magnetic field collapses, which induces a voltage spike, of around 400v (hence the small arc that tries to jump the points). That's the LT side dealt with....

As that 400v is induced in the coil primary winding it's stepped up to around 20,000v in the secondary of the coil. The coil is nothing more than a simple transformer. That 20KV then travels along the HT lead coming out of the top of the coil to the centre HT post of the distributor cap, where it travels along the rotor to one of the side posts in the dissy cap, along that lead to the spark plug attached to the other end...

Hope that helps.

Chris.......

 

[Tube1]

OK, really briefly. (Ignitions 101)

Consider it as 2 circuits, the low tension (12v) side, and the high tension side (20,000v+).

The LT side consists of (in order) the battery, the ignition switch, a ballast resistor or resistive wire, the coil, the points and condenser. (The condenser just suppresses the small arc generated across the points as they open, reducing points burn).

When the points are closed, current flows in the LT circuit and 'charges up' the primary winding in the coil (actually creates a magnetic field inside the coil). When the points open that magnetic field collapses, which induces a voltage spike, of around 400v (hence the small arc that tries to jump the points). That's the LT side dealt with....

As that 400v is induced in the coil primary winding it's stepped up to around 20,000v in the secondary of the coil. The coil is nothing more than a simple transformer. That 20KV then travels along the HT lead coming out of the top of the coil to the centre HT post of the distributor cap, where it travels along the rotor to one of the side posts in the dissy cap, along that lead to the spark plug attached to the other end...

Hope that helps.

Chris.......

Great explanation Chris, thanks. I’m an electrical engineer by trade, but obviously lack the “common sense” aspect of these older engines. Luckily I have some good news and I’ve learned a ton today.

After cleaning the points and setting dwell, The engine has solid spark. I also had to re-verify spark order by setting TDC on cylinder 1 compression stroke.

It now runs on starting fluid/drips of gas down the carb. Unfortunately I’m still facing a fuel supply issue. I do not have a fuel pressure gauge/setup on hand yet. The fuel pump seems to be erratically supplying gas to the carb supply line (it’s not consistent and barely squirts out). I connected the intake fuel line to a remote tank and it does the same thing.

I have a new fuel pump coming in tomorrow, and will swap it to see the results. Fingers crossed that’s the last roadblock in the path to getting this engine running smoothly!

 

[Eric 3.7LX Alpha 1]

Check the fuel filter on the carb inlet. It could be buggered up.

 

[Tube1]

Check the fuel filter on the carb inlet. It could be buggered up.

There’s a fuel filter on the carb assembly itself? I was under the impression the only filter is at the bottom end of the fuel pump.

 

[ScottinAZ]

if it has a GM derived carburetor, there is one in the inlet. You have the fitting from the pump that screws into a larger "hex" type thingy. Unscrew the larger hex piece and the filter resides in there with a spring. Beware of the spring.....

 

[Eric 3.7LX Alpha 1]

 

[ScottinAZ]

2jet is similar

 

[Tube1]

Another thing I need to get taken care of is changing the lower unit oil. The process looks pretty straightforward, but the one thing I need to identify is if my drive has an external reservoir or not. Are these usually located around the engine compartment area, if I do have one?

 

[Scott06]

Another thing I need to get taken care of is changing the lower unit oil. The process looks pretty straightforward, but the one thing I need to identify is if my drive has an external reservoir or not. Are these usually located around the engine compartment area, if I do have one?

Usually mounted on the engine itself small 1/2 qt translucent bottle. I didn't see one in your picture of your engine. Most Gen one drives (88 vintage) do not have one. Just open the drain and then vent plugs, when empty fill from bottom, when full put vent plug back in before removing's lube pump from bottom

 

[Tube1]

I replaced the fuel pump and am pretty sure I’m getting fuel pressure now. However, still no start. It sputters on starting fluid. No attempt to even fire while cranking regularly.

1: the filter at the carb looks clean but there was no spring in that assembly. Is this an issue? (Pic attached)

2: does the operation of the carb and linkages look normal here? (Vid attached)

 

[Scott06]

I replaced the fuel pump and am pretty sure I’m getting fuel pressure now. However, still no start. It sputters on starting fluid. No attempt to even fire while cranking regularly.

1: the filter at the carb looks clean but there was no spring in that assembly. Is this an issue? (Pic attached)

2: does the operation of the carb and linkages look normal here? (Vid attached)

If on starting fluid you don't get it to cough I'd say you don't have spark or timing is off or no compression

If you have decent spark you can keep it running by dribbling gas down the carb. Just gotta be careful in case of a back fire.....

 

[Tube1]

Edit- comment deleted, shift interrupt switch appears to be ok.

 

[Scott06]

Also… my shift interrupt switch is reading 5 ohms across the terminals when the switch is NOT depressed. Could this contribute to a no-start condition? I think it should be an open (infinite resistance) when switch is not depressed, and a short when depressed.

disconnect it and test 5 ohms is a lot of resistance but could ground out the ignition