Advice on 1975 3 Cyl Mercury 650

Here are the symptoms. Having a hard time timing the motor. It doesn't want to keep the timing it seems, it also seems like it is missing when idling. When it idles it surges. IF I can get the RPMs under 1000, it feels like it has a miss but often it will rise the RPMs on its own. Sometimes the motor will run away on its own and I have to shut it off. This doesn't matter if it is running in the barrel or on muffs. I am leaning towards a faulty trigger. I have a manual and the only test for the trigger is to test the black wire to the body of trigger for continuity. I get continuity on both the blue and the black wires coming from the trigger. When I run an Ohms test on the trigger I get 28.8 ohms from the black wire to the blue wire and the same when I run from the black wire to the body (ground). My manual doesn't have specific ohm numbers for the year of my motor, just that it needs to continuity I believe.

My next steps are to replace the spark plug wires as these are original wires. Anythings else I should be looking at or testing at this point? I hate to give up on this motor as I've put a lot of time and money into it, but at the same rate I've eliminated the easy items. I am hoping that someone here has an idea of where to look now.

So, here is what I have done so far:

Rebuilt the engine.
New power head seals all around, new top and bottom seals,
New thermostat, poppet valve,
Replaced water pump.
Replaced exhaust seal.
Changed plugs,
Carb kits, carbs rebuilt,
New main wiring,
Replaced distributor cap and plug wires (not new)
Replaced broken reeds,
Compression tested at 120 per cyl.,
Replaced all fuel lines,
Fuel filter replaced, fuel pump rebuilt,
Ohm tested stator vs. another stator, numbers are the same, continuity tested stator, tested good, no continuity between wires and body = good.
Took the switch box out and jumpered to check for spark through the switch box, got over a 1/2" jump with bright blue spark.
Getting spark to all three cylinders as tested with a timing light, but am not sure it is a good spark.

Serial Number: 3549507

Thanks,

J.T.

Two of my triples did that, and the reason was worn, way out of spec bores. There was so much blowby it would not idle below 2,000 rpms (and, if it did, it bucked about and slowly died--sound familiar?)

The only way to tell is to do a leak down test--especially on the top cylinder. (These motors are notorious for cooking the top hole.) Mine had excellent compression (!30 to 140) but also enough blowby to murder low speed running.

Jeff

PS: What's happening here--and the Power Dome motors seem to do it worse--is that the intake charge is being polluted by exhaust gas. That destroys low end power and idle.

I'm going to pick up a leak down test tonight on my way home from work. If the leak down fails, does this mean I have to replace the piston rings at this point? Or is it more serous that that? When I took the reeds off and replaced them, I looked at the cylinder walls and they seemed smooth still. However the piston themselves were a little marred. Not terribly, but enough to be noticed.

Last night I took the distributor off and manually lined up the rotor and the point and fired from the switch box through the distributor to the plug and got really good spark. It worked on all three spark plugs. I then hooked the trigger back up to the switch box, hooked power to the switch box and spun the rotor to see if the trigger would in fact trigger the shot of volts, it did not. Not sure if I grounded it correctly or if the trigger isn't working properly.

With the plugs out you should see nice BLUE spark on all three. If not...

If the block fails the leak down test, the bores are worn too far for a hone and ring job--it would need a bore and oversized pistons (at the least). Let's hope that's not the case.

Jeff

Thanks for the replies fastjeff. So I picked up a leak down test. I am getting 12% on cyl. one, 11% on cyl. two, and 12% on cyl. three. I read that on two stroke motors it is ok to have 10% loss and 15% loss on four stroke motors. I've attached the photos of the gauges on here. These gauges work two ways. I used the percentage gauge instead of the color gauge. I set the color gauge to 0 like the instructions said and that gave me the percentage on the left gauge. I triple checked all cylinders and was consistent. Do you think this is too much loss?

On top of that. I hooked everything back up and now I am not getting spark when the trigger is hooked up. I'm thinking that I have a trigger issue still. Try to turn the motor over and no spark. Jumpered the switch box by removing the trigger and get spark all the way through the box, coil, and through the distributor. Thoughts?

Okay. Good news is that the block appears to be okay.

I'm confused: You talk about the distributor, and also about the trigger coil. That motor has one or the other! Please clarify.

Note: The distributor triples have a belt driven distributor and a large switch box near the carbs. The ADI mdels have NO distributor, a trigger coil under the flywheel, and a smaller switch box near the spark plugs. Which is yours?

Jeff

Jeff-

Thanks for your help in troubleshooting this.

My motor, has a distributor with the trigger inside of it. Not under the flywheel. The later models after '75 have it under the flywheel. How this one works, is we have the switch box which takes the white/black, blue, black wires from the trigger which is integrated within the distributor body, the switch takes that signal, passes it along a green wire to the coil, the coil has a hv line to the center of the dist. cap which charges the rotor while it spins, there is only one coil for all three cylinders. The distributor is a belt driven device. As the rotor spins the windows on the rotor disk pass through the trigger sensor and the trigger tells the switch to fire. All of this I am sure you already know but for my peace of mind I'm explaining it so I make sense of it all.

Here is a few pictures of the motor setup. These were what I had on my phone.

I was noticing that you had another post that you used automotive components to use as ignition rather than the trigger assembly. I'm researching how to switch over from the trigger dist. to a MSD or other type of ignition system. Any pointers?