about to give up. still slight miss

[jfrey8899]

Ok. I have a 1973 sea swirl. Sterndrive. 140 mercruiser. Missing just barely. Carb was rebuilt. Compression test went great. So I know its not fuel or integrity of motor. Convinced it is electrical. Last night I put an inline spark tester on each plug and it flashed consistently. However when boat is being started the light on spark tester was really bright. When done starting and let it run the light still flashes consistently but about half as bright as when in process of starting. Is that normal. Now I'm pretty sure that the cap and rotor and coil are from auto part store. And I know that the condenser and points are from auto part store. So pretty sure they aren't marine grade. But since they are all new p would expect them to work anyway. Am I way off base here? I mean it is a simple GM motor.

 

[crewscontrol]

Re: about to give up. still slight miss

Based on what you said in your post, I have to agree that the "miss" is electrical. You said the points, condenser, and cap were all new. How about the spark plugs? Are they new and gapped properly. How about the spark plug wires? Are they new?

Bad spark plug(s) and bad wire(s) often only show up under a load. Also is your "miss" rhythmic? If so the problem is on one cylinder.

I know that because my 2002 Sea Ray 270 had the same problem, even though the local Sea Ray dealer told me the last thing they ever have problem with are Mercruiser spark plug wires. An arcing spark plug wire (#4) was my problem and it only showed up under a load and only over 2500 RPM and not at idle.

Often an arcing spark plug wire can only be plainly seen when running your boat at night.

 

[honeys money]

Re: about to give up. still slight miss

When starting, the resister is by-passed, and your getting 12 volts to the coil, when running it is 6 volts, but if coil is good should be no diff in spark, may be a bad resister wire, or resister.

 

[Eric 3.7LX Alpha 1]

Re: about to give up. still slight miss

What is voltage at the (+) side of the coil using a digital muiltmeter while engine is running?

Red test lead to (+) on coil
Black test lead to ground on engine, not (-) on coil.

 

[jfrey8899]

Re: about to give up. still slight miss

Which wire is the resistor wire or resistor. I'm learnin as I go. Still waitin on my manual. I don't think that it is the wire that comes out under cap and goes to coil. Is it.

 

[offcourse]

Re: about to give up. still slight miss

I've had a 70's 120hp with a car coil which ran only ok with bridged resistor wire.
Bypassing the resistor wire, supplying 12V directly to the ignition solved my problem completely.
I had to place a relais swithing the power off when not in use, since I pulled right from the battery, and risked toasting the coil otherwise (when the contacts were in closed position with the stopped motor)
Regards

 

[offcourse]

Re: about to give up. still slight miss

Remembered to mention.
I also replaced the contcts capacitor.
Very cheap, but the most important part with contact point ignition.

 

[jfrey8899]

Re: about to give up. still slight miss

which one is the resistor wire? Basically you just ran two wires straight to coil from batt with switch in middle to cut it

 

[Eric 3.7LX Alpha 1]

Re: about to give up. still slight miss

I've had a 70's 120hp with a car coil which ran only ok with bridged resistor wire.
Bypassing the resistor wire, supplying 12V directly to the ignition solved my problem completely.
I had to place a relais swithing the power off when not in use, since I pulled right from the battery, and risked toasting the coil otherwise (when the contacts were in closed position with the stopped motor)
Regards

I would advise against doing this. Get the wiring diagram and do it the way Merc. wanted it.