No spark

[Chopper0708]

Hello everyone, first post on here, looking for some help starting my boat to winterize for the glorious Michigan winter, I have a mercruiser 140 I/O I beleive it’s a 1984, s/n is 6748315. Anyway I bout the boat late summer and had a great time with her although always a little tough to start and now it’s been about 2 months since I’ve had it out and I’m getting no spark, I’ve already changed the battery, spark plugs, coil, and starter solenoid based on local advice and still no spark. It cracks but never even tries to start. Any ideas on where to go from here???

 

[Chopper0708]

Correction, the s/n is 6720563

 

[achris]

Give the points a clean. Should start straight up.

 

[Chopper0708]

Umm, I’ve changed to many parts for it to be that simple jk I scraped them a bit with a screw driver and no change but I will get some sand paper tomorrow and clean them better to try that

 

[achris]

Nope, use newspaper... And watch between the points as they open and close. You should see a small spark between the contacts...

 

[alldodge]

Sometimes points close up from either the screw loosing up, or from wear. Crank the motor to see if they do open and close

 

[Chopper0708]

They are definitely opening up, cleaned them up this morning and now I am getting good spark all the way to the plugs but still not starting, onto the fuel system I suppose. This is the first time going through this motor so I’m sure I’ll find several issues as I go deeper. Thank you for the help so far

 

[stonyloam]

OK, do not use sandpaper, it leaves grit behind and rounds off the points. Buy a points file (about $3) and clean them up. To keep a points engine tuned up you need a couple of tools, a timing light and a dwell meter. Use the timing light to set the correct ignition timing, and the dwell meter to set the points gap to make sure your coil is charging properly. You set the timing, then the dwell, then go back and recheck the timing.

 

[Chopper0708]

Stony, thank you for the info, I’ll be putting together a tool kit over the winter so thank you very much, I just researched what dwell was, I had never heard of it, but I’m also pretty novice.

 

[stonyloam]

OK chopper here are the basics of what you are dealing with: your coil is just a transformer with two sets of wire windings. A primary with a few turns and a secondary with many, many turns. The cam on the the distributor shaft opens and closes the points. When the points close low voltage current flows through the primary to ground which generates an electromagnetic field. When your points open the current stops and the field collapses. The collapsing field generates a very high (20,000 or so) voltage in the secondary winding which is attached to the center contact of the coil. This “spark” flows to the center of the distributor cap, through a carbon contact into the rotor. If your TIMING is correct your rotor will be pointed at the correct plug wire contact and the spark will jump the gap and flow down the plug wire to the spark plug and jump the gap to ignite the fuel. Usually you want the spark plug to fire just before (a few degrees of rotation) the piston reaches top dead center of the piston compression stroke. Your spec may list it as “5 BTDC”. There will be instructions with your timing light. Oh yeah, to advance the timing you turn the distributor in the opposite direction of the rotation. With a clockwise rotation, rotate counterclockwise to advance. There will be timing marks on your harmonic balancer and cam cover. Dwell is pretty simple. It is the amount of time your points are closed in degrees of rotation (one full rotation is 360 degrees). Too short and your coil may not fully charge, too long and the coil may overheat. Kind of long winded, but I hope this helps.

 

[achris]

You can read Don's full and very good description here -> https://forums.iboats.com/forum/eng...005-how-points-ignition-system-works?t=329742

On setting dwell and timing... Dwell and the gap are basically the same thing... You adjust the dwell by changing the gap.. The gap setting in the specs will get you the right dwell if the everything else is right. Most people prefer to use a dwell meter as it does give a more definitive indication of the time the points are closed (which is important for coil charging)...

I also prefer to set the gap/dwell, then the timing, as changing the gap/dwell does change the timing. If you increase the gap (decrease the dwell), the points will open earlier, thus advancing the timing... Conversely, decreasing the gap (increasing the dwell) will retard the timing.... Changing the timing doesn't change the gap, but changing the gap does change the timing, so set gap/dwell first....

Many modern timing lights have a built-in tacho, volt-meter and dwell meter, so the one instrument is all you'll need. I used to have a dwell meter, a handheld tacho and a timing light... Now I have just the one device and it's select-able for 2 stoke or 4 stroke, and for any thing from single to 12 cylinders....... ( Just realised, I can't use it on a Bugatti Veyron.. )

Chris.........

 

[Chopper0708]

Thank you for the info guys, that was a great discription and probably would have save me from changing a bunch of parts, but an education always costs you something. Thanks again and I should be able to get the boat tuned right up in the spring.

 

[QBhoy]

I’d be simply looking at the ignition sensor at the diz. It gives a signal to spark via the ignition.
Not an expert on your engine by any means and it may not even have one, but most do. Sometimes on side of diz and sometimes in it.
Being a child of the late 70’s maybe the old points engines don’t have this ? Only really worked on my own and friends engines from the late 80’s onwards with more modern ignition systems.
Used to own a classic 79 fletcher with your engine. Great thing but never worked on it. Was but a lad.

 

[achris]

I’d be simply looking at the ignition sensor at the diz. It gives a signal to spark via the ignition.....

Nope, no 'module' on a points system. The points ARE the 'module'... Well, they are what 'modules' replaced...

Points systems are dead simple...

Power runs from the ignition switch to the + side of the coil, through the coil to the points (located inside the distributor cap)... As the points close the current flows through the coil 'charging' it up, creating a 'field'. When the points open, the field collapses, and through the magic of magnetism, induces a high voltage in the primary windings of the coil. That voltage is transformed by the secondary windings and fed, via the distributor cap and rotor, to the appropriate spark plug.... As the engine continues turning, the points close again, and the cycle repeats.... The only other significant devices in the system are the ballast resistor (or resistance wire) which limits the current flowing through the coil when the points are closed, and the capacitor across the points to 'snub' the high voltage and reduce points arc.

And in essence, electronic systems are just as simple....

In an electronic system, the 'module' does exactly what the points do, switches the current through the coil, by using a 'high speed electronic switch', usually a transistor, MOSFET or SCR. The advantage of the module it that it's usually triggered by a hall effect sensor (again, magnetics) or an optical sensor, which have no moving parts, so they don't wear, or burn, and the timing never drifts (unless some klutz leaves the distributor bolt loose :facepalm.... And as it's an electronic switch, it's VERY fast. The usual 'rise time' for the spark with a points system is quite slow and a lot of energy is lost... Also the voltage produced by points is all positive, and that's what contributed the most to spark plugs wear... An electronic system has (usually) equal positive and negative, so the wear on the spark plugs is significantly less, so they last longer.

HTH,

Chris........

EDIT: Here is an excellent article explaining the same as what I have said ^^^
https://www.thoughtco.com/point-type-ignition-systems-262557