theory of ignition - troubleshooting
 

Hi all,

last week I had a new 'won't start' issue, after a season of good starts. It took maybe 20 tries before it started up, lumpy at first, and it spat some black flakes. Yesterday it nearly caught, only once in at least 20 tries. So I have some diagnostic questions. Earlier this season I installed a new battery, changed plugs, overhauled carb, checked fuel lines - facet pump is working well. I've had the dist apart and the advance springs are intact and not seized. So everything points to ignition...

1. I've had stuck valves in the past, though not recently. It would take a lot of stick valves to totally prevent ignition, wouldn't it?

2. If I want to check spark, but my ignition switch is more than an arms reach from the engine bay - do I set up some kind of parallel switch or hot wire in the engine bay?

3. Checking a spark is achieved by pulling plug wire and turning engine over while holding wire 1" from plug?

4. How do plug wires fail? Can you test with a multimeter?

5. How do points and condenser fail? How often should they be replaced?

6. I started thinking about the coil - transformers only work with AC.
How does the coil make AC or where does it get it from?

any tips gladly received
Simon

2. A remote starter, that is nice to have for trouble shooting
Autozone: Remote Starter
3. Pull out the plug connect to plug wire, rest it on side of engine head to ground it, turn over engine, there should be a strong spark in the gap of plug.

I am only have advice on those two sorry.

yeahjohn, thanks for those tips.

I also found Don's useful remarks at
from http://www.moyermarine.com//forums/s...ead.php?t=2643

" IGNITION SYSTEM: Install a permanent back-up ignition switch in the engine compartment to connect the big battery terminal on the starter solenoid directly to the primary terminal of the coil to be used any time a failure of the ignition system is suspected."

I was thinking that would have to be a heavy duty switch! Which side of the coil is the 'primary terminal'? Don's advice regarding wiring harness is well taken, though I cringe. Mine is original and very well made, by an electrical engineer, but its very complex and the widow of the builder dumped all his notes, spares, etc, before I found the boat. :(
Xmas list item 1 - wiring diagram.

"EXHAUST SYSTEM: Install a 1/8” pipe tap in your exhaust manifold outlet flange to enable measuring exhaust system back pressure."

I doubt I've got exhaust blockage, but its always possible - yipee, another engine hole adventure :(

coil receives current, dc only.
The current comes from the positive terminal, thru the coil, and to ground thru the points. The points open and a spark is produced that goes out the secondary (thick center wire)

You can have ignition on, have distributor cap off. then open and close the points with a screwdriver. This will make the coil spark. Check by holding wire 1/4 inch from block

'coil receives current, dc only.'
that's what I thought, but then how does it boost it to the purported 20,000 volts?

" The current comes from the positive terminal, thru the coil, and to ground thru the points. The points open and a spark is produced that goes out the secondary (thick center wire)"

understood

"You can have ignition on, have distributor cap off. then open and close the points with a screwdriver. This will make the coil spark. Check by holding wire 1/4 inch from block "

which wire? :)

thx
S

If you are running points and condenser you may be experiencing an ever decreasing strength of spark because the distributor cam lobes on the shaft are wearing down. As this occurs it is necessary to keep decreasing the point gap to achieve specified dwell. The specs in the manuals (.018" to .020") become irrelevant. Last year in Florida I had a distributor running an engine at .003" point gap - I know, sounds unbelievable but it was true. Because there is no source (that I know of) for new distributor cams the smart play is EI available thru Moyer Marine. I was one of the last holdouts but since going to EI my starts are quicker and smoother and require less battery drain.

Hanley
"the smart play is EI available thru Moyer Marine."
yeah, probably, but I've been waiting for the coil compatibility issues to settle out. I think I'll try a general tune up - points, rotor, condenser, and check wires and coil, this time...

OK, BTDT. Just make sure that you set your points by means of a dwell meter without regard to point gap specification; then check the actual point gap that yields your required dwell and post that number for us. This will provide useful information about the condition of your distributor cam.

Halycon you can find about how the coil makes 20000 volts on the net.

The primary side has 12 volts. It is many turns of thin wire around a center secondary (big wire).

An elctro magnetic filed builds. Then the field is broken when the points open. A spark is produced by the secondary side when the field collapses.

Aha ! - so the action of the points induces AC which makes the 'coil' function as a transformer! Makes sense. But it must be half wave rectified. Interesting.

You have the equivalent of half a square wave, since it comes from a battery and not a stator (sine wave).

The time when the current is passing thru the primary side is called the saturation time. With these old systems, this was a big thing for voltage development.

"You have the equivalent of half a square wave,"
thats' right! I hadn't thought about the possibility of that kind of pseudo AC.
interesting.
S

I see we have some physics majors aboard. Could some one explain the effect of the condenser in the system? Is it a damping force in an oscillatory system? I have experienced ignition failure from a "bad" condenser. Could some one expand on how this happens?

#1: If you suspect compression problems do a compression test with a meter.

#2 & 3: See post #2.

#4: The end fittings may become corroded and not pass electricity well. Also the insulation may break down and high voltage will leak out and short to ground. This condition is not common in solid core wires but is sometimes seen in graphite core spark plug wires. To test: Observe the engine running at night and look for shorting while the wires are in their working position. Some suggest misting the wiring with water during the test. The spark plug wires may test ok with a meter only to not work correctly when high voltage is applied from the coil.

#5: Points can fail because of the obvious stuff - corrosion, breaking, pitting ect. What's not so obvious is dwell problems. I'll say this in a slightly different way than Hanley did but we are on the same page. If you can't achieve a spec dwell with the point gap set to spec there is a problem with the points or distributor. Condensers can short out (outer can to center wire) in which case the engine will not start or run because coil - is grounded when the points are open. If there is excessive pitting on the points it means the condenser is not doing its job correctly. I file and gap my points a couple of times a year. If you run points buy yourself a points file. Only a couple of bucks at your "botique" auto parts store.

#6: Point are an off on switch. Nothing more. I don't know how AC fits into this. I'll leave that part to someone else.

BTW #1: You are not in an "engine hole". You are a member of this forum. We'll get you up and running.

BTW #2: Is your choke closed all the way when you think it is?

Hang in there.

TRUE GRIT

It helps to understand the coil as a sort of mini Tesla Coil. The condensor (old name for capacitor) can be seen as part of a reasonant system on the primary side. Also - and very important - it absorbs the voltage generated by the magentic field collapse on the primary side. Absent the condensor the points would arc big time when opening and burn out quickly.
Also note it is very unlikely for all 4 plug wires to burn out at once. OTOH the wire from the coil can very well burn out and hose up the entire operation. BTDT.

Hanley,

Simply put, the condenser stops the points from arcing. It makes the break of current flow nice and clean. It keeps the points from arcing and burning also.

Thanks everyone for useful info and support.
I teach electronics and long ago had old british cars (morris and MG) so the general layout of and old flatop 4 w/carb is not unfamiliar. But I hadn't put my electronics knowledge to work on the subject beyond basic wiring.

Since I'm going to work on it tomorrow, two simple questions are nagging:

1. testing spark. Can't I just pull the lead off the plug and turn the engine over, grounding spark to block? If so, spark gap should be ~ 1/2"? Spark should be bluish as opposed to yellowish?

2. Per Don's suggestion regarding an in-bay ignition switch bypassing wiring harness - where should it run from and to? Not running through a relay, its gotta be a heavy duty switch - is 10A big enough ?

thx
S

Testing spark.

Sure take lead and hold 1/4 inch away from a ground (block). Remember that the rubber cap that fits on the plug will hold the lead away from the ground. Many use a spark plug hooked up to a plug lead, and grounded. Any plug will do. One from an MG too.

Blue is the best spark. Nice and fat if you can get it.

You can also do a quick and dirty test to the coil. Just hook up 12 volts to the positive side. Then hook up a ground wire to the other side. (no other wires are connected) Take the ground wire and touch to ground and off ground. This will cause a spark. Maybe not the best spark, but a quick test.

---------

you just need battery voltage to the coil to run the engine.However you want to do that.

Yes to both questions. The bypass harness is custom for each boat. The auto stores do sell a handy set up with both ignition bypass and momentary button for starter, or you can cobble your own. You can usually clip on the big starter cable or other convenient live positive. Further to what rc posted; don't be fooled by a spark that is weak and will not jump 1/2". Often a spark will be present but will not be able to fire thru compression.

Found this interesting discussion: http://www.ratwell.com/mirror/users....r/ketterin.htm

thankyou all, very helpful.
Hanley, that link is nicely explanatory.
S

MMI videos
 

Money is dear to us all but a reminder our sponsor sells excellent DVD on ignition systems and video download on timing.

Hold on a minute...

"you just need battery voltage to the coil to run the engine.However you want to do that. "

Surely the starter button does two things - it supplies V to the coil and to the starter motor. Using a bypass switch from, say, starter solenoid to coil will not turn engine so points will not open and close, so no building and collapsing of magnetic field in coil, so no spark voltage... or am I missing something?

BTW - I found this old thread (via google!) on using remote starter
http://www.moyermarine.com/forums/showthread.php?t=6419

The ignition switch provides V to coil. The starter button provides V (momentarily) to the starter solenoid. The remote starter switch is used to parallel the normal starter button. Note that it controls the starter solenoid: it does not carry the full starter motor current. To check spark: disconnect plug wires (so the engine doesn't start on 3 cylinders). Connect one of them to a spark plug lying on the head. Connect your remote switch from a convenient source of 12V to the starter solenoid. Turn on ignition switch (otherwise no spark as you mentioned). Crank engine to check for spark. Good luck and have fun!

BTW: I think 1/2 inch is too far for the spark to jump. Note the normal plug gap is .035 inch!

Halcyon, it was an answer to a question, about the coil. Not about the engine as a whole.

I agree, 1/4 is best. I wonder if I typed it wrong.