OK , this inline 6 with serial number 509****, was going great guns earlier in the summer, until July 4th that was. During the middle of pulling kids in a tube it was as if a switch had been turned. The engine sputtered and coughed back to shore. We're using fresh mid-octane gas of course.
Since then I have torn apart the carbs (not especially gummed up,) soaked them in cleaner, installed new gaskets, new needles and seats. Checked to float levers to make sure they measure correctly. The fuel pump was overhauled last fall and the diaghragm is still fine (no leaks, dry on the back side).
Compressions are 115, 111, 115, 118, 112, 113. New Champion plugs were put in this spring and I have month old plug wiires in place. I followed this up with most of a can of Sea Foam thru the fuel line. It was some pretty ghastly smoke there for a while though.
I took it over to the local lake and while bothered only by some mild sputtering trying get a hole shut with the overhauled carburetors, things worked great. Cough, spit, get the boat on plane and I'm thinking I'm a regular genius. A few more turns around the lake and you guessed it, the invisible switch gets thrown and the Merc sputters back to the ramp. This time with #2 plug heavily carboned over with dry black material.
I now discover that the rectifier plastic looks "bubbly" and there isn't any continuity from any terminal to another, so I replace it and top up the battery because it hasn't been getting any sort of charge has it?
Now on Saturday the 18th my son and I go back to the little lake where this thing runs smooth and so fast it scares the 9 year old. We head back to dock and make plans to go tubing the next day.
This time it's with the wife along and the engine misses and sputters the entire time. Limp back to shore with six sets of eyeballs glaring at me and I'm feeling like a fool.
Has to be ignition right? After hauling it home, I start pulling spark plugs. Whereas before I was getting 3/8" to more than 7/16" of blue spark, all six are now showing 1/4" max spark.
Those are the symptoms, now my thoughts, even though they might be wrong: stator on this motor serves only to charge battery (it'll run without the two yellow wires hooked up to the rectifier even), so that's probably not the culprit.
The switchbox is providing electricity to the coil and distributor to operate. It doesn't determine if the spark is hot enough does it? My testing on shore confirms that it's a weak spark not an absent one. Can I say the switchbox is OK? How about the distributor?
At the moment I think it could be the coil but far more likely the battery. Let's just say that I don't have a lot of confidence in this particular battery even though it spins the motor over. Will a bad battery keep this ignition system from performing?
Check to make sure the spark plug wires have zero resistance and do not arc to ground, under load. Make sure you have +12VDC on teh red and white wires of the switchbox. The white wire comes from your ignition switch.
Make sure the green wire to the coil is not corroded, and the grounds are all good.
I made up a set of good spark plug wires, solid continuity for all six. I also went ahead and replaced the flaky battery 'cause it wouldn't hold a charge
I measured only a very low voltage drop coming from the battery back to the white and red terminals on the switch box. The green wire was clear of corrosion , firmly attached and had good continuity to the coil.
Even though the spark looked weak on the tester (1/4") it seems to run OK on the muffs this evening.
Anything else I ought to be doing? Incidentally I'm measuring only 10 vdc across the battery terminals.
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Have you checked the cap,rotor and replaced the center carbon brush, you also need to find whats causing the low voltage it takes 12 full volts to run properly, ck the connections for corrosion, the engine plug in for lose plug in tips. Then take the boat out and test run it with a voltmeter hooked to the white switchbox lug if you can keep the voltage above 12 volts you either have a problem in the alternator systm or wiring
I think I'm making progress. I had an hour before my son's Little League game this afternoon so I headed to the boat storage shed.
First thing was determining that the new VOM I was using was giving erroneous measurements. Using two others, I determined that my new battery was yielding 12.7 volts across the posts and at both the red and white terminals on the switchbox. While cranking the motor with the starter, I was getting 9.5 VDC at the terminals. I'm assuming that this means that my electrical connections from the battery to keyswitch to solenoid/starter and switchbox are at least "servicable."
I removed the high tension wire from the coil and replaced my spark tester while grounding to one of the ground straps on the rear of the engine. Cranking the starter gave me an evil blue spark that was jumping 5/8" and I could hear it snapping from six feet away. I hooked the high tension lead back up to the distributor, put the spark tester into the #2 spark plug wire socket on the distributor, I still have the weak 1/4" spark.
Laddies, I ran out of time this afternoon for further testing, but when I had the distributor cap off about six weeks ago it was surprisingly clean with no obvious cracks. I won't have a new carbon brush for a few days yet.
At this point, am I looking at trigger/distributor trouble? Suggested course of action?
John, 9 1/2 volts is not enough voltage for that system to operate properly, thats bare minimum for the system to even fire the battery CD uses a step up transformer in the switch box to produce the AC voltage for the coil (transformer) to fire the system so if it starts out with low DC voltage it ends up with low AC coming from the coil to the dist and will cause problems
John, I maybe getting lost on this problem if in a arc test you have good blue spark at the coil but not good spark at the plugs, if the trouble is only on 1 plug wire ck the wire including the cap stud for continuity also with the engine idleing use a test wire to ground and run it up and down the wire cking for arc or run it in the dark and ck for arcs it could be a wire thats arcing or a bad cap, if it's poor on all cyls it's probably the rotor. The other thing is if the spark is poor after the engine warms up and the input voltage to the switch box is less than 12 volt or more than 16 volts at 3500 RPM you have a voltage problem
If you're getting lost Laddies, it due to my poor attempts at an explanation. I'll try to summarize what I've come up with so far:
1. I have 12.7 VDC across the battery posts, 12.7 VDC while checking both white and red terminals on the switchbox.
2. While turning the motor over with the starter, my meter reads about 9.5 VDC at the white switchbox terminal. I've neglected to check the brown trigger terminal.
3. Impressive 5/8" + blue spark coming from the coil on a spark tester. (reminds me of furnace electrodes)
4. Weak spark (1/4") when I've tested each of the six spark plug wires. The wires themselves are new and I've tested each one to confirm continuity.
5. While at idle on shore with the muffs on, I can remove each of the spark plug leads and get the engine to stumble.
6. Nothing in the distributor cap six weeks ago jumped out at me.
7. Two weeks ago it ran great at the lake, the next day it wouldn't get out it's own way and I didn't change a single thing.
Next step, probably Friday, will be to pull the distributor cap and check for cracks, corrosion, etc. I will also run in the dark to see if any electricity is escaping. I also need to double check the switchbox voltages while the engine is powered up. I'm also planning to check out the keyswitch connections, bad wires
What should I look for with the rotor or carbon brush? That bright blue electricity from the coil is being dissipated somewhere.
John, I believe if you have a good hot spark at the coil and if the cap and rotor were alright it would carry on thru to the plugs, so if the sparks ability is less than half at the plugs compared to the coil lets take if a step at a time. The wires are new,were they replaced with solid core wires and well done end assemblies? Usually if the spark is poor or greatly reduced there is a common denominator in this case it would have to be a coil wire-distributer cap carbon-rotor or maybe a cap. it would be nice to know before spending the money for a cap or rotor as they are very costly, if the ign system is whats failing you, if you had a induction tach available to you I would suggest going for a ride and moving it from cyl to cyl to know if it is the ign system thats failing you or you have some other problem
Laddies brings up a good point. Did you replace the wires with automotive type resistor wires? These engines need to use the solid wire core type plug wires to operate correctly. Just a thought....
I pulled the distributor cap this morning and could not find any cracks in it which is what I feared.
The rotor had a bit of corrosion and one very small bit of pitting on the trailing edge of the blade which I removed with some 150 grit emery cloth. The leading edge was fine. I was very careful to not remove any metal, just left it shiny.
Inside the distributor cap, I found a heavy tarnish on all six of the contacts in which the spark plug wires screw into. Put the emery cloth to use again until I had shiny metal, and then followed it up with electronic parts cleaner. Before I started polishing, the tarnish was offering some resistance on my VOM. I guess I wasn't a good scientist because I didn't measure how high the resistance went.
After the cleaning I now have a really nice 3/4" spark from all the plugs, my inductive tach indicates a smooth 900 RPM from each of the six cylinders while on the muffs.
I think the next step is to head back to the lake, will try to do that this weekend.
Guys, I sure appreciate all of the advice and thoughts you've offered me. I'll post another report soon, I sure learned a lot from this exercise.
1978 Mercury 1400 with ignition trouble
Re: 1978 Mercury 1400 with ignition trouble
Re: 1978 Mercury 1400 with ignition trouble
smooth 900 RPM from each of the six cylinders while on the muffs.
Linear Mode
