1973 Merc 650-spark at coil but not at plugs. Wires check OK.

[stripes1966]

Guys I have an ignition issue that is about to drive me to the funny farm. I am working on a 1975 650 3-cyl blue stripe with a distributor and BATTERY driven ignition. Got spark from my spiffy new CDI switchbox/coil combo to the cap, but it is not making it to the plugs unless I turn the motor slowwwwly by hand. If I try to crank the motor with the key, that spark at the plugs goes away. There IS still good spark at the coil wire when cranking with the battery though. How can simply cranking the motor with the battery kill a spark that occurs when the motor is turned by hand?. Could it be a trigger issue? If it consistently sparks at the plugs at one speed, it should spark at a faster speed I would think, but that ain't happening. New NGK gapless plugs. Distributor cap shows no visible damage or carbon tracking, I cleaned it anyway. I got a new brush for the cap from OldMercs. The rotor looks OK. Rotor is pointing right at #1 TDC in the cap at 0 degrees TDC+/-. So, where on earth can a spark go if it does not jump from the rotor to the coil terminal only 1/4" away and then onto ANY of the plugs? These are copper core wires with no obvious continuity issues. Rectifier checked good as well but since it is a battery driven ignition it should not matter. Thanks for helping me keep my little bit of sanity that is left with this thing. Caps and rotors ain't cheap for these motors either.

 

[merc850]

Check the battery voltage at the red and white wires on the switchbox it should be 9.5v or higher, if the battery is weak the voltage will drop and no spark.

 

[stripes1966]

Thanks 850. Is that straight DC voltage or does it need to be measured DCVA with one of those Merctron gizmos?

 

[merc850]

Just a regular voltmeter on DC. If the battery is below 9 volts charge it with a battery charger don't boost it with another one and not the motor, the motor is meant to keep it charged not revive a dead one.

 

[GA_Boater]

And to back it up, also measure the voltage on the center of the center of the battery posts, not the connectors, while cranking. If it's higher than the switchbox voltage merc850 mentioned in post #2, clean all the battery cable ends from the battery to the motor, including the grounds.

If it's the same voltage, suspect the battery.

 

[Faztbullet]

Check ohm readings on coil wire also.....disconnected from cap should jump a 1/2 gap with a "POP"

 

[stripes1966]

I only got a chance to mess with it a bit this morning. Battery showed 12.1 V at center of terminal. Red and white wires at the trigger showed 11.0 V (white) and 12.0 (red), but this was not measured while cranking if it makes a difference. Faztbullet I also immediately suspected wiring and grounds, so I touched up all ground wires, checked the trigger wires and coil high tension for continuity and all checked fine. (The CDI switchbox/coil combo also came with a new copper core coil wire incidentally). So I turned the key to "run" position, and started turning the flywheel to try and get a spark out of the coil wire. I did. Nice fat blue one, like 1/4" jump. But, as about 3-4 minutes passed, that nice spark went away with no other changes. Coil wire spark went from nice fat blue 1/4" and consistent firing at TDC for each cylinder to very weak and orange 1/8" with very inconsistent firing. It's as if something is heating up possibly. I am starting to suspect the trigger sensor. This is hand turning the flywheel not cranking with the starter. I did notice that the distributor has a lot of vertical slop in the rotor shaft. I bet these triggers are sensitive to the separation between the wheel and the sensor. Can a later model used sensor be used if I swap the wheel? I found a 4-cyl distibutor but mines a 3-cyl. THANKS for your input.

 

[merc850]

It sounds like the trigger is faulty, any play in the distributor is no good, the shutter disk could destroy the pickup here are the instructions for removing the shaft - the rotor disk might transfer to your other distributor but a slot has to match with the brass contact on the rotor (you have to retime the engine when done). I worked on a Tbolt distributor recently be extremely careful that you don't put any force on the piece that is above the disk or you'll break it off. The rotor is molded onto the shaft so you can't just pull it off - I would try and find a manual from 1973 to see the exact methods for replacing this item. Ignore Section 8 of this page:

 

[stripes1966]

Thanks for everybody's help. I will be taking the distributor apart tonight based on the instructions and the Z-man website. To make this weirder, I got the plugs to fire. But they ONLY fire if I lay them on top of the block. As soon as I screw em' in and hit the starter the plugs lose fire or fire very sporadically. I added supplemental grounds from the back of the motor to the front but that did not help. The factory grounds at the rear of the block, two of them, are fine and shiny. This motor lost power gradually. It went from running strong, to barely running, to nada. For a while it would run, but if you gave the engine full throttle it would bog and it had very little power and would not gain RPM's.

 

[racerone]

Spark is weak.----Does spark jump a gap of 3/8" or more, yes or no ?-----Post values of a compression test too.

 

[merc850]

Mark the 3 cyl. disk with an R on the rotor side so when you put it in the new dist. it will be facing the same way. Here's the parts breakdown

 

[stripes1966]

Racerone, thanks for your help. No spark was jumping 1/4" or so at best. I have a spare trigger from a '74, if I cannot figure something out otherwise in an hour of trying tonight, I'll roll the dice and swap out the trigger optical sensor tonight for my spare. I'm surprised that nobody has looked into rebuilding these things by digging the sealant out. I have read about it being done successfully on switchboxes as the wires corrode. Might just be a corrosion issue on these things that takes em' out too. They are not really located in a high heat area. CDI used to rebuild these, but the stock that's left is very expensive and they are typically 4-cylinder units. I assume the only difference is the number of holes in the wheel, but I don't have one to compare. 850 again thanks for your help, these things are a love hate relationship indeed. Got too many new or rebuilt parts on this thing to back out now. Maybe we need to start a support group for the early 70's Mercs lol. I am printing the diagram that you kindly attached now. If the swap works, I will definitely be carefully digging the sealant out of the old trigger with pics to follow. Vintageoutboards lists a 1 month turnaround to rebuild these, so somebody must be out there doing this.

 

[stripes1966]

Well I started oh so carefully taking the distributor apart but I found the crack in the rotor in the process and broke it. I used the proper spanner wrench and the thing still broke so it was probably cracked anyway. On a whim, I check craigslist, and what do I see but a 1973 650 with a bad lower unit for $100? I call the guy, he says it was running when he parked it 10 years ago in his basement, and all seems good. Seeing this as a sign from The Almighty himself, I set out on my Holy Grail like journey for he has seen my anguish and has given me a gift! I mean what are the odds guys? An hour away? 100 bucks? It had been listed for 22 days already? So I go get the thing and all seems right with the world, it has a tight distributor with no slop whatsoever, good cap, good rotor, and lots of spare parts for me. I swapped the distributor of course, stator, rotor, rectifier, and engine harness from the donor as my harness was ratty. Attach the timing light to the coil wire and it strobes like a KC and the Sunshine Band concert. Hallelujah! Attach the timing light to the spark plug end of things, and once again, nada. This must be a religious experience, because at that point I was speaking in tongues and I'm not really sure what I said. It is said that He will not put more upon you than you can bear. As I move the timing light on the coil wire from the cap back toward the coil itself, the timing light pickup is losing signal about 8 inches from the coil center tower. Must be arcing somewhere even though this is a brand new CDI coil wire. Gonna turn out the lights tonight and see if I can spot an arc. My boats new name is Damien.