74 115HP Mercury No Spark @ Plugs HELP!!

[rrman83]

I have a 74 1150 Mercury that I desperately need help with. I have no spark at the plugs, but I do have it at the brush inside the distributor cap. It has a CDI switchbox and coil, which both test good according to tests found on this site.

I have checked all the wiring, and replaced the key switch since the wires were in very bad shape. I finally gave up and took it to a marine shop and they said the trigger was bad, which I had already replaced previously with a used one. I then bought a new one from CDI, installed it correctly, no change. Every test I can do comes back good.

The switchbox/ trigger wires have the proper voltage on them when cranking, engine grounds are good, and the white trigger wire has .90 Ohm resistance to ground. I have tried to start it with the harness removed, and jumping right at starter, same result. I do appear to have spark at the coil wire coming into the distributor, but nothing past that. Any help would be much appreciated!

 

[emckelvy]

Re: 74 115HP Mercury No Spark @ Plugs HELP!!

OK, so the spark gets out of the coil, but it doesn't make it past the distributor.

Either the distributor cap is bad, or an internal fault in the rotor is causing spark to be shorted to ground.

The cap of course is much easier to diagnose if you happen to have a spare one kicking around!

The rotor is not so simple to troubleshoot.

But you can easily diagnose an extreme failure of the rotor by checking the resistance of the rotor contact to ground with a meter. If it's grounded or shows any resistance other than Infinite, it's bad. The rotor is integrally cast on the dist shaft so you'd have to dismantle the dist and replace the entire rotor/shaft assy.

If the rotor passes a meter check try this: (very very very carefully of course!) position the coil wire (unscrewed from the dist cap) to the center contact on the rotor. Where the spring-loaded carbon brush would ride. Then check for spark energy off the brass strip where it would normally spark to the contacts of the dist cap.

He wonders to himself, "how can we do this if the rotor's spinning?". Well, you can simulate spark by doing portions of the Merc Battery CDI Troubleshooting procedure.

As follows:

"2. Disconnect all 3 distributor wires on the Port side of the switchbox (and the ?mercury switch? if present);

6. Jumper the brown and white terminals on the dist. side of the switchbox to each other.

9. Ground the black terminal on the distributor side of the switchbox - this should cause a spark each time you touch ground."

Needless to say this is with the key switch ON which supplies +12V to the white terminal on the other side of the switchbox (in addition to the constant +12V on the red terminal adjacent to the white wire).

This'll fire the ign for your evil testing purposes without danger of rotating machinery. Note I'd pull all the plug wires just for G.P. when troubleshooting.

HTH & let us know what you find on this pesky bugger..........ed

 

[rrman83]

Re: 74 115HP Mercury No Spark @ Plugs HELP!!

Thanks! I'll try all that this evening and let you know.....

 

[rrman83]

Re: 74 115HP Mercury No Spark @ Plugs HELP!!

Thanks again for the help, and here are the results.

- There is no continuity or resistance between rotor contact and ground.

- I redid the switchbox check, and then followed through with the additional steps you gave me and everything worked. I then took it a step further by in turn lining up the rotor contact with each plug wire, positioning distr. cap up on trigger, and testing for spark at the plugs. There was a good spark on each plug.

- I then reassembled everything and tried starting once again- no spark @ plugs.

Thanks in advance---

 

[emckelvy]

Re: 74 115HP Mercury No Spark @ Plugs HELP!!

Is the spring-loaded carbon contact free to move in the hole in the cap? Sometimes it gets all gunked up and you have to pull the mess out and clean both parts and hole.

Try undoing the belt and spinning the dist with the rest of the ign wiring intact and cap installed. You may be able to hear a spark event that would otherwise be muffled by the starter motor.

If the ign isn't firing at a time when the rotor contact is pointing directly at a contact in the dist cap, spark ain't gonna make it out of the cap.

This could be a result of the cap being installed in the wrong position (pretty hard to do since there's a slot lining up with the trigger assy), or maybe the copper "chopper" disk is upside down.

The windows in the disc aren't symmetrical so they'll be "off" if the disc is upside down. It's stenciled from the factory "Rotor Side" but that can fade away over the years.

Unless you're positive the disc is on the right way, it needs to be checked. If left upside down it'll mess up the timing adjustments and could burn pistons.

Well, that's all my brain cells have tonite, give 'er another shot and we'll see what happens!............ed

 

[rrman83]

Re: 74 115HP Mercury No Spark @ Plugs

Re: 74 115HP Mercury No Spark @ Plugs

Yes, the carbon contact is free to move- I just replaced it, as well as burnishing all the contact points in the cap to get rid of any corrosion.

I tried undoing the belt, and at first I had a spark at each position as I rotated it. Then I seemed to lose it for no reason, and didn't have it on any plug. You can still hear a tick as it passes each contact, but not the distinctive "pop" you hear when it is sparking.

Agreed on the cap position- pretty hard to mess that one up. As far as the disk- this is probably a no-brainer too, but I'm assuming the side marked "Rotor side" should face toward the rotor contact and not the trigger? i.e., side marked rotor would be down.

Thanks again-Jason

 

[Chris1956]

Re: 74 115HP Mercury No Spark @ Plugs HELP!!

Jason, Did you have the rotor disc out of the Dissy? If so, you need to check the service manual to make sure you put it in correctly.

Did you check the resistance of the plug wires? it should be zero. If not replace the wires or at least the wire ends at the plug. Watch for arcing from the wires to the block. If the spark is determined to be intermittent, it might be the trigger. Mine went intermittent for a week or so, before it failed completely.

 

[rrman83]

Re: 74 115HP Mercury No Spark @ Plugs HELP!!

Yes, as I said before the trigger is brand new, and came with a new steel rotor disc. The only thing I am unclear about is whether the side with the writing on it goes toward the rotor contact, or toward the trigger.

I guess there could also be a slim chance that CDI has the writing on the wrong side?

I have watched for arcing- have not seen any. I'll check the wire resistance- thanks!

 

[rrman83]

Re: 74 115HP Mercury No Spark @ Plugs HELP!!

Quick update on the plug wires. I checked resistance between distributor cap and plug end- several have .5-.6 Ohm, one 500 MOhm, and 2 have none.

 

[Chris1956]

Re: 74 115HP Mercury No Spark @ Plugs HELP!!

They should all have zero resistance. You can clean up or replace the corroded springs and any cracked boots. The one with the big resistance might need to be replaced. Try oldmercs.com, part number 84-63212A 2 for the wire replacements.

I would think if the disc was installed incorrectly the timing would be off. This might be what you are seeing. The OEM Manual tells you exactly what side is up. Unfortunately I do not have it here.

 

[emckelvy]

Re: 74 115HP Mercury No Spark @ Plugs HELP!!

The side stenciled "Rotor Side" faces downwards, towards the rotor, i.e. you would be able to see it with the rotor assembled in the dist.

 

[rrman83]

Re: 74 115HP Mercury No Spark @ Plugs HELP!!

Latest Update-
- Cleaned up the springs- only one was corroded
- No Cracked boots
- The rotor disc is installed correctly, writing facing down
- Cleaned rotor contacts more- no more "gunk"

Still no consistent spark! ????

 

[rrman83]

Re: 74 115HP Mercury No Spark @ Plugs HELP!!

I had a few weeks there that I couldn't work on this motor, but I still desp. need help. Everything is still the same as on my previous posts. I have gone through the service manuals, and have not found any new things to try. Any help would be appreciated. thanks!

 

[verno]

Re: 74 115HP Mercury No Spark @ Plugs

Re: 74 115HP Mercury No Spark @ Plugs

Yes, the carbon contact is free to move- I just replaced it, as well as burnishing all the contact points in the cap to get rid of any corrosion.

I tried undoing the belt, and at first I had a spark at each position as I rotated it. Then I seemed to lose it for no reason, and didn't have it on any plug. You can still hear a tick as it passes each contact, but not the distinctive "pop" you hear when it is sparking.

Agreed on the cap position- pretty hard to mess that one up. As far as the disk- this is probably a no-brainer too, but I'm assuming the side marked "Rotor side" should face toward the rotor contact and not the trigger? i.e., side marked rotor would be down.

Thanks again-Jason

Possible conditions, Jason. When you turn the dist, without the belt, and lose spark but still hear a click instead of the pop? Try to locate where the click is. An internally shorted coil can have a click, just like that, with no spark going to the plugs. OR, the click may be the spark passing through the plastic rotor to the steel rotor shaft. When the rotor is at fault a common symptom is that you will have spark idling but when you try to accelerate, it will act very much like fuel starvation on transition, just like if you have the idle fuel mixture screws set too lean.

 

[Chris1956]

Re: 74 115HP Mercury No Spark @ Plugs HELP!!

Gee this thread is nearly 5 years old. Is that the record?