Mercruiser 170 - Spark at distributor, not plugs

[ClintW]

Hey, all—

I’ve seen some great threads here, so figured I would try my luck. I have a 1986 Mercruiser 170 with the original ignition system (no alternator). It wouldn’t start after the winter (and bilge flooded because I forgot to take out the plug—d’oh).

The starter and battery were clearly not good, so I replaced both. Then I wasn’t getting 12v at the coil while starting, so I replaced the slave solenoid. Then I had high resistance in the coil, so I replaced that. I’ve also replaced the points, condenser, rotor, and distributor cap.

I am getting strong spark out of the coil, as tested with an adjustable spark tester and by putting the wire to the block. But I am getting nothing at the spark plugs (again as tested with adjustable spark tester). I cannot figure it out. I checked my negative at the coil, and I have voltage there while starting. I also can see spark between the points when starting. How can I have spark all the way to the distributor, then nothing at the plugs? I’m totally lost. Thanks so much.

 

[nola mike]

Spark plug/coil wire, plugs, cap, rotor are the only possible failure points. If you're not getting spark at any plug, I'd guess that the issue is at the rotor button connecting to the cap (I read this as having spark off the coil wire)

 

[ClintW]

Spark plug/coil wire, plugs, cap, rotor are the only possible failure points. If you're not getting spark at any plug, I'd guess that the issue is at the rotor button connecting to the cap (I read this as having spark off the coil wire)

Thank you. What would be causing that kind of issue? Both the rotor and the distributor cap are new. This distributor cap doesn’t have a spring loaded contact inside the cap. It’s just a solid brass little ring with a black plastic dot in the middle.

 

[kenny nunez]

The spark from coil wire to ground has to be blue and jump at least 1”.
If you are using some of the cheap auto store parts that could be your problem.
There has to be 12 v at the + or bat side of the coil when in the cranking mode. You need to verify that.
NAPA still has good quality ignition parts. Also converting to a Petronics electronic set up will also help. It is possible for the rotor to go to ground but most times when the engine has a points style ignition and has been running and will ground out when coming off plane.

 

[ClintW]

Thank you. To be sure I had enough voltage at the + on the coil, I ran a jumper wire straight from the + on the battery to the + on the coil. So it should have had 12v. Still wouldn’t spark at the plugs. I am using all NAPA parts. I have messed with the timing a bit, in previous diagnostic stuff (thinking it was just a timing issue). If it’s off, could the rotor be missing the contacts in the cap?

 

[kenny nunez]

With the points closed can you force them open by pushing on the rotor shaft? Worn out rotor shaft bushings can cause ignition problems. Even with a worn out distributor the rotor would still contact the distributor cap.
Are you getting a blue spark out of the coil wire?
What are you gapping the points at? If you had a dwell meter you would be able to watch how much if the dwell stays steady when cranking.

 

[nola mike]

I have messed with the timing a bit, in previous diagnostic stuff (thinking it was just a timing issue). If it’s off, could the rotor be missing the contacts in the cap?

No

With the points closed can you force them open by pushing on the rotor shaft? Worn out rotor shaft bushings can cause ignition problems. Even with a worn out distributor the rotor would still contact the distributor cap.
Are you getting a blue spark out of the coil wire?
What are you gapping the points at? If you had a dwell meter you would be able to watch how much if the dwell stays steady when cranking.

If he's getting a good spark at the coil wire like he says, the problem is rotor/cap/wires/plugs.

 

[kenny nunez]

Try this, pull the plugs put them on the wires lay them on he manifold, spin the engine. Look if the plugs are sparking.

 

[ClintW]

Try this, pull the plugs put them on the wires lay them on he manifold, spin the engine. Look if the plugs are sparking.

Just to respond to earlier messages, too: I do have good spark coming out of the ignition coil. I just don't have the spark at the plugs. I gapped the points with a business card, if I'm being honest, because I don't have feeler gauges. But again, I can see spark between the points, and I'm getting good blue spark at the coil wire. (Not sure it jumps an inch, but it jumps the standard ignition gap on the spark tester.)

It's like the spark just disappears somewhere in the distributor. That's why I was asking about timing. It seems to me that if the spark is happening while the rotor is between contacts in the distributor cap, it would fail to get spark to the plugs. Anyway, I'll try what you suggested--pull the plugs and try to get spark.

 

[Scott Danforth]

Get a dwell meter (borrow from local auto parts store) and set dwell properly

 

[ClintW]

Get a dwell meter (borrow from local auto parts store) and set dwell properly

The way I understand it, everything happening inside the distributor (including at the points, where I would set the dwell) is to get a good spark coming out of the coil when the magnetic field collapses. I have that good spark. I just don't have any spark coming out of the spark plug wires. Am I wrong about that?

 

[Scott06]

The way I understand it, everything happening inside the distributor (including at the points, where I would set the dwell) is to get a good spark coming out of the coil when the magnetic field collapses. I have that good spark. I just don't have any spark coming out of the spark plug wires. Am I wrong about that?

If dwell is way off you may be able to make a weak spark at coil but not be strong enough to go through rotor wires and jump at the plugs.

the dwell measures the amount of time in distributor rotation that the coil is building up energy before it fires (points closed if i recall). If dwell is off coil might not build enough energy. Do you have good connection between cap and rotor? button in cap can fall out

 

[ClintW]

If dwell is way off you may be able to make a weak spark at coil but not be strong enough to go through rotor wires and jump at the plugs.

the dwell measures the amount of time in distributor rotation that the coil is building up energy before it fires (points closed if i recall). If dwell is off coil might not build enough energy. Do you have good connection between cap and rotor? button in cap can fall out

Got it--thanks. I have a new rotor and cap, so I assume it's good. There's no button in my rotor--just a solid brass ring with a little black piece in the middle.

 

[Scott06]

Wouldn't assume because it is new it is ok. Button in cap needs to contact the rotor - if not you will lose spark

in the beginning of this video there is a nice strong spark off a coil with points ignition - you have a nice blue spark like that? Note what he says about dwell vs point gap around 11 minutes

 

[ClintW]

Wouldn't assume because it is new it is ok. Button in cap needs to contact the rotor - if not you will lose spark

in the beginning of this video there is a nice strong spark off a coil with points ignition - you have a nice blue spark like that? Note what he says about dwell vs point gap around 11 minutes

I would say my spark looks like that--just not as constant as the motor is trying to start, obviously. But I'll take a look at gap and dwell. Thank you.

 

[Scott Danforth]

The way I understand it, everything happening inside the distributor (including at the points, where I would set the dwell) is to get a good spark coming out of the coil when the magnetic field collapses. I have that good spark. I just don't have any spark coming out of the spark plug wires. Am I wrong about that?

If that is the case, you have a cap or rotor issue. However I am going to go with a dwell and timing issue

 

[racerone]

I think those had a stator and flywheel to generate voltage to charge the battery.-----Many of those 470's had a belt driven alternator kit installed.