1978 6 HP CD Ignition Problem and Question

[Bay Boater]

Working on a 6R78B (1978 6HP with CD ignition)

No spark on the top cylinder. Motor starts and runs OK on bottom cylinder, but is weak as you might guess.

I switched the two HT coils and the top cylinder still does not spark. I basically got each coil to separately run the bottom cylinder. The coils are both OK.

This motor has only one magneto coil and one sensor under the flywheel. The flywheel has two magnets. I assume this means the magneto coil and sensor get two pulses per revolution - one for each cylinder. So if even one cylinder is firing then they are both OK.

That seems to leave the power pack as the problem.

Anything I'm missing? I have read that a weak magnet in the flywheel might cause this, but the motor was running strong on both cylinders within the past two hours of operation.

Also, how does the power pack know which cylinder to fire? It gets two pulses from the coil and sensor per revolution but I don't see where it gets information about which cylinder is at TDC when the time comes. More for my general curiosity. The main thing here is to be sure the power pack is the problem before I go changing parts.

Thanks.

 

[boobie]

Re: 1978 6 HP CD Ignition Problem and Question

As a guess I'd say the power pack. Go to cdielectronics.com for trouble shooting procedures. The flywheel key keeps the ign in time with the pistons.

 

[Bay Boater]

Re: 1978 6 HP CD Ignition Problem and Question

As a guess I'd say the power pack. Go to cdielectronics.com for trouble shooting procedures. The flywheel key keeps the ign in time with the pistons.

Yeah, the power pack is where my head is just now. I got the CDI link from the forums here earlier and am puzzling through it - will run some of their tests before I actually swap out a power pack (ouch$$).

But regarding the flywheel, I understand how the key orients the magnets with the crank, but how does the power pack know which magnet is passing by at the moment?

If I cut a second keyslot 180 degrees from the original the magnets would still have the same orientation to the crank and the sensor and coil would see a magnet passing by at exactly the same time (even though it is the other, identical magnet).

The coil and sensor see a magnet pass by every 180 degrees. How does the power pack know it's for cylinder 1 or 2?

This is more of a general interest question. The real question is what else do I check, if anything (besides the CDI procedure), before swapping out power packs?

I held a screwdirver tip to each of the flywheel magnets and they all seem healthy to the touch.

 

[Bay Boater]

Re: 1978 6 HP CD Ignition Problem and Question

OK - Puzzled through the CDI checks. I can't do DVA so I can only check so much.

The CDI page seems to think my motor has more wires than it does, but what I do have is referenced. Really only two things I can check, the sensor resistance and coil resistance.

Sensor resistance, 15-50 ohms per spec, 37 ohms by my measurement.

Coil resistance, 450-550 ohms per spec, 559 ohms by my measurement. A hair high but less than 2%. Since it fires the lower cylinder just fine I doubt that's the problem.

I guess I'll head out to the boat yard tomorrow and see if I can scrounge up a power pack to at least test it.

But damn if I know how the power pack sorts out which cylinder to fire. Every time it sees a pulse from the coil and sensor, either one of the two cylinders may be at TDC.

 

[AlTn]

Re: 1978 6 HP CD Ignition Problem and Question

the OMC workshop manual has the description for the workings of the CDI ignition system, also, if you can find it using the search function, member FR described its workings ...from all you've described the pp is your culprit

 

[Bay Boater]

Re: 1978 6 HP CD Ignition Problem and Question

Replaced the pp, runs like a scalded cat. Enough about that.

I also puzzled out the mystery of how the pp knows whether it's the no. 1 magnet or the no. 2 magnet going by when it decides to fire which cylinder. I had the flywheel off again and played with a compass around the flywheel magnets.

The two magnets are reversed from each other. Where one has it's south pole the other has it's north. When the magnet goes by the sensor the little voltage pulse it gets will be positive or negative depending on which magnet went by.

The HT coil, which also gets reversed current pulses for each revolution, doesn't really care which direction the current is going. It sparks as well either way.

Slick.