Question about older V4 (CD-4) powerpacks

[CAFisch]

Hello,

I'd like to ask about known failure modes of the older (1973-1977) CD-4 powerpacks used on the V4 engines (85-140hp).

I'm trying to diagnose intermittent issues encountered with a 1973 115hp motor.

Primary symptom:
At both idle & low to moderate rpm ranges the engine will periodically kick up/down in rpm's by approximately 20 to 30 percent (no tachometer is available for actual figures). The cycling between up/down events is irregular but typically will occur within about a 10 to 30sec time frame window, and these events will also last in duration for perhaps 10 to 20sec as well.

Aside for (except for) the symptom described above, the motor will otherwise idle relatively smoothly & regular... even at a surprisingly low&steady rpm of around 250-300rpm. In other words the motor doesn't actually 'seem' to run rough or in a ragged (e.g. unbalanced) fashion in either state.

Virtually all of the typical DMM & DVA ignition system readings appear to be within range and normal. For example, ohm's readings & outputs for stator, timer-coil, powerpack outputs, ignition coils, etc. all seem to check out ok.

* Carbs have been carefully cleaned & reassembled with new kits.
* Compression is very good with variance below 10psi between cyls.
* Ignition coils are fresh without hardly any runtime on them at all.

So the main question here is this:

Is it (or would it be) unusual to have a powerpack that [might] intermittently drop spark on one or more cyls every so often (i.e. 10sec to 30sec)... but that would otherwise seem to provide good consistent spark *some-or-most* of the time?

OR... what other possibilities might exist for this type of symptom?


Thanks for your thoughts.

Charlie

[Solarman]

Could verify if spark problem, using timing light see if flashes are same on all cylinders when this occurs, or it could be a plug wire could be jumping spark to ground when this occurs, could run at night and look for an arc from plug wires when this occurs.

It is not impossible for a plug (even if new) to be shorting to the threads instead of to the electrode intermittently. Think I would check plugs first, look for brownish/burnt area on side of insulator (if plugs were new), and if you find one like that, swap it out to see if it solves problem?

Or it maybe fuel related, a very small air leak in suction side of fuel pump may cause similar symptoms, or possibly the fuel pump itself is getting a little weak?

Many possibilities, but check the first two here before anything else.

Good luck.

[CAFisch]

Thanks for the reminder(s) Solarman... I haven't done any checking in the dark to try to identify any arcing. I'll set up a barrel tomorrow here at the house for that.

And I didn't know either... that a small leak in the suction side of the fuel pump could possibly produce the kind of effect in question. As it is, I was rather assuming this was an electrical issue rather than fuel-related.

Appreciate the feedback.