Random RPM dip

[jimesh]

My A4 was running fine, then began showing brief dips (a fews seconds) in rpm almost to stalling, but never did really stall, at random intervals around 10-30 secs. Knew we had a slight leak from the fuel pump so replaced that with a Moyer rebuild. Ran OK for ~ 30 minutes after that then started showing the symptom again. Yard then checked the filters for signs of contaminated gas (negative), replaced the coil, re-checked the fuel pump installation, checked distributor and points: no change. Recently ran as long as 3-4 minutes at speed one time with no dip, now dips quite often and now stalls on about one half of these. Always starts quickly and easily immediately after stalling.

No difference if under load or idling, and does not seem to be affected by advancing the throttle or more choke, although these latter two are a little harder to be certain about since the dips are so brief.

Does not sound to me at-all like a mis-firing nor lean mixture problem--just not enough fuel, as though the throttle were pulled back momentarily. Hmm, maybe I need to check the throttle cable and linkage? Fuel vent is another possible culprit I can identify from similar threads here that we may not have checked yet, but any other advice would be most welcome.

I'm wondering if the alcohol/water separation problem at the bottom of the tank might be difficult to recognize as contamination at the fuel filters, and that's what I really have?

[hanleyclifford]

Drain the carburetor main jet passage plug at rear of bowl into a small glass container, let it settle for an hour and see if you are getting water.

[rigspelt]

Fuel or electrical seems most likely I think. Not likely the coil, since you replaced it, but as the troubleshooting goes on it might become necessary to bypass parts of the primary ignition circuit to rule out a failing electrical connection or component. Easy to visually check the primary ignition circuit wiring and connections at least.

[Kelly]

condenser

As just mentioned in the "C-30 stalling" thread, don't neglect the condenser in your trouble shooting. It's an easy to replace, inexpensive part that can create stalling, faltering RPM situations as it breaks down.

Food for thought...

[roadnsky]

Fuel or electrical seems most likely I think. Not likely the coil, since you replaced it, but as the troubleshooting goes on it might become necessary to bypass parts of the primary ignition circuit to rule out a failing electrical connection or component. Easy to visually check the primary ignition circuit wiring and connections at least.

As Rigs suggested, it's worth looking into your ignition circuit.
A "cold" solder joint or connection is what you're looking for.

A good way to isolate problems in your ignition wiring, is to run a jumper from the large battery terminal on the starter solenoid to the positive terminal of the coil.
This will bypass the wiring harness that is notorious for failing on our older darlings.
If doing this does away with the issue, you have identified your problem.
If not, you have eliminated the primary ignition circuit.

IMPORTANT NOTE: Installing the jumper wire between the starter solenoid and the coil is the very same thing as turning on the ignition switch, so you CANNOT leave that wire installed except when running the engine or you will burn out the coil or kill the batteries.

Kelly's suggestion to check (change) the condenser is also good advice, but I'd try that AFTER jumping the ignition to eliminate one thing at a time.

Finally, I notice you said you replaced the coil.
What did you replace it with? Is it the correct one?

[jimesh]

Thanks to all who responded. Kelly had it right. It was the condenser. A new one restored normal operation immediately, almost miraculously. It apparently was degrading slowly the more we ran the engine, and was causing serious erosion of the points, which mis-led the (fairly novice) mechanic for a while.

Fuel from the tank and the carb float bowl was all clean and water-free on several checks, which reaffirms my committment to keeping the gas tank topped up at all times to prevent condensation and alcohol separation.

There is one other rather subtle aspect. Somewhere in the past someone had lost the correct machine screw that holds the condenser clamp to the distributor plate and replaced it with a self-tapping one. (Unbelieveable!) This screw was *very* loose when I replaced the condenser, so perhaps it's possible that that alone was the problem. If the condenser looses its ground ( or it becomes high resistance) intermittently it would be the same as not having one and might account for the very erratic behavior we've seen thru all this troubleshooting.

[jimesh]

One other interesting point to come out of all this. I ran the primary ignition bypass test as Don and many here have suggested as part of the troubleshooting along the way. (negative of course.) However, I also noticed that the (-) connection from the coil to the dist goes around an edge of the block and could chafe on it, so I checked that also. (also negative of course in my case.) If that wire rubs thru and shorts to the block intermittently it would be the same as the points closing at random times if I understand the ignition system operation correctly. So my point is that this also seems an important thing to check at the same time as the (+) primary wiring. I hadn't noticed that mentioned by anyone else (although it's certainly possible that I missed it.)

[Mo]

Good post and troubleshooting...will store this in the back of my mind as I was thinking fuel also.