unsteady rpm

[mpohrille]

I have a 1976 FWC engine with reduction drive, electronic ignition and indigo crankcase ventilation system. Engine starts easily,idles at 650 rpm, runs smoothly but lately I have been noticing a constant vibration in the analog tach pointer, when I look at my digital tach it shows that the engine speed is constantly varying plus or minus 20 rpm. This is at all speeds.
Am I being a hypochondriac? is this a problem? IN the past the engine rpm has been rock steady.
Any thought on causes. I was thinking that one of the valves might be sticking

[Dave Neptune]

Can you feel the surging of the RPM change? Is it audible? Or is it just possibly the tach or tach connection?
If you can feel it surging it could be a coil breaking down?

Dave Neptune

[mpohrille]

Unsteady rpm

Can't feel it can barely hear it but since both tach show varying rpm it is not a tach fault

[JOHN COOKSON]

How long has it been since the last tune up?
What is the last maintenance\improvement procedure done before the surging started?
Could be a spark plug cutting out or spark plug wiring not up to par.
Try disconnecting one spark plug wire at time and see if the surging goes away.
If you feel that a valve is not closing all the way do a compression check.
Do the (alleged) surging intervals vary with engine RPMs?

TRUE GRIT

[edwardc]

What about sticky action in the centrifugal advance? If the pivot weights do not operate smoothly, it could cause the advance adjustments to become abrupt as it overcame the sticky spot. This would cause the rpm to "hunt".

[tenders]

Try the label-recommended dose of Marvel Mystery Oil (MMO) in the fuel. I use this in my engine and it smooths out some periodic valve sticking in about 10 minutes.

[Administrator]

Can't feel it can barely hear it but since both tach show varying rpm it is not a tach fault

"Both tach (sic)"?

Not quite sure how to interpret this, but are we convinced this is an engine issue and not an instrument issue?

I found this on the Internet, so it must be true:

A less common problem is a poor connection in the RPM signal wire. In this case, the needle will usually bounce around below the actual RPM reading because it will miss some of the voltage spikes. Look for corrosion, a broken wire, or a loose connection anywhere between the coil terminal and the tachometer.

Bill

[ndutton]

Process of elimination, one thing at a time

The first test I would do is defeat the PCV system by plugging or removal and see if the RPM hunting disappears. If the hunting persists, the PCV is eliminated as a possible cause and I would proceed to a carburetor rebuild including the throttle shaft seals.

[mpohrille]

Unsteady rpm

Thank u for all the great suggestions. This will keep me busy in the Winter. I will change the ignition wires, spark plugs and lubricate advance mechanism. I use Marvel mystery oi. I will also check compression. My carb was rebuilt at the beginning f the summer. I will also try your suggestion of disconnecting the PCV valve. would clamping the PCV hose accomplish this?

[ndutton]

You need to completely block off the hose that attaches to the aftermarket spacer between the carburetor and the manifold, like absolutely air tight. It might be better to temporarily remove the spacer for the test.