Shake shake shake....

[DDO]

Hello Afourians, Recently my engine shakes shakes shakes! I regularly run the engine at the dock, if we don't go out, as is the case in January/February. This is at the dock in neutral, so prop and shaft are not moving. She shakes at all speeds although a bit smoother at higher revs, maybe. This is an engine with quite a bit of click-click, sometimes rumble, but now suddenly she vibrates the whole cockpit. What has happened? Your suggestions welcome.

[scratchee]

Is it firing on all cylinders? You can try to remove the spark plug wires one at a time, while the engine is running (replace them before removing the next one). Each time you remove a wire, there should be a noticeable change in the sound of the engine, because that cylinder will not be firing. If there is no change when a wire is removed, that cylinder is probably not firing, and may cause vibration or shaking.

Disclaimer: If the Afourians were an army, I'd be a Private at best.

[JOHN COOKSON]

Did the vibration start after a maintenance\repair procedure? In particular any procedure that involved removal of the spark plug wires? If so check the firing order of the spark plug wires. It's easy to get #2 & 3 spark plug wires interchanged.
Also have a look inside the distributor cap for tracking (shorting) between the terminals.
Have a look at the spark plug wires at night while the engine is running to see if there is shorting between wires or shorting to the engine.
Feel the spark plugs. Are they all evenly warm after the engine has been running for awhile?
How long since the last tuneup? When was the advance last serviced? Kind of a long shot but since the engine seems to run a bit better at higher RPMs maybe the advance is stuck advanced position.

ex TRUE GRIT

[roadnsky]

Just to be clear...
This is while in NEUTRAL? The prop is NOT turning?

[Dave Neptune]

How long since a tune up?

Dave Neptune

[jbsoukup]

motor mount?

[DDO]

I did check the order of the plugs, made darn sure prop is still, etc. Here ya go: There is no change in the sound when #4 and then aftmost #3 wires were lifted off. I did not expect that, since the engine sounds about normal, just shakes! Replaced distributor cap since I had a new one. Same result. I'll get a rotor, don't have a new one, to replace that too. EI so the tune-up is mostly that. Now I think I can eliminate the problem probably not motor mounts.

[Peter]

Compression check probably a good idea

Peter

[Surcouf]

I did check the order of the plugs, made darn sure prop is still, etc. Here ya go: There is no change in the sound when #4 and then aftmost #3 wires were lifted off. I did not expect that, since the engine sounds about normal, just shakes! Replaced distributor cap since I had a new one. Same result. I'll get a rotor, don't have a new one, to replace that too. EI so the tune-up is mostly that. Now I think I can eliminate the problem probably not motor mounts.

Before getting into compression, I would try to swap wires + spark plugs around, to rule out wire or plug issues. But agree that this screams stuck valves, especially on 3 & 4, closer to exhaust. Is oil clean? no milky way?

[sastanley]

DDO..for now, just clean up the contacts on the rotor..I wouldn't just start replacing a bunch of parts hoping for a miracle cure, which is just like stabbing in the dark.

I second surcouf's suggestion. Double check the wiring, 1-2-4-3 on the plugs as you go clockwise around the dizzy.

The first thing i WOULD replace, are the spark plugs..I try to remember to do mine every season when I do an oil change.

[DDO]

Thanks for all the guidance. Today I will just continue with the spring tune-up and put in the new plugs, some MMO in the holes, a new rotor carefully checked for fit. Valves sound very click-e-ty today. Sigh... Happy Saint Patrick's, all, from Mrs. O'Connor... Laphroaig will have to wait until I solve this!

[DDO]

The symptoms remain the same after the above work, so I think the next step is to try "stuck valve" treatments as scattered around this forum. Favorite treatments? What are the causes of "sticking"? Thanks.

[Surcouf]

The symptoms remain the same after the above work, so I think the next step is to try "stuck valve" treatments as scattered around this forum. Favorite treatments? What are the causes of "sticking"? Thanks.

If plugs / wire tests failed, confirming no compression would be next step. Spray regularly and generously MMO / WD 40 etc.. trying to aim at valves. If you have access to the valve cover, you can try to lift a little more the stuck valves, it will help break the sticky gunk. Patience and spray...

[JOHN COOKSON]

If plugs / wire tests failed, confirming no compression would be next step. ..

+1
Confirming compression\no compression is the next step. In addition to stuck valves there may be a blown gasket between 3 & 4. I don't know common this is or even if it ever happens - something to think about........

ex TRUE GRIT

Did you confirm after the repairs that the plugs are actually firing? A crude method is to take the wire off the plug and put a screw driver into the spark plug boot and see if you can jump a arc to the spark plug while the engine is running. Wear gloves so you don't get shocked to badly.

[DDO]

Today I did my first compression test: 105 95 97 0 (aftmost). Sigh. Slopped some MMO in aftmost hole and left it for another day. I am not sure what is next. So, each of #3 and #4 spark plug can be lifted off without changing the engine run, but 3 had compression of 97. Does that tell me something more? I am assuming now I have a stuck valve and should put spray Seafoam up the carb and MMO down the hole and what else? I am actually enjoying learning about more parts of the engine. I've spent time in most other sides...