Random mis at idle, gets worse at speed

[SSTKO81]

Over the last year or two I’ve noticed a random mis at idle. It doesn’t usually start happening until the engine warms up, but then will happen every 2-5 seconds. As I idle up, it will become more pronounced. But if I get about 2500 RPM it becomes terrible to the point the tachometer bounces all over the place. - this is on a 1981 Mercruiser 140. Ignition system was upgraded away from points to a voyager a few years back and worked great initially.

 

[Bt Doctur]

could be from a bad manifold or riser leaking into a cylinder

 

[jimmbo]

How old are the Plug Wires? Could be Induction Crossfire. Is the Distributor Cap free of any Cracks, Carbon Tracks. Inside as well as Outside?
Heat on the Electronics may also be a Factor

 

[Scott Danforth]

in addition to above

could be old fuel, could be water in the fuel, could be timing is off.

 

[nola mike]

Or mechanical problem, burnt valves etc. I'd start with a compression test and check spark.

 

[kenny nunez]

I would not be surprised if your distributor was not worn out. There is only 1 bushing under the breaker plate. Remove the distributor and take it to a machine shop. They will be able to replace the top bushing and add a lower one.
Also look closely at the fly weights and pins that support them. The support pins must not be loose.

 

[SSTKO81]

How old are the Plug Wires? Could be Induction Crossfire. Is the Distributor Cap free of any Cracks, Carbon Tracks. Inside as well as Outside?
Heat on the Electronics may also be a Factor

I can confirm the plugs are a couple years old, but the wires are older, and one, I found has a cracked boot. That’ll be the place to start. Fuel and filters are good, timing is dead nuts and holds steady. Spark is nice and strong but last I checked compression is a little low at 115-120, but even on all cylinders.

 

[Scott Danforth]

Get a different gauge and check compression again

115-120 is rebuild time

 

[alldodge]

point the tachometer bounces all over the place

Disconnect the Gray (Tach) wire from the negative side of the coil

 

[SSTKO81]

Short video

 

[alldodge]

Disconnect the Gray (Tach) wire from the negative side of the coil

Having EST ignition, disconnect Gray wire on the Tach

 

[Mc Tool]

Disconnect the Gray (Tach) wire from the negative side of the coil

Wouldnt be the 1st time a faulty tacho has caused issues , the fact that the needle bounces about also points in that direction .

 

[SSTKO81]

Wouldnt be the 1st time a faulty tacho has caused issues , the fact that the needle bounces about also points in that direction .

Well I can’t imagine, it’s only 45 years old haha
I’ll give it a shot.