Slow speedometers

On my restoration project boat I'm moving on to the little things now. I have two NOS Airguide speedos that I got from Mr. Mabe some time ago (THANK YOU, Marty!). Out on the water they are:

A) very slow to respond.
B) erratic in functionality (sometimes they don't work at all).
C) give different readings at different times (sometimes the one on the left is showing a higher speed, at other times the one on the right)

What's the most likely culprit here? Before you answer I'll provide more data:

The pitot pickups, brass tubes (whatever those things are called), and the tubing all have 1000's of hours on them. I've cleaned out the plastic pitot pickups from the bottom. I might try running a wire or something down from the top, but I think they are clear.

My theory is that "stuff" is somewhere. Either in the brass tubes, or in the air lines running to the speedometers. If it is the tubes, I could remove and either replace or clean them out (or remove them altogether). If it is the lines I can purchase replacement tubing from N3 or other site sponsors.

Or, could I be missing something else? Help me, friends. I don't want to randomly replace parts hoping the problem goes away.

Kevin

I have the same problems every now and then. Take a good look at the hose where it sits at the pitot. Dry rot cracks sometimes form there aggravated by sun exposure.

Kevin,
Take a good look at the brass pulsation dampeners. If the have ever been plugged, they will hold water that will freeze and split them open. A solder job on the split is the answer.

8122pbrainard wrote: Kevin, Take a good look at the brass pulsation dampeners. If the have ever been plugged, they will hold water that will freeze and split them open. A solder job on the split is the answer.

Good thought. I wonder if I can get back there without removing the fuel tank. If not I can investigate my fuel level sender resistance levels at the same time and kill two birds with one fuel tank removal.

Nothing but problems these old speedometer systems are.