Speedometer

I have a 1975 Mustang that has the old single speedometer pitot. It's solid brass, mounted forward of where the drive shaft passes through the hull of the boat.

It works great with my Airguide speedo when I'm going over 20 mph. Often, however, when I first start out, the speedo reads '0' until I hit about 20 mph, then it jumps to the right speed. If I then slow down below 20 mph it will read all the way down to 5 mph accurately.

What is the problem? I removed it and cleaned it very well at the beginning of last season but I can't do that every time out. I've thought about replacing it with a typical stern mount pitot but I'm not excited about filling in the hole for the old pitot and I'd rather keep the boat as original as possible.

With that said, having the speedo accurate sub 20 mph is critical for kneeboarding, tricking and wakeboarding.

I'd appreciate any help you could offer.

-Mike

On the topic of gauges, has anyone had experience restoring an old Airguide Tach? If so, details would be appreciated!

Thanks! Sorry to hear about the bearing...
The speedo in my 65 barracuda works pretty well- It had a pin hole in the copper line a couple years ago causing the gauge to read very low and water to enter the bilge area.I cut out a section of copper and used compression fittings to attach the new copper line in place. Now the speedo works accurately when it works, which is intermittent-