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Author Topic:   Ailing speedometer
Chip posted 04-10-2002 08:38 PM ET (US)   Profile for Chip   Send Email to Chip  
My speedometer has'nt worked in tqwo years, and this year I'm determined to have a working speedo. I found that the pitot tubing was clogged and fixed that. Should I be able to get a reading on the speedo just by blowing into the tube? I know..they're cheap, buy a new one!
Tom W Clark posted 04-10-2002 08:49 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     

You have to blow VERY hard to get a reading. A new pitot and new tubing should do it. If the tubing is clogged, replace it.

Also, you cannot effectively splice speedometer tubing. The slice will restrict the flow to the point where the speedometer will work but will be very sluggish in its response time.

Tubing is cheap. Start fresh.

Chip posted 04-10-2002 09:19 PM ET (US)     Profile for Chip  Send Email to Chip     
Thanks Tom!
newboater posted 04-11-2002 12:40 PM ET (US)     Profile for newboater  Send Email to newboater     

I'm going to have to call you on your comment about splicing the speedo tubing.

There really is zero flow through the speedo tube. It only senses/transmits pressure, and the tube should only have air in it unless there is a problem (leak) at the instrument. If there is no flow through the tube, there are no pressure drops associated from restrictions like a splice.

Unless the hole in the splice nipple is very, very small, there should be no visible effect.

Sometimes though what is supposed to happen and what really happens are two different things. I can't give an explanation for a slow change/reading unless you maybe had liquid in the hose, or a clog somewhere else.

Dave S.

where2 posted 04-11-2002 07:32 PM ET (US)     Profile for where2  Send Email to where2     
You can however check them with a blast of compressed air from your air compressor, blowing across the pitot tube... Start with around 20psi and your blow gun at about 2" from the pitot tube. With someone watching the speedo, blow, and move the gun closer until the needle picks up.

I still prefer to put the boat in the water to test it out. Much more fun to go boating, and if the speedo doesn't work, that's a good excuse to go boating again... :)

reelescape1 posted 04-11-2002 08:12 PM ET (US)     Profile for reelescape1  Send Email to reelescape1     
Please elaborate....I have a '02 Yam 225 w/digtal gauges (on a 22 OR of course!) and the tube to the speedo has some water in seems to work OK but, I have noticed after a hard run it tends to indicate about 9 mph or so at idle. I realize this is a little different, the pick-up for this is not mounted on the transom.
Chip posted 04-11-2002 09:07 PM ET (US)     Profile for Chip  Send Email to Chip     
Thanks everybody,I'm gonna take where2's suggestion-go boating!
Tom W Clark posted 04-12-2002 01:00 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     

You are correct that a pitot speedometer does not rely on flow through the hose but none the less it does rely on a transfer of pressure from the pitot tube to the speedometer itself.

It would not seem that a slight restriction of the tubing would make any difference and this is what I myself assumed when I, at one point, extended the length of my tubing because of a relocated pitot.

It worked very poorly after splicing the line, so it is from experience that I speak.

I used the spice part that comes with an outboard water pressure gauge, which works the same as a speedometer does but with the water pressure supplied by the cooling system of the motor. According to the the instructions supplied with the water pressure gauge, the splice part is to be used if the motor is removed from the boat thus making it easy to reconnect it later. I figured if it would work for the pressure gauge it would work for the speedometer.

The result was that the speedometer was unacceptably sluggish to respond to changes in speed. This was back when I was doing a lot of slalom skiing and the speedometer was actually an important tool for maintaining the boat's speed. When I say sluggish, I mean it would take several seconds for the speedometer needle to come up to speed after the boat was already there so fine tuning the boat's speed was a hassle.

I replaced the hose with a new single piece length and the problem was solved. No kinks were in the line. Water in the tubing itself will have no effect whatsoever one way or the other. If you don't believe me, try it.

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