Vacuum Gauge
 

After driving all over town to get the fittings for the manifold tee and hose barbs, managed to get the vacuum gauge installed. Using the supplied plastic hose for now, in the spring I'll change over to copper.

Strange thing, the kit came with hose but no clamps. Do they expect one to just use a friction fit to hold? For today, since I didn't have any clamps that small, a bit of rigging wire used like a bread twist tie did the trick.

Did two runs up the rpm at the dock. One in neutral and one in gear just to see the differences.

Neutral:
RPM in. Hg
700 13-15
1000 16.5
1500 19
2000 19

GEAR:
RPM in. Hg
700 13-15
1000 13-15
1500 4

Sunday when out for a sail I'll chart some more numbers and see what shows. Then I can learn what it all means! :D


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Greg, use 1/8" tubing and fittings just like the scavenge tube. Use an extra foot or so and the needle will calm down at idle.

Watch the gage as you increase and decrease throttle. Watching will give ou an idea of what is happening in the manifold.

Dave Neptune :cool:

The plastic tubing that came with the gauge is 1/8. Autometer makes a copper line kit for this with 1/8 tubing which is what i am planning to get.

It will be interesting to see the readings tomorrow and whether the gauge settles. As it was mentioned in another topic, at idles it'll bounce around which it did as you can see by the numbers. For the higher rpm it wasn't "locked in" as I have read, just bouncing about +/- .5 either side of the numbers I gave. Or am I being overly pedantic about the phrase "locked in" :rolleyes:

Funny thing, but plastic vacuum tubing doesn't need a lot of clamping. Unless it's worn, it's sort of self-sealing. The opposite of pressure tubing, obviously. E.g. the intestinal mass of vacuum tubes in my old BMW engine just all lightly connects with finger pressure, but the car sure won't run if there's any kind of leak. It leaves me feeling sort of insecure. On the other hand, the copper tube fitting connecting the carb to the manifold on my A4 had come loose and wasn't making any connection at all, yet it kept cranking away.

I haven't had a lot of engine hours since I installed the vacuum gauge. FWIW, I did motor for a couple of hours during a mini-cruise this week and noted 10" cruising at 1500 RPM with a rather dirty bottom. Will try to remember to collect some more data.

I am over 2,000 RPM at 5" vacuum.

If you read up a little on trouble shooting with a vacuum gauge, I think you will find that the bouncing around at idle is a sign of engine trouble. Old tractor mechanics use them on many small gas engines to test things. I used to have a chart that showed a bunch of indications and what they meant. They can be a good tool. Just a thought! ____Grant.