Thu Dec 18, 2003 1:52 am
I found this in a magizine last week and in view of some of the last posts I thought I would post some of the more pertinent info.
A. If your engine is healthy, it should read a steady 18" to 22" of vacuum at sea level with engine at low idle.
B. If the needle fluctuates in below normal range it could indicate a leak at the intake manifold or the manifold gasket.
C. A regular, intermittent drop below normal indicates valve leakage.
D. Rapid intermittent dropping below normal indicates sticking valves.
E. Fluctuations increasing with engine speed indicate weak valve springs.
F. Fast vibrations, but normal vacuum indicates ignition trouble. Slow movement at normal vacuum indicates incorrect carburetor adjustment.
Bear in mind that the vacuum indication with be lower with elevation. I live at 4,000 ft and the vacuum at low idle is just below 19".
The vacuum guage is quite a handy thing!
The 1/8" square head, pipe plug on the manifold, just above the carburetor is intended I assume for diagnostics. Since this is a pipe thread and someone had posted something about a hex head bolt with a machine thread, I would think it would be leaking.
Thu Dec 18, 2003 10:24 am
A vacuum gauge is a great diagonistic tool for a carbureted engine. As long as the engine will run, you can really find and correct several problems.
A good vacuum gauge is a well spent $15.00 and after you learn to use it, you'll be suprised had smooth you can adjust an engine to run.
Thu Dec 18, 2003 8:06 pm
Every internal combustion engine is essentially just a pump.
I remember seeing a exhaust manifold pressure gauge in a float plane that I had to make a short trip in about 20 years ago. If it was important enough to put on the dash of a plane, then it obviously must relay real-time and important info to the pilot.
Regarding the hex head machine screw, there is such a thing as a hex socket pipe plug - this may have been mentioned, I didn't find the original post on that issue.
1/8 NPT has 27 threads per inch. As a hex socket plug can be fairly short in length, I suppose it might be mistaken for a 3/8-24 UNF, or a 7/16-20 UNF hex socket sew screw.
The only way to be sure is to measure it, of course!
Thu Dec 18, 2003 10:26 pm
Thanks for the tech specs Donny! This will definitely help me with my fine tuning once I get this darn thing started again!
Fri Dec 19, 2003 12:09 am
Donny M wrote:
The 1/8" square head, pipe plug on the manifold, just above the carburetor is intended I assume for diagnostics.
Actually, one of the purposes of that plug was to power an old fashioned can style vacuum milker. There was an adapter to fit in the place of the plug that was a ball type valve, and the hose from the milker hooked onto it. It may even be listed in the special attatchments section of the parts book.
Tue Sep 13, 2005 5:14 pm
This thread needs to be put in the how to section of this board. Perhaps titled engine diagnosis and tune up.
The hex plug in the intake manifold takes a 1/4 x 1/8 hose barb (about $1.00). Used to install your vacuum gauge.
Site below provides a number of vacuum gaauge readings.
Tue Sep 13, 2005 8:08 pm
The vacuum gauge in the plane is there because many of the other instruments are run by vacuum - and a failure of the vacuum pump(s) could lead to a partial or complete failure of the autopilot, horizon indicator, directional gyro(s) and other nasty, nasty stuff.
Tue Sep 13, 2005 10:42 pm
allenlook wrote:The vacuum gauge in the plane is there because many of the other instruments are run by vacuum - and a failure of the vacuum pump(s) could lead to a partial or complete failure of the autopilot, horizon indicator, directional gyro(s) and other nasty, nasty stuff.
Manifold pressure gauge gives important information for adjusting the variable pitch prop. Gyro instruments can and often were supplied by a venturi. Most of my experience is with aircraft equipped only with turn and bank indicator, airspeed indicator, and a finicky compass. The rest is fluff.
Tue Sep 13, 2005 11:19 pm
Great info Donny and Eugene...Thanks
Still don't understand how to hook up a vacuum gauge to the manifold though. Does it thread into the plug hole
Tue Sep 13, 2005 11:38 pm
Will take the wife to the shop tomorrow morning to photograph my hooking up a vacuum gauge.
When I was a kid, just yesterday, we had vacuum gauges installed under the dash. The general idea was to keep the needle in the green - high vacuum - good gas milage - low vacuum foot to the floor. And gas was 30 cents a gallon.
Wed Sep 14, 2005 12:01 am
I didn't have the gauge Eugene, but used my wipers to teach me when the engine sound had enough vacuum...
Remember .27/gal. at Star Service.
Wed Sep 14, 2005 9:07 pm
I can't resist this!
For you youngsters
- when I started driving gas was 16.9 cents a gallon, the old slow rpm long stroke engines got 20+ MPG, and we weren't plagued with all the "improvements".
Before I was even driving tractor - several years prior to car driving age, gas was delivered by a guy who measured it by known volume containers and the dumped it into the farm underground tank. No pump or hose on the tank truck.
Now for the real revelation - Do I wanna go back there? Only to visit!
Wed Sep 14, 2005 9:41 pm
I remember you gave the guy in the station a dollar and the question was 5 gallons and change or shoot the buck. He cleaned the windows and you got free glasses or green stamps. I enjoy every minute of today and every minute of then. If I went back I would miss the wife and kids. Life is such a learning experience. The more I learn the more I learn I don't know yet and there is so much less time in which to learn.
Wed Sep 14, 2005 10:15 pm
I wish I could have said it that well, Bill!
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