Vacuum Gauge on a 4 cyl flathead

jethrow4 · Mar 17, 2015

I want to hook up a vacuum guage to 4 cyl
flathead that uses a one piece intake/exhaust
manifold and an updraft carburetor. I'm
considering drilling and taping the manifold
for a threaded fitting. This manifold has the
embossed "Oliver" unlike plain manifolds with
a raised boss.
Alternative would be to use a carburetor raiser
plate but there is not enough clearence beween
the bottom of the carb and the magneto. Also
not sure if the readings would be accurate that
close to the carburetor throat. I'm looking to
diagnose an intermittant miss and check the
overall condition of the engine.
Has anyone ever tried this would it be
effective?

jimg.allentown · Mar 17, 2015

A vacuum gauge will tell you something about the overall condition of the engine, but not much about an intermittent miss. Vacuum at or near the throttle plate may be the most accurate place for checking.
BUT.... you can tell a lot about the general condition of the engine by looking at the crankcase vent. Not much blow by? Engine is in good shape. Lots of blow by? Engine getting tired. Personally, I would not go to the trouble of installing a fitting just for that purpose. A compression test will also tell you a lot. Condition of the exhaust will also tell something - is it oily? Sooty? Or relatively clean with a light coating of exhaust deposit?
An intermittent miss can be caused by many things -
Carbon buildup on the valves.
Weak valve springs.
Worn valve guides.
Distributor/mag issues - like worn shafts, etc.
Lean mixture.
Timing retarded.
Leaky ignition wiring.
And, the list goes on. Some of these things will show up on a vacuum gauge, but most will require some secondary diagnosis to pinpoint.

Txturbo · Mar 17, 2015

sounds like a lot of trouble just to add a vacuum gauge for troubleshooting. A wet and dry compression test will tell you what you need to know as far as engine condition but nothing in regard to an intermittent miss. That would be an ignition/ timing problem. Wait till it's dark and crank it up. Look for stray sparks as you move the plug wires around with a non conductive tool.

old · Mar 17, 2015

Cletarc OC-3 right?? A miss like that can be many things and while a vacuum gauge may help it is not likely to be worth the trouble to install. Plug wires can cuase that as can a bad plug or bushings in the mag or a valve that sticks from time to time

black · Mar 17, 2015

When I suspect a vacum leak , I just sniff with an unlight propane torch to see if the miss will stop , if not the it's something else .

Larry --ont.

jeffcat · Mar 17, 2015

Yes plug wires, plugs ,and or a bad cap where the spark runs around and cross fires. Well loose shaft in the magneto too.

Vacuum Gauge on a 4 cyl flathead

jethrow4

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jimg.allentown

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Txturbo

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old

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black

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jeffcat

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