Re: vacuum reading
Acquire/borrow/rent a screw-in compression gauge.
Remove all spark plugs after bringing engine up to operating temp.
Hold throttle to wide open position.
Unhook ignition power supply, either the power wire or unhook the wire going to dist from coil.
Make a chart 1-3-5-7 on one side, 2-4-6-8 on the other side.
Screw in gauge into spark plug hole and crank engine. Stop cranking when gauge reaches a max indication. Write reading down next to each cylinder number on chart made.
Continue for seven cylinders.
1. Overall readings should be within 10% from lowest to highest.
2. One low cylinder out of range may be broken rings, misadjusted/burnt valves. Squirt oil into low cylinder, compression comes up, bad rings - oil sealed broken rings--compression stays the same - misadjusted valves or burnt valve , or maybe a worn cam lobe not opening valve as much as other valves.
3. Two adjoining cylinders same reading and lower than the rest - blown gasket between the cylinders.
4. If compression readings are good, might be an ignition problem for one cylinder, leaky manifild gasket. A quick check of exhaust manifold temps with a temp gun should show 500°F temps min. Any reading lower and you have a dead cylinder.
Be sure to release WOT when done.
Fluctuating vacuum reading may be due to loose intake, leaky hose - large hole, worn cam lobe. Might remove valve cover and witness all rockers moving the same distance during cranking. Any running engine should have a solid vacuum reading regardless of camshaft installed unless you have a 0.700" lift cam at 500 rpm idle - not quite possible.
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