Vacuum test Help

[nateo]

ok, so I've been having some ocassional backfiring through carb. I took carb apart and inspected it, not dirty at all. I put it together with new gaskets and adjusted the idle/air mixture screws and backed off 2 1/2 turns from the bottom. Set idle screw to 650-700 rpm (seems to fluctuate a tap bit not sure if this is normal). I then set the base timing right at 8 degrees.

I bought a vacuum gauge and the necessary fittings to hook up to intake. Ok so here's my question. Should the neddle on the gauge be very steady? My seemed to vibrate bounce. Also it fluctuated between 9 and 10. Is that normal? When I adjusted the mixture screws up or down it didn't seem to have an effect on the gauge or notice a difference in sound of engine.

Engine runs a lot better now, no major backfiring but I have noticed it missing a few times here and there. Hard to tell for sure because even before I did any work sometimes it would run pretty decent and sometimes it would run like crap.

 

[dubs283]

Engine runs a lot better now, no major backfiring but I have noticed it missing a few times here and there. Hard to tell for sure because even before I did any work sometimes it would run pretty decent and sometimes it would run like crap.

sounds more like a fuel supply issue, junk still in the carb bowl, possible blockage of the fuel pick-up in the tank intermittently

vaccuum issues are usually constant - not intermittent

 

[NHGuy]

try this

CHART OF VACUUM READINGS​
Smooth and steady idle (800 to 1200 RPM)	Between 17 to 21 inches	Engine is in Good Condition, but perform next test to be sure.
Open and close throttle quickly	Jumps from 2 to about 25 inches	Engine is in Good Condition.
Smooth and steady idle	Steady, but lower than normal reading	Worn rings, but perform next test to be sure.
Open and close throttle quickly	Jumps from 0 to 22 inches	Confirms worn rings.
Steady idle	Intermittent dropping back 3 or 5 divisions and returns to normal	Sticky Valves. If injection of penetrating oil into intake manifold temporarily stops pointer from dropping back, it's certain the valves are sticking.
Steady 3000 RPM	Pointer fluctuates rapidly, faster engine speed causes more pointer swing	Weak valve springs.
Steady idle	Fast fluctuation between 14 to 19 points	Worn intake valve stem guides. Excessive pointer vibration at all speeds indicates a leaky head gasket.
Steady idle	Constant drop	Burnt valve or insufficient tappet clearance holding valve partly open or a spark plug occasionally miss firing.
Steady idle	Steady 8 to 14 inches	Incorrect valve timing. It must also be remembered that vacuum leaks and/or poor compression can result in a low vacuum reading.
Steady idle	Steady 14 to 16 inches	Incorrect ignition timing.
Steady idle	Drifting from 14 to 16 inches	Plug gaps too close or points not synchronized..
Steady idle	Drifting 5 to 19 inches	Compression leak between cylinders.

Engine Speed Reading Indication of Engine Condition