Hoping not to be lambasted by other members, I will offer a way to check whether the intake gaskets are sealing or not. I was thinking about this the other day because of all the posts I see on this and other forums concerning internal vacuum leaks.
The best way to eliminate leaks in the first place is to make sure the mating surfaces of the heads/intake are parallel in both planes when assembling the motor like this:
Occasionally, you may experience a vacuum leak into the intake ports from the crankcase of the motor due to the intake manifold / cylinder head interface not being machined parallel. No amount of propane or carburetor cleaner will find such a leak. The best way is to insure the elimination of such a problem while building the motor.
Here's how I set up the intake manifold/cylinder head interface to prevent vacuum leaks from the crankcase to the head ports.... Measure the thickness of a new intake manifold gasket. Get flat washers or shims that will measure that thickness. With the manifold off and the mating surface on the cylinder heads de-greased, put a dab of RTV on the washers/shims and stick them on each corner bolt hole on the cylinder heads. Let the RTV set up. Stuff paper towels into the ports to keep debris out. Make up 16 pea-sized balls of modeling clay. De-grease the intake manifold at the ports. Place the balls of clay on the top and bottom of each port of the manifold, squishing them down well so they stay in place. You want them to be thicker than the shims/washers that are RTV'd to the heads. With your fingers, coat a little oil on the heads where the clay will meet the heads to keep it from sticking to the heads. Now carefully place the manifold into place on the heads and use bolts on the four corners to just snug the manifold down until you feel resistance against the shims/washers. Remove the manifold carefully and measure the thickness of the clay at all 16 positions with the depth function end of your 6" dial caliper. You'll know pretty quickly if the manifold/head interface is square. Record the measurements on the manifold with a permanent marker like a Sharpie. The widest measurement will be the standard to which you will want your machinist to cut the other positions on the manifold to make it square with the heads, thusly sealing up the motor.
Now, with the motor already assembled, I was trying to envision some way to check the gasket seal. Here's what I came up with:
What if you made a steel or aluminum plate to cover the carb mounting hole in the intake. Drill one 0.453" hole in the plate to accept a tubeless tire valve. Drill another hole and tap it to accept a 0-15 psi pressure gauge. Back off all intake rockers. Use a bicycle tire pump and attempt to slightly pressurize the manifold plenum/head ports. If you couldn't get any pressure built up to stabilize the gauge, wouldn't that indicate that there is lack of seal somewhere?
Like I said, please don't flame me for this, I'm just trying to come up with a workable solution for Joe Scratchrubber who may not even have shop air at his garage.