TR, to gain proper baseplate vac signal when setting up a carb, since there are two barrels on the carb, is it suggested to drill/tap two holes, one for each barrel, on the adapter? This may make setting a carb properly a difficult task since the two throttle bores do not mix together until the plenum.
No, not at all.
Pressure differentials (Vacuum) mixes fairly quickly in the plenum so there really isn't a reason to drill the spacer unless you don't have access to the plenum vacuum.
Most carbs have baseplate vacuum sources that sample BOTH Venturis so they are mixed before you ever see the nipple on the carb base plate.
A vac reading in each would be necessary for setting each needle, unless the two ports were to be tee'd together...
Nope, always move the needles TOGETHER, exactly the same amount.
The ONLY TIME the needles shouldn't be moved together is if you have split plenum (Dual Plane) intake and you have a air/fuel ratio indicator or exhaust 'Sniffer' probe in each side of the exhaust.
Always move the idle mixture needles TOGETHER when you are tuning with a vacuum gauge.
For determining valve issues, is drilling and tapping a port at each runner close to the cylinder head recommended?
Not really, since there are many other ways to see if each cylinder is doing it's job or not...
And taking an intake off to drill/tap is a ROYAL PAIN IN THE AZZ!
A $20 temp gun will tell you if all cylinders are exhausting the same temperature,
A leak down tester or compression gauge will tell you if you have leaking valves or piston rings,
Vacuum gauge will tell you if you have bad valve guides (and you never just pull ONE cylinder... you always do all cylinders when you do a valve job!)
Although pretty expensive (around $1,000), we have 'Bore Snakes' that can curve right down the intake and have a look at the back sides of the valves to see if there is buildup of deposits on the valve, gaps, leaking gaskets, ect...
The only time I tap intake runners is to install mass airflow sensor or individual pressure sensors when we are doing dyno testing.
It's MUCH easier than it used to be, since most intakes have bosses for fuel injectors now! No welding up our own bosses anymore!
I would believe that to be a good place for cracks to develop due to the faster heating/cooling of the alluminum manifold compared to the cast steel head and their close proximity to eachother. But then again, multi port injectors are commonly placed around that same area.
Yup, depends on if you build up a boss for support there before you drill...
Cast iron intakes are MUCH more prone to cracking than aluminum when you drill them!
Most of the time you are just checking for leaking valve guides...
If you find a leaking valve guide with a vacuum gauge,
You are more than ready for a valve job and head rebuild!
If you just find an engine misfire with the vacuum gauge,
99 times out of 100 it's a bad spark plug or plug wire.
Spark plug quality was so bad about 20 years ago, we RARELY got a set of 8 good plug in the same box...
Usually picked up 10 plugs for a V-8 engine,
And I'm still seeing a bad plug in about every other tune up we do around here.
People think I'm going overboard by resistance testing the plugs before I put them in,
And checking firing voltages after they are in, but with 1/8 of ALL your power/fuel being lost out the tail pipe,
I think it's a REAL good idea to send that vehicle out the door with all cylinders firing correctly!
NEVER, EVER, UNDER ANY CIRCUMSTANCES PRY ON THE CENTER ELECTRODE OF A SPARK PLUG!
YOU WILL BREAK THE CENTER CARBON RESISTOR IN THE PLUG BODY AND RUIN THE PLUG!
Use a pair of needle nose pliers to open the gap!
NEVER EVER 'TAP' THE GAP CLOSED!
Again, baning the plug can break the center carbon pile in the plug!
IF you drop a plug during install, THROW IT AWAY.
It's not worth $2 to loose 1/6 or 1/8 of all gasoline you burn until the next tune up!
If you get a box of plugs with a crushed corner,
REJECT THE BOX!
You can bet it was dropped before it got to you, and you can also bet the plug it landed on is shot!
REMEMBER TO USE A DAB OF 'NEVER-SEIZE' ON THE SPARK PLUG THREADS!
'Never-Seize' is Zinc or Copper based, and it helps the plug get a proper electrical ground, along with keeping the plug threads in the head where they belong!
Run a dedicated ground wire to your engine head(s)...
Spark plugs are receiving between 10,000 and 45,000 volts!
Make sure they are getting a good DIRECT path to ELECTRICAL 'GROUND'!