I agree with Jody. Try another gage first, at least you'll know if low compression is really the problem that way and not another culprit. 74 is just too low, especially if you're pulling 15" of vacum at idle. That tells me it's breathing okay and you'd probably be wasting time doing the dynamic check. My 331 with a .539 lift solid cam and a huge plenum crossram pulled around 7" at idle, and idle was set very high on this car, it wasn't a street car. 15" leads me to believe you're breathing fine at idle. Make sure the engine was up to operating temperature before doing the test.
Here's the dynamic check anyways.
1.Write down all the cylinders static (cranking) compression figures.
2.Hook up all your plugs except one. Put the compression tester in that cylinder and remove the Shrader valve from the tester. You can leave it in and bump it every couple of revolutions to get the same effect. Ground out the spark plug lead for the cylinder being tested so you do not damage the ignition module.
3.Start the engine and record the reading while engine is running and write it down.
4.Next snap the throttle as quickly as possible using the carb throttle linkage, not the accellerator. You want to snap it all the way open and closed without allowing it to accelerate. This allows the engine to take a big breath of air, but closes back to idle before it can respond.
5.Write down the reading noted during step 4.
6.Do this for all cylinders.
Dynamic (running) compression should be half of what the Static (cranking) compression values are, say 70-90 versus 140-180 for example. The snap throttle value in step 4 should be 80% of Static (cranking) compression.
If Snap value is much lower than 80% of Static then you have a restricted intake or supply problem. anything from an intake lobe or rocker or lifter. If Snap value
is much higher than 80% you have an exhaust restriction, same thing here exhaust valve restricted, plugged convertor or exhaust, collapsed lifter. what you'll need to determine is how many of the cylinders are high or low and that will tell you if it's an individual cylinder problem or one bank of cylinders has a problem or if they're all high or low together. The results will give you a starting point on where to look.
Like I said normal Static compression chekcs only check the sealing ability of the pistons and valves, not the breathing. Vacum gage tests look at how the entire engine is breathing, not individual cylinders.
You say you're reading 15" vacum at idle. If atmospheric is 29.92 for a standard day, (I don't know what the exact baro was the day you checked it, but 29.92 is considered the standard for horsepower and thrust ratings at sea level) the difference 29.92-15=14.92. This is what the engine is drawing in. 14.92" of mercury = approximately 7.5 psia. If you compress it at I'm guessing 9:1 considering flat tops and 76cc chambers, 7.5 x 9 = 67.4psi and this is what you'd expect to see on your Dynamic, running, compression check.
The snap test causes the manifold vacum to drop further and the air pressure in the cylinder increases.
Since all your readings are fairly close 8-10% I don't think the dynamic test is going to teach you anything, because I don't think it is an individual cylinder problem you have. I'd try a different gage and maybe look at you compression height of your pistons. Possibly ALL your valves are lashed too tightly or your cam timing is way off? It's hard to say.
[This message has been edited by SY1 (edited 09-06-2002).]