 1966 Mustang, 289, 2V, rough idle-low vacuum corrected
Perhaps someone can benefit from my experience to help solve a rough idle and low vacuum problem that eluded me for some time.
I had my original 289, 2V engine and heads completely overhauled by an engine machine shop (recommended to me). Immediately after I started the engine, I had trouble with rough idle and low 10-11" Hg vacuum at idle (should be 18" Hg according to spec). This continued for 3 weeks while I looked for vacuum leaks and double checked “almost” everything, like carburetor, valve timing, cam lift, valve stem clearance, etc.
Compression on all cylinders was within spec at around 150 psi – which I initially though meant all the valves were okay.
Finally I did some cranking vacuum tests. First with the carburetor still on (throttle plates closed and vacuum openings taped or plugged), I got 5-6" Hg vacuum with just the starter turning the engine (coil grounded). According to my manuals, the vacuum should have been 10" Hg for a pre-emissions 1966 engine. Second, with the carburetor off and the intake manifold ports plugged (simple tapered plugs from ACE Hardware), I repeated the cranking test and got 9-10" Hg. I didn't know what the vacuum should have been with the carburetor off, but felt it should have been much higher than 9-10".
Long story, short; – The engine builder installed valve springs that were 40 lbs in lieu of 80 lbs at assembled height. Plus a lot of carbon was built up on the valve seats by this time.
With new springs and reconditioned valve seats, I installed the heads and repeated the cranking vacuum test with the carburetor off. This time I got a cranking vacuum of 20+” Hg.
The engine idles smooth now with a vacuum of 15-16” Hg. This still might be a little low, but I have not yet fine tuned the idle mixture screws and ignition timing. Plus I do have a mild performance cam so the vacuum could be an inch or so lower than the 18” specified at idle.
So if you can’t track down the reason for a rough idle and low idle vacuum, I recommend checking the cranking vacuum with the carburetor off. | 
