[HO455]

Over the last 2 weeks or so I have been doing some more tuning on the old Qjet. The secondaries were super slow to open, so slow sometimes I wasn't even be aware they had opened. Being that the carburetor is a 1978 vintage I would be surprised if it they weren't slow to open. Those were the years of lean carburetor mixtures and low numerical gear ratios, every thing you need to get an engine to detonate itself into scrap metal. So GM used choke pull offs with a very slow bleed off to help keep engines alive. Today with my motor, these conditions don't exist, so I want quicker secondaries!
So I used a vacuum tester (Similar to photo # 1) and hooked it up to the choke pull off to get my base line. To hook it up I removed the hose to the choke pull off where it hooked to the carburetor (Blue arrow photo #2). I then hooked that hose to the clear hose from the tester (Yellow arrow photo #2. Please forgive the bad photo the connection of the black hose from the pull off and the clear hose from the vacuum pump is behind the guage. Dhoo!) and then pumped until the pull off was fully retracted (red arrow and notice 8 inches of vacuum photo #2). Once I opened the vacuum release I timed how long it took for the pull off to return, i.e bleed off. (The choke pull off has a diaphragm and a spring inside and vacuum compresses the spring. Remove the vacuum and the spring returns to the normal state. (See photo 3 red arrow and zero vacuum) By repeatedly operating the pull off and timing it I came up with a base line number of about 3.0 seconds to bleed off.
That means every time I punched the throttle I had to wait 3 seconds to get full throttle. In comparison a top fuel dragster covers the 1/4 mile, (Well 1000 feet these days.) in 3.6 seonds. Painfully slow!
The speed in which the vacuum bleeds off is controlled by an orifice inside the pull off just visible through the tube where the vacuum hose hooks up. (Photo #4) The larger the hole the faster it bleeds the vacuum off. As you can see it is a very small hole. So small in fact I was unable to accurately measure it. My best guess is that it is about .010. (The amount of work it took to drill from .0135 to .0145 was noticeably less than when I drilled the .0135 hole that and the small change in bleed off between .0135 & .0145 leads me to my estimate of the original orifice size.)
To modify the pull off I needed to drill the orifice larger using my smallest drill bit, a #80 .0135. First I removed the pull off from the carb and clamped it in a small vice. Having chucked the #80 in a pin vise I slowly drilled the orifice out to .0135. (It took like 10 minutes. Being very careful not to break the bit off in the orifice. ) Hooking up the tester back up I measured the bleed off time. Now it was 2.2 seconds. Progress in the right direction but still more to go. So I went to a # 79 bit .0145. The .0145 gave me a slightly faster 2.0 bleed off. Next up was .016 a #78 bit. Now that gave me about a 1 second bleed off. I decided that was good for now and reinstalled the pull off on the carburetor for some on the road testing.
The last photo shows my results and next to each result is the bit used (they look like pencil lines on the paper.) I actually made a video of the bleed off times only to find out I can't post them.