Had a Victor jr intake on my 74 [see my sig]. The car ran very well and up to my expectation with regard to the total package. The rpm limit was around 5800 - 6000 rpm. Which would not be the ideal range of this single plane intake. Over the winter I needed to pull my intake and check my roller lifters for ware, so I decided to replace the V jr for a RPM Air Gap intake and get an increase in low end torque and better driveablility. Maybe even and increase in my Q mile mph. I finished the swap leaving everything else the same. On Sunday in between snow storms I test drove the car for the first time. I was very disappointed. The car stumbled coming off idle. It did not pull as well as the Vjr from 1000rpm in 4th gear. I checked the vacuum at idle and it registered 17 in instead of the 19-20 in I am accustom. I will fool with setup and see if I can find a vacuum leak and check the carb and timing, but I am ready to change back. Should have let a good running comination be. Any suggestions on why or how to fix?
Ed

 

I used a single plane intake on my various 355 ci for over ten years. Then for an experiment I tried a couple of dual planes "RPM" and "Stealth" The only thing that I noticed was jetting leaner on the primary side because of increased carb signal. I had port size mismatch and went back to single plane.

IMO - You have something else going on because vacuum should not change. You will need to lean the idle cuircut and drop the primary jetting 1-2 numbers.

With your vacuum gauge connected and motor at temp. I leave the vac advance connected for the highest smoothest idle. My Demons have 4 corner idle screws so I set then all to 2 turns out because I know that with my 1/8 inch holes drilled in the primary throttle blades that at @1 1/2 turnes out is going to be correct. I fire the motor up. I turn the idle speed up to 1100 rpm by cranking on the secondary plates.

Then in equal small units I crank all of the four corners. As you get closer to perfect A/F ratio the idle and vacuum both go up. So you crank the secondard idle speed screw down to keep the rpm at 1100 or slightly less.

Once you get the highest reading you just crank the primary idle speed screw down to loose a couple hundred rpm and then the secondary screw to get back to your typical idle. Mine just happens to be 850 rpm.

Just as a foot note I've driven with my vacuum gauge attached and I never see 20 inches unless i'm using compression braking coming off a mountian.

 

gkull, I can always depend on you for a good direction. I know you dont like V jr intakes, but you cant believe how nice this combo ran. I know I did not give this set up a chance as I only drove it about 7 miles. I was really checking for leaks and loose fastners. But I could not keep my foot out of it and needed to see how much difference there was.
My distributor is a Mallory unit without vacuum advance. I was going to lean the carb out, check for loose fittings and retorque the intake. I know that the vacuum of 20 in is high for a performance motor, but this thing pullsa strong vacuum even while I am driving. I had to put a 10.5 power valve in the carb. I also got 17 mpg on the highway without a vacuum advance on the dstributor. I will continue to analyze this new set up. It is difficult this time of the year as the weather here has been terrible for driving a vette.
Ed