Vacuum Source - 1958 Dual Quad

[Thomas H.]

In the process of tuning my freshly rebuilt dual quad 283. Went to connect my vacuum gauge.............

No external vacuum source anywhere?

Ideas??

Tom

[Richard M.]

Re: Vacuum Source - 1958 Dual Quad

Tom, On the rear of the Primary carb is a Pipe Plug. Remove that and use a adapter to connect to the gauge hose and when done just replace the plug. That port is full time manifold vacuum.

Rich

[Thomas H.]

Re: Vacuum Source - 1958 Dual Quad

Tom, On the rear of the Primary carb is a Pipe Plug. Remove that and use a adapter to connect to the gauge hose and when done just replace the plug. That port is full time manifold vacuum.

Rich

Hi Rich,

Thought that would be the case, but didn't see one last night when I was poking around for a source. I know the pipe plug you refer to as I had left it out of my 60 when I rebuilt that carb - didn't run very well, but it also didn't take very long to find the source of the problem either!

Sounds like the "correct" carbs I bought may not be so correct............. Great.........

I'll take a look again tonight.

Tom

[Frank D.]

Re: Vacuum Source - 1958 Dual Quad

If you find you DO have a vacuum fitting in either carb then this Paragon piece is the way to go: https://www.paragoncorvette.com/p-35...m-fitting.aspx

It is in two pieces so you can install/remove it with the carb on the car...
If your car had the dual point distributor with no vacuum advance the there may not be a vacuum 'take off' on the carbs....
Bob Kunz drilled one into the base of my dual quad setup for me to use...

[Thomas H.]

Re: Vacuum Source - 1958 Dual Quad

Nope, no plug on either carb. Maybe shouldn't have one? I don't want to put one in if there shouldn't be one.........

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[Thomas H.]

Re: Vacuum Source - 1958 Dual Quad

If you find you DO have a vacuum fitting in either carb then this Paragon piece is the way to go: https://www.paragoncorvette.com/p-35...m-fitting.aspx

It is in two pieces so you can install/remove it with the carb on the car...
If your car had the dual point distributor with no vacuum advance the there may not be a vacuum 'take off' on the carbs....
Bob Kunz drilled one into the base of my dual quad setup for me to use...

Neat idea........

[David Z.]

Re: Vacuum Source - 1958 Dual Quad

Tom
Here is the fitting that Frank referred to above. I've had this in our '60 since we swapped the vac advance distributor for the dual pt that was in it when we got it.
Dave Z

Paragon Fitting 755.jpg

[Richard M.]

Re: Vacuum Source - 1958 Dual Quad

So I'm unsure now about the 1958 WCFB's. I thought they all had the rear fitting on the base. If yours are not supposed to have the fitting, there is yet another solution.....

Install one of these hollow studs in the manifold to replace one of the carburetor mount studs, typically at the rear of the Primary carb. When you're done with it just replace the original stud and back to normal.

http://www.zip-corvette.com/catalog/product/view/id/8047/s/58-62-carburetor-intake-mount-hollow-stud/

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[Frank D.]

Re: Vacuum Source - 1958 Dual Quad

Right where I was going Rich - CC has them too...

http://www.ecklerscorvette.com/corvette-vacuum-advance-carburetor-stud-hollow-300hp-1962.html?cm_mmc=google-_-pla-_-25-249009-1-_-NA&gclid=CKPqrovwnskCFQQJkQodUisEUA

[Richard M.]

Re: Vacuum Source - 1958 Dual Quad

Yes it's a little bit of a pain but I think it'll be the solution.

Tom, If you get one just make sure the hole in the intake you use is not a blind hole. I know I have photos somewhere of a 2x4 intake backside but i cannot recall if they're blind or open or a combo of both.

I'd also hacksaw-cut a screwdriver slot on the top of the hollow stud to make it easier to get in-and-out whenever you want to connect your gauge to it in the future.

I did a base engine '59 a few years ago and it previously had a vacuum advance distributor on it. The (NTFP) configuration it had used that hollow stud. Worked nice.

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[Frank D.]

Re: Vacuum Source - 1958 Dual Quad

I added RPO-242 to my '61 (California PCV option) so my carb legitimately had a vacuum fitting in the base...
You can run a wire down the carb stud hole and measure to see if it makes it into the intake runners; it should be pretty obvious.

Just don't forget to cap off or remove that hollow stud when you're done tuning...or you'll have a killer vacuum leak...

If you have the Winter's "figure-8" manifold there is also a theory that these two bolt holes (see red arrows in pic) lead to a vacuum source so you could
possibly rig something up there too. I've never confirmed it but have heard it quite often!

[Thomas H.]

Re: Vacuum Source - 1958 Dual Quad

Frank,

I do have the Winters manifold - those holes on my intake are flat bottomed.

Tom

[Thomas H.]

Re: Vacuum Source - 1958 Dual Quad

Yes it's a little bit of a pain but I think it'll be the solution.

Tom, If you get one just make sure the hole in the intake you use is not a blind hole. I know I have photos somewhere of a 2x4 intake backside but i cannot recall if they're blind or open or a combo of both.

I'd also hacksaw-cut a screwdriver slot on the top of the hollow stud to make it easier to get in-and-out whenever you want to connect your gauge to it in the future.

I did a base engine '59 a few years ago and it previously had a vacuum advance distributor on it. The (NTFP) configuration it had used that hollow stud. Worked nice.

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Rich,

That thought crossed my mind. My 60 has this setup for the vac adv on the distributor. (BTW, the set up on the 60 is stock, so that hollow stud was how it came from the factory).

Tonight I'll check the carb stud holes and see if any can be used as a port.

Had the car running last night on the engine analyzer looking for the cause of a misfire above 2k rpm - looking at the spark waveforms it appears that my new reproduction coil is a bit on the weak side - go figure, another questionable reproduction part. Condenser (new also) tests a bit on the leaky side. Could be a combo of both. I have a new NAPA coil I'll put in tonight along with a new condenser to see if it improves anything.

Thanks,

Tom

[Thomas H.]

Re: Vacuum Source - 1958 Dual Quad

I
If you have the Winter's "figure-8" manifold there is also a theory that these two bolt holes (see red arrows in pic) lead to a vacuum source so you could
possibly rig something up there too. I've never confirmed it but have heard it quite often!

Well what do you know.......... The rear most threaded boss is flat bottomed, but the forward one isn't!. Open right into the #7 intake runner.

Guess who was running the engine with it not plugged and wondering why there seemed to be a misfire......... Duh!

I'm going to plug it for now and continue on my tuning. Still have a coil problem that I hope to resolve tonight. Maybe I'll take my first drive around the block tomorrow!

Tom

[Frank D.]

Re: Vacuum Source - 1958 Dual Quad

Yup - I've had a couple of C1 pals that discovered those two weird bolts were the cause of some vacuum problems....several theories about what they were for. The NAPA IC-12 coil works great on these cars and reasonably priced...