The below drawing shows a cross section of the carb body through where you are wanting to drill the hole and tap the existing hole. As you can see, the existing hole is 3/4" deep with a 3/8" ID. I used a standard tap and tapped only the first 1/2" of the hole.
Using a gasket making sealant I screwed in a fitting like the one shown in the following picture. 3/8" with a 3/16" barb.
Wouldnāt it be easier to put a TEE in one of the vacuum hoses?
I believe the correct answer is the location of the source of the vacuum is important.
Got it. So itās not full manifold vacuum that you want.
Correct. You want ported vacuum - ie the vacuum source needs to be blocked off when the throttle plate is in the idle position. Hence the need to drill only a small (1/16") hole which is about the thickness of the throttle plate, and the drilling need to be precise, so that it comes out at the point that is covered by the throttle plate when the throttle is closed. If you have a programmable electronic distributor like the 123 TUNE+, you can mimic the effect by using manifold vacuum and programming the vacuum advance to only kick in above idle speed - I set the cutoff for 1000rpm. That way, I was able to put a T into the vacuum pipe that feeds the front bypass valve on the 1969 model year Stromberg.
Thatās looks smaller in the pic than 3/8 for the base- the hole on the manifold doesnt look that wide
Youāre most likely looking at 1/8ā MIP. Pipe size doesnāt actually correspond to hole size. How that all came about could be the internal diameter, or maybe some strange thing thing like the width of rail track
No it wonāt. The fitting you show is described as 3/16 in. ID X 1/4 in. MIP. That is, the barbed fitting is designed for 3/16" Inside Diameter pipe which is probably rather too large, and the threaded end has a 1/4" Male Iron Pipe (MIP) thread. 1/4" MIP is the same thread as 1/4" NPT which has an outside diameter of 0.54" - much too large for this application. Maybe @John_Walker can comment on where he sourced his fitting? If I had to guess, Iād say that 1/8" NPT might be what you want (OD of 0.405" - ie a little over 3/8"). On my Strombergs, that hole (which wasnāt drilled through) was already tapped with some thread form, but as I didnāt use it (see my earlier post) I didnāt try to measure it to see what might fit.
There ya go learnt something new, never heard of this definition before.
Iād never come across MIP (male) or FIP (female) before either, so I had to look it up. Apparently, the thread dimensions are the same as NPT.
Good to hear Iām not alone with this thenš¬
I bought it at Lowes. In my post #21 above, in an effort to avoid confusion, I gave the actual measured dimensions of the fitting I used, which may be different from the size designations of the manufacturer.
For this discussion it might be helpful to see how Jag & SU approached the vacuum advance port on a S1.
Note where the port is in relation to the butterfly spindle and therefore the throttle plate.
Also note the size of the fitting.
I have a BB in the port to block it off. My Mallory distributor uses mechanical advance only.
I donāt think thatās going to work?
If my memory serves correctly, that line thatās blocked off on your manifold goes down to that little Doohickey on the carburetor. The carburetor, at that point does not provide vacuum.
The rubber tube that is blocked should go to the top of the bypass valve, which is what the second rubber tube is connected to in your picture. Where does this second Rubber tube go to now (itās out of the frame of the picture).
Some similar discussion and pictures in this thread -
I agree. The mechanic did this. First would be to see if sucking on the tube to distributor would advance anything. Then to put advance tube from distributor to the hole on the first carb intake where inside the butterfly valve would be for proper venturi effect
Early in acceleration. At least thatās how I understand it. Thanks guys.
Iām afraid not. Iāve labelled your photo below:
Prior to their modification, brass Pipe A was connected to the Stromberg Bypass Valve using a rubber elbow. I am presuming that Hose B that disappears out of sight goes to the vacuum capsule of your distributor - is this the case?
So, what they have done, is:
plug the line that carries the manifold vacuum (Pipe A) so it isnāt used for anything
Connect the Stromberg Bypass valve input to the distributor vacuum capsule
This will do nothing except, disable the bypass valve (a bad thing), and not feed manifold vacuum to the distributor.
Before I can recommend how you can fix this, I need to know what sort of distributor is fitted. If it is the original Lucas distributor with a vacuum advance unit, the source of vacuum must be āportedā (see earlier in the thread for a description), so you canāt use the full manifold vacuum from Pipe A for this purpose. You will have to drill the carb and fit a fitting as described earlier. If you have a 123 TUNE+ distributor fitted, you can mimic ported vacuum using the manifold vacuum from Pipe A by programming the 123 in the manner I described earlier. If you also wish to continue to use the Stromberg Bypass Valve, you need to reconnect it to Pipe A - I used a T in that pipe so that I could use a single source for both jobs.
If you let us know what distributor you have, we can walk you through what has to be done.
Might be an idea to have a look inside the bypass valve - there is a diaphragm that sometimes disintegrates and allows air to leak in . It`s a neat bit of kit when it works , but after 50 + years , it might need replacing ( I just disconnected it from the vac tube and tightened the spring adjuster to max to seal off the intake pipe ) .
Thanks Dave. Where on the body of the distributor is the model number? So I know if the vacuum advance even works.
