My understanding is the only difference between ported and manifold vacuum is the location of the source in relation to the throttle plate in the carburetor. Ported vacuum comes from a tap up stream of the throttle plate and manifold vacuum is sourced from behind the throttle plate according to the discussions I’ve read on the internet.
My reason for bringing this up? I received my Petronix distributor last night and the instructions said to connect the vacuum advance to a ported vacuum source. To the best of my knowledge the Stromberg carbs don’t have a ported vacuum tap. So, any suggestions on how to obtain ported vacuum from the Stromberg setup?
FWIW John…when I installed the EDIS, Ray’s instructions are to
install a T where the brake vacuum valve is on the reservoir so that the
system could read when the engine is under load.
Ray might want to comment on whether this location would suffice for your
application.
John;
I have a '70 E Type with Z-S carbs in original configuration except I removed the secondary throttle plates this spring. I had the distributor changed from vacuum retard to vacuum advance when I had the distributor rebuilt a couple of years ago.
The front carb has a vacuum port on the under side of it’s manifold flange that serves as the vacuum source for the distributor. I think this is a ported vacuum source.
I’m pretty sure the vacuum port that was used for the vacuum retard distributors (on the underside of the flange that connects to the manifold) gets full manifold vacuum, not ported vacuum (i.e. they are on the wrong side of the throttle plate). If John’s car is anything like my '69, he probably doesn’t have this port anyway, as the 69’s used a distributor without vacuum advance or retard. However, the earlier '69 carbs (at least mine anyway) do have an undrilled port on the top of the manifold flange that I believe was used for ported vacuum in other applications. The port is tapped with what may be a 1/8" NPT thread:
You can see the threaded ports in square boss in the center of the flange above the throttle plate of each carb. A detailed photo below shows the threads in the port clearly:
In this photo you can see that there appears to be solid material below that port, which could be drilled to provide say a 1/16" hole through to the area just the other side of the throttle plate. The hole might need to be angled back to ensure it emerges on the right side of the plate. I found some of this information in a post by Andy Blackley back in 2011. You may find more with a similar archive search.
There are three main vacuum pickup points associated with the ZS carburettor and the easiest way to see them is on the final 1974 cars.
The first conventional one will be manifold vacuum - measured after the throttle butterfly and being what the cylinders actually see.
The second is the ported vacuum location as described above by David. If you look at the emissions carburettors with EGR for 1974 then you’ll see that this is where that connects and you’ll see a small drilled hole (from memory about 2mm diameter).
Lastly, the v12 used a vacuum retard capsule which operated at idle only and the pickup for that was on the bottom of the LH rear carburettor.
The Ford EDIS used by Ray and others will have an ignition map covering the whole range of vacuum from atmospheric to full vacuum. It’ll expect to see manifold vacuum, not anything ported or ameliorated by other factors.
John W, This topic was posted on the forum at least 5yrs ago by a Brit with a Daimler double six ( ie V12 powered saloon ), and sorry to say his name eludes me…maybe someone will remember.Its also likely that his webpage is gone anyway. Based on my partial recollection he drilled a hole in the carb body on the upper lip by the throttle plate so that it became effective immediately the throttle moved. That’s the history part. Now to stuff I know from the ZS carbs from a 1974 XJ12 saloon, which were given to me 15 years ago atop a spare V12 engine that I dismantled for parts.Each of the 4 is unique is some manner, depending on its particular job! One has ports at both 12 and 6 oclock re the throttle plate.The upper one is 3 or 4mm wide and just visible with a fully closed throttle, and i think its for the dizzy vacuum. The lower hole is 1mm and 4mm inside the closed plate.I don’t know what this is for!? I can’t post pics here as the logic is beyond me…just getting signed up was a task! If you wish send me an email RegardsJohn H
Someone is working on a guide for posting pictures, in the meantime use the upload button in your composer window (where you type your message), it looks like this:
Thanks to everyone. This is really good info. I’ll be messing with the distributor installation this afternoon.
And thanks to Gunnar and the whole team who put the new forum together. The ability to add pictures directly to the posts like David Langley did really enhances the knowledge transfer.
I have a 1970 E-type with the secondary inlet manifold and dual Strombergs. Somewhere in the car’s past, someone installed a later model Lucas electronic distributor with a vacuum unit (I presume it is an advance unit rather than retard). Vacuum to the distributor comes from the rear of the inlet manifold (near the firewall). There is also a vacuum source behind the front carb throttle plate which goes to the thermo switch on the rear of the air cleaner box. The car runs fine with this setup other than I’m chasing a high-speed miss which I don’t think is related to the vacuum system.
You mean a blue cap XJ6 one with separate AB 14 amplifier box? That should be vac advance. The way to tell is if the pipe stub connection for the vacuum tube is facing the distributor (retard) or pointing away from the dizzy (advance). It is .possible that someone strobed your car to a stock idle advance while hooked up to manifold vacuum. That would make it mechanically very retarded to come out stock under ‘incorrect’ full manifold idle advance). If so, it would likely be timed pretty late at higher revs, because the retarded distributor would have no compensatory vac advance under load, only at cruise, when it would be timed similarly to an engine with no vac advance.
Peter - Your description of my distributor is accurate. It has a blue cap and a separate amplifier box. The pipe stub points away from the distributor. I’m getting ready to have the distributor analyzed since I don’t know the condition of the bearings or internals. I measured the timing and received the following results: at idle ( approx 500 rpm) with the vacuum disconnected and plugged the timing was about 10 degrees ATDC. With the vacuum connected the idle increased to about 900 rpm and the timing was about 10 degrees BTDC. Again, with the vacuum connected at 3,000 rpm the reading was about 12-14 degrees BTDC. At 4,500 rpm the timing was about 20 degrees BTDC. How does this sound to you?
Sounds lije vac advsnce ported wrongly to manifold vacuum. If you can’t rig up ported vacuum, blank everything off and strive for 34-35 BTDC at 4500 rom
As Peter suggests, I just blanked the port on the '70 intake manifold, and don’t use the vacuum advance(retard), at all…Just use the centrifugal advance, in the dizzy…Car runs great
this way.
Edward
I know you are responding to Peter’s post. But I removed the secondaries and the shafts. They say it helps and perhaps the car feels livelier, but I have no hard data to prove it.
