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Tue Jun 20, 2023 9:33 am

I am running 40 HPMX's on my 1776 in my 56 bug. I was considering running a vacuum advance distributor on my setup. Do the 40 HPMX's have a vacuum port? I have read that dual carbs don't generated enough vacuum to work a vacuum distributor. Oh I would be running a 123 distributor.
Thanks in advance for any suggestions/comments
Bruce Ebling

Tue Jun 20, 2023 12:20 pm

MeeferMadness wrote: I am running 40 HPMX's on my 1776 in my 56 bug. I was considering running a vacuum advance distributor on my setup. Do the 40 HPMX's have a vacuum port? I have read that dual carbs don't generated enough vacuum to work a vacuum distributor. Oh I would be running a 123 distributor.
Thanks in advance for any suggestions/comments
Bruce Ebling

For that distributor you need to grab a vac signal from the manifold. I teed #3 and #4 together and used M4 barb fittings at the base of each carb barrel below the butterflies. Works well.

Tue Jun 20, 2023 1:12 pm

Most of the dual Webers made in Italy, Spain or China don't provide enough ported vacuum to work the canisters on an SVDA distributor. Ported vacuum is created when you open the throttle/s up. You don't want to connect the canister to manifold vacuum.

There are shops they can modify your carbs to get the right amount of ported vacuum though.

Tue Jun 20, 2023 1:14 pm

wcfvw69 wrote: Most of the dual Webers made in Italy, Spain or China don't provide enough ported vacuum to work the canisters on an SVDA distributor. Ported vacuum is created when you open the throttle/s up. You don't want to connect the canister to manifold vacuum.

There are shops they can modify your carbs to get the right amount of ported vacuum though.

Agreed, but he said he is using a 123 distributor so that wants manifold vacuum. It doesn't have a vacuum can but has a built in MAP sensor.

Tue Jun 20, 2023 2:49 pm

MeeferMadness wrote: I have read that dual carbs don't generated enough vacuum to work a stock vacuum distributor.
There, fixed it for you but... it doesn't apply to you as you are not using a stock VW distributor vacuum pot.

Tue Jun 20, 2023 6:13 pm

Vacuum signal is generated from the duration of the cam. Small cams such as a W-100 or W-110 produce enough vacuum signal to work. When you get W-120 & larger cams the signal is too weak to actuate the vacuum canister.

Wed Jun 21, 2023 8:22 am

Been running Vacuum advance on multiple engines with E-120 ish cams for years with dual Dellortos and it works great.

I know that the Spanish IDFs need to be modified so they have a good vacuum signal, I think the HMPXs work as delivered.

As I always recommend, put a vacuum gauge on your carbs and test

Wed Jun 21, 2023 8:44 am

Just because there is vacuum there, don't think you are done. You need to know how much vacuum is there at the engine speed and road speed you want it to be. Then you need to match that exact signal to a vacuum pot that will deliver the degrees of advance that you want with that specific signal strength. I am not aware of any "one step, bolt on and forget" solution outside of a matching stock carb and distributor.

Or run a manifold pressure sensor, a digital program and tune it on the fly in actual real life conditions.

Wed Jun 21, 2023 9:33 am

oprn wrote: Or run a manifold pressure sensor, a digital program and tune it on the fly in actual real life conditions.

This is what the OP has, so this is the route he must take to make his 123 ignition distributor work, assuming it's the style with BT or USB capability.

Wed Jun 21, 2023 12:15 pm

boxer74 wrote: oprn wrote: Or run a manifold pressure sensor, a digital program and tune it on the fly in actual real life conditions.

This is what the OP has, so this is the route he must take to make his 123 ignition distributor work, assuming it's the style with BT or USB capability.
That.