TheSamba.com :: View topic - Do you need to hook up vacuum advance with dual carbs?

Tue Jun 21, 2016 7:50 am

I am looking at putting some EMPI dual carbs on the my 1600 and I am wondering do I need to run vacuum lines from the carbs to the vacuum advance on the distributor?

Tue Jun 21, 2016 8:15 am

What distributor are you running? I have Dual Dell 36 DRLAs with my 034 connected to the left carb. Runs great and is nice and smooth.

Tue Jun 21, 2016 9:06 am

DesertBob wrote: What distributor are you running? I have Dual Dell 36 DRLAs with my 034 connected to the left carb. Runs great and is nice and smooth.

I believe it is the original stock distributor. Here is a pic.

Tue Jun 21, 2016 10:51 am

Yep, you still need to hook up one carburetor to that distributor vacuum line.

Tim

Tue Jun 21, 2016 11:06 am

Tim Donahoe wrote: Yep, you still need to hook up one carburetor to that distributor vacuum line.

Tim

Thanks!

Tue Jun 21, 2016 3:01 pm

What model Empi carbs are you planning to run? I'm pretty sure they make a few different ones and not all have a vacuum port to work with vacuum distributors.

I've never run a dual carb setup but that looks like the mid-70's SVDA distributor (small vacuum can). These required a more vacuum (lower pressure) to properly advance the timing. The earlier '60s SVDA (large can) vacuum advance used much lower vacuum (higher pressure) to fully advance. Depending on how much vacuum advance your carb's vacuum port provides and WHEN it provides it, you may not be able to mix and match distributors.
The large vacuum can distributors will go to full advance too quickly when matched with intake or ported vacuum from the carb.
The small can distributors are expecting ported vacuum. If you connect then to venturi vacuum they won't advance at all (or VERY little).

On the positive side... even if your carb doesn't provide the correct vacuum, you should be able to run that SVDA distributor with the vacuum disconnected and just let the mechanical advance do all the advancing (similar to 009 distributor). This is not ideal, but should work well enough, depending on the model of the carb.

Do you have the specs for the vacuum produced by the carbs you plan to buy? If you post the model maybe someone here has worked with this combination?

Tue Jun 21, 2016 3:44 pm

ashman40 wrote: What model Empi carbs are you planning to run? I'm pretty sure they make a few different ones and not all have a vacuum port to work with vacuum distributors.

I've never run a dual carb setup but that looks like the mid-70's SVDA distributor (small vacuum can). These required a more vacuum (lower pressure) to properly advance the timing. The earlier '60s SVDA (large can) vacuum advance used much lower vacuum (higher pressure) to fully advance. Depending on how much vacuum advance your carb's vacuum port provides and WHEN it provides it, you may not be able to mix and match distributors.
The large vacuum can distributors will go to full advance too quickly when matched with intake or ported vacuum from the carb.
The small can distributors are expecting ported vacuum. If you connect then to venturi vacuum they won't advance at all (or VERY little).

On the positive side... even if your carb doesn't provide the correct vacuum, you should be able to run that SVDA distributor with the vacuum disconnected and just let the mechanical advance do all the advancing (similar to 009 distributor). This is not ideal, but should work well enough, depending on the model of the carb.

Do you have the specs for the vacuum produced by the carbs you plan to buy? If you post the model maybe someone here has worked with this combination?

They are EMPI EPC 34. Only reason I am thinking of going down this road is

1. The current carb is a no named brand that I cannot keep tuned.
2. A replacement carb was quoted @ 300 for just the carb
3. The dual setup is being sold by a coworker and it is practically brand new. It has been on his car only for setup and he is offering me the set for 225. He wants to go back to a single carb.

Tue Jun 21, 2016 4:49 pm

Tim Donahoe wrote: Yep, you still need to hook up one carburetor to that distributor vacuum line.

Tim

No.. dont do this,your vacuum advance plate will pulse.run an equalizer hose between both carbs vacuum ports and tee into it for distributor vacuum...you should run an equalizer even if you run a mechanical distributor,it helps with tuning..

Tue Jun 21, 2016 6:46 pm

johnnypan wrote: Tim Donahoe wrote: Yep, you still need to hook up one carburetor to that distributor vacuum line.

Tim

No.. dont do this,your vacuum advance plate will pulse.run an equalizer hose between both carbs vacuum ports and tee into it for distributor vacuum...you should run an equalizer even if you run a mechanical distributor,it helps with tuning.. ?? helps with tuning??? :shock:

Tue Jun 21, 2016 7:52 pm

mark tucker wrote: johnnypan wrote: Tim Donahoe wrote: Yep, you still need to hook up one carburetor to that distributor vacuum line.

Tim

No.. dont do this,your vacuum advance plate will pulse.run an equalizer hose between both carbs vacuum ports and tee into it for distributor vacuum...you should run an equalizer even if you run a mechanical distributor,it helps with tuning.. ?? helps with tuning??? :shock:

helps to balance vacuum side to side..

Tue Jun 21, 2016 8:02 pm

I T'd off the vacuum from these carbs to an 034 like yours, a few years ago, plus ran a balance tube between the manifolds, had to drill and tap this.

Frankly ran better, in my opinion, with a 009.

Tue Jun 21, 2016 8:20 pm

johnnypan wrote: mark tucker wrote: johnnypan wrote: Tim Donahoe wrote: Yep, you still need to hook up one carburetor to that distributor vacuum line.

Tim

No.. dont do this,your vacuum advance plate will pulse.run an equalizer hose between both carbs vacuum ports and tee into it for distributor vacuum...you should run an equalizer even if you run a mechanical distributor,it helps with tuning.. ?? helps with tuning??? :shock:

helps to balance vacuum side to side..

My Kadrons idle smoother, sync easier, and have better off-idle response with the balance tube fittings plugged completely off for whatever reason, so i keep 'em that way. :-k

I know it flys in the face of common practice but works for me. I know of guys who can't get dual 1BBL setups to tune worth a damn without the tube, so, go figure.

I get slightly less jumpy vac advance with the ports connected through a tee, but I'm experimenting with running it off just the left carb and I haven't noticed any difference in daily use. total vacuum pull is the same too, just ever so slightly more discernable jumpiness on the vac gauge.