taking vacuum advance from manifold

Hey everyone,

I picked up a pair of rebuilt HS-4's off eBay. The pair that had been on my car was taking vacuum advance from the rear carburetor; my new ones don't have that port.

I am using the pertronix distributor; can I take vacuum from the manifold? Is there a simple way to put a port on the manifold or will I have to have somebody install one?

Thanks for the advice,

Eric

never seen a hs4 without a port, but then i haven't seen many of them. in order to do what you suggest you WILL need to have your distributor recurved and possibly a new advance unit. otherwise you'll never get the timing right.

a much more suitable solution would be to install a vacuum port on the new carbs.

The later model HS4s don't have vacuum ports. You can tee off of any vacuum line that runs from the manifold - there must be at least one vacuum port there?

Thanks,

Sorry for the ignorance but what/how would I tee off. The manifold has several bolts on it, would I just replace one with a plug of some sort and run the line through that?

-Eric

jeff, if his distrubtor was set up to use ported advance wouldn't it mess things up to run off manifold? you da distributoer expert here. maybe i'm wrong.

Drake,

It's not the original setup, it's the complete pertronix distributor. To be honest I'm not sure where exactly it's supposed to get the advance from, it was running fine from the carbs.

So I guess that's another question, where should I be getting the advance from?

With the Pertronix distributor, it'll run better with ported vacuum from the carb. It'll still run better with manifold vacuum than with no vacuum advance at all, but not optimal. Actually, manifold vacuum versus no vacuum may be a wash in this case, since the Pertronix distributors offer 20 degrees of advance at the crank. I used to modify them to cut the amount of vacuum advance down by half in manifold vacuum applications. My question is WHY would you get rid of the stock carbs for the later moedl without the port? Not a good idea!

Thanks guys,

I suppose this is a case of buyer beware. I picked them up off ebay, they were listed as hs4's from a 1970, I assumed they would be the same as what I was swapping out.

-Eric

I ran my pertronix off manifold vacuum for a while, it sucked (pun intended), the main problem was that it was at full advance at idle, that pushed the idle speed way up, I adjusted that down using the idle screws but then pulling away from standstill the engine would stumble and hesitate badly as the advance suddenly dropped off.

I installed a port in my rear carb and it runs a whole lot better.

It also seemed to me when I ran the vacuum to the manifold, it seemed too strong and pulsed the distributer. I took it back off and I am assuming once I get around to porting to the carb, it wont be as strong at idle and wont pulse. Just a theory, I am not sure when I will get to it.

You really need a special vacuum advance module to work with manifold vacuum. It needs to start advancing the timing at a relatively high level - 10" rather than 5-7. That way it drops the advance faster when you hit the gas, for less hesitation. For the 45D distributors, the '75 model year had the right unit. For 25D distributors, '73-'74 had the right unit. They can be retrofit to other years though! :-)

Pulsing is another issue. That has more to do with where the vacuum port is located on the manifold and what size the vacuum line is. The smaller the vacuum line, the steadier the signal. Also, the further you are from the center of the manifold, the better the signal, due to less crossover gas pulses from carb to carb.

Any carbs from AUD465 and newer (late 1970) don't have a vacuum port. That's where you need to talk with Jimmy Hilton to make that modification!!!

Pennsylvania MG Wrote:

No ported advance on the carb? sounds like you got yourself a set of AUD465's

I have a set of 465's on my '80. Manifold vacuum. Runs nicely, but maybe this explains the slight hesitation I get when taking off from a stop.

IF you decide to hook up a port on your new carb it should be very easy to take both rear carbs to a machine shop, point at one and say this is what i want on the other in EXACTLY the same location.

Thanks again everyone, Jimmy had been recommended but I'm a sucker for eBay.

I don't have them at hand but I am pretty sure they're 465's.

-Eric

gooser Wrote:

Another picture

Hap, that looks pretty straight-forward. After making that change (from manifold vacuum to carb vacuum on an HIF4-equipped car), how badly would the Pertronix advance be affected?

R>

Thats a better question for Jeff than me. Ocassionally Jeff and I will calberate on a job, where he does the distributor and I do the SU carbs, that was the case on the vaccum ported HIF4, Jeff set the distributor up for vaccum port and when I was rebuilding the carbs, I install the vaccum port.

Rick, the Pertronix really isn't affected at all. That's a separate deal, liek points. No affect from where the vacuum is taken. Only the amount of advance at idle changes (decreases) due to a reduction in vacuum at idle from the new port.