Another 396 Question

[Joel T.]

I was reading a thread which goes back to January of this year on the L78 396 vacuum advance.. There was a reply in the thread which essentially said that the 396 was like the 1963 340 engine in that vacuum at idle was not sufficient to fully pull and hold the OEM VAC... with the indication being that the 396 had full time (manifold) vacuum to the can...

The 4150 which is on the car now I picked up at Carlisle back in 2010.. professionally rebuilt, all correct casting numbers and a date which works for my car. It appears however that it has ported vacuum in that observed vacuum (via gauge at the front metering block) is zero at idle and rises as the throttle is open. Another 4150, a Service Replacement, had observed vacuum at idle.

Question.... Is the vacuum port on the L78 4150 supposed to be "ported" or "manifold".. and if it is incorrect, can it be changed, or should I tap directly into the intake?

Thanks,

Joel

[Jim T.]

Re: Another 396 Question

The vacuum that goes to the vacuum choke unit can be used if your carb has this vacuum unit. A plastic "t" can be used.

[Joe R.]

Re: Another 396 Question

I was reading a thread which goes back to January of this year on the L78 396 vacuum advance.. There was a reply in the thread which essentially said that the 396 was like the 1963 340 engine in that vacuum at idle was not sufficient to fully pull and hold the OEM VAC... with the indication being that the 396 had full time (manifold) vacuum to the can...

The 4150 which is on the car now I picked up at Carlisle back in 2010.. professionally rebuilt, all correct casting numbers and a date which works for my car. It appears however that it has ported vacuum in that observed vacuum (via gauge at the front metering block) is zero at idle and rises as the throttle is open. Another 4150, a Service Replacement, had observed vacuum at idle.

Question.... Is the vacuum port on the L78 4150 supposed to be "ported" or "manifold".. and if it is incorrect, can it be changed, or should I tap directly into the intake?

Thanks,

Joel

Joel,

A hole can be drilled in the carb to provide full time vacuum advance. I plan to do this to my carb for my L72.

Joe

[Joel T.]

Re: Another 396 Question

Thanks guys, I appreciate the feedback... Joe, whenever you do your carb, if you post a picture, that would be great. In the meantime, I've sent a note off to the guy who did the rebuild back in 2010 to see if he can do something.... Regards, Joel

[Clem Z.]

Re: Another 396 Question

the "D" shaped passage outlined in red is the vacuum feed to the metering body. the small hole shown in the throttle bore must be plugged and another same sized hole drilled below it into the "D" shaped hole underneath the butter fly when closed.

[Joe R.]

Re: Another 396 Question

Joel.

OK, will do. Meanwhile, if you get yours done, please walk us through it, step by step, with pictures. I'm not completely picking up what Clem is throwing down, probably because I know absolutely nothing about carbs.

Joe

[John H.]

Re: Another 396 Question

I'm not completely picking up what Clem is throwing down, probably because I know absolutely nothing about carbs.

Joe

Joe -

A "ported" vacuum source (as in Clem's photo) has the round vacuum port exposed ABOVE the closed throttle plate, so there's no vacuum signal to the metering block (and from there to the distributor vacuum advance line) at idle; that port only becomes exposed to manifold vacuum as the throttle plate opens. Converting it to a full manifold vacuum source requires re-drilling that hole lower in the bore, so it's exposed to full manifold vacuum (below the throttle plate) with the throttle plate closed.

[Joe R.]

Re: Another 396 Question

John,

Thanks for the explanation! I think I understand now.

Joe

[Joel T.]

Re: Another 396 Question

Hi Clem;

You probably know this off the top of your head... Aside from fixing the carb (thanks!), when the L78 was introduced back in 1965 did the carb provide manifold vacuum, or was it ported? I think Duke or someone indicated that carbs went to ported vacuum in the late 60's? I'm still trying to figure out what was the original set up... Thanks! Joel

[Wayne M.]

Re: Another 396 Question

.... Is the vacuum port on the L78 4150 supposed to be "ported" or "manifold".. and if it is incorrect, can it be changed, or should I tap directly into the intake ? ...

Joel -- the L78 vac port does not have a ported spark advance. This didn't start until Controlled Combustion System [C.C. S.] was introduced with the 1968 model year.

Here is an example, of a model 4150, List 2818 ('64 L76) carb. A L78 List 3124 base plate is essentially the same except for bore sizes. First pic repeats the view of Clem's shot, ie. top view of the base plate (that bolts to the main body of the carb), except that Clem shows a ported carb. Second pic is of the bottom of the base plate, taken to show the 2 small holes that take the manifold vacuum directly from the intake, irrespective of position of throttle plates. [The big holes are for the screws]. The small hole furthest from the screw hole feeds full manifold vacuum seen at the carburetor interface into a corresponding hole in the main body, which connects directly to a long slot in the metering block that terminates at the fitting on the right side, to which the tube connects and feeds the vac. advance of the distributor.

Third and fourth pics are views of the bottom side of the base plate, with throttle plates fully open, then fully closed. You can see the 3rd pic that, even if the plates were fully closed, there is NO vac port hole above them. The hole that you see when throttle plate is closed is the idle fuel feed; the slot that becomes exposed as the throttle plates open is the idle transfer slot.

[Joel T.]

Re: Another 396 Question

Wayne,

Thanks! So what I probably have here is a carb made up of various parts with the base plate coming from a later 4150.... Interesting... I have a Service Replacement unit at home too which I need to look over...it may be different... Should I expect a decent carb rebuilder to be able to make this 'switch" as Clem has outlined.

Thanks again!!

Joel

[Joe R.]

Re: Another 396 Question

Hi Clem;

You probably know this off the top of your head... Aside from fixing the carb (thanks!), when the L78 was introduced back in 1965 did the carb provide manifold vacuum, or was it ported? I think Duke or someone indicated that carbs went to ported vacuum in the late 60's? I'm still trying to figure out what was the original set up... Thanks! Joel

Joel,

I believe that the L78 had full time vacuum advance, the L72 and the L71 had ported vacuum advance.

Joe

[Wayne M.]

Re: Another 396 Question

....So what I probably have here is a carb made up of various parts with the base plate coming from a later 4150.... Interesting... I have a Service Replacement unit at home too which I need to look over...it may be different... Should I expect a decent carb rebuilder to be able to make this 'switch" as Clem has outlined.....l

Joel -- I attach a pic of definitely a ported spark advance; a late '69 center carb of a tripower set, List 4055-1. View is from the bottom of the base plate, which is opposite of Clem's shot. I circled the factory drilled hole that exposes the spark advance circuitry to vacuum when the throttle plate opens, but not at idle. Also, I've circled the interconnect to the small hole in the channel which goes (only) to that nipple by my finger, which is where the distrib advance hose connects. Notice that this spark advance hole is only in one of the two primary bores.

So you'd have to search for that "other" hole, higher up in the bore. If not there, then you have a full manifold advance system. You may be able to see this with the carb on the car, holding the throttle open and looking down. I have my spare List 3124 on the bench, so easy for me (heh heh). I still stand by my readings of the Chevrolet manuals; ie. that C.C.S. didn't start until the 1968 model year.

Now the problem that you (and I, with my '65 L76, and others) have with high valve overlap engines is loss of idle manifold vacuum, requiring adjustment to the idle set screw (to increase idle RPM) which then exposes the idle transfer slot and screws up the response of the idle mixture screw (paraphrased from the Holley 4150 & 4160 Carb Handbook). Solution is to drill a 1/8" hole (or smaller) in the throttle plates (same side of shaft as idle holes/slots). The extra air allows closing of throttle plate and a return to idle adjustability. I haven't done this (yet ?).

[Clem Z.]

Re: Another 396 Question

Joel -- the L78 vac port does not have a ported spark advance. This didn't start until Controlled Combustion System [C.C. S.] was introduced with the 1968 model year.

Here is an example, of a model 4150, List 2818 ('64 L76) carb. A L78 List 3124 base plate is essentially the same except for bore sizes. First pic repeats the view of Clem's shot, ie. top view of the base plate (that bolts to the main body of the carb), except that Clem shows a ported carb. Second pic is of the bottom of the base plate, taken to show the 2 small holes that take the manifold vacuum directly from the intake, irrespective of position of throttle plates. [The big holes are for the screws]. The small hole furthest from the screw hole feeds full manifold vacuum seen at the carburetor interface into a corresponding hole in the main body, which connects directly to a long slot in the metering block that terminates at the fitting on the right side, to which the tube connects and feeds the vac. advance of the distributor.

Third and fourth pics are views of the bottom side of the base plate, with throttle plates fully open, then fully closed. You can see the 3rd pic that, even if the plates were fully closed, there is NO vac port hole above them. The hole that you see when throttle plate is closed is the idle fuel feed; the slot that becomes exposed as the throttle plates open is the idle transfer slot.

the hole closest to the screw hole is a vacuum drilling for the power valve. the other drilling will allow full vacuum to metering body if your base plate has the slot in it and if not you will have to mill in the slot because without the slot the hole will be closed off by the carb to manifold gasket