In a previous thread I mentioned that some carburetors are set up such that the distributor vacuum advance sees full manifold vacuum even at idle. This, according to factory service literature, was done to give better driveability in certain vehicle configurations with automatic transmission. And there's at least one vendor of performance ignition systems for Mopars who advocates running unported manifold vacuum to the distributor. So I decided to try an experiment on my '73. This car has a Carter BBS carburetor, a 1976 distributor, and no EGR. The carburetor's vacuum advance port does not have vacuum at idle. So I capped that port and connected the vacuum advance to manifold vacuum. Left it there for a couple of weeks to get a good, solid idea of the changes. One immediately obvious change was an increase in idle speed; I had to back off the idle speed screw.

Driveability did not improve, it got worse. Specifically, the car's pickup sagged slightly but detectably immediately after the 1-2 and 2-3 shift, and light-throttle acceleration in 2nd and 3rd gear (well short of causing the trans to kick down) was noticeably mushier. Once I put the advance hose back on the carburetor and readjusted the idle speed, the sag and mush went away. I've still got ping to take care of at 65+ mph under steady throttle even with high-test gas; that wasn't affected by the vacuum hose placement.

My guess is that the driveability improvement with manifold vacuum spark advance is specific to the underhood configuration. Mine differs substantially from the setups on which manifold spark advance was used; I have no EGR, my base timing is about 8°, etc. Still, I can't help wondering why I remember seeing manifold vacuum at the spark port of various more-or-less stock carburetors on my '62 Lancer back when it was my dad's car. I'm pretty sure the vacuum porting particulars depended in the pre-emissions era on manual/automatic transmission, but back then I think it was manual-trans cars that got porting closer to manifold-vacuum-at-idle.

Haven't dug the receipts out of the glovebox to calculate mileage yet, but I can't imagine better mileage having accompanied the poorer driveability.

FWIW, YMMV, BYOB.

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Hmm.
The 390 (2100 Autolite & C-6) in our float uses manifold vacuum as well (my sole Ford history). I've been told most or all Fords were that way but haven't researched that.

I have noticed it's more sensitive to excess advance causing overheating than my slants are. Some cause/effect relationship perhaps?

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I get that almost same feel. Except the spark control computer uses manifold vacuum and base timing set at 7.

This is driving me nuts as this forces me to stomp on gas often to take off if I must go cleanly instead of a sleepy take off.

Meaning to get around to modify the 1.8L VW hall effect mechanical distributor with vacuum can to weber's ported vacuum and use bosch ignition module to drive coil.

Cheers, Wizard

Dan,
At what rpm do you start to see vacuum on that port?

I have found this response similar with other carbs on the SL6.... and also on other vehicles.....
That is why recurving the distributor and tuning the vacuum advance can makes for better drive-ability (responsiveness and punch) and mileage.

Basically when you plugged into the full vacuum instead of the ported vacuum you pre-advanced the engine (by the increase in idle) and threw away some of your total advance that you would normally feel in the pedal. It makes for a very flat feel when the vacuum can has already advanced. You may have thrown away 10 to 20 degrees depending on your vacuum can.

I like to have that feel back in the pedal during acceleration and have control over it. As an example I run my VC-208 (11R can) screwed in 3 turns just so it doesn't come on too early. It is plugged into the ported vacuum fitting). It provides a 22 degree advance in timing that I can feel as the throttle blades are opened up. My initial timing is still at 8 degrees for a 30 degree hit plus another 20 degrees out of the 15R governor and spring combination.

It is a balancing act between tuning the pod and the distributor springs to get the nice pedal feel and responsive acceleration for your particular engine build. On a stock engine it is even more difficult to find that perfect balance because of the low compression. Most stock motors from the factory have very lack luster performance. With my 9.5 to 1 compression motor and the E254 can it is much easier to get a good steady acceleration with out having to mash the throttle. I can gently accelerate and get a nice punchy response. It is a delight to drive with the modified motor and recurved distributor. As a result the mileage is much better because I am not pushing it.....since the pedal is so responsive to the slightest movement.

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74 Swinger, 9.5 comp 254/.435 lift cam, 904, ram air, electric fans, 2.5" HP2 & FM70 ex, 1920 Holley#56jet, 2.76 8 3/4 Sure-Grip, 26" tires, 25+MPG

Ted which is it(in or out)?

I'm installing my recurved distributor per previous suggestions & with VC-208 now and need to make this adjustment in the right manner

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1965 Dart 110k, 225, Carter BBD Super Six, 2 1/4 single exh., sbp manual scarebird front disc, 7 1/4 rear 2.94 sure grip, 14 x 4.5 OEM wheels, 833 OD with hyd. throwout bearing, HEI, electric fan, ram air/heated air, Accusump. http://plymouthcarclub.com/

Turning to the right is in......clockwise......all the way in is full on.......see Doc's 4th .jpg showing the recurve instructions. There is a good picture of the direction and using the 3/32 allen. You can start from full on (which is screwed all the way clockwise) and soften its engagement by screwing the adjuster out. Start with it fully screwed in per the instructions. I adjust it with a timing light. Using the ported fitting I bump the throttle to see how quickly it engages. You can tune it to come on quicker if you have some really stiff springs or delay it later if you are running medium to light springs. I don't like it to come on too quick or it could cause pinging when using light springs....
The best test is to take it for a drive after documenting the advance. See what it feels like....Some folks create a simple table and log the advance at several rpm points. If they don't like it and make a change, it is a quick comparison.

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Aggressive Ted

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74 Swinger, 9.5 comp 254/.435 lift cam, 904, ram air, electric fans, 2.5" HP2 & FM70 ex, 1920 Holley#56jet, 2.76 8 3/4 Sure-Grip, 26" tires, 25+MPG

It may not apply here, but I have an AFB I have been planning to use. There are two ports, one is timed, one is supposed to see manifold vacuum. The manifold port is actually timed slightly later than the timed port.

I too have seen two barrels that have vacuum at the timed port at idle. Sometimes I am sure it is the engine is out of tune and has the idle setting cranked up, but sometimes I think it is just how the carb is drilled or cast

Hi Dan, sorry for activating this thread again. I know that several car makes has used manifold vacum to the distributor, many Ford models had it, as I recall. You wrote: I don't know if you read it, but in my thread about optimal water inlet temperature, I mentioned the TIC-valve, I don't know if it was placed in the radiator tank or on the engine: On some engines, the TIC valve was fed ported vacum from the OSAC valve (Orifice Spark Advance Control), which used a tiny orifice to delay the ported vacum to the distributor when going from idle to part throttle by 10 to 27 seconds, depending on the application, to aid in control of NOx. The TIC valve by-passed the OSAC valve when feeding the distributor with manifold vacum, and returned to ported vacum (zero at idle) from the OSAC valve when the engine cooled to normal temp. Not very useful on engines with electric fan, of course!
Olaf

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True. I'm working on a Mopar, though.
Neither. This is a '73 Dart, therefore not equipped with a TIC valve.

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