Vacuum Advance Question

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Jun 1, 2020 | 01:47 PM

#16

Christmas

Logistics Pro

Joined: Jun 2018

Posts: 3,556

Likes: 444

The 70's were a terrible time for mfgr's to try and meet new emission standards. Big block Vetts that only made 170 hp and the death of the FE.

Jun 1, 2020 | 11:48 PM

#17

F-250 WARHORSE

Cargo Master

Joined: Nov 2009

Posts: 2,364

Likes: 118

From: starship enterprise

that amount of timing in a vacuum advance is possibly OK. I like to tune in 20-25* advance from vacuum. you are in the 30* range. I have always ran a spark delay valve, as well. sometimes it is less good when cruising flat and coming up on a hill, or starting from the stoplight directly uphill, a direct vacuum might help, but as far as flat ground the spark delay valves make things come into a nice transition from no timing to lots of timing. driving for a few miles at 40 mph or so, you find yourself just slightly easing off the gas pedal all the time to maintain a speed, as the timing comes in. I feel that as the timing comes in at once, I am feathering the throttle to keep speed and that varies vacuum and that varies timing. On some of the vacuum canisters there is an allen screw hidden inside that a small allen wrench will adjust spring tension against the vacuum. That will change the amount of vacuum it takes to pull the timing in and can also change the timing amount to a certain degree. Distributor tuning is an artform that can be addicting... be careful.

Jun 3, 2020 | 01:29 PM

#18

Tedster9

Post Fiend

Joined: Apr 2002

Posts: 19,311

Likes: 97

From: Waterloo, Iowa

The problem the average DIYer I see, is people don't understand at a basic level how vacuum advance works. It is a load dependent device. The distributor itself, on the other hand, the weights and springs, are based on RPM. They are completely independent of each other. When one is pulling in more ignition advance, the other is backing off, and vice versa, at least most of the time. At cruise in high gear, on flat ground there may be as much as 50+ degrees of ignition advance on any factory OHV V8. This is why vacuum advance was invented in the first place basically, there's no practical way to use weights and springs alone to achieve that. It also helps for all around driveability and smoothness, and engine will run much cooler in stop and go driving.

An engine doesn't "need" vacuum advance but there is no good reason not to, and will lose 3 to 4 hwy MPG without it. That's quite a bit, at least on a percentage basis, on a pickup. With a sharp tune in both carburetor and ignition curve 25 mpg is doable in 50s and 60s cars. Why, you can actually drive past a gas station once in a while!

Jul 2, 2022 | 02:23 PM

#19

RTT

Fleet Mechanic

Joined: Apr 2017

Posts: 1,391

Likes: 106

From: COSMOS

The blue retard vacuum piece needs a ported vacuum connection. I saw a couple times here that there is no ported outlet on this carb. I have not read complete thread...........so forgive me...
I would triple check that the hoses are in proper locations including a ported one. Very easy to get confused.......
Also I have researched that part extensively and on a truck with out any OTHER emission parts ( like my 71 ) the piece appears to be a good thing to have working correctly and will not rob any power, and aides in better engine rpms & health and gas mileage !!!!!!!!!!!