Vacuum Advance was not hooked up? What can I expect MPG wise?

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cvan1971man

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Nov 15, 2019
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Arlington, TX
My son has been getting about 14MPG from his FJ40.

I was looking at the electronic ignition in the distributor and realized that the vacuum advance had never been hooked up correctly. It was always sluggish when accelerating, and now I know why. The "Fork" was not connected to the knob / c clip at all.

I have seen online where someone stated that the vacuum advance being hooked up correctly can have a 10 to 30% increase in MPG. I don't see getting a 30%... going from 14mpg to 18MPG, but was wondering if it will make a big difference when driving at highway speeds for longer trips, i.e. when my son drives 2.5 hours home on the highway.
 
10% gain would seem a max. All the vacuum advance does is put the cherry on top of the mechanical advance, which does most of the advancing.

You might notice slightly sharper throttle response, but not much else. If your son is getting 14 mpg driving an FJ40, I doubt you could hope for more.
 
Best I usually get is 16 highway driving the limit or just under

14 normal driving

Heavy foot is more like 12

I have electronic ignition on stock vac advance dizzy … high output coil… headers… weber 38

Your not going to be too much better then what your getting now
 
cvan1971man said:
My son has been getting about 14MPG from his FJ40.

I was looking at the electronic ignition in the distributor and realized that the vacuum advance had never been hooked up correctly. It was always sluggish when accelerating, and now I know why. The "Fork" was not connected to the knob / c clip at all.

I have seen online where someone stated that the vacuum advance being hooked up correctly can have a 10 to 30% increase in MPG. I don't see getting a 30%... going from 14mpg to 18MPG, but was wondering if it will make a big difference when driving at highway speeds for longer trips, i.e. when my son drives 2.5 hours home on the highway.
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Wow, a sluggish FJ40. Never seen that before :)
 
SteveH said:
10% gain would seem a max. All the vacuum advance does is put the cherry on top of the mechanical advance, which does most of the advancing.

You might notice slightly sharper throttle response, but not much else. If your son is getting 14 mpg driving an FJ40, I doubt you could hope for more.
Click to expand...

^^^^^^^This^^^^^^^

Vacuum advance is unique to automotive engines or applications where the engine isnt under constant load. It advances the timing when there's no load when the vacuum is higher, like coasting or traveling on flat ground when there's light throttle input. It smooths out slight variable inputs. It does help improve fuel consumption. Service engines or certain race application engines dont have vacuum advance.
Mechanical/centrifugal advance is the primary advance system. This is why you disconnect the vacuum advance when setting timing. Initial + Mechanical advance= total advance. On my sbc I recurved my distributor so mechanical advance is all in at the highway speed i like to run. My initial timing is set where it won't ping.
 
From the late 70s until 1990 I tried all the tricks from headers, ignition, offenhauser manifolds, water injection, etc, before I finally found a option in 1990 that increased my mileage from 12 highway to 16 consistently. It was a TBI350. The cherry on top was that it came with 80 extra horses. The previous carbed 350 just added ponies but not much mileage
 
put me in the probably not going to see an improvement in mpg and might even notice a possible decrease in mpg.....since it will likely have a tad bit more get up and go, the foot might get heavier.....camp.
 
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MDH33 said:
Post a picture. Might be vacuum retard and disconnected to run better.
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Yeah still do not know if it is even a vac advance… I was wondering this as well in his other thread about the diaphragm being bad

Maybe @cvan1971man can post a picture of his dizzy

Kinda odd to have the c clip come off from inside the distributor… would think it was intentional
 
JohnnyC said:
Yeah still do not know if it is even a vac advance… I was wondering this as well in his other thread about the diaphragm being bad

Maybe @cvan1971man can post a picture of his dizzy

Kinda odd to have the c clip come off from inside the distributor… would think it was intentional
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The c clip didnt come off, i had to pull it off to get the horse-shoe part of the vacuum advance under it.
 
It is a Vacuum Advance

Screenshot_20260110_161902_Gallery.webp
 
I've heard it said that not running a vacuum advance puts more wear on the engine. I've never been able to visualize how that was true. Maybe there's slightly better efficiency at light throttle/cruise because you're igniting the lean mixture a little earlier and the flame front is so slow?

Yeah, 14mpg is real good .
 
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numby said:
I've heard it said that not running a vacuum advance puts more wear on the engine. I've never been able to visualize how that was true. Maybe there's slightly better efficiency at light throttle/cruise because you're igniting the lean mixture a little earlier and the flame front is so slow?

Yrah, 14mpg is real good .
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I don't think it's a wear problem. Service engines are under load constantly. A boat engine is either idling or pushing water, a dragster is wide open, then you have industrial use, like operating a machine.
 
pb4ugo said:
Mechanical/centrifugal advance is the primary advance system. This is why you disconnect the vacuum advance when setting timing. Initial + Mechanical advance= total advance. On my sbc I recurved my distributor so mechanical advance is all in at the highway speed i like to run. My initial timing is set where it won't ping.
Click to expand...

So you're not running any vacuum advance?
 
No, I run vacuum advance. It offers some efficiency, and smooths out some slight variable throttle inputs like when your on and off the throttle going down the highway.
 
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