Flathead Vacuum Question

Hotrodvideo

Hotrodvideo

Senior Member
I have an 81A eng with later crab style distributor, the SW vacuum gauge needle hardly moves and it never goes into the yellow upon deceleration.

I tried 2 different gauges and they both do the same thing.

Is this normal or does it sound like I have a leak or does the distributor need adjusting?

BTW, I'm using a T fitting off the Edmunds intake with one hose going to the dist and the other to the gauge. I have the same setup on another car and it works fine.
 
Have you tried a separate - hand held type of vac gauge? You might have a leak / crack in the hose from the manifold to the gauge.
 
Make sure the rubber tube is not blocked or pinched. You could place the guage close to the T fitting and use another tube for a test. It would seem the gauge is not responding very quickly. There is an orifice at the back of the gauge if the small hole in it is partially blocked it could make the gauge slow to react. Without the small orifice the gauge moves too quickly.
 
Perhaps there is too much leakage in the vacuum break piston bore, or the seal between the dist and front cover --or the adapter if you are using one
 
Block off the line to the distributor and check it. As noted, your vacuum brake is probably leaking and so not working.
 
On the helicopter engines, one of the engine gauges is an MAP indicator or manifold air pressure and it is graduated in inches of Mercury (Hg). At idle with the throttle closed we seldom get lower than 10 inches Hg with low loading. Once a heavier load is applied and anything off idle the reading goes up to around 15 inches HG. These are air cooled motors with continuous flow fuel injection but this should be close to an average for higher compression OHV internal combustion engines with lighter than average flywheels.

Flatheads are relatively low compression and might not even get to 10 inches HG at idle with low loading. Don't forget that the flathead V8 engines are always pulling a relatively heavy flywheel so there is always some form of load there. As was mentioned previously, any manifold leakage will increase the MAP to some extent depending on how big the leak is.
 
Even with a cam, my 39 idles at 15" hg. I think your numbers are a little off base. As the load increases, the vacuum decreases.
 
The pressure increases with load true but as the throttle opens the manifold preasure goes only one direction as the plate opens wider. On a standard day at sea level, the barometric pressure is 29.92 inches of HG. With wide open throttle, the pressure can't get any higher than that but the vacuum pressure is not low at 29-inches. It's just moving a lot of air through the engine. When the throttle is closed it would be around 15" HG which is fairly high vacuum or "Low Pressure".

The reason the helicopter engine gets down to 10"HG at idle with the rotor disingaged is that is has only a very light impeller for a flywheel with a steel weight ring added so that it will idle at a decent rpm (1,450rpm). This engine is also 10:1 compression OHV. The flathead idling at 15"HG seems reasonable to me.
 
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When the manifold pressure is below atmospheric, we call that vacuum. At 10" hg vacuum, you would have a MAP sensor reading of 19.92. (atm press of 29-92" - 10" vac = 19.92). In a normally aspirated engine, the vacuum will never be zero, nor will the MAP be 29.92 because there will always be some restriction in the intake system.
 
If you have a blower or a turbocharger, you can get more than atmospheric. As far as the MAP gauge is concerned, it is just a pressure reading bourdon tube instrument that is calibrated for that purpose. How accurate it is a 9 or 10 inches HG, I don't know. It's accuracy is more important up around the power limitations levels which is 27" Hg for this particular engine/transmission combination. A person would have to use a calibrated manometer with a flow bench to check it for sure but the readings I mentioned are not for a car engine either. We have no digital sensors on the helicopters, just direct reading gauges and some electrical temperature sending units for electrical movement gauges.
 
All great arguments! I think his crab's o-ring is leaking at the timing cover and/or his elevation is so high (Denver) that that it would drop the Hg drastically.

Lonnie
 
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Edmunds? Does that means you have twin carbs? Your vacuum will drop off with twins.
Just out of interest. My little '37 21 studder, stock cam gets a perfect 21 "Hg at idle.
 
Yes, we are pretty high up here in Denver and I do have dual carbs on the Edmunds intake.
The distributor gaskets are new, they should be ok.

Is there supposed to be a gasket on the vacuum brake adjustment? The schematic from Ford does not show one.
 
I'm also at mile-high altitude and I can say I've never seen an engine pull more than 19" up here, and only that on a big-displacement V8 (like a 350+ ci) with no PCV.

What value is the yellow zone on your gauge?
 
Hotrodvideo said:
Yes, we are pretty high up here in Denver and I do have dual carbs on the Edmunds intake.
The distributor gaskets are new, they should be ok.

Is there supposed to be a gasket on the vacuum brake adjustment? The schematic from Ford does not show one.
Click to expand...

There should be a gasket under the large nut.
 
The adjustment of the screw on the vacuum brake doesn't make much difference with today's fuels. It was originally designed to prevent preignition knock with the old low octane fuel. Same is true with the slider on the side of the distributor. We set these up on our Sun machine for today's fuel. The piston is usually loose enough to allow vacuum leakage, and the grommet seal between the timing gear cover and the distributor is commonly both hardened and misshapen enough to prevent a good seal. The important part of the distributor is the weights and springs under the breaker plate. These must move freely, and the leaf springs have to be intact and with proper tension.
 
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