Vacuum guage

miniceptor86

miniceptor86

Senior Member
I got a vacuum guage and plugged it into the 51 Merc in my ‘39 Ford. I had the carbs set pretty close by ear. Set the timing by the gauge and ooo did that change the sound of the old beast. Most importantly it pulls a nice healthy 20 inches and very steady. Happy
 
Have you driven it yet? I've no idea what a '51 Merc wants, but typically more modern engines need to have the internal advance mechanism tailored for that on the far end or it will be Ping City. Flatheads don't want or tolerate near as much ignition advance as OHV engines either so I'm curious.
 
Well I’ll keep an ear out. And I’ll get out the timing light. What’s the dwell? I got one of those dwell tach thingies that I’ve had for next to ever.
 
Dwell is a more accurate way to set breaker points. Theoretically when the points are set with a feeler gauge to spec the dwell is also at the correct spec. Ford cautioned this only applies when installing new points, once they start to wear after run-in, using a feeler gauge alone would be inaccurate as the point faces are no longer flat.
 
qTalking vacuum gauges. All combustion engines are just air pumps. How much air you move effects fuel. probably heard this saying before, there is no replacement for displacement. Spark and timing is independent somewhat to vacuum, if it's not controlled by vacuum. Internal engine pressure is not always effected by compression, pretty much the internal vacuum is around 18-20 from 32-53 and beyond. Your highest vacuum reading will be at idle.


air-fuel-spark.


If you are not optimizing best vacuum, you may have a leak or bad rings/valves/etc. No amount of spark will help that. Certainly all has to be in concert. No one thing makes the other not matter.
 
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Thanks for the info. I’ll see how the points setting lines up against the desired 27 degrees and see how close the pointer is to the timing dot and report back. Sure sounds better at idle. Before it had just a little cackle at idle, now it’s a very smooth almost humming sound. I don’t know which way I moved it from the original position because as soon as I loosened the clamp and checked to see if it moved I lost the initial orientation.
 
I just went through the same thing today! Dropped in a Mallory duel point and used a vacuum gauge to dial it in, sounded great at idle but test drive it pinged as soon as I put my foot to it. Took 3 adjustments to get rid of pinging, now it dies when I put it in gear? still playing with it?
 
setting ignition with vacum gauge will give you a starting piont often it will be advanced from stock you still have to workout your total advance and at what rpm remember todays fuel has almost nothing in common with the fuel of 60 years ago other than its name
 
backrodebob said:
I just went through the same thing today! Dropped in a Mallory duel point and used a vacuum gauge to dial it in, sounded great at idle but test drive it pinged as soon as I put my foot to it. Took 3 adjustments to get rid of pinging, now it dies when I put it in gear? still playing with it?
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This sounds odd to me. Are you sure the advance mechanism is free and working correctly?
 
One other thing. I put a dual point Mallory in one of my engines a while back. It didn't run very well, so I checked the point gap, which agreed with the instruction sheet. On a hunch, I dug out my old dwell meter and checked the dwell, and it was way short. After carefully setting the dwell (which can be a fussy procedure), the car ran great. I believe that I had the wrong instructions or they were incorrect; either way, I now always use the dwell meter to set the points.
 
Fussy is right! Especially if the distributor is tucked back tight up against the firewall and you haven’t done this often. At least it’s easy access to later model flathead’s.
 
So the dwell is between 26 and 27, I’ll call it 26 1/2, I’ll not mess that up. What I supposed was timing bump on the crank pulley that I painted yellow is not insight using the front passenger side cylinder. I moved the distributor and the timing light but still no dot.should the pointer be at about 11 looking from the front? I do see some advance from the distributor. Not sure how many degrees, the scratch that was visible moved maybe an inch and a quarter when I revved it up to about 2000 rpm.
 
"26 and 27"? I am away from all of my documentation, but I thought the dwell should be around 36 degrees.
 
To add to my post before. If your engine vacuum is not tuned it will have an effect on all systems like vacuum dizzy advance and wipers.


If your engine vacuum is 15. It will effect your vacuum advanced dizzy. Dwell will just fine tune it more.


I would achieve good solid vacuum readings from the holes in the motor first (like adjusting carbs) then tune a bit.
 
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I just looked up dwell with a search on this site and 27 is the right number. Next project is to find out why the pimple that I painted yellow is nowhere in sight of the timing light flash.
 
backrodebob said:
I just went through the same thing today! Dropped in a Mallory duel point and used a vacuum gauge to dial it in, sounded great at idle but test drive it pinged as soon as I put my foot to it. Took 3 adjustments to get rid of pinging, now it dies when I put it in gear? still playing with it?
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Use a timing light to get an accurate setting! A vacuum gauge is NOT a timing tool! Use it for diagnosing for burned valves or intake leaks, or to drive for best economy. If you try to set the timing to the highest vacuum, it will be too far advanced.
 
Sorry; the 36 degrees is for a dual point Mallory. I believe 27 is correct for a stock distributor; (I lost track of who was posting what.) Seeing that the timing mark can't slip (like on a harmonic balancer), are you sure you have the timing light lead on the correct (#1) plug wire?
 
Is it possible to check dwell with the distributor off of the engine; and not having any kind of distributor machine? The idea of tweaking the points adjustment on an early flathead if you have to run the engine between adjustments is quite discouraging.
 
tubman said:
Sorry; the 36 degrees is for a dual point Mallory. I believe 27 is correct for a stock distributor; (I lost track of who was posting what.) Seeing that the timing mark can't slip (like on a harmonic balancer), are you sure you have the timing light lead on the correct (#1) plug wire?
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Right front? Or passenger side front for the disoriented? Oh and the pointer at 11 o’clock looking from the front?
 
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