vacuum gauge as diagnostics tool

[freak]

In the quest for fine tuning I read...

http://www.fordgarage.com/pages/vacuumtune.htm

...and decided to check my engine. Although my gauge isn't as nice as the one pictured I believe it works good enough. I connected to the windshield wiper port and I get a fast fluctuation between 17 and 21. The fluctuation is present at all rpm's. So to any of you who are experienced in vacuum gauge diagnostics, does this sound like possible valve guide trouble? My primary complaint is loping which I cannot adjust out by the (professionally rebuilt) carb even at a very low (~400rpm) idle.

[Tom Wesenberg]

The 4 cylinder will have an even flicker of the needle due to a suction every half turn of the crankshaft. As long as the flickers are evenly spaced your valves are probably OK. How does the needle read when you have the engine at fast idle, like 1200 or so RPM?

OK, I see you said it was present at all RPM's. Try shorting the plugs to ground and see if any cylinder has little to no effect.

[BILL WILLIAMSON]

Have you checked the compression? Just off hand, "might" be a burned valve, or a valve adjusted to tight to close completely?? Does any cylinder have a complete miss? "Might" be a slightly sticking valve. A compression or a leak down test would be the first test to rule out compression problems.
Check it in the dark, "might" be the conductor going to #4 cylinder, in the distributor body, arcing out the top of the body, to #3 plug strap, I'm not sure if that would give you jumping guage readin, or not. If it's the arcing problem, it'll cause a miss, BUT it's usually erratic, NOT a constant miss. Just throwing out some possibilities,---As it's been a looong, tiring day & my "brain"?? is TIRED!!---Bill W.

[pooch]

A cheap undamped vac gauge will always fluctuate every time a valve opens.

You are looking for irregularities as the engine goes thru its 4 cylinders firing.

A fast regular flicker of a cheap gauge is normal.

If you want to learn and play with a vac gauge, set revs to about 1200 and then move spark lever, the results will impress you.

[freak]

Ok... thanks everyone. I'll be doing a lot more testing. The vacuum gauge seems like a great diagnostics tool but there is no doubt that I need a better one.

[Mitch//pa]

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i never used a vacuum gauge in my proffesional life since the late 70's early 80's. i do own one mainly for testing fuel pressure on mechanical pumps... IMO there are more accurate ways to diagnose drivability and mechanical engine problems.....

[pooch]

I don't know about model A engines and vac gauges but I do know that it is the best way to set optimum timing on a later engine.

Set revs at about 1200 RPM so the bob weights are advanced and set timing to maximum vacuum and back off just a touch .

This will get your fuel mix to the maximum pressure at 17 degrees after top dead centre where maximum power is obtained.

Maybe I should have tried this on my own A before I sold it.

It would have given me the best place for the spark lever at cruising speed without guessing .

[oak29]

I'm helping a friend with his A. We've had to replace the head gasket - it was blown - recon the distributor (cam and bearings shot) and rebuild the carb (jets missing from original!). It's running loads better but there's an underlying misfire and she's slow in pulling under load. We've timed the engine with a timing gun against one of those graduated scales bolted to the block, and it staying solid and the adv/retard lever is working fine. We've replaced plugs, dist cap and rotor with known working parts to eliminate any shorts on the electrics. We've done a compression test and all 4 pots are fine at c65 with a variance of 2 between them.
We've put a vacuum guage on and the readings, like freak's above, are flickering between 17 and 23. When you open the throttle and hold it at higher revs, it drops and stays between 10 and 17. When you read the diagnostic charts for what the readouts mean, it says a flickering reading like this indicates worn valve guides or head gasket issues. Two questions. 1.if you had a head gasket issue, wouldn't it show up in the compression test? 2. is the flickering needle perhaps a 'feature' of a cheaper guage? 3. I've been been reading about leak down tests. Could you use a pressure limited (say to 70) compressor to pressurise each cylinder (at TDC) to do the leak test; if not else what could you use? (a tyre pump?).
We're working on the car again tomorrow so any quick advice - particularly on Q3 - would be welcome. Thanks John

[Crankster]

Originally Posted by Mitch//pa

i never used a vacuum gauge in my proffesional life since the late 70's early 80's ... IMO there are more accurate ways to diagnose..

Too bad this feller was banned he could have shared his "special sauce" with the rest of us amateurs for diagnostics. Oh well.

[Werner]

Guten Abend John,


I have mounted such a vacuum gauge because it is ancient and beautiful. A stylish device of its time. Nothing more! Because the benefit for diagnosis is extremely doubtful.

I would rather see on the basis of the spark plug insulator stone color, whether the engine burns too lean or too rich the mixture.

[Crankster]

Originally Posted by Werner

Nothing more! Because the benefit for diagnosis is extremely doubtful.

Is that observation specific to the Model A engine for some reason? I can't speak to that, however for the more modern engines they are easily one of the most useful diagnostic tools ever invented.

[freak]

Originally Posted by Crankster

Too bad this feller was banned he could have shared his "special sauce" with the rest of us amateurs for diagnostics. Oh well.

How do you get banned from Ford Barn? I rarely see anyone argue too bad or anything.

[Redbird]

You can gauge your fuel mileage running a vacuum gauge. Keep it in the green or high green as you drive, and you get the most mileage.

[burner31]

Originally Posted by freak

How do you get banned from Ford Barn? I rarely see anyone argue too bad or anything.

Here's how...
Several years ago someone asked for pictures for Mocha Brown (paint color)
I googled and found a picture of one Ms. Mocha Brown, she must have been a criminal because it looked like a mug shot, I posted the picture with a single word (Found)
I was banned for thirty days
I guess I see humor differently then others, but thinking back, I still chuckle over that one.

[katy]

I once got banned for 30 days for using a word that the censors thought was inappropriate, I thought it was humorous.

[Aretino]

cancel culture

[oak29]

thanks for feedback. We've sorted the problem. We rechecked the timing and points; gap had opened up to 25+ thou. We corrected it; and messed around with the timing in the end giving it a bit more advance. We also balanced the idle jet and gas valve for smoother running. She's ticking over much better and pulling better now.



Feedback on vac guage and readings; first I suggest you get another model A that running well to compare against and take a video of the guage readings. We used my Ford Model A as the comparison. Taht way we could see what was normal flicker and a flicker caused by a real fault! At normal idle, the needle is flickering all over the place, so don't read too much into those readings. At a step above fast idle, the needle settles down in the green zone but there's still a bit of flicker. When you blip the throttle it falls fast to 5 and then picks back up into the green. The meter we used is a $35 one from a good manufacturer. Found the vac guage straighforward to connect and a good way to tune things.

[old31]

Originally Posted by burner31

Here's how...
Several years ago someone asked for pictures for Mocha Brown (paint color)
I googled and found a picture of one Ms. Mocha Brown, she must have been a criminal because it looked like a mug shot, I posted the picture with a single word (Found)
I was banned for thirty days
I guess I see humor differently then others, but thinking back, I still chuckle over that one.

That's funny, and i don't see anything wrong with that.