Tuning with vacuum gauge

[Deleted User]

I would like some feedback on tuning Hudsons with a vacuum gauge, i.e. adjusting carbs and setting the timing. If you have done so, have you found it efffective, any tips you could pass along to me. I have a 54 308 with Hydramatic and Twin H.

Thanks for any help you can provide,
Doug

[Deleted User]

You need a UNI-SYN to start with, and set both carbs the same and then the linkage. The vacuum gauge is then used to set the idle adjusting screws to the highest vacuum reading, then recheck the carbs again. Timing set on long mark on flywheel, then check that the centrifugal is working and that the vacuum advance is working. Walt.

Jon B

I tried using a vacuum gauge to set my Terraplane's timing, and the engine starting pinging so I gave up!

hudsn47

When tuning with the vacuum gage, where exactly is the best and easiest place to hook it up? Thanks, Doug.

onerare39

Doug,
I found this article a few years ago, hope this helps.
John Forkner



The best way to set the spark timing is by a vacuum gauge. The reason being that method takes into consideration the condition of your engine and your elevation/local conditions.

How to do this on a Hudson.

Attach the vacuum gauge to a source of full intake manifold vacuum. Do not attach the gauge to the vacuum advance line coming from the carburetors, as this is ported source and not full manifold vacuum. Attach the gauge to the line coming from the intake that supplies the windshield wiper motor, or going to the two-stage vacuum assist fuel pump.

Disconnect the vacuum advance line from the distributor and plug it off.

Crank the engine and watch the gauge, the needle reading should be steady. It should be reading somewhere between 18 and 28.

Loosen the 7/16" bolt that holds the distributor in the block and rotate the distributor counter-clockwise and look for a rise in your gauge reading. If it begins to miss and lower the vacuum reading, move it clockwise. Keep moving the distributor slowly in both directions until you get the highest, steady vacuum reading possible.

Once you have the highest reading your engine will achieve, move the distributor clockwise until you drop 1" Hg off the gauge. For example, if your engine will make 22" Hg - lower the timing until it reads 21" Hg. The reason for this is that the 99% of the mechanical advances I've taken apart are worn at the cam plate and let the mechanicals fly out too far at max advance. This initial retard of a couple of degrees makes up for that over-advance on the high end and has no effect on the engine in the driving rpm ranges. Being a couple of degrees retard at starting makes cranking a bit easier as well.

While you have your vacuum gauge hooked up, check your carburetor idle mixtures as well. Turn the idle mixture screws in and out one at a time while watching the vacuum gauge. Turn one idle mixture screw clockwise while watching the gauge, when you exceed the ideal lean limit of that screw - the gauge reading will lower. Then turn counter-clockwise until it reaches its peak reading. Repeat this for both screws. Lower the idle speed screw and repeat the idle mixture screw process. Your goal is to lean the idle mixtures only enough to achieve maximum vacuum at the lowest possible, smooth idle rpm. It takes a few repetitions to get it right, but eventually any slight turn of the idle mixture screws clockwise results in an immediate vacuum gauge response.

If adjusting your idle mixture gains an appreciable vacuum gauge increase, repeat the timing procedure, then double check your idle mixtures one more time.

Adjust your idle speed back to where you're comfortable with it.

Now hook up your timing light and just shoot at the timing marks. If the marks are jumpy at steady idle, you have some wear in your distributor. Either the mechanical advance or the main shaft bushing. Make a note of where your timing is according to the light and check for proper mechanical advance by slowly increasing the engine speed.

Hook the vacuum advance back up properly and go Hudsoning.

7XPacemaker

John,

That is a good set of instructions. The name of the game is trying to get the engine to run as close to the stoichiometric mixture as possible. While NO engine will run at the perfect stoich mixture, with the aid of a vacuum gauge gets us as close as we can. This helps with fuel mileage and overall performance. Retarding the timing a little bit after max vacuum will keep the engine detonation out of the equation. If the timing is not retarded slightly after max vacuum is achieved, the car will probably not restart after it is shut off- the timing will be too far advanced. It may take a little time to find the "sweet" spot, but it is definately worth it in my opinion. As long as the gauge is hooked up to a constant vacuum source on the manifold, you shoud be good!

RonS

I've tuned my Hornet with the vacuum. I combined Walts uni sync manner and Johns. Backed off 1 hg at the finish. The car runs very well. Just to add one thing,,, ie, your vacuum reading will be lower by 1 hg for every 1000 ft elevation. Keep this in mind for you guys who live in mountain areas. My elevation is 2700 ft, so I will be about 3hg lower with all things equal.