Adjusting timing using vacuum gauge

[tom'sB2B]

I want to adjust my timing using a vacuum gauge. I have an electric wiper motor. Where do I attach the vacuum gauge? What vacuum numbers should I be looking to achieve?

thanks guys

[Bradley S.]

I don't remember where I got this information. Probably off the net partialy and partially from one of the great minds here on the forum. This is how I timed my truck engine with vacuum. Remember to set the points dwell first, then time.

Brad

Timing Engine Using Manifold Vacuum

Older engines can't always be tuned or adjusted per their original guidelines. Timing chains can slip over the years, harmonic balancers can move, and the bottom line is you are no longer able to verify where top dead center is using the old methods. Plus, with today's fuels, it's more difficult to reach the point where your old engine performs both efficiently and reliably without sacrificing performance. Following the steps below, you will be able to minimize pinging, have reliable starting under all conditions, and easily get maximum performance from your engine using today's fuels.

1. Connect a vacuum gauge to a vacuum source directly on the intake manifold. Do not attach gauge to ports being used by any other device. Plug any vacuum lines you remove from manifold to make this connection.

2. Loosen the bolt securing the distributor so you can turn the distributor to adjust the timing.

3. Start the motor, let it warm up to normal operating temperature, and stabilize at normal hot idle. Using the idle speed adjustment at the carburetor, set the idle speed as low as possible without stalling the motor. Disconnect the distributor vacuum advance and plug the line.

4. Keep in mind when adjusting the timing, you have to turn the distributor clockwise to advance and counter-clockwise to retard.

5. While watching the vacuum gauge, slowly turn the distributor clockwise (advance timing) and look for a maximum reading on the gauge. For instance, if the reading on your gauge peaks at 18 inches of vacuum, that is the maximum for your engine. (The normal range could be from a high of 20 inches to a low of 15 inches, depending on the internal condition of your motor). The reading will drop off if you advance too far, so turn the distributor counter-clockwise (retard timing) to go back to the peak reading on your gauge.

6. Once you have established the peak reading, retard the timing (turn distributor counter-clockwise) to reduce the reading by about 1 inch to reduce ping. You may need to reduce the reading by 1.5-2 inches total, or even more, depending on the fuel available in your local area. Let your engine's performance guide you. If you had the same 100 octane fuel in your tank that was available when these cars were new, you would most likely leave the timing at the peak vacuum reading, but even when these cars were new, there were issues with fuel. Old Technical Service Bulletins (TSBs) exist that recommend retarding timing due to engine ping, so this was a concern even back when the cars were relatively new. We've included a chart on Engine Vacuum Gauge Diagnosis (link opens a new window, disable any pop-up blockers to view) to help you determine what the gauge readings mean.

7. Stop the motor and tighten the distributor hold-down bolt. While you still have your vacuum gauge hooked up, it is a good time to set the idle mixture needles on the carburetor. The adjustment is normally made with the two screws at the front base of the carburetor, depending on model. Turning the screws in will cause a richer mixture, turning the screws out will cause a leaner mixture.

8. Start by turning the two screws in fully then backing them out about 1 1/2 to 2 turns. Start the motor and check the vacuum gauge. What you want to see is a maximum steady vacuum reading. At the beginning stick to adjusting just one screw at a time to achieve maximum vacuum. As you turn the screw in you will see a reduction of vacuum on the gauge. When you turn the screw out you will see an increase in vacuum but only to a point. If you get the mixture too lean the vacuum gauge needle will start bouncing around. Your goal is to have maximum steady readings. Then repeat the process for the other screw. To fine tune, adjust both screws slightly while watching the gauge.

9. Once you have achieved a satisfactory idle mixture, set the idle speed to specification, then remove the vacuum gauge and reconnect any vacuum lines you removed. Don't forget to reconnect the distributor vacuum advance.

Test drive your car and listen for any ping under light acceleration as well as under heavy acceleration. Additional retarding of the timing may be necessary if excessive pinging occurs. You should hear a very slight ping under light acceleration, and none under heavy acceleration. Your engine is now optimally timed to run on today's lower octane, no lead fuels.

[tom'sB2B]

Great info.! And easy to follow. Can't wait to try it out.

thanks Brad.

tom

[Martin53Cranbrook]

Awesome info, this looks like just what i needed, now i just need to pick up or borrow a vac gauge and get this done.

[Dave72dt]

A gauge that can be mounted in the cab after timing the engine can be used to help adjust your driving habits for getting better mileage. Maintaining higher readings on the gauge gets you the best mileage.

[DollyDodge]

I was looking through the old posts on vacuums, and this is the only one that seemed to help me a little, but they never really answered where to hook the vacuum. I installed vacuum gauge in my B-2-C this weekend, but the only vacuum line is to my carb for the vacuum advance and the gauge reads 0 at idle, once I start to accelarate then the gauge moves, it hangs in the 15-20 range at a slow cruise, drops really low going down hill, and rises going up hill. I don't know much about vacuum, but what I have read says the connection should not be where I have mine (closed vacuum at idle to the vacuum advance), but I don't see any other option. Any help would be appreciated as usual. I also installed a voltmeter. You can see how I mounted them in the photos. I like gauges. One thing about having the vacuum attached where it is the gauge moves a lot . I was able to mount them in the radio slot without having to modify the dash. The last picture is of the truck at idle, you can see the vacuum is low.

[greg g]

There are a couple places on the intake manifold where there are casting areas that if there are no vacuum wipers, be drilled and tapped to accept a fitting for attaching the gauge. If its just for a Test and tune, remove the vacuum hose to the wipers and attach the gauge. If you have wipers, the line can be fitted with a T and the gauge can run off the side of the T. manifold vacuum on a well maintained engine at idle should be steady at 19 to 21 inches assuming a stock cam and proper timing.

If you go the the second chance garage web site they have a great sectio with reading and interpreting the vacuum gauge's reading with animated gifs showing the varying movements of the gauge and what they mean.

If you use the vacuum signal from the carburetor, you will have reading that are basically opposite of manifold reading.

Typically manifold vacuum will be high and steady at idle, will drop quickly at WOT, and will regain at partial throttle opening like at moderate cruise speed.

[Dodgeb4ya]

The vacuum guage needs to be connected at the rear of the intake manifold. Your current conection is above the throttle plate-thats why you have a reading only when you are above idle speed. There is a plug to remove for your guage connection. It should read 17-21" of vacuum.

Bob

[DollyDodge]

My old truck has electric wipers so I can't use that option and I will look on the rear of the manifold. I didn't see any plugs when I last looked, but I will look more closely, if there isn't one I will tap the manifold when I take the engine out for rebuild in Dec. Thanks for the info.

[Barry Maxwell]

"Reading your vacuum gage" from the following link may be helpful to some.

www.classictruckshop.com/clubs/earlyburbs/projects/vac/uum.htm

[DollyDodge]

Cool information. I looked last night and sure enough there was a plug in the rear of the manifold, and in the kit that came with the Westach Vacuum gauge there was a brass fitting that fit the manifold port perfectly. So I hooked the gauge up last night. Engine idles at 17 cold, 18 warm. It drops to 10 and below when going up a steep hill, and around 20 coming down hill. Running along on flat ground it seems to hold at 17. I am just tickled to have the gauge hooked up correctly and working. Thanks for all the help.