Vacuum reading opinion

[3twinridges]

We are building a 52 F1 and have it far enough along to get it out of the garage for a few test runs on our street. It has been started a couple of times to full temp so I could torque the heads down and today was the second test drive. I shot a video of the vacuum reading. Should I be concerned about the reading? It’s an 8BA bored 60 over with the 4” crank with all new internals.

https://m.youtube.com/watch?v=rAP51t...ature=youtu.be

JB

[petehoovie]

Stock cam?

[Black Fifty]

I'm no expert, but it seems reasonable. I think 18-21 is typical, maybe a bit lower or not as steady with a bit of cam. My two cents.
Mike

[3twinridges]

It’s running a new 1CM cam. It pulls like a mule even with a 3.54 rear end.

JB

[petehoovie]

Needle steady between 17 & 21 is a normal reading. I think you're fine with that cam....

[Drbrown]

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My stock '47 with 35,000 on rebuild shows a steady 19 on vac gauge when at running temp and carb set right. Per petehoovie's post, key is a steady needle indicator.

[19Fordy]

3twinridges; That reading is just fine and dandy. I am getting 20 on a stock 8BA with a Holley 390.

[Ol' Ron]

Your Idle AF will have an affect on the amt. However, whit see is pretty close. Check your AF in cruise, that's important Low 14 and up.

[Flathead Fever]

It should be pretty steady. You might have a problem and you might not?

If each cylinder is pulling in the "exact same" amount of air the needle will stay perfectly still because the vacuum does not change as each piston takes its turn sucking. If for some reason one cylinder or more is pulling in less air it causes the needle to momentarily drop because the amount of air coming through the intake lowered during that intake stroke

If the needle is steady but low the problem is something that is effecting all of the cylinders the same. Timing, low compression.....

If you see a steady, systematic drop. Something is "mechanically" wrong with one cylinder.. Burned valve, hole in a piston, broken valve spring, worn cam lobe...… When you have a big systematic drop the next test is to do a compression test.

If one cylinder does not fire because of an ignition or fuel problem the crankshaft rotation will slow for just a fraction of a second. Mechanically that cylinder is good and was able to create a good strong vacuum because another cylinder was firing and pushing our non-firing piston through its intake stroke, just like what normally happens. When its turn comes to fire and it does not, that slows the crankshaft rotation speed for just a fraction of a second that it would have an effect on how much air was drawn through the intake causing the needle to fluctuate just a little. As Ron said above, if the air fuel ratio is not correct they might not all be igniting creating the same pressure.

Something is not 100% the same in one cylinder as the others or all the cylinders are not consistently creating the same pressure on the pistons. The second part could be an air fuel mixture problem like Ron mentioned.

Adjust the air fuel mixture using a propane bottle. Add a little propane and if the rpm goes up the engine is too lean. Add propane and if the idle goes down the engine is too rich. Add "a little" propane and the rpm does not change the engine is at its happy spot. Gives the A/F mixtures screws another 1/8 turn rich for all around driving conditions.

This chart says a 1 to 2-inch constant fluctuation can be caused by spark plug misfire or a slight valve leak. A spark plug might not fire the cylinder because there was not enough fuel molecules between the center electrode and ground for the spark to ignite. Spark plug "not firing" does not always mean that it did not make a spark. Make sure your air fuel ratio is correct

Shut off the engine pull the plug wires and just set them back on the plugs so they make contact but can be easily removed. Start the engine and with the best insulated plyers money can buy lift one wire off at a time and see what effect it has on the rpm and the vacuum reading. Compare the rpm drop and the vacuum drop of all the cylinders. If they are all the same you probably don't have a problem.

Warning, never remove a spark plug wire on a running engine with electronic control modules or computers. The voltage is always looking for a way to get to ground. If it cannot jump the gap when you pull the wire it will find another way to get to ground. It will backtrack through the plug and it might just find a path to ground through the electronic module. With electronic ignitions they make a special dummy spark plug that you clamp to a ground and connect the plug wire to it before checking the cylinder. Its not the same as trying this test with a grounded loose spark plug. You need to buy the tool. They are made internally different.

Here is website describing how this special spark plug works. Its only for testing HEI ignitions.

https://easyautodiagnostics.com/misc...i-spark-tester

[3twinridges]

I adjusted the AF mixture and got a more steady reading and a tad higher. I think I need to dial the carb in a little more and make sure my timing is dead on one more time.

https://m.youtube.com/watch?v=1hDNGXsH558

JB

[flatjack9]

What is your idle speed? It will make a difference in your vacuum readings. I certainly would not lose any sleep over your present reading. Drive and enjoy.

[3twinridges]

interesting question and I have wandered that myself. what is the best way to get my rpm since I am 6 volt?

When I set the idle is was barely enough to keep it running.

JB

[flatjack9]

Isn't there an adjustment screw on the throttle linkage? Also, what is the initial advance?

[Flathead Fever]

Originally Posted by 3twinridges

interesting question and I have wandered that myself. what is the best way to get my rpm since I am 6 volt?

When I set the idle is was barely enough to keep it running.

JB

Idle speed has nothing to do with it being 6-volts. What do you mean when you said, "I set the idle"?

Normally you would just turn the idle speed screw in a little and the idle speed should come up. Turning the screw opens the butterfly just a little allowing the engine to get more air and fuel which raises the idle speed. More air and fuel in the cylinder creates more pressure when it is ignited. More pressure pushing on the top of the piston makes it move faster.

Normally on an automotive carburetor you have two idle air fuel mixture screws. Your idle speed adjustment screw and also a fast idle speed adjustment for when the engine is cold and the choke is on.




If you want the deluxe idle speed adjustment use the steps below.

Make sure the fuel is not old. Most places are saying this new fuel will start to go bad in six-months. Old fuel will effect the idle speed.

1. Always set the distributor dwell (point gap) first.

2. Then set the timing. The dwell (point gap) changes the timing but the timing does not change the dwell. That is why dwell is set first and then timing. 4 degrees before top dead center is the stock timing setting on a flathead. How you check the timing depends on what year flathead you have.

3. Adjust the idle air fuel mixture's adjusting screws. Timing will effect the idle air fuel mixture but the idle air fuel mixture will not effect the timing.

4. The last thing is to turn the idle speed screw to the correct rpm.

[40cpe]

I "think" he meant that most tune up gauges are 12v, hence hard to get an idle speed reading with them.

A spare 12v battery set on the floor next to the engine would power the 12v tach, timing light, etc.

[svmsr1044]

What effect on vacuum reading and/or timing would a mild cam such as an Isky 77B have on an otherwise stock 8ba?


Thanks,
Steve

[BUBBAS IGNITION]

Check the vac reading at different rpms , if it jerks at all rpms one or more valves havent seated yet!!! May take a trip or two to get a perfect seat seal...

[1948F-1Pickup]

Can't see the second video but the first one just shows the need for a little
bit of tuning on the idle mixture screws.....

[Flathead Fever]

Originally Posted by BUBBAS IGNITION

Check the vac reading at different rpms , if it jerks at all rpms one or more valves havent seated yet!!! May take a trip or two to get a perfect seat seal...

heck out this article on camshafts and vacuum. Its Comp Cam talking about Chevy motors.

https://www.hotrod.com/articles/ctrp...-vacuum-rules/

[Merc Cruzer]

How much would high altitude effect the overall reading?