Vacuum testing questions, stock 21 stud
 

Hello 'Barners, I have two questions about vacuum testing and values.

The engine is a stock 21 stud, helmet dist, stock fuel pump, water pumps in heads, real stock.

Did a vacuum test today, engine cold, gauge attached to the line for the wipers, right off the intake manifold. Readings between 15 and 16 inches at idle which I estimate at 500-600 rpm.

What are the appropriate values? Did I do the test correctly?

Thanks in advance for your input.

-VT/Jeff

Re: Vacuum testing questions, stock 21 stud
 

Looks good to me if it was steady,I think 15 in is very good.

Re: Vacuum testing questions, stock 21 stud
 

Does this help?
http://www.secondchancegarage.com/public/186.cfm

Re: Vacuum testing questions, stock 21 stud
 

Here ya go. I believe you want around 18lbs.

Good one.
http://i201.photobucket.com/albums/a...geReadings.jpg

Maybe easier to read? Not as detailed as above chart.
http://3.bp.blogspot.com/_3gMQry4o_J...00/vacuum1.jpg

One more....
http://www.my5oh.com/tech/vacdiagram.jpg

Re: Vacuum testing questions, stock 21 stud
 

The reason for the test was to use the vacuum gauge to set the initial timing at idle.

We understand the way to do this is hook up the gauge and move the set screw on the side of the distributor upward, advancing the spark until a peak is detected, then move the screw back down to reduce the vacuum by 1".

What we found was there was a constant reading of about 14" when we started, and by advancing the screw we gained 1" of vacuum. At that point the screw was at the top of the slot and no further advance was possible.

We made a test run and the engine 'seemed' to be a shade happier, there was no pinging even though we tried to make it ping. We noticed a slight drop in engine temperature while running under normal conditions.

My opinion (not based on scientific evidence) is that the typical flathead as we use them could stand more advance. The fuels are better and the roads are better. Getting the engine to run at the proper advance point also helps it to run more efficiently and thus cooler.

Has anyone ever modified the top of the slot in the distributor so the adjustment set screw could be moved further up - thereby advancing the timing somewhat more? IS there any other way we should approach this? The distributor is a Ford genuine rebuilt which was sent to Skip H. for checking - he said it was OK.

Sorry for the long post -

Henry

Re: Vacuum testing questions, stock 21 stud
 

Im getting 20" of vacuum.. My idle is slightly higher than 500rpm tho'

Re: Vacuum testing questions, stock 21 stud
 

What you say is especially true at higher elevations. At 5,000 feet where I live I run full static advance on the distributor and full vacuum advance and get more power and runs better. No pinging.

Re: Vacuum testing questions, stock 21 stud
 

Vacuum leaks, timing, carb settings, all will all effect vacuum reads... or am I wrong. Low readings doesn't mean major engine problems but maybe a tuning issue. If you have blowby or low compression readings, then yes.

Flatheads always run hot when retarded. When is it too much advance? and when does it hurt performance? and what is to say that the dizzy is setup right too? Do point gaps effect advance settings? Inquiring minds want to know.

Lean condition will also cause a flathead to overheat too. Well a host of things will.