Vacuum gauge measurements

[goodnigh]

Have been having carb issues with poor idle and performance when cold. Cold here means 60F. Turned all four idle screws back to stop

then opened 1 1/2 turns each. Engine purrs but vacuum guage

slowly oscillates between 15 and 16 in Hg. Timing is solid at 16. Could the flucuating vacuum be due to a cam? When this

engine was rebuilt it was rumored to have a cam installed.

mike

 

[72HCODE]

could be.

go here

http://www.secondchancegarage.com/public/186.cfm

click scenario #3

anything like a mild cam is going to give you about 15-18" of vac.

with tuning you should be able to come up a little more from 15"

 

[goodnigh]

could be.

go here

http://www.secondchancegarage.com/public/186.cfm

click scenario #3

anything like a mild cam is going to give you about 15-18" of vac.

with tuning you should be able to come up a little more from 15"

Excellent demo, my gauge looks just like scenario #3.

It bounces between 15 -16. We were having issues with

the idle screws so I cranked them all in and back out 1 1/2 turns

just like they said to do years ago. I may fiddle with those a bit

to try for more vacuum. But as you know the enemy of good is

better. My mechanic had them set different from each other but

Holley says they should all be the same. If you turn one a quarter

you should turn them all. As far as the cam, I don't know what it

has. My mechanic measured intake valve lift to be 0.58".

mike

 

[72HCODE]

Basically with the vac gauge hooked up. You begin adjusting the bleeds till the vacuum rises without effecting idle Rpms while under load with a automatic.Usually you lean them out at idle that raises the vacuum. I get about 17" at idle after tuning and adjustment. So with the car in drive foot on brake you start adjusting the bleeds evenly. 1/8 a turn at a time, until the vac rises but the rpm do not. Leaning raises idle.

Then you go for a ride and usually you end up opening the bleeds back up 1/8-1/4 a turn to richen it.

Next step would be tuning the power valve. If you maintain 15" in gear, then you divide by 2 add 1 . So you would install a 8.5hg power valve for performance or keep the 5hg valve for fuel economy. Some people install a dual stage valve that opens half way at 8.5 then fully 5.5 this is to overlap the engine going lean under hard acceleration when vac drops off. Larger hg rating means it opens earlier.

You could also tune the accelerator pump.

After that you lead into secondary tuning if you have a 4 barrel.

Basically your tuning an engine to use more fuel at certain points to cover lean spots and make more power which drops the mpg.

could be.

go here

http://www.secondchancegarage.com/public/186.cfm

click scenario #3

anything like a mild cam is going to give you about 15-18" of vac.

with tuning you should be able to come up a little more from 15"

Excellent demo, my gauge looks just like scenario #3.

It bounces between 15 -16. We were having issues with

the idle screws so I cranked them all in and back out 1 1/2 turns

just like they said to do years ago. I may fiddle with those a bit

to try for more vacuum. But as you know the enemy of good is

better. My mechanic had them set different from each other but

Holley says they should all be the same. If you turn one a quarter

you should turn them all. As far as the cam, I don't know what it

has. My mechanic measured intake valve lift to be 0.58".

mike

 

[goodnigh]

could be.

go here

http://www.secondchancegarage.com/public/186.cfm

click scenario #3

anything like a mild cam is going to give you about 15-18" of vac.

with tuning you should be able to come up a little more from 15"

Went out today and fiddled with the idle screws.

I was getting 15.5 and the gauge slowly oscillated.

I turned the screws 1/8 closed at a time until I got

a steady (shakes a bit) 17.5, turn them all one 1/8

closed again and the vacuum dropped to 15 with rough

idle. Backed off 1/8 , and all is fine. Found the carb's

sweet spot.

mike