Timing and Reading Vacuum

[71GTXdragpack]

Hi all,
I'm trying to set the innitial timing on my 71 GTX. My dad always recommends checking the manifold vacuum along with setting the timing. I tried to connect my vacuum gauge to the hose from the carb to the distributor but it doesn't show vacuum until the engine is reved. The other hose on the carb goes to the brake booster but it much larger diameter vs my vacuum gauge. Is that where I should be checking manifold vacuum?
Any assistance is appreciated!
thanks,
Karl

 

[AR67GTX]

Some carbs have a vacuum port above the thottle blades ( which you seem to be on) and provide ported vacuum - when the throttle is open. Other carbs have a vacuum port below the throttle blades and show manifold or full time vacuum, even at idle. And some carbs have both on them. What carb is on your engine? Sometime there is a divorced choke on the engine with a full time vacuum connection to a small choke pull off vacuum can.

 

[71GTXdragpack]

Some carbs have a vacuum port above the thottle blades ( which you seem to be on) and provide ported vacuum - when the throttle is open. Other carbs have a vacuum port below the throttle blades and show manifold or full time vacuum, even at idle. And some carbs have both on them. What carb is on your engine? Sometime there is a divorced choke on the engine with a full time vacuum connection to a small choke pull off vacuum can.

I'm pretty sure it's the original carb which I think is a Carter. Pic is for reference, I installed the vacuum gauge on the left side port which I believe is the ported. As you can see the right side port is larger diameter and goes to the brake booster.

 

[HALIFAXHOPS]

Need to t off of the large one or is there a nipple somewhere on the intake manifold? Sometimes under the back of the carb on the manifold there is a tapped port with a plug you can get a adapter to put in.

 

[HALIFAXHOPS]

Wait just looking the tube for the choke pull off is manifold vacuum.

 

[AR67GTX]

Ok - to check idle vacuum you need to get a plastic vacuum T and short piece of vacuum tube. Pull the vacuum hose off of the choke pull off and put the T in line using the short piece of vacuum tube to connect it back to the choke pull off. Then hook your gage up to the T. When done you can either remove the T or leave it on and plug the one outlet with a vacuum cap.

 

[AR67GTX]

I use manifold vacuum to adjust the idle mixture screws generally, although more often I use a tach. Adjust the mixture screws to get the highest vacuum at idle. I don’t use vacuum for setting timing. Idle vacuum state can sometimes be used to diagnose an engine problem like a burned valve or vacuum leak, etc.

 

[71GTXdragpack]

Need to t off of the large one or is there a nipple somewhere on the intake manifold? Sometimes under the back of the carb on the manifold there is a tapped port with a plug you can get a adapter to put in.

there is a nipple at the back of the manifold that supplies vacuum to the air grabber. I tried connecting the gauge there with no vacuum. I have been hearing a whistling sound coming from under the dash, looks like my air grabber switch hose nipple has broken off the body. Tried installing a used air grabber switch I had and it's leaking through the body also. Might just bypass it to fix the vacuum leak.

 

[HALIFAXHOPS]

Plug it for now. I think Tommy at parts hound can rebuild them.

 

[turbine68rt]

I would source a manifold nipple that's branched for your booster as well as the air grabber, then get your pcv system corrected by using the larger center port going to the pcv valve.

 

[69a100]

I just disconnect the power brake booster hose at the manifold and use that port. Good Luck

 

[sam dupont]

Plus, my IH FSM's say to give it one degree advance for every 1,000 feet of elevation. Living at 4,000 feet, I find the difference to be noticeable.

 

[71GTXdragpack]

I just disconnect the power brake booster hose at the manifold and use that port. Good Luck

the hose connection on the gauge is much smaller than the brake booster hose. I'll get a step up connector and check from there. Thanks for the advise!

 

[AR67GTX]

Why would you mess with the brake booster port? Use the choke pull off nipple. The gage tubing is probably the same size.

 

[HALIFAXHOPS]

Thats what I would do if you dont have a tee. Warm up so the choke is open then put the gauge to that manifold port.

 

[Bee1971]

If it’s a Factory 1971 Carter AVS

There is a full time vacuum port nipple located in back bottom , small line port like you need

Went or connected to the air cleaner for the vacuum pods on air doors

 

[turbine68rt]

If it’s a Factory 1971 Carter AVS

There is a full time vacuum port nipple located in back bottom , small line port like you need

Went or connected to the air cleaner for the vacuum pods on air doors

From what I can see, that carb number is for a '70 440 mag. (4738)

 

[71GTXdragpack]

If it’s a Factory 1971 Carter AVS

There is a full time vacuum port nipple located in back bottom , small line port like you need

Went or connected to the air cleaner for the vacuum pods on air doors

Hi all, I used the vacuum port nipple at the back of the manifold and adjusted the carb to 19 inches. The engine is purring so thank you all for you help. Now I'll tackle the timing, hope to get about 12 degrees advance and total of around 40.

 

[sam dupont]

19 is about as good as it gets. Pop open a cold one.

One of my dad's friends was a line mechanic at the International dealership. They would take the big trucks to a highway leading out of town to set the timing. It's a steep hill two miles long, so they could put a load on it, stop and adjust, repeat. They would advance the timing until they got a ping, then they would back it off just a little bit. That way they knew if the owner put a heavy load on it and was in a hurry, it wouldn't detonate the motor.

They sent a mechanic up to Limestone Country to set the timing on a '65 Scout Turbo. The dealership was at 3,000 feet, the property was over a mile. They couldn't guess correctly at the lower elevation.

 

[71Beeper]

40 degrees total on a healthy 440 is high.
Try to keep it around 32 to 34 degrees.