Using a vacuum gauge

[adileo]

Hi folks

My first attempt at using a vacuum gauge to adjust the dual carbs on my 39 coupe.

Currently my vacuum line only goes to the helmet distro. Wipers have been disconnected.

Do I disconnect the vac line to distro and connect vac gauge direct to manifold? Or do I need to get a Y connector and have both the gauge and distro vac connected together to adjust carbs?

[weemark]

disconnect the linkage on carbs then plug the vacuum line into the base of each carb and tune them individually. Once you have balanced them individually connect the linkage etc. back up and check nothing has changed.

[adileo]

The only vac outlet I have is on the manifold.

[37 Cab]

My dad used one of these back in the day.

https://www.amazon.com/Edelbrock-402...chronizer+tool

[V8 Bob]

Disconnect the linkage, run all 4 idle mixture screws in and then out 1 1/2 turns. Now start the engine and sync each carb for equal air flow and adjust for a low speed idle. With a vac gauge connected to manifold vacuum, slowly adjust the mixture screws for the highest vacuum levels.
Now re-sync the carbs with the desired idle speed and connect the straight linkage, and re-check sync.
You want the idle rpm to be low enough to allow mixture adjustment, but running a big cam may prevent this, so just stick with syncing and leave the screws 1 to 1 1/2 turns out.

[adileo]

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Bob thanks but....

Do I disconnect the vac line to distro and connect vac gauge direct to manifold? Or do I need to get a Y connector and have both the gauge and distro vac connected together to adjust carbs?

[flatheadmurre]

Are we talking vacuum gauge or syncronizer here ??
Vacuum gauge doesn´t work on syncing carbs...

[adileo]

I would like to adjust fuel mixture.

[Ronnieroadster]

Originally Posted by weemark

disconnect the linkage on carbs then plug the vacuum line into the base of each carb and tune them individually. Once you have balanced them individually connect the linkage etc. back up and check nothing has changed.

To balance each carb you use a Unisyn gauge not a vacuum gauge. Also the carb linkage must be left on the carbs the proper lingage between carbs should have a left hand and right hand threaded adjustment rod between the arms on each throttle shaft. The carb ballance can now be adjusted using the Unisyn gauge to get the throttle shafts in a position where both carbs are pulling the same amount of air thus becoming balanced. After the carbs are balanced the idle mixture screws are then adjusted using the vacuum gauge the idea is to get the highest vacuum reading possible when adjusting the idle mixture screws. Most times getting the perfect setting takes a few try's and after running awhile the adjustments will need to be fine tuned again that's what I have found when tuning so many flatheads I have lost count.

[V8 Bob]

Originally Posted by adileo

Bob thanks but....

Do I disconnect the vac line to distro and connect vac gauge direct to manifold? Or do I need to get a Y connector and have both the gauge and distro vac connected together to adjust carbs?

I would either tee into the line or find another vac source, as you don't want to alter the mechanical advance. (You don't want the vac brake to work)

[V8 Bob]

flatheadmuure,
Both. You use the vacuum gauge for idle mixture adjustment, and Uni-sync for throttle blade position, and engine idle.

[adileo]

Guys sorry all I want to do is adjust fuel mix it's running lean.

Do I disconnect the vac line to distro and connect vac gauge direct to manifold? Or do I need to get a Y connector and have both the gauge and distro vac connected together to adjust carbs?

[TagMan]

If I were doing it, I'd disconnect the present vacuum line and "T" it, so the system is as it was. Why introduce another variable ? Not a big deal - just do it.

[flatheadmurre]

Originally Posted by adileo

Guys sorry all I want to do is adjust fuel mix it's running lean.

Do I disconnect the vac line to distro and connect vac gauge direct to manifold? Or do I need to get a Y connector and have both the gauge and distro vac connected together to adjust carbs?

If you´r running multiple carbs you first need to get them syncronized.
Or you could be running one leaner then the other due to poor syncronization giving you different mixtures to the cylinders(not sure what setup youre running here ?)
After that you can adjust mixture.

[JSeery]

Idle mixture adjustment isn't going to do much for a rich or lean condition! The overall mixture is controlled by the jetting in the carbs. The idle mixture adjustment is mainly to smooth out the idle and very low speeds when the idle circuits bypass the normal jetting (when the throttle plats are closed or very near to closed).

[keith oh]

Bravo JSeery for bringing up the Idle mixture fact. Well stated.