Vacuum lines where do they go

[jimmy78]

Looking at the carb on my 350 5.7l I have almost all the vacuum ports plugged off.

The only ones connected are PCV valve to large port on front of carb. And turbo 400 to intake manifold. Hei distributor to small port on front carb.

There is a vacuum actuator pot on the side of the carb that is capped off that seems like it should be plugged into something.

I will post pictures soon and hopefully the wise people on here can help me straighten it out.

 

[mtbadbob]

Is there a port on the carb base plate?

 

[jimmy78]

There is one on the left front corner (looking towards the firewall) that’s capped off yes.

 

[mtbadbob]

That vacuum pot should be for your choke, not exactly sure on what vac port it should be connected to. You should be able to find a vac diagram on this site somewhere.

 

[59840Surfer]

The HEI vacuum must get its signal from manifold vacuum - but it has to go through a thermostatic vacuum control valve first so it won't advance until the engine's warmed up sufficiently for the oxygen sensor(s) to start talking to the computer. On some small-displacement engines, there may be a vacuum delay valce to keep the EGR from dumping too hard and heavy into the combustion chambers.

As systems got more complicated, there were added a lot of checks-n-ba;ances via computers and scripts to modulate the EGR.

Manifold vacuum is needed to compensate for throttle load on the engine - as the throttle opens, the vacuum drops until the RPM rises high enough to be able to handle the extra advance and not ping or knock.

If you are using the EGR --- and you should because it lowers fuel consumption and lowers the need for higher octane to keep the combustion temperatures low and not destroy the engine.

Many people have zero idea of the purpose and the value of the EGR and just merrily rip it off --- but having it operate is a good idea --- but ...

It has to not be allowed to come ON during warm-up and is only actually needed during cruise speeds --- and that's where it gets the mileage. .

The vacuum supply for the EGR needs to be qualified through a thermostatic ported valve that is coolant-compensated - and that vacuum CAN arrive through a vacuum amplifier to make sure there is sufficient vacuum to keep the valve ABLE to operate during cruise --- uphill and/but not so much for downhill.

Just beware that there are different types of vacuum available -- ported, manifold and modulated, to name just three (there's more) --- so picking the correct port for whatever you want to use them for, makes a difference.

MANIFOLD - is ALWAYS ON and is taken from sources from inside the manifold or plenum, below the throttle body or carburetor - although some AFBs, Central Ported FI and Q-jets have such ports included in the baseplate castings --- so beware!
PORTED - is usually taken from above the venturi or throttle plate and it is usually co-ported with another bleed-port to drop the signal and keep it from swamping' the signal to not override the application of vacuum as too aggressively or too slowly.
MODULATED - MAY be a small hole or slot in the "tip-off" position of the bore of the throttle body where the plates or butterflies rest at idle -- to ONLY apply vacuum just as the throttle makes the first few degrees of rotation.

PCV and Power Brake taps are always to receive full available engine vacuum.

Those transmissions with vacuum modulators can tap from manifold vacuum for its purposes .... hot - cold or any temperatures in between.

ECS systems are usually manifold/plenum values so they can turn the control valve ON - but the actual drawing of the vapors is thermally controlled and usually only at medium RPM, boulevard cruise --- not accelerating or decelerating.

Any other time that raw fuel gets into the engine combustion processes can blow mufflers into basketballs or blow the tailpipes off the vehicle --- think 1966 Chevy and Ford Pinto or Merc Capri 2.0-2.3/2.6 2.8 Decel Valves!

 

[jimmy78]

The HEI vacuum must get its signal from manifold vacuum - but it has to go through a thermostatic vacuum control valve first so it won't advance until the engine's warmed up sufficiently for the oxygen sensor(s) to start talking to the computer. On some small-displacement engines, there may be a vacuum delay valce to keep the EGR from dumping too hard and heavy into the combustion chambers.

As systems got more complicated, there were added a lot of checks-n-ba;ances via computers and scripts to modulate the EGR.

Manifold vacuum is needed to compensate for throttle load on the engine - as the throttle opens, the vacuum drops until the RPM rises high enough to be able to handle the extra advance and not ping or knock.

If you are using the EGR --- and you should because it lowers fuel consumption and lowers the need for higher octane to keep the combustion temperatures low and not destroy the engine.

Many people have zero idea of the purpose and the value of the EGR and just merrily rip it off --- but having it operate is a good idea --- but ...

It has to not be allowed to come ON during warm-up and is only actually needed during cruise speeds --- and that's where it gets the mileage. .

The vacuum supply for the EGR needs to be qualified through a thermostatic ported valve that is coolant-compensated - and that vacuum CAN arrive through a vacuum amplifier to make sure there is sufficient vacuum to keep the valve ABLE to operate during cruise --- uphill and/but not so much for downhill.

Just beware that there are different types of vacuum available -- ported, manifold and modulated, to name just three (there's more) --- so picking the correct port for whatever you want to use them for, makes a difference.

MANIFOLD - is ALWAYS ON and is taken from sources from inside the manifold or plenum, below the throttle body or carburetor - although some AFBs, Central Ported FI and Q-jets have such ports included in the baseplate castings --- so beware!
PORTED - is usually taken from above the venturi or throttle plate and it is usually co-ported with another bleed-port to drop the signal and keep it from swamping' the signal to not override the application of vacuum as too aggressively or too slowly.
MODULATED - MAY be a small hole or slot in the "tip-off" position of the bore of the throttle body where the plates or butterflies rest at idle -- to ONLY apply vacuum just as the throttle makes the first few degrees of rotation.

PCV and Power Brake taps are always to receive full available engine vacuum.

Those transmissions with vacuum modulators can tap from manifold vacuum for its purposes .... hot - cold or any temperatures in between.

ECS systems are usually manifold/plenum values so they can turn the control valve ON - but the actual drawing of the vapors is thermally controlled and usually only at medium RPM, boulevard cruise --- not accelerating or decelerating.

Any other time that raw fuel gets into the engine combustion processes can blow mufflers into basketballs or blow the tailpipes off the vehicle --- think 1966 Chevy and Ford Pinto or Merc Capri 2.0-2.3/2.6 2.8 Decel Valves!

This is a lot a good information
I am not very technically skilled when it comes to engines and there systems but I see “computer “ a lot in there. With my 78 the closest thing to a computer is the vacuum system.

 

[Finkaire]

California if that helps

 

[59840Surfer]

This is a lot a good information
I am not very technically skilled when it comes to engines and there systems but I see “computer “ a lot in there. With my 78 the closest thing to a computer is the vacuum system.

Does it have HEI (I think so - 'cause it came out in i972-73 or so in CalEPA and CARB stats and the Feds weren't far behind. .

My '86 just misses it - but I wish it had FI --- but ... no!

... and since I know your '78 doesn't have FI, you can eliminate the 4-wire connex behind the distributor. I think so, anyway in a Light Duty Truck.

IMO --- the worst emission systems were in the 1968-70 GM. Ford and Mopar systems --- and the Euroboxes weren't far behind either!

 

[jimmy78]

Here’s my carb set up where does the choke vacuum go to?

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[mtbadbob]

Is that nipple at the top right of carb the same size? I just can't recall exactly what it should hook to. I would think off the intake somewhere, but not 100% sure.

 

[jimmy78]

You must be registered for see images attach

 

[jimmy78]

Is that nipple at the top right of carb the same size? I just can't recall exactly what it should hook to. I would think off the intake somewhere, but not 100% sure.

I looked into that position but the ports are drastically different sizes. There is no port on the top that will fit the hose size the choke needs. Also I noticed a vacuum pot on the back of the carb that is wide open.

 

[Ricko1966]

That vacuum servo on back gets manifold vacuum from the bottom rear of the carb,by the brake booster port.The choke doesn't get vacuum,that's for a hot air tube on the intake manifold,if you don't have it,you can switch to electric choke or trial and error your choke setting until you find something acceptable,meaning you can still start it cold but it opens fully when warm,in actuality you don't even need a choke,it just makes cold start and cold drivabilty better. So error on the side of getting it open,not on keeping it closed. If you don't have all your original stuff,and original vacuum schematic. Vacuum advance can go straight to manifold vacuum,you could run ported,but you'll lose the advantage of having the extra timing at idle. The egr ideally should go through a thermo vacuum switch and ported vacuum. You can get away with just straight ported vacuum to egr if you have no thermo switch,but you'll have to baby it until it warms up. If you have a vacuum gauge finding vacuum is easy ported will have no vacuum at idle,but vacuum with the throttle open. Manifold vacuum will have vacuum at idle.

 

[Ricko1966]

Is that nipple at the top right of carb the same size? I just can't recall exactly what it should hook to. I would think off the intake somewhere, but not 100% sure.

Top right is the bowl vent and goes to the charcoal canister,and I would hook it up

 

[Turbo4whl]

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In this picture it looks like you have two PCV valves, one in each valve cover. You should not have two valves, just one. The other valve cover should have a breather, or hooked to a tube with the breather in the air filter housing.

Two PCV valves could lean out the carb's fuel mixture.