C3 vaccuum question

[rick spears]

i am working on a 70'roadster...and don't seem to have enough vaccuum to operate the headlights and wiper door. (could possibly be a leaky vac tank also, but not sure)

my question is this: how much vac should i expect to see at the intake fitting at idle? can timing affect this? if so, how do you gain vac? advance or retard? thanks in advance, rick

[Joe L.]

Re: C3 vaccuum question

Rick-----

You don't say which engine you have. However, in any event, you should have somewhere between 14-20 inches of vacuum at idle. Anything in that range will be enough to operate all the equipment in the vacuum system, assuming the rest of the system is tight and in proper repair.

[William C.]

Re: C3 vaccuum question

Also, make sure you are checking from a manifold connection, rather than fron the carburetor.

[Jack H.]

Common causes include:

(1) Vac leak(s) at various points in the system (get a vac troubleshooting manual, read, and step through the system point by point).

(2) Mis-routing of various vac hoses (confirm with a vac system schematic).

(3) Plugged vac filter and/or worn vac check-valve (these need periodic replacement and few owners are aware of the need to do preventive maintenance).

[rick spears]

Re: Common causes include:

thanks eveyone for the responses, i will check all of that out! rick

[Pat M.]

Re: C3 vaccuum question

Hi Rick. When tracking leaks I have found it very helpful to connect a remote vacuum source (such as another running car) to the main vacuum line coming from the manifold. This way, it will be quiet enough for you to listen for and pinpoint leaks around vacuum relays, connections, etc.

Good Luck, Patrick Moresi #45581

[Denny J.]

Re: Common causes include:

rick
Jack knows his stuff. # 3 So let me give you a starting point.Drivers side in the back is a white round plastic part with 2 hoeses from the manfold,take that off if you can see threw it replace it,next on the same line is a metal check valve 1 hose in and 2 out if you can blow threw it replace it.start there than work back.

[rick spears]

Re: C3 vaccuum question

wow.....that's a good idea! thanks

[Jeremy D.]

Re: C3 vaccuum question

Also, I used a propane torch (unlit), just cracked the valve open a hair, and ran it along all my vacuum lines & connections and listened to the engine...if the RPM's increase, you found a leak/crack/etc. I had a '70 coupe that had a small crack in the actual vacuum reservoir under the front fender....that was a nightmare to trace! Good Luck!

[rick spears]

Re: C3 vaccuum question

one last question....and, btw, this is a 350/300hp car. does the timing affect the vaccuum production? thanks, rick

[Jim T.]

Re: C3 vaccuum question

Rick reference your question about advancing or retarding the timing on your 1970 350/300HP convertible. If the original factory configuration of vacuum from the carb to the distributor has not been altered, the vacuum hose from the carb will go to a electric solenoid mounted to the intake. There will also be a vacuum hose from the solenoid to the distrubutors vacuum advance.
The electic solenoid recieves input to control when the distributor vacuum advance receives vacuum.
The input comes from two sources, the transmission and the temp sensor in the passenger side cylinder head.
When you set/check ingnition timing, remove the vacuum hose from the distributors vacuum advance and plug the end. At normal operating temp and automatics in park and 4-speed cars in nuetral there will be no vacuum through the electric solenoid anyway.
Set timing and reconnect vacuum hose to distributors vacuum advance.
I am a original owner of a 70 350/300. First modification was to bypass the electric solenoid inhibiting full time vacuum advance. In park engine would always have very good vacuum, usually about 21 dropping down to about 17 or 18 when the automatic was put in gear without air conditioner on. Usually set my ignition timing about 10 degrees BTDC. Running full time vacuum to the distributor vacuum advance really picked up the idle speed. In city only driving the MPG increased about 3 mpg. Interstate mileage was not really affected. 70 automatics have full time vacuum advance with it shifts to 3rd when vacuum to the distributor vacuum advance is controlled by the electric solenoid.
Check your pull down switches that raise and lower your wiper door and headlights to make sure they are not affecting your vacuum problem.
To check if the storage tank is leaking, by pass the tank with vacuum hose changes going direct and see if there is a change.
How did your 69's and 73's vacuum operated headlights work?

[rick spears]

Re: C3 vaccuum question

jim, thanks for the input. i checked the vac at the intake tonight. 16.5 at idle, so well within the tolerable range. i now suspect the regulators up front. i isolated each one, and heard "sucking" from each when the headlights were accutated.

my 73' was a complete frame off resto, and when i completed it the system worked like new! (since i replaced everything)

my 69' worked ok, but not great. pretty lazy when opening and closing the headlights and wiper door, but still workable. i have 2 70 models right now. one is a roadster and one coupe. the coupe is relatively low mileage 350hp car that is unmolested, and the vac system works like new on it......thanks everyone for your input, rick

[Mike S.]

Re: C3 vaccuum question

check your power brake booster also for vacuum leaks. Mike