Question about External Vacuum Pump
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Joined: May 2018
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From: Tennessee
Question about External Vacuum Pump
I have a 1979 with a 350 and Thumper Cam. On idle the manifold vacuum runs about 9" and bounces up to about 15" on quick throttle bounce. The brakes seem to be just a little weak, but not what I call a major issue. However, the lights are terrible. The car has new actuators and I've check for vacuum leaks within the headlight system. The large metal tube vacuum reservoir holds vacuum really well. Possibly the issue is either there is not enough vacuum to operate the actuators or the valves that operate the actuators may be weak or faulty. I'm thinking adding an external pump would take care of these issues.
any ideas?
thanks!
any ideas?
thanks!
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Joined: May 2006
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what is your initial timing with vacuum advance disconnected?
is this an automatic or manual transmission?
what is the idle speed in gear for an automatic or in neutral for a manual?
which Thumper cam?
what carburetor are you using and do you have the correct power valve/spring installed?
is this an automatic or manual transmission?
what is the idle speed in gear for an automatic or in neutral for a manual?
which Thumper cam?
what carburetor are you using and do you have the correct power valve/spring installed?
Last edited by MelWff; May 23, 2018 at 03:50 PM.
Le Mans Master
Joined: Apr 2011
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From: Hermosa
doesn't sound like the acutators are at fault. I run 10" and headlights work fine. do the headlights come up when above idle producing more vacuum?
What Mel is getting at is that your state of tune may be to blame for the low vacuum but we can't know without more details. Dual plane intake? Cam specs, timing specs, vac advance manifold or ported vacuum etc.
What Mel is getting at is that your state of tune may be to blame for the low vacuum but we can't know without more details. Dual plane intake? Cam specs, timing specs, vac advance manifold or ported vacuum etc.
Race Director
Joined: Apr 2009
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From: Charlotte NC
I have a 1979 with a 350 and Thumper Cam. On idle the manifold vacuum runs about 9" and bounces up to about 15" on quick throttle bounce. The brakes seem to be just a little weak, but not what I call a major issue. However, the lights are terrible. The car has new actuators and I've check for vacuum leaks within the headlight system. The large metal tube vacuum reservoir holds vacuum really well. Possibly the issue is either there is not enough vacuum to operate the actuators or the valves that operate the actuators may be weak or faulty. I'm thinking adding an external pump would take care of these issues.
any ideas?
thanks!
any ideas?
thanks!
DID you check to make sure your vacuum check valve is GOOD and the vacuum storage tube can actually hold vacuum???
Are you 100% POSITIVE that the actuator relays are also good.
There are two positions for these and if you want the system to WORK..they have to be sealed in BOTH positions.
You also MUST take into account your HVAC vacuum system inside the car....if it is still intact. IF you have vacuum leaks there...then it also can be a problem.
DUB
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Joined: May 2018
Posts: 52
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From: Tennessee
doesn't sound like the acutators are at fault. I run 10" and headlights work fine. do the headlights come up when above idle producing more vacuum?
What Mel is getting at is that your state of tune may be to blame for the low vacuum but we can't know without more details. Dual plane intake? Cam specs, timing specs, vac advance manifold or ported vacuum etc.
What Mel is getting at is that your state of tune may be to blame for the low vacuum but we can't know without more details. Dual plane intake? Cam specs, timing specs, vac advance manifold or ported vacuum etc.
Haven't checked the timing, but I suspect it is retarded just a little. Just a little hard to crank. The idle seems a little low. Maybe I can get those specs. tomorrow. The Thumpr cam has duration 279/297 and lift .479/.465. 350 with auto 700R4 tranny MadDog Stage 2 with 2200 stall converter and Edelbrock #1405 Performer.
Thread Starter
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Joined: May 2018
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From: Tennessee
Yep, the filter, check valve, the vacuum reservoir tank, all good. I think I'm going to measure the vacuum at the tank itself, then at the actuators. If it takes the same amount of vacuum to open them as it does to close them, then I'm leaning toward a problem with the vacuum relays. Yesterday, the right side closed immediately and the left was a little sluggish.
Race Director
Joined: Apr 2009
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From: Charlotte NC
I am going top BET with about 100% certainty the it is in the actuator relays. One of them..if not both.
BE AWARE that new ones can leak also. They should NOT leak in both positions of the relay. I have had MANY that leak when the lights are UP..but are good and sealed when they are down.
DUB
BE AWARE that new ones can leak also. They should NOT leak in both positions of the relay. I have had MANY that leak when the lights are UP..but are good and sealed when they are down.
DUB
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Thread Starter
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Joined: May 2018
Posts: 52
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From: Tennessee
I am going top BET with about 100% certainty the it is in the actuator relays. One of them..if not both.
BE AWARE that new ones can leak also. They should NOT leak in both positions of the relay. I have had MANY that leak when the lights are UP..but are good and sealed when they are down.
DUB
BE AWARE that new ones can leak also. They should NOT leak in both positions of the relay. I have had MANY that leak when the lights are UP..but are good and sealed when they are down.
DUB
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Joined: May 2018
Posts: 52
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From: Tennessee
Question......should a Mityvac hand pump be able to open and close the actuators that are connected to the lamp assembly? It seems like it takes an awful lot of pumping just to make a small movement and these are new actuators. Also, the springs that are mounted in the assembly. Is the vacuum suppose to be strong enough to overcome the spring tension or what part do they even play in the operation if there is constant vacuum in opening and closing.
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Joined: May 2018
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From: Tennessee
Just a FYI about the lights. It appears that the seal for the opening action on the passenger side actuator was cracked. I took it out of the loop and the driver side opened and closed good. Guess there was just too much vacuum loss when the lights were on and that caused the driver side to only open about half way. New actuator on order.
