Vacuum advance over advancing
10-14-2013, 12:13 AM
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Vacuum advance over advancing
Looking for some help here.
4 Years ago I bought a stock 1968 Dodge Charger with 383 Premium 4 BBL. Somebody had change the carbi to a Webber/Edelbrock, I checked and its the correct replacement for this car. The vacumm advance was not connected, so I tried connecting it to the port referenced in the carbi manual, but it pulled the vacuum advance full on at idle, so I assumed it was manifold vacuum. I tried all the ports and they did the same, so I assumed the carbi did not have a ported vacuum port. I managed to get a fully build AVS, once again the correct orginal carbi for the car. It came with a manifold gasket with a large square hole, rather than the 4 round hold that align to the manifold. The carbi I pulled off had the same gasket, the gasket looked like a Thermoquad gasket. I tried the re con AVS with the gasket it came with, but it also had vacuum at idle on the vacuum advance port. (left hand side next to the mixure screws.) i sourced the correct square bore gasket and this then gave me close to 0 vacuum at idle, but it still pulles the vacuum advance almost fully on when I rev it to arpound 1000rpm and it has a significant surge at lower RPM. I have striped and check the distributor and everything is 100% to spec. I have set the point gap at 17" and set the fixed timing to 5 deg before TDC. I have run out of ideas, why would there be so much vacuum on this port?
4 Years ago I bought a stock 1968 Dodge Charger with 383 Premium 4 BBL. Somebody had change the carbi to a Webber/Edelbrock, I checked and its the correct replacement for this car. The vacumm advance was not connected, so I tried connecting it to the port referenced in the carbi manual, but it pulled the vacuum advance full on at idle, so I assumed it was manifold vacuum. I tried all the ports and they did the same, so I assumed the carbi did not have a ported vacuum port. I managed to get a fully build AVS, once again the correct orginal carbi for the car. It came with a manifold gasket with a large square hole, rather than the 4 round hold that align to the manifold. The carbi I pulled off had the same gasket, the gasket looked like a Thermoquad gasket. I tried the re con AVS with the gasket it came with, but it also had vacuum at idle on the vacuum advance port. (left hand side next to the mixure screws.) i sourced the correct square bore gasket and this then gave me close to 0 vacuum at idle, but it still pulles the vacuum advance almost fully on when I rev it to arpound 1000rpm and it has a significant surge at lower RPM. I have striped and check the distributor and everything is 100% to spec. I have set the point gap at 17" and set the fixed timing to 5 deg before TDC. I have run out of ideas, why would there be so much vacuum on this port?
10-14-2013, 08:45 AM
#3
Mopar Lover
Welcome, you have my interest and I'd like to help.
When you say left side do you mean when your standing in front looking at the engine, if so that should be it for AVS and AFB.
Have you used a vacum gauge to confirm amount of vacuum at rpm or just seen it at the timing mark with a light.
I do have a 1970 manual with all the mechanical and vacuum timing increments for reference if you want, but a tach and vacuum gauge would be needed.
It shows 2.5 degrees timing at idle for 1970. Another manual I have says 5 degrees for 1969.
Something I failed to mention, the ported vacuum opening in the throttle bore should be above the throttle blade at idle. As you open the blade it exposes this opening to full vacuum then pulling the canister diaphram to add advance timing. The different base gaskets will have no affect on this.
I'm wondering if you have the blades more open to achieve idle rpm that may be compensating for another unrelated problem.
If you attach a tube to the one your useing and blow through it you should hear an audible difference when you open the throttle blades slowly. Engine not running or carb off as a bench test.
When you say left side do you mean when your standing in front looking at the engine, if so that should be it for AVS and AFB.
Have you used a vacum gauge to confirm amount of vacuum at rpm or just seen it at the timing mark with a light.
I do have a 1970 manual with all the mechanical and vacuum timing increments for reference if you want, but a tach and vacuum gauge would be needed.
It shows 2.5 degrees timing at idle for 1970. Another manual I have says 5 degrees for 1969.
Something I failed to mention, the ported vacuum opening in the throttle bore should be above the throttle blade at idle. As you open the blade it exposes this opening to full vacuum then pulling the canister diaphram to add advance timing. The different base gaskets will have no affect on this.
I'm wondering if you have the blades more open to achieve idle rpm that may be compensating for another unrelated problem.
If you attach a tube to the one your useing and blow through it you should hear an audible difference when you open the throttle blades slowly. Engine not running or carb off as a bench test.
Last edited by Coronet 500; 10-14-2013 at 01:21 PM.
10-14-2013, 10:04 AM
#4
YOUR Weber carb may not have a ported vacuum Leave it disconnected The AVS should have a ported vacuum connection. Other wise you could have a bad vacuum advance unit
Last edited by TVLynn; 10-14-2013 at 10:07 AM.
10-14-2013, 10:07 AM
#5
Or maybe the carb idle is way too lean causing it to "want" a large throttle opening.
10-14-2013, 11:57 AM
#6
Mopar Lover
DC-
Quick reality check here.
How does the car run, how does it feel? Back in the day, most folks believed that big blocks didn't need the vacuum advance. I've run my 440 with and without and really can't tell a seat of the pants difference. Unless you're racing on a regular basis, that seat of he pants feeling is really all that matters.
Forget what it's "supposed to be".
If the engine has been modified at all, factory specs no longer hold. If the engine is bone stock (never rebuilt) the odds are good that with carbon buildup, timing chain stretch and usual wear and tear, the original factory specs may be just as useless.
The only issue will be if you aren't getting enough total advance and again, I'd go by the seat of the pants rule.
Archer
Quick reality check here.
How does the car run, how does it feel? Back in the day, most folks believed that big blocks didn't need the vacuum advance. I've run my 440 with and without and really can't tell a seat of the pants difference. Unless you're racing on a regular basis, that seat of he pants feeling is really all that matters.
Forget what it's "supposed to be".
If the engine has been modified at all, factory specs no longer hold. If the engine is bone stock (never rebuilt) the odds are good that with carbon buildup, timing chain stretch and usual wear and tear, the original factory specs may be just as useless.
The only issue will be if you aren't getting enough total advance and again, I'd go by the seat of the pants rule.
Archer
Last edited by Archer; 10-14-2013 at 07:32 PM. Reason: oops - that should have been can't, (in bold). kinda changes the meaning a little
10-15-2013, 12:05 AM
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Welcome, you have my interest and I'd like to help.
When you say left side do you mean when your standing in front looking at the engine, if so that should be it for AVS and AFB.
Have you used a vacum gauge to confirm amount of vacuum at rpm or just seen it at the timing mark with a light.
I do have a 1970 manual with all the mechanical and vacuum timing increments for reference if you want, but a tach and vacuum gauge would be needed.
It shows 2.5 degrees timing at idle for 1970. Another manual I have says 5 degrees for 1969.
Something I failed to mention, the ported vacuum opening in the throttle bore should be above the throttle blade at idle. As you open the blade it exposes this opening to full vacuum then pulling the canister diaphram to add advance timing. The different base gaskets will have no affect on this.
I'm wondering if you have the blades more open to achieve idle rpm that may be compensating for another unrelated problem.
If you attach a tube to the one your useing and blow through it you should hear an audible difference when you open the throttle blades slowly. Engine not running or carb off as a bench test.
When you say left side do you mean when your standing in front looking at the engine, if so that should be it for AVS and AFB.
Have you used a vacum gauge to confirm amount of vacuum at rpm or just seen it at the timing mark with a light.
I do have a 1970 manual with all the mechanical and vacuum timing increments for reference if you want, but a tach and vacuum gauge would be needed.
It shows 2.5 degrees timing at idle for 1970. Another manual I have says 5 degrees for 1969.
Something I failed to mention, the ported vacuum opening in the throttle bore should be above the throttle blade at idle. As you open the blade it exposes this opening to full vacuum then pulling the canister diaphram to add advance timing. The different base gaskets will have no affect on this.
I'm wondering if you have the blades more open to achieve idle rpm that may be compensating for another unrelated problem.
If you attach a tube to the one your useing and blow through it you should hear an audible difference when you open the throttle blades slowly. Engine not running or carb off as a bench test.
10-15-2013, 12:07 AM
#9
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Thanks Coronet 500, Yes the port I am using is on the left if you are standing at the front of teh car looking at teh engine, its the only port of that size on teh carbi. What you are saying about the position of the of the throttle blades in relation to the port might be the problem, there is not much else it could be. I will try your trick of blowing into the port. Thanks for your reply
10-15-2013, 12:13 AM
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The engine runs great on 10 or so degrees, have not tried it at 12 or 14. At the moment the timing advances to around 25 degrees at around 1500rpm
10-15-2013, 12:18 AM
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DC-
Quick reality check here.
How does the car run, how does it feel? Back in the day, most folks believed that big blocks didn't need the vacuum advance. I've run my 440 with and without and really can't tell a seat of the pants difference. Unless you're racing on a regular basis, that seat of he pants feeling is really all that matters.
Forget what it's "supposed to be".
If the engine has been modified at all, factory specs no longer hold. If the engine is bone stock (never rebuilt) the odds are good that with carbon buildup, timing chain stretch and usual wear and tear, the original factory specs may be just as useless.
The only issue will be if you aren't getting enough total advance and again, I'd go by the seat of the pants rule.
Archer
Quick reality check here.
How does the car run, how does it feel? Back in the day, most folks believed that big blocks didn't need the vacuum advance. I've run my 440 with and without and really can't tell a seat of the pants difference. Unless you're racing on a regular basis, that seat of he pants feeling is really all that matters.
Forget what it's "supposed to be".
If the engine has been modified at all, factory specs no longer hold. If the engine is bone stock (never rebuilt) the odds are good that with carbon buildup, timing chain stretch and usual wear and tear, the original factory specs may be just as useless.
The only issue will be if you aren't getting enough total advance and again, I'd go by the seat of the pants rule.
Archer
10-15-2013, 12:20 AM
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10-15-2013, 03:44 AM
#13
First forget about the vacuum advance Leave it dsconnected for now. Get the mechanical advance set first with 34* total advance. Then work on the vacuum advance High vacuum with advance you may get 50 degrees in cruse
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