So very stuck...

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Plug a vacuum gauge in to the port on your carb, tape it to your windshield and and go for a drive.
 
So, hypothetically I could hit 45* maybe even 50* at cruise because of this curve and this isnt something to be to concerned about as the spark should hit sooner so that combustion burns at the right time.
 
TT5.9mag said:
Vacuum isn’t an all in one time deal. It’s constantly variable with load and throttle opening. So the can will add timing based on that. But it will be “all in” at a certain vacuum number.
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Yeah I see that.
 
So I'm gonna slowly turn up my can, test and adjust until it pings at cruise. Then I will back off 2* and test... if it still sounds a little pingy I'll back off another 2* on the can and test. That should give me my full timing range. And I'll put in the medium spring (running 1 light 1 medium spring currently) and test again to see how the can responds. It may be my current spring is the best but I'll test just the same.
 
Optimize the power timing first and leave it alone. Then bring in the vacuum advance and tune it for cruise mileage and efficiency. If the engine rattles when VA is coming in, modify the VA not the curve in the distributor.
 
My timing itself is set. I'm working on how much timing I can pull from my can, but was wondering if because its ported, does that mean that the vacuum signal the can receives is variable... does vacuum movement control how much advance is added. The more the blades open the more pull the can sees, so adding timing as it goes. If this is the case I can just open the can up to 15* and the motor will only use what it needs. I rarely see over 2200 rpm as at highway cruising speeds I'm doing 1600 and passing maybe 2200 if I'm really on my pedal. Doesnt take much for this truck to move now.

If this isnt the case and timing comes in too soon then I'm at 22*(initial) + 15*(the can) almost immediately. Seems like this isnt the case or doesnt have to be.
 
R3N0 said:
Okay, so first off I want to start this by saying I'm not asking to start an argument about which is better "ported" or "manifold". So let's not do that seen to many.

My question: when ported does the motor only use the amount of advance needed? As in when the motor is pulling say 1800 rpm does it only use the amount of advance needed at that point or does it try to use all advance regardless?
So if I turn my can all the way up as the advance comes in will it only pull the advance it needs or will I get the max 15*(on the can) the moment vacuum starts pulling?

My truck really likes the ported side, but I have kept it at its minimum of 8* to be safe and not get pinging. But that means at cruise I'm getting almost 30* should I attempt to push it till it pings and then back off a couple of * on the can?
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Let's keep this simple
If your engine likes the advance operating from the "ported" carb connection, keep it there.
Some engines like more initial timing can and are connected to full manifold vacuum.
Do what works for your application there a no hard fast rules.
It will be a happy balance between the VA adjustment and your all in timing set buy the distributor.
For example the VA adds 10* advance on my 340 operating on ported vacuum.
My all in mechanical timing is 34*
So 34* + 10* = 44* at steady cruising speed while the engine is producing 11 inches of vacuum, no ping no knock under acceleration or cruise mode.
 
R3N0 said:
My initial and total timing are set. What I'm asking is when ported does the carb act like a computer, basically only allowing as much timing needed at on time. Since the vacuum only comes in after acceleration, does this change affect how much timing is pulled? So if you open the can wide open but put a stiffer spring in, your initial timing should curve with the amount of vacuum being applied by the position of the throttle plates. Am I saying this correctly?
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I'm not sure why You want to change the spring, the allen screw should allow enough range of adjustment, unless it's some odd option app.
Ported is going to respond to the airflow/throttle position balance. If You're at 1/3 throttle on the level cruising @ 2800rpm, more air will be flowing & the vacuum will be higher(light load = more vac adv.), than 1/3 throttle climbing a slight grade at 1850rpm(heavy load = less vac adv.).
 
R3N0 said:
Got it, that's what I was asking.
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You're not talking screwing with the mechanical advance springs are You?!?! If so, no no no, leave that sh*t alone, You only need to adjust the screw in the can. The arm on the can controls the total the can add to the base & mechanical, if adjusting the diaphram spring tension to max is allowing light pinging or a slight surge, get a can with a lower advance arm.
 
No I'm not talking about the mechanical advance springs, my timing is set at 22* initial and 36* all in. I'm talking about the can setting. The screw and spring inside. I was wondering if by changing that spring I could slow the rate at which the can comes in... but I dont think I can access it. I was also asking that if the distributor is ported then does the carburetor act as the computer and only pulls the amount of vacuum needed for the advance it needs at that time. Due to the fact that as the blades open more vacuum is produced, would it slowly raise the vacuum. I know that eventually the timing will be all in. But up until that point if the carburetor is acting as the computer it will bring in the vacuum according to the amount of throttle used and creating the "curve" or balance between firing and ignition. Man I can see the process in my head but I cant seem to say it correctly here. Maybe I'll make a video and try that way.
 
Killer6 said:
I'm not sure why You want to change the spring, the allen screw should allow enough range of adjustment, unless it's some odd option app.
Ported is going to respond to the airflow/throttle position balance. If You're at 1/3 throttle on the level cruising @ 2800rpm, more air will be flowing & the vacuum will be higher(light load = more vac adv.), than 1/3 throttle climbing a slight grade at 1850rpm(heavy load = less vac adv.).
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So I can open my can all the way to the 15* it allows, as my rpms dont go high enough for the timing to be all in. My O/D and L/U make sure of that. My timing comes all in at 2200 rpm. But my cruising rpms are at 1600. I dont race this truck so I really never see 2200 rpms unless passing when I turn off the O/D and L/U. And it's not for that long. Almost all of my shifting happens between 1700 and 1800 rpms. Opening up my can all the way allows the motor to use what ever timing it would need without ever reaching a point where it would start to ping.
 
When you use the term "spring" I automatically think of the 2 springs affecting mech. advance.
Use the term screw or va. When talking vacuum. And yes the va curve can be adjusted with the screw. In reality you are adjusting the rate when adjusting screw.
R3N0 said:
No I'm not talking about the mechanical advance springs, my timing is set at 22* initial and 36* all in. I'm talking about the can setting. The screw and spring inside. I was wondering if by changing that spring I could slow the rate at which the can comes in... but I dont think I can access it. I was also asking that if the distributor is ported then does the carburetor act as the computer and only pulls the amount of vacuum needed for the advance it needs at that time. Due to the fact that as the blades open more vacuum is produced, would it slowly raise the vacuum. I know that eventually the timing will be all in. But up until that point if the carburetor is acting as the computer it will bring in the vacuum according to the amount of throttle used and creating the "curve" or balance between firing and ignition. Man I can see the process in my head but I cant seem to say it correctly here. Maybe I'll make a video and try that way.
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Got it thank you so much. Sorry for using wrong terms to explain this. Like I said I can see it in my head but putting it to paper as it were is a bit harder for me.
4spdragtop said:
When you use the term "spring" I automatically think of the 2 springs affecting mech. advance.
Use the term screw or va. When talking vacuum. And yes the va curve can be adjusted with the screw. In reality you are adjusting the rate when adjusting screw.
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The carburetor does not “control” how much vacuum there is. Vacuum is generated by the engine running and the carburetor restricting the airflow. At different throttle positions and different load situations you’ll have different levels of vacuum. That’s why I said tape a vacuum gauge to your windshield, it will help you understand what’s happening and when. Vacuum is not rpm dependent, you can be at 6000 rpm wot and have zero vacuum, or 6000 closed throttle and peg the gauge. Vacuum is load dependent.
 
R3N0 said:
No I'm not talking about the mechanical advance springs, my timing is set at 22* initial and 36* all in. I'm talking about the can setting. The screw and spring inside. I was wondering if by changing that spring I could slow the rate at which the can comes in... but I dont think I can access it. I was also asking that if the distributor is ported then does the carburetor act as the computer and only pulls the amount of vacuum needed for the advance it needs at that time. Due to the fact that as the blades open more vacuum is produced, would it slowly raise the vacuum. I know that eventually the timing will be all in. But up until that point if the carburetor is acting as the computer it will bring in the vacuum according to the amount of throttle used and creating the "curve" or balance between firing and ignition. Man I can see the process in my head but I cant seem to say it correctly here. Maybe I'll make a video and try that way.
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You are over thinking this, these are simple systems doing simple tasks.
Like others have suggested get a vacuum gauge and watch what it tells you in differant driving modes. When you do this you will have a much better understanding.
Keep it simple, small adjustments, one change at a time and you'll get there.
 
R3N0 said:
No I'm not talking about the mechanical advance springs, my timing is set at 22* initial and 36* all in. I'm talking about the can setting. The screw and spring inside. I was wondering if by changing that spring I could slow the rate at which the can comes in... but I dont think I can access it. I was also asking that if the distributor is ported then does the carburetor act as the computer and only pulls the amount of vacuum needed for the advance it needs at that time. Due to the fact that as the blades open more vacuum is produced, would it slowly raise the vacuum. I know that eventually the timing will be all in. But up until that point if the carburetor is acting as the computer it will bring in the vacuum according to the amount of throttle used and creating the "curve" or balance between firing and ignition. Man I can see the process in my head but I cant seem to say it correctly here. Maybe I'll make a video and try that way.
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Why can't You access the adjuster? It's a small allen wrench, is the hose nipple pointed right at/against something close?
 
If you are making 400 hp with 9:1 CR, engine must have a sizeable cam in it. That means less idle & cruise vacuum.
So trying to get a stock VA to 'work' could be an exercise in futility. Buy a cheap Chinese HEI dist, which comes an adj VA unit that has a softer spring [ Have used heaps of these, zero problems, can buy here for under $100; new adj VA unit costs almost as much & all you get is a VA unit....].
Adjust VA Allen Key fully CW [ softest spring setting ]. You do NOT use this spring to limit the amount of VA applied. That will result in erratic operation. The amount of VA is controlled by the how far the actuator moves [ it has about 30* total ]. With the HEI design, it is very easy to make a stop that fits under the mounting screw to limit plunger travel.
For the centri curve, the weights/springs are under the rotor & easy to modify.
 
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