the trick to vacuum advance

[sleepar]

Most but not all advance pots have an adjustment you have to feel around inside the nipple on the pot.. it's usually 3/32 Allen head... with a vacuum gauge on the lower manifold port on the carb and a cap on the timed port adjust timing and carb to get the most vacuum at idle... and lock down distributor. then put the cap on the manifold vacuum and plug the distributor line on the vacuum advance test drive hard and if a mis or lag and if you have under 24" vacuum back out the Allen screw a turn... reconnect and test drive same as before.. if it gets better go another turn if worse go in a turn go in one direction till it gets better then gets worse then go back to better and a 1/2turn more. The advance is set. If still a lag adjust the metering rods till better then the accelerator pump shot to the best conditions if still a miss how ever slight you may have too low a vacuum to operate the pot. Sometimes people will hook it up to manifold vacuum because it's stronger... but this is not the best solution... with big cams and low vacuum a mechanical advance distributor is the one... the springs can be changed and not always in pairs... as long as the engine has its max vacuum at idle it's in tune... the misses are either a slight advance curve adjustment or slight fuel adjustment... Holley double plumper is the easiest came to dial in because of the cams for the pump shot and there is was a green one that was squared off in profile known as the mopar one.. but you can cut you're own cam..... and the power valve on a Holley has a few sizes..... the electronic has a reluctant gap and the points have a gap and a dwel spec..... I like a .016 gap and will set it wrong according to how the lock screw pulls it when tight a .020 gap usually tightens to 16 but some i set to 13-.014 because they will open up and settle in on a new distributor.... rarely will they tighten up.... as for the dwell I guess by the duration and vary the gap to get the dwel by ear.... I mark the hold down with some white out... the timing will vary stock to wild 8°-22° on average 12° being most common...

 

[Photon440]

By 'replicator gap' did you mean 'reluctor gap'? Allen head should be 3/32. Sometimes it's slotted for a screwdriver.

Good points though, not everyone knows that some vacuum advance is adjustable.

 

[sleepar]

By 'replicator gap' did you mean 'reluctor gap'? Allen head should be 3/32. Sometimes it's slotted for a screwdriver.

Good points though, not everyone knows that some vacuum advance is adjustable.

Thanks I need to proof read more... I figured I would throw it out there...

 

[Xp29h]

It's funny you threw this out there. I just went away from vacuum advance to mechanical. Just tested last night and Working good but still have a little flutter/miss at slow "no real load" once I get onto it a bit it goes away. Going to play with springs and curve and carb adjustments as you mentioned. Thanks for info!

 

[383man]

I just want to know what a "Holley Double Plumper 850" Is ? LoL !

You know you can not always set the timing for highest vacum on some cars. My buddies 340 was built with the stock 10.5 pistons and it has no quench and a small .480 lift cam. If I set the timing for the most vacum at idle it pings like crazy. He does not want to run race fuel and I have to back his timing down some so it wont ping so bad. And of course it looses power then and is at the point of almost hesitating. I told him he either needs to run race gas or have me build the eng right to run on pump.

I am guessing you mean you set your points at .016 as the reluctor setting is .008 to .010. One other thing I see most of the time is if an eng actually has a missfire when cruisin it wont be timing it will be fuel or ign as timing will be a loss of power and hesitation when driving along. Or as you stated timing can cause a backfire when you hit the gas hard if to retarded but a dead miss at steady cruise will not be timing as most of the time it is an ign miss or to lean.

Myself I dont use a vacum advance because my Mallory dist does not have one and I set my dist up with a fast timing curve. If it had a vacum advance I would have used it to see how well it would work with my combo. But it really helps alot if you build your eng to run on pump gas if you want to run just pump like I do. Good quench , aluminum heads and the right cam and valve timing to control cyl pressure to run pump. My buddies older 340 was not built for todays pump and it just wont run as good as it should on pump because of that. Thanks for the post Ron

 

[Photon440]

I just want to know what a "Holley Double Plumper 850" Is ? LoL !

A Double Plumper

 

[sleepar]

Nice plumper... it's the one with the big piss stream... maybe a double fluffer is better... LOL spell checker

Comes in a high polish pink and will turn purple if you use a 5/16 line

 

[383man]

I could use spell check myself many times. Just funning with ya Sleepar. Ron

 

[Meep-Meep]

Just one question. Are we going to have to pay for this advice?

 

[Dragon Slayer]

I appreciate the post, unfortunately the way it is worded it is a little hard to understand what you are recommending. The folks that understand all the principles already know it. Those that are learning, can't use it easily or at all.

I am at the stage where I am trying to tune my car. Carb is 69 roadrunner Carter AVS carb on a 70 RR engine that is stock (assumption) but with 69 Air Grabber set up (oval cleaner). The only two vacuum ports on this carb are the one going to the distributor and the large one going to the PCV valve. There is a port off the intake at the rear that feeds the power brake booster and the air grabber canister.

The car wasn't running very well and I was running some race gas through to clean it up. I am not ready to take the carb off for a rebuild yet, though I think it probably needs it. I did change the hoses out, and just recently changed the PCV valve cover grommet. I think it was leaking as the car is performing better now.

I set the idle mixture screws at 2 turns out. Initially one was 1.5 out and the other 4 out. I adjusted the curb idle to 750 rpm. The choke seems to work fine. The exhaust heat valve spring is broken, so the flap can turn randomly, if a flap is in there.

The timing is about 5 deg BTDC. The dwell is reading about 19 deg (Duty cycle 44%) with the hose from the vacuum advance removed and installed on a vacuum gage. Vacuum was about 13 and fluctuates at idle.

Are these readings ok? Dwell is out of spec low. What is the best way to tune the carb idle mixture. Where should I get a vacuum reading at? The plugs have a nice tan color, and the wires, cap, rotor and points look fine. I realize the condition of the distributor plays a big part in this too.

Right now the car starts easily, idles fine but might be considered rough (I don't know if the car has an upgraded cam or not). The popping out of the exhaust when gas is let off, is now gone, but I do have a miss starting off at low RPM in first gear (3.54 dana) and get a miss if I punch it at low rpm. What do I need to do to the distributor and carb to maximize the tune? Thanks, G

 

[sleepar]

I would rebuild the carb... Or get a new one... then the distributor or new one the timing chain and its slack has a lot to it and the cam. Once you have been through the components then it's easier to set the time and carb because you're not over or under adjusting for a worn or sticking part(s) ... anything questionable 're build it.

The popping sounds like a power valve or metering rod out of adjustment.... you will spend more time on trying this or that than the rebuilding....

 

[coloradodave]

School me, sleeper. How does a power valve or metering rod "come out of adjustment"?

 

[Dragon Slayer]

Well, after a few google searches on tuning, and reading the white papers on one of the builders website. I got the car tuned and removed all the issues. I found that going richer on the idle mixture removes the exhaust popping. I set the dwell to 28-29 deg and than advanced the timing for a strong idle. It is off the timing scale so it would be about 20deg BTDC if the timing markers are right, but I could go more without any knocking. I brought the idle back down, checked dwell again and idle mixture. Wound up at 20deg or so BTDC timing, 29dwell, and 3 and 1/8 turns out on the idle mixture screws. Idle at 750RPM. No hesitation, smooth power in any gear when WOT, and no hesitation at low RPMs. No knocks or pings. I use a mix of 1/3 110 OCT and 2/3 93 Oct gas. Starts easily hot or cold and idles well. Will drive for a day or two and pull the plugs to check. Going to leave well enough alone for now. This is a cruiser not a drag car. So I am pretty happy right now. G

 

[sleepar]

School me, sleeper. How does a power valve or metering rod "come out of adjustment"?

Back fireing, foreign materials, age,

- - - Updated - - -

Back fireing, foreign materials, age,

Glad it's working out... the harmonic balancer can slip and give a false reading...

 

[coloradodave]

Back fireing, foreign materials, age,

- - - Updated - - -

A Holley power valve can FAIL but neither a power valve or metering rod can "come out of adjustment". The older Holleys were prone to power valve failure if you got a good enough 'backfire' up through the intake manifold. There is a widely known, easy fix for that. The newer carburetors come with the fix in place. Metering rods (Carter & Edelbrock) simply move up and down depending on engine vacuum working against a spring. You can change the springs to alter this movement. Information is only as good as it's accuracy...

 

[383man]

A Holley power valve can FAIL but neither a power valve or metering rod can "come out of adjustment". The older Holleys were prone to power valve failure if you got a good enough 'backfire' up through the intake manifold. There is a widely known, easy fix for that. The newer carburetors come with the fix in place. Metering rods (Carter & Edelbrock) simply move up and down depending on engine vacuum working against a spring. You can change the springs to alter this movement. Information is only as good as it's accuracy...

I agree with this completely. Never saw a metering rod come out of adjustment. Might have seen a metering rod piston bind once in the bore but even that is not vey likely. I like metering rods because you can do alot of tuning with them. And Dave is dead on about the old Holley power valves blowing out. You sure cant adjust them. And as Dave said the newer Holleys have the backfire check ball in the vacum passage to the valve. Ron

 

[sleepar]

Good to know were all in agreement on this... the new fuel is 99% of all the carb problems and most don't know the vaccum advance can be tuned I DON'T KNOW EVERYTHING just throwing ideas out there on the common stuff I have come across..
The rods and power valves can be changed and adjusted to suit the cam of the engine its all trial and error and i have a tackle box full of various distributor and carb and auto trans linkages and springs....

 

[383man]

Good to know were all in agreement on this... the new fuel is 99% of all the carb problems and most don't know the vaccum advance can be tuned I DON'T KNOW EVERYTHING just throwing ideas out there on the common stuff I have come across..
The rods and power valves can be changed and adjusted to suit the cam of the engine its all trial and error and i have a tackle box full of various distributor and carb and auto trans linkages and springs....

There's the problem !! A tackle box is for fishing as you are confused since you need a toolbox to work on cars ! LoL !
Just funning with ya. Ron

 

[sleepar]

I've actually gone out fishing and got all my gear in the boat only to discover I had the wrong tackle box...
Some on here can't tell if I'm serious or jokeing with out the smiley... so I won't use it... I like it when they pick me appart on here. I learn from it..
They should have a way of counting how many times these guys have been told to kiss someone's *** on here... Colorado Dave being in the lead...