Curious about my distributor

[OregonNed]

When my truck was tuned up a year or so ago, they replaced the distributor entirely, and plugged off the vacuum advance port. Is there a reason they may have done this? Should I connect as proper to my carburetor? (1406)

[Sklptrljay]

Yes, unless it's a full hi-performance application you should plug the vacuum into the manifold vacuum port of your carb. Your losing idle and cruising advance without it.

[OregonNed]

Thanks for the response on this. I've been working on sorting out some issues while my truck is down waiting to finish me bed project. One was getting the carb figured out to reduce the issues I am having with and overly rich mixture issue, and this was something I wanted to address too. My truck is definitely just a cruiser.

[geezer#99]

When you hook it up to manifold vac your idle will speed up.
You'll need to turn it back down.
Your richness is likely due to too much fuel pressure.
You'll need a fuel regulator set at 5 psi. Eddy carbs don't like more than 5 psi.
Do you know what your initial timing is?

[OregonNed]

I'm upgrading to an Edelbrock fuel pump next week. I unfortunately do not know the answer to the timing. I'm unfamiliar with dists enough to check. Someone had mentioned on another forum that timing may need a little advance based on an example of one of my plug electrodes

[geezer#99]

Upgrade to a fuel reg instead. That way you know you're controlling the pressure. I've heard of eddy pumps putting out more than their 5.5 psi rating.
Your timing is part of your richness. You need enough initial to allow you to close the primary throttle and cover the idle transfer slot. Otherwise you're idling on the carbs power circuit.
There's a write up in here about that. On a holley but all carbs work the same.
http://www.crankshaftcoalition.com/w...EI_distributor

Do you have an hei or a points style distributor?
If you're not sure, just post a pic of your motor.

[OregonNed]

Great article! My old distributor was junk, but the new one put in has an external coil. I will post photos on this site tonight...

[OregonNed]

Here are a few pics. It seems to be EFI but external coil

[OregonNed]

Next

[OregonNed]

Air cleaner size. Maybe not efficient enough

[davepl]

Originally Posted by geezer#99

When you hook it up to manifold vac your idle will speed up.
You'll need to turn it back down.
Your richness is likely due to too much fuel pressure.
You'll need a fuel regulator set at 5 psi. Eddy carbs don't like more than 5 psi.
Do you know what your initial timing is?

True, but don't do that.

The vacuum advance of the distributor goes to a special PORTED vacuum source on the carb that only has a vacuum signal during throttle tip-in (part throttle load). At WOT and at IDLE there should be NO vacuum advance. If there is, its usually because the carb's idle screw has been turned up so far that the slots are uncovered internally. You can usually verify that by discovering that the idle mixture screws no longer do anything.

If you were to connect the vacuum advance to manifold vacuum, you'd get vacuum advance at idle (and the symptoms described above).

Vacuum advance isn't about idle quality or max power, it's about part-throttle cruising, throttle response, emissions, and driveability.

Race motors don't need it because they operate in two zones - putting around at idle and running at WOT. Street vehicles need it. I figure if the auto makers had it in 1965, keep it. They didn't NEED to do anything, so if a vehicle had it, it's because it's really a good idea. This applies to vacuum advance, chokes, choke pull-offs, fan shrouds, thermostats, and various other things that our trucks run SO much better with, but that people sometimes remove.

[OregonNed]

Thanks for the reply. My first thought was to connect to the higher port on the front of the carb - passenger side after following be recommended procedure with the mixture screws - then followed by adjusting idle speed if necessary.

[rsgt]

Plugging your vacuum advance into a direct source will allow it to engage at idle, which is good for a number of reasons. Much like cruise conditions, engines run leaner at idle than they do under load. Again, this means the mixture burns slower and needs an earlier spark to optimize the burn. Ensuring that the mixture has a complete burn before leaving through the exhaust port also helps the engine to run cooler at idle. All carbureted cars were set up with direct vacuum to the distributor before more stringent emissions requirements reared their heads.

[Sklptrljay]

There is a good thread/article about using manifold vacuum and not ported. Check this out.

http://67-72chevytrucks.com/vboard/s...d.php?t=481245

[68 P.O.S.]

Originally Posted by Sklptrljay

There is a good thread/article about using manifold vacuum and not ported. Check this out.

http://67-72chevytrucks.com/vboard/s...d.php?t=481245

x2, never use ported, this thread explains exactly why.

[OregonNed]

Where can I pull non-ported from? Is that the lower front port on an Edelbrock? Back next to the distributor there is a tee installed in the manifold with one side feeding the transmission and the other is plugged off. Can I connect there?

[RichardJ]

>> All carbureted cars were set up with direct vacuum to the distributor before more stringent emissions requirements reared their heads. <<

That is flat out incorrect.

The reason a lot of guys use manifold vacuum is for use as a band-aid to prevent off-idle pinging.

As Geezer said, when you connect the full manifold vacuum to the dist, the idle rpm will increase. The reason the rpm increases is because at idle, manifold vacuum is very high. The vacuum advance canister pulls the dist to max vacuum advance. You then have base timing + vacuum advance timing.
As soon as you step on the gas peddle, the manifold vacuum drops to near zero. Vacuum advance timing drops out and the timing drops back to the initial /base timing.

You are using the vacuum advance as an off-idle vacuum retard unit.


ALL old carbureted tune-up manuals tell you to set the timing by first disconnecting and plug the vacuum source to the distributor vacuum canister.

A misadjusted carb may have some ported vacuum at idle and as you need to adjust base timing before you adjust the carb, you plug the vacuum source first.

[davepl]

So GM, Fords, and Chrysler got it all wrong, and "don't ask Summit or Jeg's about it – they don’t understand it."?

I think me, GM, Ford, Chrysler, Summit and Jegs will be content setting up our cars "wrong" like they did at the factory.

[68 P.O.S.]

Originally Posted by OregonNed

Where can I pull non-ported from? Is that the lower front port on an Edelbrock? Back next to the distributor there is a tee installed in the manifold with one side feeding the transmission and the other is plugged off. Can I connect there?

When looking at the front of the Edelbrock you'll see 3 ports just below the idle mixture screws. The large one in the middle is for your PCV valve, the small one on the left is ported, and the small one on the right is manifold vacuum (where mine is hooked up). You could hook it up to the tee on the manifold if you wanted, but that's where the power brake booster is usually connected to...depends on the vehicle.

[davepl]

I'm working from memory here but I don't think the ported is technically below the mixture screws. It's the right hand one (looking at the front of the carb) as you say, but I think its up a little because it is, after all, ported and therefore above the plates, unless there's a passage internally.

Hang on, here's mine, you can see where its hooked up (it's the metal pipe that then goes to a rubber short hose to connect to the carb):

[rsgt]

Early quadrajets do not have ported vacuum source, only full vacuum.

[Sklptrljay]

I copied and pasted this from a thread on a Camaro board. Just more fuel for the fire.....

Now, for the rest of us: Vacuum advance.

Ported vacuum advance was, in fact, used by many makers of performance and stock engines in the past. Reason, the fuels were so good that the burn temperatures of the combustion chambers actually lowered as upper rpms were reached, and the added timing from ported was needed to heat the burn properly. In those days, the amount of degrees added in the cannister were fairly large, with some engines having 20+ degrees, but this number was usually not attained, as the total amount of vacuum added by the ported port was lower than that needed to get the vacuum advance to pull all the way in.

Today's pump vended fuels, and their additive packages, or lack thereof, have just the exact opposite of the earlier fuels, they always burn much too hot, even stuff like the Sunoco 280 have this problem. Add vacuum advance into the lace most succeptable to detonation, upper from mid to upper rpm ranges, and you can take detonation to the bank, you're gonna get it.

The only time today that ported timing is beneficial, to a point, is on Exhaust Gas Recirculation designed engines. These engines recirculate partially burned gasses from the exhaust system into the intake tract again to be re-burned and reduce emissions output, and the partially burned fuel needs the added combustion chamber heat to more completely burn the exhaust fuels recirculated.

In today's non-EGR performance engines, stock to performance, we do not require any upper rpm added timing for burn temperature control, in fact, it is detrimental to proper running.

So why use a vacuum cannister at all? Well, there is a simple silver lining to vacuum advance, engines as we are working with like timing at idle in the 18 to 24 degree range, but this isn't all that easy to do with some parts packages. How do we do this? Well, first, we need to think of the vacuum advance as not that, but an idle timing supplement to the initial timing.

As we now know, the idle timing can now be raised to a decent level at idle, to help with a stable idle, pull against a bigger than normal cam, pull against a converter and help cool the engine better from more complete idle fuel control. There is a catch, though, too many degrees of supplemental idle timing can be very counter-productive.

If we use the 24 degree figure, we can do things like set the initial timing at, say, 14 degrees, starts easily, then add the 10 extra with the cannister. OK, how do we get the 10 crankshaft degrees into the 20 degree cannister? We use either a Crane scroll plate to stop the advance pin that conects the diaphragm to the point/pickup plate to restrict plate movement down to the number of degrees we wish different from the static initial timing and the total idle timing.

OK, now that we have the number of degrees restricted, do we want to be able to change the vacuum level the canister comes in goes out with? Yes. We do this in two different ways, we look at the information of the stock cannister pull rate, and select the proper one for our vacuum levels, or we add the adjustable cannister from Crane, with the scroll plate that comes with it. Adjust away, you will find the correct amount of added timing for idle to make your engine run correctly.

Well, doesn't the idle added vacuum timing stay in the system when the gas pedal is hit? No, actually. It is immediately dropped as the off-idle vacuum goes away, and the centrifugal advance acts as usual . Only 180 on this is an engine that has no idle vacuum from a realy radical cam duration, and then, the vacuum actually rises after the engine is brought off idle. We just wouldn't use any kind of vacuum advance on an engine this radical anyway. Static idle timing would also be jumped up to 18 or so with the redical engine anyway, to try to stabilize the idle quality. Once again, adding initial timing helps the idle, just depends what you have available to help.

All HEI distributors manufactured for GM vehicles as production ignitions have way too much advance, mechanical degrees and vacuum degrees, and usually cause mucho problems in our performance and stock engines until they are set up correctly, and have full intake manifold vacuum sourced.

99 percent of all vacuum advances have way too many degrees of timing available and need to have the total number reduced to work with full manifold vacuum, as the manifold vacuum will pull all the degrees available into the cannister, not like the partial vacuum pull available with ported vacuum sources does at rpms.

Now, to dispell some myths about dyno tests and ported vacuum advance. Dynos always use more jetting to keep the chamber, engine and water temps cooled off, and ported timing just counteracts the rich mixtures of the rich jetting meeded for the dyno. In all the experiences I have had with crate and built performance engines, ported vacuum advance has been simply not productive, no matter what the engine did on the dyno.

If you are forced to use ported vacuum for your supplemental timing surce, then the curve and initial are not correct, and/or the total amount of vacuum supplemental timing is too large foruse as an idle supplemet from full intake manifold vacuum.

[OregonNed]

Well, following the advice I added the vacuum line between distributor and manifold vacuum. I made a few adjustments following and took a spin. Couldn't go far, just around the block and some driveway revving and idling. I will say for sure that the exhaust is not eye watering like it was - more like I'd expect. Pulling the throttle at
The carb I also notice more responsiveness and no longer a brief hesitation at all (I suppose that is probably improved by what I did). I'm fairly certain the the previous owner dinked with it some so I ordered the stock out of the box jets and metering rods to install for now. Thanks for all of th great advice! One I drive it more I'm sure I'll see
The difference more. I'll also probably get the timing checked too at some point.