Here goes....

I have the Mallory HEI distributor...here's a link to it. http://store.summitracing.com/partdetail.asp?autofilter=1&part=MAA%2D8548201C&N=4294908216+115&autoview=sku

Anyways, it has an adjustable vacuum advance on it. I lost the instructions and they're not listed on Summit's web page.

I can't remember but what is the adjustment screw inside for? Is it to compensate for different vacuum signals and such? When turned clockwise or counter clockwise does it move the reading up or down?

Also, something I have noticed...on my 402 it has an XE284 cam. In gear the engine idles rough at 600 rpm....take it out of gear and it continues to idle rough for a few minutes before the rpms shoot up. I have found if I remove the signal from the vacuum advance makes the engine idle rough again like if it were in gear.

I believe what is happeneing here is when it idles rough there's little if any vacuum signal but when you take it out of gear it begins to build up vacuum and thus starts pulling on the vacuum advance and therefore causing the engine rpms to shoot up....am I right? Is this normal? Or do I need to change something? Perhaps adjusting the vacuum advance will solve this problem?????

How much vacuum do you have at idle? If you're going to use manifold vacuum for your VA can, you want to assure that the VA can is fully actuated at idle.

When cold the engine makes 12-13 hg at 1100-1200 rpm (park/neutral) and less than 5 at 500-600 rpms. (drive) I've yet to test it warm but it does have a smoother idle when warm versus cold.

Why does the rpm drop so severely from neutral to drive? I would expect 100-150 rpm drop at the most.

Why does the rpm drop so severely from neutral to drive? I would expect 100-150 rpm drop at the most.


It could be doing the same as mine and losing the vacuum in gear that was keeping it idled up.

I think it's due to what Camaro fever says....the vacuum.

I thought it might've had something to do with running a 3000 stall converter as opposed to a 3500....but then again when the vaccum isn't present there doesn't seem to be as much as an rpm drop.

Run the vac adv off the ported vacuum if you can run enough intial timing to idle decent in gear,not detonate,and still start the motor hot .

It's definately vacuum thats issues that's causing the erratic idle but also keep in mind if youe mech advance is comming in early like at 900 idle rpms that will also affect the idle and make it rise up if the mech kicks in advancing the timine thus increasing the idle too.

So check to see what rpm the mech adv starts to come in too as this can definatel play a part in your erratic idle too.

Scott

when it idles rough there's little if any vacuum signal but when you take it out of gear it begins to build up vacuum and thus starts pulling on the vacuum advance and therefore causing the engine rpms to shoot up....am I right?

That is what is happening. It's kind of a positive feedback thing...low rpm means low vacuum which reduces timing which causes lower rpm...higher rpm (smoother) means more vacuum which means more vac advance which means more rpm. To avoid this feedback loop ( self-licking ice cream cone?) you can plug the vac advance into ported vacuum. Then you will have vacuum when the throttle is open above an idle. Idle will not be as smooth but it won't surge or "hunt".

Yeah ported vacuum for the advance will stop the hunting...

But I question if it is the correct fix the problem. What I would recommend doing is hooking up a hand vacuum pump/meter and figuring out exactly what the vacuum advance configuration is. This looks like a plain HEI distributor and I would bet that the vacuum levels are mismatched between the can and the motor. You need full vacuum advance at 1-2" less than you have at idle, in this case apparently idle in gear.

I had a no-name distributor that looks just like that and the vacuum can was all in at 15". Adjusting it would bring it all the way down to... 13". Piece of junk. I now run a can that is all in by 9" and idle around 11" and have a much happier combination.

Note that you will also need to validate the total amount of advance the vacuum can provides is right. Many, perhaps most, pull in too much. My theory is they may have compensated for slow factory advance curves with extra vacuum advance at cruise. You want somewhere in the 12-16* crankshaft range (6-8* distributor).

The ported thing would work expect theres a problem...the carb I currently have (Holley 750 cfm model 82750) has two small ports.....both are full manifold vaccum! Seriously! I've had other Holleys that had both full manifold and timed spark ports but this one has two full manifold ports.

I even hooked up the vacuum advance to the port the instructions for the carbureator said to but it's full manifold....same as the other smaller port that my vacuum modulator for my TH350 is connected to.

I can hook my vacuum gauge to either one of these ports and get a reading at idle.

I guess I could just cap off my vacuum advance but since this is a street/strip car I figured it's better to have it.

I was thinking that turning the screw inside the vacuum advance would compensate for problems like what I'm having.....???

Speaking of turning screws... what way is more and what way is less ???

This is a very common symptom of having the carb throttle blades opened too far at idle. I used to have this problem before I figured out to use a lower vacuum level advance can and have it add ignition advance at idle. After that I was able to close the throttle blades back down.

Note that having the throttle blades open up too far has other side affects like making your idle mixture screws non-functional because you are idling on the transition slots rather than the idle ports.

A hand vacuum pump/gauge is extremely useful in figuring out and tuning the vacuum advance can.

How can you tell if the blades are opened too much? How do you close them? Reduce the idle setting on the carb?

Well... I used to use vacuum on the ported side as an indicator :)

You can also remove the carb and check the appearance of the transfer slots under the throttle blades. You should have no more than a square at idle. If it stretches into a rectangle the blades are too open.

Fixing the problem is tricky. This is why people sometimes drill small holes in the blades. The gotcha with this is that it is irreversable and requires trial and error to get the holes the right size. It is also claimed that most situations do not require the holes even when they are put in. The point being to get the ignition setup correctly *first* before resorting to drilling holes and modifying the carb.

When I first put my 402 engine together I set it up on ported vacuum. It would not idle unless I opened up the throttle blades a bunch. It seemed to then idle too high (should have been a clue: idle increased when ported vacuum kicked in). I have a Crane HR296 cam, so I am well into the range of cam specs that cause idle/timing issues. I set my timing using the total timing method so my initial was 16-18*. As a means to close the throttle blades and idle I made up a controlled/adjustable vacuum leak. I would reduce idle on the carb until the ported vacuum went away and then set idle speed by opening up a needle valve on the vacuum leak. This worked after a fashion but my vacuum at idle was in the 7-8" range.

I did some reading up on the net about ignition setup and tuning. Then I borrowed a friends hand vacuum pump/gauge and spent an evening playing around. I figured out that the "adjustable" vacuum can I had on there was really not very adjustable and started at 5" and finished at 13-15" depending on adjustment. It also provided too much advance. For grins I checked out a vacuum can on an old distributor and found it started at 3" and finished at 9" and provided 22* advance. Hmmmm... so I converted to manifold vacuum, ran the 3-9" can, limited the advance travel to 12* (which changed the range to 6-9") and WOW! I no longer needed the controlled vacuum leak kludge. My vacuum at idle increased to around 11". My off idle hesitation was reduced. It was a beautiful thing. BTW this means I idle around 30*.

So what I suggest is get the engine warmed up and see what your vacuum is at idle in gear with the headlinghts on. In other words you want worst cast with the engine loaded because your vacuum will be lowest under these conditions. Make sure this is a reasonable idle speed because vacuum will vary significantly with engine speed on the low end. Take that vacuum level and find a vacuum advance can that is fully advanced at 1-2" less than that. Put it in and limit the vacuum advance travel so you get 12" total advance (if the can provides more). Crane makes a nice little eccentric to do this or you can make up an L shaped limiter using .030 stainless steel strip from K&S metals (commonly sold at hobby stores). Hook this up to manifold vacuum to provide full vacuum advance at idle. See if this does not make it work better and allow you to close the throttle blades (you can use the ported vacuum as an indicator here).

We had somebody awhile back try an expirement to see just how much advance their engine liked at idle. The idle speed kept increasing up to 40*.

Thanx Steve. My vacuum can is adjustable...I just need to find out which way does what when you turn the screw. Also you wouldn't have a pic of the idle transfer slots would you? I kinda have an idea on what you might be talking about.

http://www.chevelles.com/showroom/data/500/transfer_slot.jpg

Well since no one could tell me which way to turn the screw inside the vacuum advance I took it upon myself to try it.

I turned the screw pretty much all the way out ccw. Then I took a vacuum reading and adjusted the idle down some.

The engine doesn't seem to idle as high now. In park it will go up to 1200 in park but eventually drop back down to 900-1000. Put it in gear it drops another 300.

Not sure if this will hurt or help economy/performance but the idle doesn't seem as erratic.

Reason it out: how could they build an adjustable vacuum can? It is going to have a can/cylinder with a vacuum hose connected to one side. There is going to be a diaphram on the other end connected to the rod coming out. And finally there will be a compression spring inside providing resistance for vacuum to overcome.

So to make adjustable the preload on the spring must be varied by moving one end or the other. The diaphram side is like easier and more consistent with the adjustment screw. Which suggests tightening the screw will move the spring seat away from the diaphram and increase spring preload and thus vacuum levels.

It really is WAY simpler to tune this stuff with a hand pump so you can associate vacuum numbers with movement. Then you can just turn the screw one way and see what happens. Sure beats twisting and running and twisting and running...

Cool. I just picked myself up a vacuum pump a few days ago and I will try that. I guess what I need to do is remove the distributor cap and pump until I see what hg the actuator starts and stops moving?

I figured they made it adjustable so for low vacuum signals it would take less vacuum to actuate it.....and vice versa.

Yep, that is what you need to do. And yes the adjustable is supposed to increase/reduce the required vacuum level to actuate the advance.

You can also remove the carb and check the appearance of the transfer slots under the throttle blades. You should have no more than a square at idle. If it stretches into a rectangle the blades are too open.

Question.....I just did this, yes the slots were a little rectangular looking so I backed the scre off until they turned into a square....should the little hole for the front full manifold port be not viewing when adjusted correctly?

I think the "squared" up setting is just a ballpark, once you have it squared you can actually use the idle screw to adjust idle. I mean, perfect square, too little square, too much square can drive a man nuts. Heck, the new 870 Holley I just got from Holley is not even showing the slots exposed at all. Bottom line, is: Square them up, and if you have to turn the screw the idle screw in or out a little once its running at gonna hurt nothing, if that was the case Holley would of put some sort of limiter on there (maybe they should of).

The problem is that people just really crank the screw in and really throw that setting off that's the issue.

I got mine squared up, then I adjusted the rear plate so the squares were exposed, engine still doesn't want to idle and I have it timed right too.

What does your timing setup look like now?

Initial timing?
Total timing?
idle timing?
cam timing?

I found that when I did not have enough idle timing I had to open up the throttle blades too far. This turned on the ported vacuum port and screwed up the transition slot carb calibration.

20 initial 38 total, the other two I dont know. I have the cam set str8 up if that's what you mean.

Did you get the vacuum advance setup to be all in at idle?

Your cam will probably require more than 20* advance at idle. I run a HR296 in my 402 and it much prefers 30* over 20*. I run 18* initial plus 12* from the vacuum advance on manifold vacuum.