What do you guys with vac advance do to get a clean start?

Maybe my dizzy is just old and sticky, but my new 412 likes to run at around 35 degrees idle. If I run 10* initial, and vac direct to the manifold I have to play with the pedal to get it started until it will get enough vac advance to idle on it's own.

If I crank out the initial to like 25 or so, it will start on the first turn and idle fine, but then I get around 50* at idle. My adjustable vac advance won't let me turn it back enough (clockwise) to run 25 degrees of initial and minimum vac advance at idle.

Maybe I answered my own question, and should plug the vac advance into the ported vac port..

5.7kid, any thoughts?

The vac advance does seem to react ok, because if I stab the throttle and watch it with a timing light it will go to 10* when vac drops like you'd expect.

This is what happens when an EFI tuner builds a carb car after not touching one for 15 years

-- Joe

 

Ok tried that.

So I moved the vac to the ported vac port on the carb, and I set 25 degrees initial. So it will crank about a half turn and fire, idle around 900rpm. So it's 25 degrees idle.

I guess the problem with this is, I don't know what the mechanical limit is. Guess I have to do some googling now. I changed the weights and springs using a mr. gasket set, but the goal there was to get all the advance in by 2800. I'm guessing the total-advance is limitted by (or not) some set screw or simply the total travel of the weights?

Idle is pretty lopey. Can't really tell how much of it is the cam. I can tell that the exhaust is not blowing black smoke nor do my eyes water. If I crank the idle mixture screws clockwise (lean) it will drop the idle speed, and eventually stall. I guess I should probably really do this with a vac gauge.


-- Joe

 

Joe,

Nothing wrong with running 50* at idle. I run 55* with mine. You can limit the HEI's mechanical advance by inserting a screw in one of the holes in the arm...you will need to grind the head of the screw to get the mechanical you want. I have a GM performance HEI, and it only has 15* advance. Most stock ones have 20-25*. I have 22 initial, 15 mechanical and with vacuum I have about 55* at idle. It runs great and keeps the plugs nice and clean. My only problem now is it wants to continue running after I turn the key off. I may need to limit the vacuum to 50* or so to stop that.

 

Hey,

Thanks for the reply.

I'm going to have to look at one of my distributors to get an idea of where you are putting the screw. I imagine you just check total advance by spinning it to 3k or whatever with the light?

I'll keep tinkering. I had to wire up a sunpro cheap tach for now because the stupid factory cluster is junk. tach reds too high, fuel reads too high, coolant doesn't read at all, etc.. Then when I took the cluster out, I noticed the speedo cable is broke too.. hah. what a day.

It runs though

Thanks!

-- Joe

 

If you pull the rotor, you will notice some holes in the arms. They are in a good spot for inserting a small bolt (not screw!) with a locknut on the underside. Then you grind the head to adjust the advance. You will need heavier advance springs after you do this (more weight means quicker advance). I check my mechanical advance by using a protractor or just measuring the sweep of the arms (s = R*theta gives you radians and then convert to degrees). You can calculate the degrees and divide by two to get the advance as seen at the balancer. Then I check total by reving the engine and using a dial back light. I have springs in my advance that are too light right now. The timing really bounces around at idle. I need to put one stiffer spring in to see if that stops.

 

I was just looking at a spare dizzy. what about putting a screw/bolt in the arm rather than the weight? I noticed that if you put a screw there, the weights can only travel 1/2 the distance they used to travel. ??

-- Joe

 

Personally, I've only ever seen the advance slot modified to change the total amount of mechanical advance available (not familiar with the screw/bolt mod). Aftermarket distributors have bushings that fit into the advance slot to provide varying advance values.
The following link has a brief discription of the modification. Chevy High performance did a one page article strictly about the same thing. Unfortunately, I only have it in hard copy. Can't scan it but I could take a pic if you're interested.

http://www.chevyhiperformance.com/ho...438/index.html

As for the ported vacuum vs full manifold vacuum. I've had this debate before, (right 57kid, Atilla?). I've only ever used ported vacuum. Car starts instantly. Idles well with 16* initial.(The ported vacuum doesn't enter into it). The mechanical advance is limited to 18* and it's all in by 3000.

 

Mechanical advance on a HEI is limited to 18*, or on yours?

If I try to run less than 25* I have to play with the pedal to get it started, and that's just not cool. It will lope like nothing else at 20*, but at 25* it seems happy. So I set the initial to that and plugged the vac into the ported.

However, 25 + 18 is way too high for any motor at WOT, so I need to find a way to limit it. I'll check out your link, thanks!


p.s, in my head the vac advance to direct manifold vac sounded like a better plan. Run a small initial combined with the mechanical to get 36* at WOT, and have the vac advance have a strong idle advance however on crank there isn't enough advance to light through the lean crank/idle AFR. No bvac at crank, and even if it kicks it stalls before it gets enough vac to ramp out the adavnce.

I also don't want to do 4 shots of the pedal and a pig rich idle either.

So I guess we're doing ported vac + more initial.

-- Joe

 

HEI's have factory advance ranging from 20-22 (I've seen claims for as high as 25).
I've limited my mechanical by modifying the advance slot as shown in the link. (Or better still in the CHP article).
At cruise, I have full vacuum advance (10-12 IIRC) so at 3000 there's a max of 48 -50. WOT is 34.
I've talked with guys who prefer the full manifold vacuum source. The only trouble they describe is a slower response to idle speed (because the manifold vacuum helps to keep the rpm's up). Your sig shows a 288 duration cam so I can't necessarily apply my results to your situation as my cam is only a 276. 13-14" of vacuum at 750 rpm. In my case, the engine runs fine with only the 16* initial.

 

Bolt in the arm...that's what I was talking about. I can't claim ownership of that mod. The guys over on chevelles.com pointed it out to me. I cut a stock HEI down to half the total mechanical. Now, with the GM Performance distributor, I only have 15* total. I am happy with that. I have never used a ported vacuum source for the advance. I want to have as much advance as possible at idle. Mine is at 55* right now, but I may try to limit it back to 50* so the car turns off easier.

 

IF you take the advance can/valve/diaphram thing off the dizzy, the lever that it connects to the reluctor or whatever it's called, if you move that with your fingers it requires.. some effort. I compared it with another dizzy I have on the bench and it.. It kind of binds.

DO you think that the problem is the stupid thing is bound up so much that it's not bring the advance quick enough when the motor catches? It seems to me that the problem I'm having is an odd one. I've run motors with cams around the same size that idled, not great, but idled at 10-12*. This thing pukes.

-- Joe

 

It shouldn't really require much effort to move it, it should just slop around. The whole mechanism is pretty simple to rebuild if you want to see if cleaning it up fixes it.

 

I guess I'll play with it more tomorrow. I'm half tempted to buy a new dizzy, looks like with cap and coil they are like $50 to my door. Would need to change the weights/springs and my adjustable vac module but might be worth not having a 30 year old dizzy..

Thanks!

-- Joe

 

I feel your pain. I finally got sick of playing with the $25 dollar one I got out of someones basement. I broke down and bought the GM Performance unit (it was $185). It was money well spent. If you need the melonized gear (factory roller cam), it already comes with it.

 

I thought you needed the special gear with billet steel roller cams? I was under the impression factory rollers used a normal iron gear.

-- Joe

 

Joe,

It depends. My roller had a cast iron gear pressed onto it so it would be compatible with the factory distributor which has the melonized gear. If it is a straight steel billet, you are stuck with a bronze gear or a composite gear (I think comp and lunati have them). The bronze gears are somewhat sacrificial and may only last 10,000 miles (or less). Whichever you use, go to comps website and look up the oiling modification you can do to the distributor housing. This will especially help a bronze gear last longer.

 

The combination I went with, which seems to work is:

1) Vac plugged into the ported port
2) Initial timing set to 24 or so degrees
3) Screw in the dizzy limit mechanical advance to around 10 degrees.

I can grind the screw if I need more advance out of it. I put the screw in the hole behind the rotor screw hole, and put a nut on the bottom. It limits how far the arms can scissor out.

What a pain the the butt.


-- Joe

 

Did you notice that the extra weight of the screw makes the mechanical come on faster? Big shock when I had the two lightest springs installed (all my mechanical was in at a 1000RPM idle).

 

Hrmm. Did I? Not really. But I'll tinker some more.

Did you put your screw in the same spot I did? I put it in the hole just behind the outter hole where the rotor screws to. I then had to grind the bottom of the rotor just a hair to get it to fit over the screw.

I've got the car 'tuned' to the point where it idles around 12hg (as opposed to 5 or 6), around 900 rpm, at 24 degrees. I have to pump the carb a few times and often have to feather it while cranking to start. If I change the idle mixture screws AND add more advance it will start easier, but has a hesitation when transitioning to open throttle. The way I have it 'tuned' right now, I can snap the throttle like a quick blip and the motor will rev to 4k before your eyes can react.

The only thing that I'm a little unsure of is, the needle on the vac gauge (tool, not like a real car gauge) bounces. Perhaps that is just the big cam. When the tuen was worse it would bounce between 6 and 8 like a guy shaking a can of spray paint, and now it kinda 'shakes' between 11-12hg. I adjusted the valves twice so I don't think it's a tight valve.

-- Joe

 

I've replaced my HEI with a MSD small cap billet piece. Adjustable mechanical and vacuum advance limits. It should make the recurve process very simple. It doesn't hurt that I still have an old Sunnen distributor machine to do my work on. It makes the process of swapping springs and weights and checking the curve easier on the back!

 

The distributor machine is nice, but I even got tired of that and went with an electronic-advance distributor. :P

 

I've though of that upgrade myself.
Maybe the next one.

 

That's where I put mine. I also have that slight hesitation when blipping the throttle. I idle at 52* with 13" on the vacuum gauge (1000RPM idle speed). I can live with that (I think some more carb tuning could cure it). However, with so much advance at idle, my car doesn't always shut off when I turn the key off. I wish I had an electronic retard device that I could activate just for start up and shut down.

I had my best track pass two weeks ago after getting my WOT AFR and timing fixed. I went 11.89 at 113.5MPH in a 3000' DA.