QJ Vac Advance port

Buzzbomb · Oct 29, 2011

ss3964spd said:
I can hear, and see on the tach that the idle RPM is fluctuating. Among other issues I need to figure out what is causing the fluctuating idle; either the distributor is or the carb is. At this point I'm leaning towards the distributor because, if I remember correctly, when I shoot the balancer with the TL the mark is moving.

Which springs do you have in the distributor? And are you using the stock weights? You can sometimes get a "hunting idle" from using too light of springs to where your mechanical advance is coming in too close to idle causing fluctuation. The stock weights are actually the best weights, but you can use the spring kit from the Mr. Gasket GM points recurve kit for the springs. It also might be worthwhile to make sure the weights you're using are clean and not sticky.

Schurkey, good points about the ported vacuum. It makes a lot of sense that each port is "timed" differently for various reasons. Thanks for clarifying that..

ss3964spd · Oct 31, 2011

Finally had time to dig in deeper. With the engine warmed and idle set at 750 ish and initial timing at 15*. A dial back light was used for readings:

1) Timing is fluctuating by 2 or 3 degrees.
2) Connect vac adv to full manifold source, drop idle speed back to 750. Timing jumped to about 34*. Vac adv is adding nearly 20*
3) Move vac adv from full vac source to "ported/timed source. SAME result as #2 above.
4) With vac adv connected to either source timing was still fluctuating by 2 to 3*.
5) Vac adv can is pulling in all that advance with just 12 ish In/Hg of vac. I did not check the minimum vacuum needed for the can to start adding.
6) With initial still set to 15* and the vac adv disconnected, removed the weights and re-installed the springs (to try to keep the dist cam stable). Timing still fluctuating by the same 2 to 3*.
7) Re-install weights, vac adv disconnected. Appears the mechanical advance is starting to come in just above 800 RPM - maybe 850.

So, it certainly appears that my points distributor has some problems and I think I need to get that straightened out before I can make any real progress with the SMI QJ. Was interesting that the traditional timed port was making the same amount of vac as the manifold source. The mixture screws do have a little effect in that If I seat them the engine will die but I only need to un-seat them by 1/2 a turn to keep it running. Anything more than 1/2 makes almost no difference - unless I turn them out 3 or 4. While messing around with the timing I did notice that if I disconnected the vac advance and left the port open the engine pick up RPM and seemed to run smoother.

Time to go distributor shopping.

Dan

71350SS · Oct 31, 2011

Just because the vacuum port is physically located on the carb body above the throttle blades does not always mean that port will always have ported vacuum.
Case in point, on most quadrajets I've ever worked on the 3/16th vacuum nipple located directly in front of the carb and about 3/4of the way up has manifold vacuum at idle and this is where most factury air cleaners EFE valve would get its vacuum from. So it is possible that the port you're using is supposed to have manifold vac. and no amount of adjusting will get the vacuum to where you want it to be.

77 cruiser · Nov 1, 2011

How much vertical movement do you have on your dist. shaft? If they have the stock thrust washer I've seen as much as an 1/8 inch. I like to reduce that to about .015.

ss3964spd · Nov 1, 2011

Thanks John.

I don't know Jim, I haven't actually measured it but I certainly will when I pull it out.

Based on some very positive feedback I think I've settled on a brand new Pertronix distributor with the Ignitor III, their matching coil, and perhaps a set of their 7MM wires to replace the reproduction set I currently have.

Dan

ss3964spd · Nov 14, 2011

I went with a brand new cast alum distributor from Pertronix with their Ignitor III module.

I installed the new dist yesterday. Ran a temp wire from the "Fused Ign" blade on the fuse block, terminated all wires ends with crimped connectors covered in shrink tubing. Started it, warmed the engine, disconnected the vac adv and set the base timing to 17*. Connected my hand held vac pump to the adv and whoa - the thing was adding close to 20* at idle - which is just too much. Between base, mechanial, and vac adv the engine seriously bucks on decel.

I could have sworn Pertronix said the vac adv was limited to about 10* and that the can was adjustable. It's definitely NOT limited to 10* and for the life of me I can't see how it's adjustable - no amount of probing with various sized allen wrenches through the vac port finds anything.

Yanked it back out. Old dist had a mechanical travel limiter installed for the vac adv so I removed it and installed it in the new dist. Got to where I thought it was close and reinstalled. Still too much. The limiter is basically a cam and I was able to rotate it with the dist still installed. Got it down to about 12*.

The vac adv limiter works - kinda. The issue is it works by NOT allowing the vac adv mechanisum to return to its no advance state. The adv can is progressive - meaning initially it takes little vacuum to make it start advancing and as more vac is applied it pulls in more advance. The result is this; before I installed the limiter the vac adv was pulling in 20* at idle. With it installed it's pulling in roughly 0. The cam limiter should have been designed to be installed on the other side of the vac adv arm so that it limits the travel in the other direction - as in Shurkey's pictures.

I'm going to fuss with it some more. My old dist DOES have an adjustable vac can on it. I should have installed it but I wanted to see how the existing one would work. Now I know.

I did drive the car for about an hour. The bucking on decel is mostly gone now but it's sluggish just off idle. I don't yet know what the centrifugal advance curve looks like but my guess is that it's very slow.

Dan

Schurkey · Nov 14, 2011

ss3964spd said:
Started it, warmed the engine, disconnected the vac adv and set the base timing to 17*.

Likely too much; but that much is common if not "optimum". Excessive initial timing is often used to crutch poor idle resulting from lean idle-fuel passages and rough camshafts.

ss3964spd said:
Connected my hand held vac pump to the adv and whoa - the thing was adding close to 20* at idle - which is just too much. Between base, mechanial, and vac adv the engine seriously bucks on decel.

I could have sworn Pertronix said the vac adv was limited to about
10*

10 distributor degrees, or 10 crankshaft degrees?
10 distributor degrees would be 20 crankshaft degrees.

ss3964spd said:
Old dist had a mechanical travel limiter installed for the vac adv so I removed it and installed it in the new dist. Got to where I thought it was close and reinstalled. Still too much. The limiter is basically a cam and I was able to rotate it with the dist still installed. Got it down to about 12*.

The vac adv limiter works - kinda. The issue is it works by NOT allowing the vac adv mechanisum to return to its no advance state. The adv can is progressive - meaning initially it takes little vacuum to make it start advancing and as more vac is applied it pulls in more advance. The result is this; before I installed the limiter the vac adv was pulling in 20* at idle. With it installed it's pulling in roughly 0. The cam limiter should have been designed to be installed on the other side of the vac adv arm so that it limits the travel in the other direction - as in Shurkey's pictures.

Sounds like the limiter cam included with the Crane adjustable vacuum advance.

Best solution is to disregard the packaged instructions, and rework the vacuum advance to put the limiter cam on the other side of the pull-rod. Drilling and tapping one hole in the vacuum advance arm can do that. Truthfully, it's as easy to braze or mig-weld the tab in place than to drill, tap, and play with the adjustment.

ss3964spd · Nov 14, 2011

Schurkey said:
Likely too much; but that much is common if not "optimum". Excessive initial timing is often used to crutch poor idle resulting from lean idle-fuel passages and rough camshafts.

The cam is 234/244 on a 112 in a 489. Not huge but definitely not small.

Schurkey said:
10 distributor degrees, or 10 crankshaft degrees?
10 distributor degrees would be 20 crankshaft degrees.

Good question, I don't think they specified. Since all I have to work with is a timing light I always think in/work with CS degrees.

Schurkey said:
Sounds like the limiter cam included with the Crane adjustable vacuum advance.

Pretty certain it is.

Schurkey said:
Best solution is to disregard the packaged instructions, and rework the vacuum advance to put the limiter cam on the other side of the pull-rod. Drilling and tapping one hole in the vacuum advance arm can do that. Truthfully, it's as easy to braze or mig-weld the tab in place than to drill, tap, and play with the adjustment.

Since I don't have any welding/brazing equipment I'll see what nuts/bolts/hardware I can scrounge up to move that cam to the other side.

Dan

ss3964spd · Dec 13, 2011

An update for those interested.

I ended up removing the vac adcance can the Pertronix distributor came with and installed the adjustible one from my old distributor. I also installed the mechanical advance springs and weights from the old distributor.

Still too much vac advance so yanked it back out. Decided to make my own vac advance limiter by drilling/taping the vac adv can mounting plate, a scrap of flat aluminum into which I cut a slot, and a couple of washers and a small allen head bolt.

The good news is with this set up the vac adv is limited to about 12 degrees and I now have vac adv at idle. Also good is that I can adjust it by sliding the stop forward/backward. Bad news is the mechanical advance is still slow - I get no mechanical until around 1500 which is making the engine feel lazy and unresponsive off idle. Next I'll try different combinations of springs and weights. Also bad is that the engine is still bucking on decell so I may have to pull out some more initial and vac adv.

Pics attached.

Dan

magicrat56 · Dec 14, 2011

Dan,good to see your not giving up!i think that your real close with sorting out the ignition system,how about base-lining the curve now by driving the car without the vacuum can plugged in. see if the bucking ends if its still there i would start making some carb adjustments.maybe try bringing up the metering rods a 1/4 turn at a time; would start by going in with apt screw counting the turns to seat it so you know where your starting from.might really be time to take the top off and see whats in there. bet you'll find "k" metering rods. these are stepped rods and really can act as an on/off switch. plain tapered rods give a better range of adjustment. keep at it your closing in,keep us posted!

ss3964spd · Dec 14, 2011

Thanks Joe.

Yeah, I think I'm fairly close now with the ignition - definitely going in the right direction. Presently it's at 17 degrees initial + 11 or 12 vacume at 800 RPM. I haven't driven it yet with the vac disconnected to see how it behaves. I also need to pull and re-gap the plugs from .35 to .40 or so for the hotter ignition system.

Actually, that was going to be my next question regarding the bucking on decell. How much, if any, can the carb contribute to that? If I elimiate the vac adv and it's still bucking does that then point to carb adjustment?

I know the carb is rich at idle. This one has no APT. I also know the primary rods are all the way down into the jets at idle so the power piston spring isn't too stiff. I think my first step with the carb is to get SMI on the phone and see what can be done to clean up the idle.

Dan

bowtie0069 · Dec 14, 2011

While trying to sort out an apparent too high fuel pressure
problem

I was surprised by that comment, when I ran a Q-Jet I usually bumped the pressure up to about 9psi to keep the bowl full. If I left the same pressure used when running a Holley, it would empty the bowl about the 1000ft mark. The car was running 11.80's at the time, so it wasn't like it needed a huge volume of fuel, but it cost me some money on a couple races lost due to it nosing over.

1972 El Camino · Feb 22, 2012

Interesting thread. I'm wondering why so much initial timing is needed?
Putting a timing light on my dist, it gives 25 degrees of crank shaft advance when full in. I had another dist that have too much at 36 fully in, turned out it was missing the limiting bushing)

I set my initial to 9 degrees idling at 600rpms = 9+25 = 34 total timing.

With the vacuum can disconnected I test drive the car, it runs good with no detonation. (Also reading cliff's Rochester book I set my SMI quadrajet mix screws at 6 turns out from being seated which got rid of the bump I had in my idle.)

Next I swapped out my vacuum advance for a b25 advance can that gives a total of 16 degrees of crankshaft advance when fully advanced. (the can that came on the Cardone dist gave 20 degrees of crankshaft advance, way too much!)

So I ended up with 16+25+9 = 50 degrees total timing, works with my stock 350 using manifold vacuum. My idle is at 725-750rpms with the vacuum advance hooked up.

Oh and a dial back timing light is a must have!

I'm going to install the crane adjustable advance can next just out of curiosity.
Cool the way the limiter for the advance can also will limit how much centrifical advance is happening as well. ( this will allow me to get more initial timing if i want it.) if you read through instructions for the Crane vacuum advance kit it explains how it does this as well as limiting how much advance the advance can gives.

For what it's worth,
Mark

QJ Vac Advance port

Buzzbomb

ss3964spd

71350SS

77 cruiser

ss3964spd

ss3964spd

Schurkey

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magicrat56

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1972 El Camino

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