[MrWrestlingII]

I am resurrecting an old thread/conversation regarding a PVA vacuum source on my quadrajet. Below are copy and pastes from some of the old thread and also a PM that I had with Cliff.

https://forums.maxperformanceinc.com...d.php?t=752961


I finally pulled the carb (pn 17056228) and am wondering if this vacuum port can be modified to use as a PVA source? One of the pictures shows the vacuum port bypass slot.

From old thread...
Cliff-
Later model q-jets will also have up to two other "ported" sources, both are in the baseplate, and neither one is suitable as a source for the vacuum advance. This happens because they either have a bleed-off port, or high source location, so do not provide FULL manifold vacuum to the distributor right off idle.

MrWrestlingII
After looking at the carb, Cliff's explanation makes sense. I've got three vacuum ports available, two on the base plate (ported) and another on the main body (manifold). Vacuum at the ported sources lags behind the manifold source as the throttle plates open. I don't believe there's a good ported source on this carb.
The three ports I'm describing are labeled in one of the pictures.

Cliff-
Neither one of those baseplate sources is suitable to operate the vacuum advance. The one on the lower left can be modified to work as in most cases the source location is correct, but it has a vertical slot cut in (bleed off port) to delay and reduce the vacuum signal as the throttle angle increases.

From PM...
MrWrestlingII
You indicated one of the vacuum ports on the baseplate could be modified to work as a pva source. I'm wondering what needs to be done for this mod? I haven't taken the carb off yet to look at the bleed off slot, would it be as easy as filling the bleed off slot with Marine-Tex?

Cliff-
On most later q-jets the ported vacuum to the EGR can be modified by filling the bypass slot with epoxy, just takes a few seconds and you will have a good ported source for the advance.
Some models used a high source location in lieu of the bypass slot, so they will not work for the vacuum unit.

[MrWrestlingII]

Here's a pic from the throttle opening side...

[Cliff R]

Just block the "bleed-off" hole and use the lower right vacuum port.......Cliff

[MrWrestlingII]

Thanks Cliff. I epoxied the hole up and will report back on the results.

[Cliff R]

They either used a round hole or "slot" for bleed-off in that location to "kill" signal for ported vacuum. In almost all cases the source location is low enough it will function as normal ported vacuum showing all the vacuum right off idle just like the dedicated distributor ports would have.

If the location was placed higher they didn't use a bleed-off port as it wasn't needed. The idea was to provide ported vacuum for the EGR but not too much of it and it diminishes with increased throttle angle/engine load.

This is why it is imperative to provide the Vacuum Advance with a well located ported vacuum source IF you want it to behave just like manifold vacuum just nothing at idle and coasting.

It's also why adding ported vacuum requires a very specific location for the supply so it's ALL in with the slightest movement of the throttle right off idle.

As it relates to this topic, it is now 2019 and lots of folks still do NOT understand this deal (vacuum advance) and at least 2-3 times a week I have to explain to folks who call in here for help with tuning their new engines how it works. It's ALWAYS the same story....."my engine builder told me I didn't need it", but they go on to tell me how much fuel it uses and requiring jet/rod combinations WAY outside the "norm" to be happy, etc. That shouldn't be a big surprise, instead of having 42-48 (or more with some set-ups) degrees timing at cruise you've only got around 30-36, so it's NOT going to effectively burn a leaner mixture.....DUH!........Cliff

[MrWrestlingII]

Cliff,

As always, thanks for your help and advise.

Your trick for blocking the “bleed off” hole worked as described. Using a multi port vacuum gauge, with one port hooked to MV and another to PV, the PV port mirrors the MV port once the throttle is cracked open. On the PVA source, after adjusting the throttle linkage screw to get 800 rpm (manual), the timing showed 11 degrees on the balancer. On the MVA source timing is at 23 degrees. Timing is dead stable on both PVA and MVA. Idle on the PVA source is a bit lumpier, otherwise I don’t notice much difference.

I have a question about setting the idle mixture screws. With both screws 3 turns out I started on the driver side screw, going ½ turn at a time and watching the vacuum gauge (hooked to MV). I didn’t notice any change in vacuum or idle until ½ turn from seated, and backed it out to 1 ½ turns from seated. On the pssngr side the idle changed at 1 turn from seated but I was seeing nozzle drip until it was backed out 3 turns from seated, so I left it there.

I am wondering if I have enough control over the idle circuit and if my process to eliminate the nozzle drip is ok? In its current state the motor idles good, doesn’t smell and drives great with phenomenal throttle response.

Vacuum at 800 rpm 13.5

Notes from the carb build (third recipe), you supplied the rebuild parts
PN 17056228
Idle Tube- supplied
Idle down channel- .054 to .059
Lower idle air bleed- .075
Upper idle air bleed- .066
Upper idle air bleeds in air horn- not on this model

Motor-
461 CID
Cam- 230/236 1st gen SD stump puller hyd roller
1.65 HS rocker arms
Around .620 valve lift (if I remember right)
72cc eheads (old style)
22cc piston dish
Zero decked
10.25 calculated compression including head gasket thickness

[Cliff R]

What intake manifold are you using? What gasket is under the carb...4 hole, fully open, etc?

[MrWrestlingII]

I am running a Performer RPM intake with your thick open purple gasket.

[MrWrestlingII]

Yesterday I removed the idle mixture adjustment screws and blew compressed air into the openings. With the engine warmed up I got a better adjustment response with the idle changing at about 1 full turn from seated. But there was still nozzle drip on the pssngr side until the adjustment screw was backed out 3 turns from seated. I tried bumping up the timing from 11 to 14 degrees with the idle readjusted to 800 rpm but still had the pssngr side nozzle drip issue.

The next time I have the motor warmed up I will look at this using the MVA source. My previous tuning sessions on the MVA source were with the air cleaner installed so I wasn't looking for nozzle drip. I don't doubt that I've had this issue since I rebuilt the carb and motor a few years back.

Also of note, with PVA the motor has dieseled a couple of time when shutting it down. It hadn't done this using MVA.

[TedRamAirII]

Maybe time to enlarge the ByPass Air passage.

[Cliff R]

Did you check the size of the idle tubes?

I only drill them if requested.

When using a fully open gasket you may end up with some stager in the adjustment screws. I usually balance them to the same settings before removing the carb when I’m adjusting them here......Cliff

[soupman]

I am resurrecting this thread, I believe this is important since many of us run qjets and want to hookup a ported vacuum source. I my case, I have a 1978 qjet # 17058278, I took some pictures of my horizontal slot. I would like to use this as my ported vacuum source but unsure if I need to fill a passage with epoxy as the above thread. It appears this horizontal slot goes to a port on the passenger side outboard of the idle mixture screw. I blew air and used carb cleaner and appears the air and fluid only exited at the horizontal slot, I didn’t see or feel anything from the vertical slot. Do I need to epoxy anything, or is this a dedicated ported source? Attached are 2 pictures with the throttle blades at idle then opened a 1/4.

Thanks, hopefully Cliff will chime in, I used his kits and book and this carb is responsive, very happy with it.

[Cliff R]

Test the source by putting a vacuum gauge on the fitting just left of the mixture screw. If you don't see FULL manifold vacuum right off idle then the slot is either too high or it has a vertical "bleed-off" slot located outboard of the horizontal slot....,

[soupman]

I tested this port. No vacuum at idle, but if I just barely move the throttle vacuum starts to read, about 5”. If I open the throttle more to 1/8 open, vacuum goes to 12, which is the max vacuum reading at idle from a manifold port. If I blip the throttle higher, rev the engine quickly, vacuum drops quickly, down to 10. I think this port will work for my PVA source. Any thoughts?

[Cliff R]

If it's showing the same vacuum as a manifold located source then it will work fine.

Some EGR sources are located slightly higher so they never show the same readings as a manifold source.

Q-jets came both ways, higher location sources or they used a bleed-off slot. The ones with the slot work perfectly after you block it off.

One can also block of both and relocate the supply at the correct distance above the throttle plates to supply FULL manifold vacuum right off idle. I do this here and pretty good at it. Matter of fact I'm adding ported VA to two Pontiac HO carburetors this weekend. I carefully measure for the supply hole and gently spike into the material with a very sharp punch. They I drill the hole and work the bit back and forth to widen the open vs having to make it bigger. It's not all that difficult but does involve exact precision.......

[soupman]

Ok, thanks Cliff.

Yes, if I Rev up to 1500rpms, with no load, then this port shows the same as the manifold vacuum, so I am good to go. I understand what you are doing adding the new port if necessary but I don’t have the precision to do it here, I only have a hand drill, probably need at a minimum a drill press to be accurate. Your probably so good at adding these new ports you could use a hand drill, I don’t trust myself, haha.

Btw, carb is perfect, a/f ratio verified and right on, off idle response is great, I see no reason to ever change this over to fuel injection. Thanks for all the help.