Anyone familiar with Q-jets?

[rfoll]

Earlier Pontiacs had a bathtub shaped combustion chamber. Not the kind of thing to promote good swirl and fuel mix. This required more initial timing.

[Geoff2]

The Pontiac 'bathtub' chambers had the same initial timing [ 6* ] as the later open chamber heads, both had 20* added at idle by MVA.

[rfoll]

My comment was about the nature of that type of chamber, not specifically how Pontiac set timing. More modern chambers require much less timing, initial and total.

[MarkR]

Cliff Ruggles has a book on Q-Jets and Modifying them. Danny Ashley was into q-jets at one time I believe most Stock SS stuff. Look up Lars Grimsrud he had a bunch of articles on Q-jets and was very helpful.

[novadude]

The Ruggles Book is good, but be careful with the Recipes, as Cliff gets a bit aggressive with the drilling, making the idle circuit far richer than it needs to be for most applications. The Doug Roe book is a GREAT resource.

[My427stang]

I wanted to circle back and let you know where we are.

In the end, at least for the short term, I increased initial timing and got to something acceptable, but the bottom line is, I need to see what this thing does when the weather supports. It's a beauty of a 67 GTO and Nebraska weather / salty roads has it garaged for the season, but it sounds pretty good right now

What I did yesterday was:

1 - Verified TDC on the balancer using a piston stop (it was accurate)
2 - Put a thermostat in it (had it out during cam break in, wanted to make sure I was tuning with the car as it would run)
3 - Checked total timing (The HEI has a very quick curve, 17 degrees centrifugal all in by about 2500)

At that point, I knew that what I wanted to do wasn't going to work as planned without a bunch of initial. So for the short term, I set the initial at 17 (34 total) I think the engine will be happy with a bit more, but I am concerned with quick advance that it would ping.

However, I still pulled the carb because I wanted to see how the vacuum source worked (ported or not) and if the carb had idle bypass orifices in the baseplate

The vacuum advance port was indeed a ported vacuum source, there is a small horizontal slot above the throttle plates that feed the port. The carb did NOT have any bypass idle holes, and I didn't drill any because I saw I was very close. FYI as the car was set up (prior to yesterday), the throttle blades were exposing the port with initial at 12 BTDC.

I fired it back up and at 17 initial, I am just at the point where ported vacuum shuts off idling at 780 per my timing light. It runs well, but time will tell, because the idle seems slightly low for me, we'll have to see how the car does in traffic.

Some observations though.

- If the car doesn't start hard, I will likely add another 2 degrees of lead to get total a little closer to what I expect it would want.
- I expect I will likely have to slow down the advance based on gearing and how the car is used
- However, if after I add initial timing, if I close the primaries at all at idle, the idle screw doesn't have enough spring to hold a load on the screw. Not a difficult issue of course, but odd that it has such a short spring. (I don't expect this to be any concern, because I'd prefer to get it to about 850 rpm anyway, just an odd observation on this Q-jet, maybe the wrong spring because if you close the primaries all the way, it is loose)
- The vacuum advance adds about 20, so I expect that I may limit that later
- An interesting thing is that before the cam swap, this car had a slightly smaller Lunati cam and the timing was set to 6 BTDC (factory setting) by the previous mechanic. That means, with the carb the way it was, it would start at 6 BTDC, idle at 26 BTDC due to vac advance, but at WOT (no vacuum) would have only had 23 degrees of lead until the vacuum started building again. Assuming it's happy on pump gas, I think it will be much happier now.

In the end, it sounds great, but until I can get it on the road I can't really evaluate how happy it will be

I really wanted to go inside the Q-jet to see the effects of the bypass, and then run a more standard timing curve, but given the fact that I was able to get the ported vacuum source to shut off, I am happy enough for now assuming the curve is not too quick for a heavy-ish car with 3.55s.

[Tuner]

You can lengthen the idle speed screw spring with a pair of diagonal wire cutters. Pinch between the coils so the tapered jaws spread the coils. Work around the spring so the ends stay relatively square to each other.

If that Pontiac has original heads and compression ratio it might be happier with 30-32 degrees total .... and with today's gas not until 4000+ RPM. If you can get to a distributor machine shoot for 18 initial, start advance 1200, 26-28 at 3000, 32 at 4000, 34 at 6000. The late 60s Pontiac heads with machined combustion chambers don't need (or want) as much timing as Chevy heads.

[F-BIRD'88]

You don't need to drill any holes for extra idle air. This is all done thru the PCV valve. The carb was designed to work with a PCV valve.
Even on a race only car that does not use a PCV the PCV hose can be connected to the air cleaner base with a simple fixed restriction orifice to set air flow rate.
You don't need to compromise the needed base initial idle timing that this cammed up motor will want. Get a auto cranking start spark retard box to allow easy hot motor starting, with the needed aggressive increased idle base timing.

[pdq67]

Make sure that the REQUIRED air pressure is in all 4 tires as well.

If the REQUIRED air pressure isn’t there, the tires will either wear in the center or on the outsides.

Good advice and most people today are too damned lazy to check their own tire pressures!

The comments about the starter and tires have nothing to do with a Q-Jet, but still good to know, imho.

pdq67

[My427stang]

You can lengthen the idle speed screw spring with a pair of diagonal wire cutters. Pinch between the coils so the tapered jaws spread the coils. Work around the spring so the ends stay relatively square to each other.

If that Pontiac has original heads and compression ratio it might be happier with 30-32 degrees total .... and with today's gas not until 4000+ RPM. If you can get to a distributor machine shoot for 18 initial, start advance 1200, 26-28 at 3000, 32 at 4000, 34 at 6000. The late 60s Pontiac heads with machined combustion chambers don't need (or want) as much timing as Chevy heads.

An easy and logical fix...and should have been obvious, but in years of carb work, I have never had to do that...thanks!

As far as total timing, it should be stock 400 HO compression, which is supposed to be 10.75:1 SCR, but I am not sure how accurate those numbers are and didn't build the engine. This was supposed to be all stock, but when we took out a bad cam (ate a lobe), we also found a Crane cam and rockers that were hitting the retainers, it could be anything. I have no idea of where deck height is, no idea of what they used for head gaskets, however, one thing I do know is it should be in a far better place than it was before. I won't race to more timing, and I agree the curve likely needs to slow down.

You don't need to drill any holes for extra idle air. This is all done thru the PCV valve. The carb was designed to work with a PCV valve.
Even on a race only car that does not use a PCV the PCV hose can be connected to the air cleaner base with a simple fixed restriction orifice to set air flow rate.
You don't need to compromise the needed base initial idle timing that this cammed up motor will want. Get a auto cranking start spark retard box to allow easy hot motor starting, with the needed aggressive increased idle base timing.

I gotcha man, don't worry, I know it will like more initial, but the original Q-jets for these DID use an idle air bypass. So far I do NOT intend to use it because I think it's going to do OK, but the base 400 and 400 HO, both had idle air bypasses and the one with the bigger cam had bigger orifices, so don't worry, not going to fix something if it drives well, but it's not like I am going willy nilly into the carb

You and I are on different levels with how much cam this has though, I don't consider this much cam, and certainly doesn't need a retard box, this is a 400 with a 280 adv HFT. This cam, using all specs other than .050 is milder than Pontiac delivered it, it shouldn't need any crutches, but the carb is a 1976 carb, and needless to say, 1976 Oldsmobiles didn't run nearly as much overlap or advertised duration as a 67 RAIII or 400 HO.

Again though, I don't intend to drill the carb if we are close enough, the car sounds great, just need to get it on the raod and see how it does.

[427dart]

When you get some good 60-70 degree weather setup a A/F data gauge and see where your idle and cruise a/f ratio is while on the road.
I'm sure the 1967 HO and Ram Air cammed Q'jets were much richer on the idle circuit than the late 70's carbs.

Going back thru my old '67-69 issues of Super/Stock magazine Royal Pontiac used to bump the timing up a good bit in their Bobcat kits.
Of course this was with Sunoco 260!

[D.S.R.E.]

If you were close to me "Portland Oregon" id invite you over to get it squared away, i build custom Pontiacs and Q-jets, I do a lot of 75-79 800cfm Q-jets for guys making up to 600hp, Cliff Ruggles is Legit and is where I get most of my parts when I need new Stuff, good luck with your car, you have gotten good advise, fwiw if it's idling too fast with the throttle blades closed its because you have too much initial timing from my experience.

[F-BIRD'88]

What you are not accepting is the fact that the cam you have now has more overlap and more effective overlap (area) and thus more exhaust dilution at idle than any of the Pontiac OEM cams.. All the Pontiac cams were ground on wider LSA's than your cam
and have less overlap. Your cam will idle with more lope and have lower manifold vacuum too. It will want more initial base timing than the Pontiac oem cams.

[Geoff2]

Pontiac, for reasons known only by Pontiac, used a very odd method of rating the adv duration of their cams. Probably for advertising hype, the 'Mine is bigger than yours syndrome'.
Most 65 & later cams had the adv duration measured at 0.002" on the opening side & 0.008-9" on the closing side, & these were at valve lift. The result was inflated figures that made the cams look larger than what they really were.
The RA3 cam, also known as the 744 cam, had 301* adv duration on the intake . At 0.006" tappet lift, where Comp & others quote the adv duration, the 744 is about 282*.

[My427stang]

What you are not accepting is the fact that the cam you have now has more overlap and more effective overlap (area) and thus more exhaust dilution at idle than any of the Pontiac OEM cams.. All the Pontiac cams were ground on wider LSA's than your cam
and have less overlap. Your cam will idle with more lope and have lower manifold vacuum too. It will want more initial base timing than the Pontiac oem cams.

I got it, but I also think that you are assuming that I don't understand overlap and the other valve events. All you say is true, it's got a decent cam in it, matched well to use, but it's just a 280 @ .006 lobe on 109, with ICL at 107. I am not saying it's a stocker, I am saying it's not a drag motor, and certainly doesn't need a retard box or starter interrupt Overlap at .006 is 60 degrees.

That being said, what I don't understand is the reluctance to accept that there is a tuning circuit available in the Q-jet, put there by the designers for this reason, and for this specific application. I may not need it, and all indications are that I am close enough to let this one leave, but why would I not want to use every means at my disposal for the next Q-jet tuning event that comes in. Don't mistake exploration for troubleshooting. The carbs that have this circuit should be a good thing, it saves from guys drilling holes in throttle plates, exposing the transition circuit and all kinds of other things we see botched all the time

Now I came to the forum to talk about it, so when someone tries to help, I am the last guy to throw stones, but I just don't see the logic of trying to scare me away from the internals of a Q-jet, when those passages were appropriate for the original application and can solve some problems.

BREAK BREAK - Thanks Geoff, needless to say I didn't have the original RA3 cam to compare .006 events, it had a goofy little cam in it, real late ICL, wide centers, and a bunch of exhaust duration and lift. Certainly an adequate grocery-getter cam, but missing a lobe and mismatched to the combo. FWIW, the cam that came out of it was rated at .004, so very similar conversation for my follow on choices, as the cam was smaller than advertised numbers indicated.