Q-Jet acceleration Question
 

So, my 455HO (470cid) runs great, get about 15mpg when drive easy. But when hard accel, say from 30mph, Pull out pass someone, WOT itll starve for fuel after a second. Then kinda come back, If I ease off a little, she fine.
Also, If I slowly ease into her, she fine all way up to top speed, 6000RPM for long Hwy run. AKA, dusting that BMW who thought he was fast on the 2 mile straight.

I have Elec fuel pump, Holley Blue, and a .110" inlet seat.
Would you think its the small inlet seat not letting enough in when I really accel hard? Letting bowl drop and fuel slosh to uncover jets? Ir pump not keeping up to the acceleration demands?

Only happens on a Hard launch or accel when there lots of 'push you into the seat' G forces.
I can burnout for days no issues either

I'm far from an expert, but fuel uncovering the jet seems like a logical place to start. And the simplest thing to check first is if the floats are set high enough. Don't go and just raise the floats though. Setting them too high causes other problems. Just make sure they are within the upper end of the acceptable range. If they are, then move on to one of the other potential and more difficult problems.

I'd say check the secondary air valve adjustment?
It's probably flopping open to quick. There is a little spring adjustment on the top. Or could be the vacuum dashpot also maybe.
If you can see fuel squirt out of accelerator area when opening throttle immediately, then check what I said.


:)

A instant good reaction to backing off the throttle a little with a problem like your stating you have is far more likely to be due to air valve adjustment related much more so then fuel supply related.

What size fuel line

That was my first thought too, but he says “after a second….”
When I’ve had issues with the upper air valve too loose, it bogs instantly, then comes back around.
But it doesn’t hurt to try, as all you need is an Allen and zero parts thrown at it.Well, unless the spring is broken

Just a thought because Ive had this and its easy to check.

What flavor of vacuum is your distributor hooked to?

Both Qjets engines Ive had in the past would briefly stumble when you stab it when hooked to full manifold. Swapped over to ported vacuum and the problem went away.


Im not a truther one way or the other on vacuum source. Im much more in the "try and see" camp with each engine. But Ive never had a quadrajet that liked getting the throttle punched with full manifold.

I would use a .135" inlet valve and float set at 13/32" with about 7 psi.
With 1/2 turn preload on airvalve spring use a DA or CE secondary rod set.
With 3/4 turn preload on airvalve spring use a CC secondary rod set.

Using a ported source for the vacuum ignition advance requires larger primary throttle valves openings and a richer idle circuit calibration wich adds more fuel to the primary circuit going rapidly to WOT compared to using full manifold vacuum wich may or may not be noticeable.

Fuel line, 1/2" braided line, Pump at tank
Vacuum advance is ported

I'll try tighten the spring a little. Too hot outside right now and shop is full so can't bring her in

The problem is for him is that running a .135” in the carb is useless because that Blue Holley regulator he’s running only has a .110” in it.

You can’t fit a battleship through a lock in a canal that can only accept a 30 ft fishing boat.

Pontiac used the 0.135 fuel valve in the 455 carbs.

Pontiac did not use the regulator.

Personally not a fan of the 0.135 FOR THE STREET ! We had a number of customers complain of flooding at idle. Unless the customer demands the 0.135, we include a 0.130 in our repair kits.

Back to back dyno runs on a stock RAII engine with the 0.135 valve and the 0.130 valve yielded virtually identical torque / HP

I would want a 0.130 valve in the carburetor, and fix the pressure issue, either with a different or modified regulator or a different pump.

One of the issues of using an electric pump ON THE STREET is that the maximum pressure from an electric pump will be at minimum engine usage (idle) whereas a mechanical pump is keyed to the engine RPM, and minimum pressure / volume occurs at idle. I am not throwing rocks at electrics pumps (I use them on my street vehicles), simply mentioning the characteristics are different and must be considered.

But the 2-mile 6000 RPM run seems to suggest the fuel inlet is NOT the issue. As has been mentioned before in this thread by others, I would play with the air valve spring adjustment.

Jon

Change the seat to a .135” and set the float to 5/16.

I’ve been running Cliffs .140” needle and seats for years on the street with no issues. This is with a stock large canister fuel pump with a return at 6.5 to 7.5 psi. I believe he now has them up to .149”!

All of you folks advocating that the OP needs a bigger needle and seat,( or should try a bigger one ) for Gods sake please read or re-read his second paragraph in his first post.

[QUOTE=zippy;6624488]So, my 455HO (470cid) runs great, get about 15mpg when drive easy. But when hard accel, say from 30mph, Pull out pass someone, WOT itll starve for fuel after a second. Then kinda come back, If I ease off a little, she fine.


remove the secondary hanger and rods. chuck the rods up in a drill, spin and file them down to at least .040" on the tips- a long taper. reinstall rods with hanger into the qjet. Then push the secondaries fully open, measure from the top of the carb nearby, to the top hook of each secondary rod. bend the hanger arms (not the rods themselves) so that dimension is 21/64" from top of carb at wide open- this will be the same dimension as the richest "B" hanger of which in 45 years, I've only seen and obtained ONE. start car, happy motoring.
btw this entire procedure is in Pete McCarthy's first SuperStock book in the Qjet section. you can file them to .040" for starters, but if need be even to .030" tips. Pete Mc. also used rods with only .018" tips on his SS class cars. you can make .020", .030", and .040" tips from rich junk rods from other vehicles, use them as cores to file new rods from. then tune as need be to the engines needs.
I'll add one trick found myself recently - my friend's 1972 455 w/7M5 heads had an aftermarket universal electric choke bolted onto it by previous owner, and the dashpot pulloff vacuum pot to control secondaries was completely missing. I cranked up the tension on secondaries from 1/2 turn of tension on the small spring, to 3/4" tension. also filed the rods to .040" - and adjusted the secondary hanger to 21/64" at WOT. the car is now a rocket- my point being you really don't even need the dashpot If the rods are rich enough to make the transition. this is only with a Pontiac Qjet with the square cutouts in front of secondary air valves, and big fuel pullover fuel passages in top front of secondaries on lid.
HAPPY MOTORING

https://streamable.com/xdz1qy

It works. It's a 4 speed with no dashpot, filed rods to .040" bent hanger to 21/64" at WOT. Took owner for ride kicked it open through gears he got really quiet. Got back asked him what do you think ? He said he thought he was going to die, he's impressed. This is nothing new the info has been out 45 years it's just people forgot it.
'72 455 bone stock in a '70 Lemans Sport. Owner is lifetime motorhead now age 75.

Here's just a few of his other cars . My next door neighbor, I hung out with his kids since we were 15
All the Chevys are rotisserie restos
'57

70 SS454 M22 tribute
70 blue 350 4bbl M21 he bought new 1970 when he was 19
Both ragtops

Last

FIRST, I would verify accelerator pump capacity. A weak pump might provide enough extra fuel for the initial jump, but not enough for the full transition to the main and auxiliary circuits. Not uncommon for the accelerator pump to deteriorate in modern fuel.

SECOND, I'd verify the air valve operation (spring, and vacuum break(s) as applicable) as other folks have stated.

THIRD, I'd verify that the secondary "accelerator pump" drillings/tubes are functioning properly. As the air valve cracks open, it should draw fuel from the float bowl, up the tubes to the discharge ports on the casting beside the air-valve. I've seen Q-Jets where the tubes have fallen out of the air-horn casting, and one where the float bowl had debris at the bottom, plugging one of the "wells" that the tubes sit in, so the well couldn't properly fill with fuel.
Photo (and it's not a great one) shamelessly stolen from The Internet.
https://squidsfabshop.com/gtotrain/images/2275bg.JPG

If it runs good to 6000 RPM, the fuel pressure, inlet seat, and float-level are not the problem at throttle-stab.

No. HOLLEYs "uncover the jets" on the secondary side, because they're facing the wrong way during acceration. Some Holleys on Fords are mounted BACKWARDS, so they uncover the primary-side jets for the same reason.

Q-Jets don't, because the jets are at the bottom of the float bowl, not angled 90 degrees from vertical.

Kinda sounds like you don't have enough fuel in the tank and aggressive driving sloshing the fuel is uncovering the pickup for a second.