BigBlockk

Yes the LS5 had the large bleeds. Sorry I misled you earlier. Any more I just assume that every Quadrajet has been hosed because I see so many of them. I just got one off of ebay. When I opened it up I found that the power piston was popped out of its bore, the pin on the bottom of the piston had about 1/4" cut from it and the power tip on one of the primary metering rods had been bent 90 degrees when they had tried to insert it into the main jets.

I just shake my head!!!

That is why Quadrajets get a bad name and that is why I was able to picked this one up for $7 plus $12 shipping.

People just don't realize that these things are so precise that sometimes the smallest alteration can really screw them up. The idle circuit has 3 air bleeds in it alone. You change any one and it can really mess things up.

With the small bleeds the 72 main may be a good place to start. It may be a good idea to lock the secondaries out and just drive the car on the primaries to possibly isolate the problem. If it lays down going up a hill check to make sure there is a power piston spring under the power piston. Yes I've seen that too.

If the plugs smell like gasoline I would say there is a good chance that the carburetor has been messed with internally. When I say that I mean that the internal passages and orifices may have been altered. If that is the case it would do you good to look at this thread.

http://forums.corvetteforum.com/c3-t...effective.html

You're going to need to study Roe's book and understand it completely. It's been a while sense I've looked at it but I think something like the first 24 pages has a walk through on carburetor function. For general stuff like jet/rod changes and idle system problems this section of the book will answer 80% of the questions asked on this board. Once you understand it of course.

The original camshaft in this engine was rather mild, so I don't think this carburetor would have had idle bypass air in it but it will need it with the cam you have now. The engine may sound like it is idling fine but it may be idling on the mains.

With what you have described there could be anything wrong with this carburetor. Come to think about it I have also seen the secondary jet orifices drilled out which "only" destroys them.

All of the things I've mentioned here I have seen in the flesh and I bet Lars has too.

Good luck.

BigBlockk

Later.....

Shark Racer

You can't use the jet/rod pair off the later carb because the larger air bleed is adding a lot more air to the mixture and thus an increase in jet size is required to maintain proper AFR.

73ss

[QUOTE=Bobs69BB;1583848902]FasterRat, carb is 7029215 which according to the Prince book is correct for a 69 427/390. Stock jetting was 71 45 AX from the Lars list.

73SS, gas and return lines and hoses are new, along with the fuel pump. Am getting "woosh" when cap removed. Will check needle and seat dimensions.

If you are getting the "woosh" sound that is most likely the problem. When this happens you are drawing a vacuum on the tank. The fuel pump can no longer pull fuel from the tank fighting this vacuum. This started happening to my '68. At about 4000 rpm's into the secondaries, The car would fall on it's face. I poked at the check valve on the cap and it fixed the problem. No more "Woosh". Car would then pull hard to 6+ with no problems. My '68 has the large style cap. Not sure what a 69 has for a cap.
On a similar note my '73 chevelle was doing the same thing. Turned out a mud-wasp built a home inside the vent line for the tank. Found this after having the sending unit rebuilt, new fuel pump, etc. Again the "woosh" sound when removing the gas cap was the give-away. Both cars are q-jet equipped. Take it for a test run with the cap off or loose..

Bobs69BB

Got a chance to do a short test drive, I am getting up to about 4500 RPM now ...

The midrange is a lot meatier, and idle has improved as well. Also, no cloud of smoke on hot restart.

Here is what I have done so far:

1) fixed leaking primary well plugs and clipped needle valve properly to the float
2) adjusted secondary butterflies at full open to align with the baffle rather than 90 degs, per Lars paper. Did this by exoxying a small aluminum stop to the throttle plate. Adjusted the primary butterflies to 90 deg.
3) Turns out my linkage was restricting the secondaries to about 30-40 degrees of opening. Did surgery on the throttle cable, cut back the sheath about an additional 1/2", so now I get full opening of primaries and secondaries at WOT
4) Backed off the spring on the secondary air valve to 3/4 turn
5) Enlarged the idle screw opening from 0.081 to 0.085, one side was not responding to adjustment, now it is.

I think the most improvement has been from the throttle cable and the secondary air valve. I tried removing the gas cap, a little improvement. Is the cap supposed to maintain some pressure, and if so, how much?

I compared all the orifices to the Ruggles book (great Qjet book BTW), the only thing that stands out is that the fuel inlet is 0.125, which is probably too small for this engine. Is 0.135 the best bet without changing out the mechanical fuel pump? I saw the comment about 0.149 is probably too far.

Thanks again for all the comments, they have really helped!

Faster Rat

I was going to suggest the infamous 5 minute / no charge Stingray high performance tune up...but figured you were way beyond that point.

Bobs69BB

Yes, one should always start from the obvious and work backwards!

What was not obvious is that the new repro accelerator cable is not the correct length to get the needed travel. I had to shorten the plastic sleeve over the cable and the cable sheathing to get full travel. So yet another reminder that just because it is shiny and new, doesn't mean it is right!

Still looking for more RPM. My next step is to enlarge the fuel inlet. But the fact that I now get 500 more RPM and it fades in a similar fashion I guess points to more fuel needed. So I'll see where the inlet gets me, then go to checking fuel pressure.

It was very instructive to pull the Qjet apart and study it. Very clever engineering!

gcusmano74

Get yourself an Air/Fuel ratio gauge and then you will know exactly what is happening. Otherwise you are just guessing. Jegs and Summit sell them for around $200 and change, last time I looked.

Boswell

I am not a Quadrajet guru or a Chevy engine specialist so this can be taken with a grain of salt. I went through a similar problem 2 years ago with my 69 small block. With no warning it began to stumble at or slightly above 4000 rpm. Over several weeks of crossing things of the checklist it ended up being the fuel pump. Simple fix, and the engine ran great again. This may or may not help you. I just did not see it on your checklist and I overlooked my pump for other issues.
Sincerely,
Norm

LastBigBlock

Think I have the same problem. My 1974 454 TH400 accels great up to about 3500 rpm then feels like it is running out of gas. Seems to fall flat right after shifting up to 3rd on accel, but also dies when I keep it in 2nd. all ignition system is spot on. wondering if I need to get into the carb details or if there is something else obvious I am missing. Just got this car and the last guy has records of recent new fuel pump.
engine was rebuilt with high performance cam, but dont have details. runs good if I ease into it, but all I want to do is punch it. Any advice?

Shark Racer

Pretty common for the secondary air valve spring to be adjusted loose or worn out. Does it feel like you just hit a wall or simply a lack of response?

LastBigBlock

feels like it accels great then hits a wall.
Wondering if the secondaries are opening at all.
air valve spring is providing resistance, but I have not tried adjusting it.
I think I may have a choke problem with the secondary lockout lever not moving forward enough when hot to allow the secondaries to open. Need to try some adjustments, but not sure how everything is supposed to look in the first place.
already adjusted choke and pull off per Lars recommendations, but not sure how to adjust the secondary lockout lever so it works cold and moves when hot.
here are pics cold and hot. Does anyone see anything way out of sorts here??
as you can see on the hot side view the lockout lever is still in the way of the secondary throttle shaft pin.
cold side view:

cold top view:

hot side view:

hot top view:

Shark Racer

When hot, can you open the secondaries by hand? The photo makes it look like it's open enough to open.

How easily do the air valve doors open and shut (pressing with your finger)?

LastBigBlock

So I warmed the car up in the garage real good and it appeared the secondary throttle plate lock out lever was not interfering with the throttle plate opening, but I wired it out of the way to make sure and took a drive. Same result. The secondaries open nicely and I get great accel up to about 3500 rpm then it just dies, like it is out of gas. It starts sputtering and wont accel as long as I keep my foot on the accelerator.
The seconardary air valve has a nice spring resistance and the choke pull off is working nicely so I know it is not slamming open and swinging back and forth. Do I just need to do a rebuild or is there something else causing it to die off? Any ideas would be appreciated.

Shark Racer

Do you know what the float level is at?

When you move the air valve doors, are the secondary metering rods appropriately lifting out of the jets? (checking for a worn secondary air valve cam)

LastBigBlock

I have not gone into the carb yet. Trying everything on the outside first before I get into a rebuild or taking it apart. Also, this is a rebuilt carb off of a 78 chevy, so I am wondering if while I am at it I should try to find a 74 matching numbers carb. I would rather just get this one running and wait on a new carb, but hate to throw good money after bad.

the metering rods are attached to the hanger and are moving up when the secondary air valves are opened up.

The acceleration in the secondaries is fine until I hit 3500 rpm, like it runs out of gas.

7t9l82

check your float, and while your in there the inlet valve ( needle and seat) have a small opening in the rebuild kits available today and GREATLY restricts fuel flow. it could be your running out of fuel as the Q jet has a tiny fuel bowl. Cliff Ruggles OR Lars can get you a bigger fuel inlet.

Shark Racer

If this is a stock engine, I wouldn't expect the needle inlet to be causing a huge issue, esp all the time. Maybe higher up in the RPM band, but it shouldn't soak the float bowls dry the instant you cross over to the secondaries.

If that carb is off of a 78 vehicle, then it looks like a truck carb. Going to a 74 carb will net you pretty much the same carburetor (trucks used the 4Ms longer, passenger cars switched to M4M, at least on Vettes, in 75). I would actually look for a 75+ M4M (non computerized) QuadraJet. I prefer 77-78, 79-80 went to tinier idle mixture screws that are a bit of a pain to deal with but not a deal breaker.

LastBigBlock

thanks for the info. I will check the float level and needle inlet size as soon as I get the courage to crack into the carb.
This is an R4-4MV, truck and marine carb.
If you are saying I can get it to work like the original I will stick with it.
I only hit the wall after 1 or 2 seconds of accel. It will jump from 1500 to 3500 RPM instantly, then starts stalling and sputtering.
I keep reading Lars notes and am starting to understand a little.

John Holmes

Very common for the plastic air valve cam to be worn down and not lift the metering rods adequately
and
The feed holes for the seconday wells get plugged over time. This will cause exactly what you are describing and I have not seen it mentioned up to this point.

LastBigBlock

I ordered the Quadrajet book, rebuild kit and primary rods from Cliff's High Performance today. He said the needle and seat assembly were probably too small, but really needed to get into it to see. Maybe over Thanksgiving I will see if I can figure this thing out.
Thanks for all the feedback.