[RocktimusPryme]

I have an 800 cfm quadrajet on the 455 olds in my 62 Catalina. Ive talked about the motor some in other posts. Its a 70s smogger turd, but runs generally okay. It has a baby surge occasionally that I think is probably linked to a vacuum leak inside the brake booster. Ive probably put 60-80 miles on it.

The carb itself while dirty, seems like its functional. I took the time to clean up any vacuum leaks except the one in the booster. Its a hot air choke, that looks like its working normally.

Heres my issue.

Maybe two weeks ago, I was working on it in the garage with the engine running. I reved the engine (I think I was checking timing) and when I released the throttle it wouldn't come below about 1800 RPM. I had it set at about 800 RPM before that. Okay so I reduce the idle set screw and it bring it back down to where I wanted it, below 1000.

Its been like that for a little time. Yesterday I load up the dog and take it for a drive. After about 8 miles, I go around a turn and when I release the throttle the engine dies. I pull over and start it up, but the idle RPM is so low it will barely run. So I dutifully pop the hood, remove the air cleaner (choke was fully open) and adjust the idle set screw back the other way. Now it's running normally again at about 800 RPM.

What could be causing this severe idle speed flip-flop with the carb? Normally I would think that throttle itself was sticking, but in both cases it was resting against the idle set screw. If it was sticking, it wouldn't have came back to the screw.

[400 Lemans]

Maybe check the pcv valve is not sticking. Check for weak distributor springs and wd40 the weights if there dry and binding.

[RocktimusPryme]

Originally Posted by 400 Lemans

Maybe check the pcv valve is not sticking. Check for weak distributor springs and wd40 the weights if there dry and binding.

Advance springs, I didn't think about that. Good idea. Ive never even opened the HEI.

When I got the car it was on full vacuum. I switched it over to ported, just because its been my experience that qjets run better that way. Its set to about 12* initial.

[Holeshot71]

Another thing that could have happened is the secondaries could have stuck open a tiny bit and caused a vacuum leak.

[george kujanski]

Agree... throttle blades stuck more open. Could also be worn throttle shafts which cause the blades not to close properly.

George

[RocktimusPryme]

Ive had my questions about secondary operation in general. I mentioned earlier that this engine is a dog. Which based on what it is it should be. But I have to question if any engine that is 455 cubic inches should be this much of a dog. It wont even spin the tires. So I was wondering if the secondaries were functioning correctly.

I know that Ive seen several "Who to send a Qjet to threads", but Im going to ask a more targeted version of that question.

Who is a good option for quick turn around non restoration rebuilds?

Ive done a little searching and saw some good options, but they tend to be 3-6 months and do full on re-coated restorations. Which is great, but I do not care about that stuff on this car Just clean it, rebuild it and send it back to me functioning like new even if it doesn't look brand new anymore.

I have thought about buying the basic kit from Cliff site and doing it myself but if we are into things like work throttle shafts and all I just don't know that I trust myself. I would be fine just replacing the needle/seat assembly, accelerator pump (which leaks a little) and putting in new gaskets. But beyond that I would rather just have someone who has done it before do it.

[77 TRASHCAN]

Is any of the choke linkage possibly hanging up???
A little dirt in choke linkage can be a problem.

[Cliff R]

I'd do the build yourself, saves money and in almost all cases it probably just needs a good cleaning, lubrication, and some better parts installed to hold up in this new fuel.

The main causes of high idle then returning to normal are the fast idle cam not dropping all the way down, or the secondary throttle shaft stuck/hung open just a tad.

A good cleaning while it's apart and a little light oil in the right places during assembly is likely all that it needs.......

[ZeGermanHam]

Originally Posted by RocktimusPryme

Who is a good option for quick turn around non restoration rebuilds?

You might consider removing it from the car and inspecting it yourself before seeking a rebuild. If there is no excessive play or binding associated with the throttle blades, it's probably not the cause of your symptoms. That said, my mind also went to something binding with the choke mechanism, so personally that would be the first place I'd look.

To me, it seems doubtful to me that it would be caused by vacuum leak elsewhere, as your symptoms are intermittent but a vacuum leak outside the carb would be a constant.

In any regard, I rebuilt a Q-Jet for the first time a few weeks ago for my brother's '67 Impala, and I found it to be a very easy process. Much simpler than rebuilding a Holley 4150 style carb. If you feel it needs a rebuild, you might want to consider doing it yourself. Not hard.

[steve25]

If the inner advance shaft on your distribution ( the one with the rotor on it that gets moved by the motion of the weights) is hanging up due to old grease then the timing will go up when the motors rpm goes up, but it will not go back down.

This a easy thing to check with a timing light.
Bring the motor up to 3500 rpm so you have a good amount of advance kicking in and then drop the motor back to idle and see if the timing hangs up .

This seems to get more and more common these days since even the newest factory distributors are some 41 years old now!

[Half-Inch Stud]

Dizzy (HEI?) advance sticking? return springs probably too flimsy, rusty. Sounds like the Mech ADV benefits from firmer springs.

[RocktimusPryme]

Originally Posted by steve25

If the inner advance shaft on your distribution ( the one with the rotor on it that gets moved by the motion of the weights) is hanging up due to old grease then the timing will go up when the motors rpm goes up, but it will not go back down.

This a easy thing to check with a timing light.
Bring the motor up to 3500 rpm so you have a good amount of advance kicking in and then drop the motor back to idle and see if the timing hangs up .

This seems to get more and more common these days since even the newest factory distributors are some 41 years old now!

Ill check it when I get home and see how the distributor looks. To be fair there is another complete Olds HEI in the back seat, which should have activated my old car Spidey Senses lol.

Its pretty intermittent so Im not sure I will be able to induce it at will. Ive had the car for a few months and this has only happened twice. But I will take the cap off and look at it. Spray a little penetrating oil in if it looks gnarley in there.

As for the carb, I was originally planning on buying one of the most basic kits, that come with the bigger needle and seat because I do plan on adding some more power in the future. But that was when I thought it was good, just a little used.

Now that I think it might have more severely worn parts that scares me a little since Im far from a Qjet expert. Plus I read all the horror stories about even pros getting them wrong and then Cliff having to fix their work. Then again, Ive rebuilt engines for planes that managed to stay in the air, so in theory I can rebuild a carburetor.

[Greg Reid]

I had rebuilt a few q-jets in my life but they were all just basic disassemble, clean, reassemble and install jobs. I never truly understood how they worked until I read Cliff's book. I had owned the Doug Roe book for years but it was Greek to me until I read Cliff's book. It is well worth the money and time to get that book and read it. Once you get it, you'll be able to rebuild your carb with no problem since you do obviously have some mechanical ability. Definitely go with one of his kits as opposed to the auto parts store version as well.
As for the throttle bushings, he's also got everything you need for that. I have built four carbs using his recipes and instructions and they all have run like fuel injection.

[Kenth]

Originally Posted by RocktimusPryme

Advance springs, I didn't think about that. Good idea. Ive never even opened the HEI.

When I got the car it was on full vacuum. I switched it over to ported, just because its been my experience that qjets run better that way. Its set to about 12* initial.

Some engines likes full manifold vacuum to the vacuum advance.
If carb is not set up for (richer idle/low speed circuit) ported vacuum (less pressure difference between atmosphere and intake vacuum) you may run into lean running issues.

[Cliff R]

I think what really happens here is that some engine like a LOT of timing at idle speed, others not so much. The need to run a lot of timing varies from engine to engine but absolutely and for sure what quickly sends one down that path is lowering compression, followed closely by installing a "bigger" cam on a tighter LSA.

Sure, everyone loves to hear some "attitude" in the idle note and uneducated/novice hobbyists associate that with improved engine power and vehicle performance. In reality having "rough" idle simply means to me that you are bleeding off valuable cylinder pressure at idle speed, pulling unburned intake charge over into the exhaust, lowering engine vacuum, hurting throttle response right off idle and requiring more timing and fuel at idle speed to make the engine happy........FWIW......

[Cliff R]

I think what really happens here is that some engine like a LOT of timing at idle speed, others not so much. The need to run a lot of timing varies from engine to engine but absolutely and for sure what quickly sends one down that path is lowering compression, followed closely by installing a "bigger" cam on a tighter LSA.

Sure, everyone loves to hear some "attitude" in the idle note and uneducated/novice hobbyists typically associate that with improved engine power and vehicle performance. In reality having "rough" idle simply means to me that you are bleeding off valuable cylinder pressure at idle speed, pulling unburned intake charge over into the exhaust, lowering engine vacuum, hurting throttle response right off idle and requiring more timing and fuel at idle speed to make the engine happy........FWIW......

[RocktimusPryme]

This is going to be small sample size. But FWIW I’ve had two vehicles with q jets at some point. Both my Pontiacs. One with a bone stock used up olds 455 and one with my built 462.

Both of them when on full manifold if I stab the throttle to the floor would bog. Put it on ported or metered whatever you want to call it, and the bog issue went away in both cases even though the engines couldn’t be more dissimilar.

[bhill86]

For what it’s worth, I do consider myself mechanically inclined but carbs intimidated me. I did a quick rebuild on a Holley 3310 for the heck of it before I decided to tear apart my Qjet for some Cliff improvements. They’re really not hard. Hard part seems to be knowing what to do, what works and what doesn’t. Well, Cliff took care of that part for me and I did the rest to include throttle shaft bushings. It’s fairly idiot proof. There are some videos on YouTube if you’re so inclined. Will give you an idea of what you’re getting into.

[Kenth]

Originally Posted by RocktimusPryme

This is going to be small sample size. But FWIW I’ve had two vehicles with q jets at some point. Both my Pontiacs. One with a bone stock used up olds 455 and one with my built 462.

Both of them when on full manifold if I stab the throttle to the floor would bog. Put it on ported or metered whatever you want to call it, and the bog issue went away in both cases even though the engines couldn’t be more dissimilar.

Stab the throttle to the floor and you will have almost zero vacuum in the intake. This disables the vacuum advance, regardless of full or ported source.

[RocktimusPryme]

Originally Posted by Kenth

Stab the throttle to the floor and you will have almost zero vacuum in the intake. This disables the vacuum advance, regardless of full or ported source.

I understand that, but in both my cases something in that momentary transition from light throttle with max vacuum to full throttle and no vacuum caused a stumble.

Its been a long time since I experimented with it on the Firebird, but I feel like there was a weather element with it too. Like it would stumble if it was cold but not hot or vice versa when on full manifold. In either case when it was on ported the stumble went away.

I recognize that results are going to vary, but I definitely took note when the Safari did the exact same thing that my Firebird did when I had it on full manifold.