voscreature

Lars,

You are right as usual. The quality focused commercial carb builder decide it was best to dump the inserts in the high speed air bleeds. And then he probably decided to drill out the idle down air bleed as well.



However, I must admit that I don't fully understand the difference between the "high speed airbleed" and the "low speed airbleed", that exits in the primary circuit bore. Both exits in the bore at almost the same place...

Given that the idle down restriction is 0.052" and the air bleed just next to it is almost 0.100" (reservation for not being a carburator design engineer) my gut feeling is that it's waaaay to large to pull any fuel at all from the idle channel. That large bleed ruins the entire idle system.

I noticed that in the air horn it almost looked prepared for one more idle down airbleed - but to the best of my knowledge it looks like a blind hole on my airhorn:



So I guess my best way of wasting more time (and Lars time) would be to:

- Fabricate a brass insert for the high speed air bleed and drill a bleed hole of 0.xxxx".
- Fabricate an insert for the idle down airbleed and drill a bleed hole of 0.0xxxx". This will be a bit challenging.

Martin

(thanks Lars for still following this)

voscreature

Finally got some time this evening to fabricate brass inserts to close the large air bleeds. It must admit that this was a bit difficult.



Both holes are 0.039" in diameter.

I was very very close to open up the idle down restriction a bit, but I'll wait with that until I have tested this change.

We'll see what happens.

voscreature

Carb back on car this evening. Started with idle screws 3 turns out.

As usual, start easy and warms up perfectly.

Now I can close the throttle plates so ALL NOZZLE DRIP IS GONE! Haha!! Finally. Dry as a desert in the venturis.

Only problem I still have is that the car idles with both idle screws fully closed, it even runs smoother than when having them backed out a few turns (I can get the car to idle perfectly at 500 rpm if I want!). The idle speed goes UP say 75-100 rpm when closing both screws completely. My conclusion to this is that I made the restrictions TOO small so I pull too much fuel through the idle circuit so the small transition slot opening alone can keep the car idling. It smells fat from the exhaust also, and tends to smoke a little black. But it idles extremely stable, without hesitations, and when closing the throttle it settles down immediately on the correct idle speed without surging or hunting at all. General driveability is also "perfect", no hesitations, flat spots etc.

I guess my best bet is to open up the air restriction that allows air into the idle fuel stream at small increments until I get the optimum idle 2-3 turns out. Intuitively that feels like it should be possible?

The air bleed holes is currently only 0.039" (1.0mm).

/Martin

redvetracr

a 10 second search for "Chicago Corvette complaints" would have saved you a ton of aggravation, Chicago Al has been screwing people for years.

Shark Racer

Congrats. I'm not sure what the original air bleed size should be, but you seem to have licked your problem!

It may be worth while to just drill it out a little bit at a time until you find something that works perfectly.

voscreature

This weekend I took the time to start enlarging the brass insert in the air bleed channel that goes into the idle circuit. The following summarizes my work:

- Started with 0.039".

Result: Idle was smoothest and highest with both idle screws turned in all the way. I guess it gets the fuel from the transition slot openings only. 0 nozzle drip so nofuel from there.

- Enlarged bleeds to 0.043".

Result: Perhaps a very, very small idle speed reduction with both screws all the way in. No distinct change from 0.039".

- Enlarged bleeds to 0.047".

Result: Idle slows down when both screws all turned all the way in. Not much, but definitely the idle goes down.

- Enlarged bleeds to 0.051".

Result: Significant idle speed drop/roughness when both screws all the way in, bordeline to engine stall. Idle smooths out very nicely with screws approx. 2.5 turns out.

General observations: No nozzle drip at all. It is possible to have the engine idling very stable and smooth with throttle plates closed entirely, but idle speed is very low, 400-450rpm. If I screw in the idle screws engine dies more or less instantly. Setting idle at 650rpm make the car run very, very smooth and nice. It has never (in my posession) been remotely so stable in idle, easy to start and absolutely no dieseling tendency at shutoff. No driveability issues either, runs great.

The ONLY thing now that disturbs me is that I can turn both screws OUT more or less all the way without any observable reduction in idle speed. That disturbs me. I would have expected at least some idle speed reduction when fattening up the idle mixture.

I will hook up the exhaust gas analyzer tomorrow and plot CO and lambda (AIR belt off) as a function of idle screw turns in 1/2 increments.

Not sure if I should consider enlarging the throttle body holes where the idle screws sticks in, to try and compensate....hmm...suggestions welcome.

Martin

voscreature

So, today I hooked up the exhaust gas analyzer and did the following measurements.

Car fully warmed up. Idle screw on throttle plates adjusted to approx. 600rpm. No nozzle drip at all.

- Both screws fully closed. CO = 0.21 vol%, lambda = 1.464. Engine barely runs.
- Both screws 0.5 turns opened. CO = 0.64 vol%, lambda = 1.274.
- Both screws 1 turn opened. CO = 2.40 vol%, lambda = 1.063.
- Both screws 1.5 turns opened. CO = 4.03 vol%, lambda = 0.973.
- Both screws 2 turns opened. CO = 5.33 vol%, lambda = 0.939.
- Both screws 2.5 turns opened. CO = 6.66 vol%, lambda = 0.910.
- At 3 turns and more no change from 2.5 turn readings.

I adjusted both screws to get approx. 2.5 vol% CO, and approx. lambda 1.0, approx. 1.25 turns out. Car runs extremely stable, with the best idle I ever have had on this car.

With this I close this thread, and accept the results. I can't see any arguments why I need to have more adjustability than what I have.

So to summarize this long story:

From the beginning, after installation of Chicago Corvettes rebuilt carb:

Car did run a bit rough, had a tendency to diesel at shutoff and had a very low an unstable CO of 0.20 vol%. Idle was surging, hesitating, and sometimes the car could die when coming to a stop. Idle had a tendency to not settle down good either. Hard hot-starts. Slight pinging at partial throttle also.

The following is a summary of the issues/fixes I have been able to fix over here with lots of help in this thread, especially from Lars:

- Power piston nylon retainer was cocked in the bore causing the PP to bind. Easy fix.
- Incorrect gasket betwwen float bowl and throttle plate. Blocked air bypass holes. Correct gasket installed.
- Plugged air bypass holes. Knocked out lead plugs.
- To lean primary jets/needles for my setup. New jets/needles installed, 74/43.
- High speed air bleed missing. Fabricated brass insert.
- Low speed air bleed missing. Fabricated brass insert.
- Idle circuit air bleed holes too large for idle circuit to work. Fabricated brass insert and enlarged bleed hole to 0.051".
- Lowered float level to 12/32".
- Throttle plate to manifold, missing gasket between carb and SS plate. Installed missing gasket.
- Fine tuned power piston depressed height according to Lars specs.
- Enlarged the idle down tubes to 0.039".
- Also installed a B20A can for better vacuum advance and switched to manifold vacuum. But with carb running more properly ported vacuum is still OK.

I think that was all. Perhaps I missed something.

With this I have found closure on my idle nightmare over here. No nozzle drip anymore.

Again, thanks everyone, and Lars in particular, for all the help. I appreciate it alot.

Martin

Shark Racer

These guys need to stop effing quadrajets up. I have a 17058228 carb that is (mostly) unmolested and the idle bleeds were probably around 0.050... and this is one of the carbs where everything ends to be on the large side!

Glad you're back in business, and this thread is GREAT. Really demonstrates what people do to these things and also helps with an understanding of the air bleeds.

larrywalk

What a great thread! Thanks for the numerous postings and finally solving the problems.

lars

Good job with the relentless effort and work on that carb! Now you'll be able to tell people, "Yeah, those commercially rebuilt carbs aren't as bad as Lars says - they just need a little tuning."

Lars

jotto

Great job Martin and Lars!

Just goes to show, although difficult, it is possible to diagnose and sort carb problems on a forum.

Kudos to Lars for his patience and sharing his wealth of knowledge and also to Martin who listened and did what needed to be done. A job very well done!

voscreature

Thanks again everyone. I did check the old "truck" carb that was on the car when I got it. It was molested in similar ways, with plugged air bleeds, brass float, air bleeds similar in size as my Chicago Corvette carb. It seems like most of the generic commercial carb builders molest in similar ways....

Anyway, I have been driving the car for a couple weeks now on weekend nights, and again I must say: It runs very very nice. Idle is extremely predictable, absolutely no dieseling what so ever, easy to start etc. etc. I'm amazed on the improvement. And I'm sure that it could be tuned much more if time allowed and Lars was here.

Now I must address other problems with the car, season is almost over. Perhaps a week or two left. My main problems I will dig into is:

1) Transmission leaks quite a lot from the seal around the slip yoke. Strange since I replaced that 2 years ago, but it was never a 100% leak free seal....hmm.....
2) Throw out bearing wines like crazy.
3) Rear crank seal leaks.
4) Possible worn valve guides/seals, when I replaced the intake manifold gaskets I looked into the intake ports and noticed that the valve stems had a lot of oily goo buildup. Perhaps time to pull the heads and give them some attention....

and on it goes....

Martin