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69Lobster Rebuilt Rochester carburator too lean, 69' 350/350
Guru Newb
11 Posts
Member since:
2011/4/21 19:05



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Hello Friends,

I have replaced the carburetor with an overhauled unit from Eckler's and I am unable to adjust the mixture. It is too lean.

The carburetor, that was installed in my car was incorrect for the type. According to the numbers it belonged to a truck.

The book calls for a 7029207

I ordered a replacement rebuilt carburetor Eckler's pn A5918.
The Eckler's catalog indicates, that it will fit 350ci/300hp, but they told me, that it will also fit my 350hp.

I installed it, but I am unable to get the mixture adjusted. It is too lean.
I have checked the two jets inside and they are different. one number 71 and one number 74 (strange).
Also the choke, which is new, is not working good.

Maybe my engine has a performance camshaft and needs a different carburator? I have no way to know if it does.

The carburator I removed is pn 17065439 had an electric choke
The jets on this one are 73's

Any suggestion?

Thank you very much
Stefano
Posted on: 2011/6/6 16:44
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Durango_Boy Re: Rebuilt Rochester carburator too lean, 69' 350/350
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Columbia, MO
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Before you get too crazy with possibly fixes, search far and wide for vacuum leaks. A vacuum leak, even a small one, can create a lean condition.

Also, how have you determined you have a lean condition? What are the signs you're reading to tell you this?
Posted on: 2011/6/6 16:49
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69Lobster Re: Rebuilt Rochester carburator too lean, 69' 350/350
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Thank you Durango,

I have verified the vacum. No leaks found.

At idle the engine is running lean, the mixture screw was adjusted to the max reach and the engine runs better, but not enough.

There is too much air and not enough fuel. It is running too lean and I can’t increase the fuel mixture ratio.

The smoke test unit indicates 0.3%, which is the limit of the stoichiometric ratio. You can tell by the smell of the exhaust gas, that it is lean.

Stefano
Posted on: 2011/6/7 14:26
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Durango_Boy Re: Rebuilt Rochester carburator too lean, 69' 350/350
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Columbia, MO
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Okay good the smoke test was one of the 3 I was going to suggest.

Have you pulled and read all 8 plugs? White is lean, black is rich, and brown is perfect. Read the plugs to determine if it's bank specific or a whole condition.

Just to gather more info I would also perform a leak down test and a compression test per cylinder.
Posted on: 2011/6/7 15:10
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Matatk Re: Rebuilt Rochester carburator too lean, 69' 350/350
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Why did you change the carb in the first place? Is it because you want it to be period/motor correct? I'm wondering if the reason there is a different carb on there to begin with is from internal engine changes that you don't know about.

How did it run with the old carb?

Being that I know about less than zero on carbs, I won't tell you how to choose jet size and all that nonsense.

Matthew
Posted on: 2011/6/7 16:31
_________________
2002 EBM convertible, Magnusson supercharger, cam, headers, etc.
1989 Corvette...RIP
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69Lobster Re: Rebuilt Rochester carburator too lean, 69' 350/350
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The reason why I changed the carburator is, that the engine was chocking in acceleration.

I had never used the car before since I bought it in 1990, other tnan to move it to different storage places and shops.
I did not know the cause of the chocking and replaced all the items, that were worn; spark plugs, ignition wires, coil and distributor with an electronic ignition kit, air cleaner filter, fuel filter, etc. and after I opened the carburator to clean any dirt in it and looking for new gaskets for it I realized, that it was not the original model, that belonged to the engine, I decided to replace it with the correct type and install the original choke system.
After all this work I run the engine again and found out, that I am unable to get the mixture reach enough.
I am also thinking, that maybe the carb I removed, could have been installed in conjunction with a cam performance kit, but I am not sure about it and I have no contact with the previous owner.
It would make sense, that the engine was pumped up since the car was also modified with the headers, fender flares and larger tires.
I also removed the oil pan to see if there was any sludge in it, I always do that on older cars, that I do not know.
I did not find any sludge and noticed, that everything inside on the bottom of the engine was very clean, as if the engine had not run many miles since an overhaul.
Now I am thinking to put back the old carb or maybe first try to replace the jets in the overhauled carb with bigger ones.

Any suggestions are appreciated.

Thank you
Stefano

I found strange, thet they installed two different size jets in the overhauled carb.
Posted on: 2011/6/7 23:52
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tjpreul Re: Rebuilt Rochester carburator too lean, 69' 350/350
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Columbia, MO
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What is it idling at, be sure to have it around 600 when trying to adjust the mixture screws. Put them both back at 1 1/2 turns and try to move the idle down. Then readjust the mixture screws to the smoothest point. You might have to move the idle down as the engine smooths out and tries to raise back up.

As for the two different jets. They either put back in what they received it with, or couldn't read. I would match them up to have a better starting point for adjusting.

The bogging issue with the first carburetor can be caused by a few things. Engine timing, accelerator pump lever not adjusted proper or a cracked/misadjusted float.

There are several adjustments/metering rods on the quadrajet and can be tricky. However many people say they aren't any good. I had one on my '77 Nova and had good luck with it (once adjusted).
Posted on: 2011/6/8 3:43
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Vegas wasn't built on winners.

'92 auto, MSD, Banski, Taylor wires, SS lines, C5 rims, 3.07 gears, white gauges, seats from a '96.

Motor: Forged rotating assy, Probe pistons, CC valvetrain, AFR heads, Procharger P1SC
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Matatk Re: Rebuilt Rochester carburator too lean, 69' 350/350
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Great document out there / by Lars:

http://www.corvette-restoration.com/r ... ers/Q-Jet_Carb_Tuning.pdf

His article states p/n 7029207 is for a 69 350/325 and p/n 7029202 for a 69 350/300 at and 7029203 for a 69 350/300 mt. In those cases the primary jets would be 66/67/67, respectively. I can't easily find the secondary jet, but I know on other cars it wasn't uncommon to have two different jet sizes.

I'm sure when they rebuilt this one, they just used what was in there. Makes you wonder what else is wrong/broken/missing?

Matthew
Posted on: 2011/6/8 11:49
_________________
2002 EBM convertible, Magnusson supercharger, cam, headers, etc.
1989 Corvette...RIP
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69Lobster Re: Rebuilt Rochester carburator too lean, 69' 350/350
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2011/4/21 19:05



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Very interesting Matt,

I will investigate about the jets, strange, that the ad does not indicate the 69 Vette with the 350/350 engine.

Do you know someone who has installed a performance camshaft? It would be nice to know if he had to replace the carb or the jets to get mor fuel in the engine.

Stefano
Posted on: 2011/6/8 12:29
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69Lobster Re: Rebuilt Rochester carburator too lean, 69' 350/350
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2011/4/21 19:05



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Thank you very much. I will try all this again.
Stefano
Posted on: 2011/6/8 12:31
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Matatk Re: Rebuilt Rochester carburator too lean, 69' 350/350
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SW Chicago Burbs
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2008/1/7 0:00



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Quote:

69Lobster wrote:
Very interesting Matt,

I will investigate about the jets, strange, that the ad does not indicate the 69 Vette with the 350/350 engine.

Do you know someone who has installed a performance camshaft? It would be nice to know if he had to replace the carb or the jets to get mor fuel in the engine.

Stefano


I can almost guarantee you need bigger jets that stock since you already stated you have headers/exhaust. A cam would probably require more yet. There are some books out there on tuning quadrajets, unfortunately I don't have one.

Matthew
Posted on: 2011/6/8 12:45
_________________
2002 EBM convertible, Magnusson supercharger, cam, headers, etc.
1989 Corvette...RIP
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