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Carburetor problems?


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I experience some real problems with my 56 Cadillac CdV, no Buick, but you guys know everything.

 

The car needed new mufflers and pipes and my mechanic replaced them. Then he wanted to fine tune the two four barrel Carter carburetors to make the engine run a little smoother and the odyssey started.

In the course of the last 3 months he rebuilt the carbs, adjusted the linkage between them, replaced the intake manifold gasket due to a vacuum leak, replaced the points by electronic ignition and much more I guess. We had all sorts of malfunctions, some really scary, some less. But he did not give up, nor I. 

 

The end of the story is that the car works reasonably fine when you accelerate, but at constant speed runs way too lean. When it is cold it runs better than when it is hot. 

 

What needs to be done now? Any idea would be highly appreciated. 

 

Next issue is that the left cylinder bank seems to run not as smooth as the right. Compression was good on all eight cylinders before this thing started. Timing good, no vacuum leaks, good fuel pressure. 

 

Any advice what we can try now? My mechanic is lost. He is a good guy, but we need some help now. He is at the end of his knowledge..... 

 

Thanks everybody! 

 

Hans 

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Edited by Hans1965 (see edit history)
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I cannot see the linkage but believe that all of the factory 2 four barrel set-up ran a progressive linkage.  The car starts and runs on one carbs primaries and

the other carb would not have chokes because the throttle butterflies were closed on that carb. 

When you are running lean,  that is equal too little gas or too much air.     Make sure the main butterflies are closed on the secondary carb. 

Edited by Barney Eaton (see edit history)
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On ANY factory 2x4bbl set-up, there is a primary carb and the secondary carb.  In normal driving, the primary carb is what runs things AND has the automatic choke on it.  Looks like you have TWO carbs with automatic chokes, there!

 

The secondary carb will have an idle speed and mixture screws on it, I suspect, to keep fuel flowing into the engine to help ease the transition into when the secondary carb comes into play.  With, as mentioned, a progressive linkage that engages the secondary carb past about 1/2 throttle, typically.

 

Check the carb numbers to ensure you have the correct carbs for a factory Cadillac 2x4bbl application for the particular model year.  If not already done, you might check the national Cadillac-LaSalle club website for any technical resources they might have on your vehicle.

 

But, to me, making sure that the carbs are correct, or reasonably correct for the application is important.  Just as getting the base idle set correctly.

 

Please advise,

NTX5467

Edited by NTX5467 (see edit history)
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I would get access to a five gas exhaust analyzer, a ignition scope, and start doing a cylinder power balance test AFTER you recheck the compression. Also, I would use propane to check for manifold leaks, and I don’t trust the electronic ignition to be correct out of the box. It’s all about the basics. Twin carbs can be a real pain in the ass. Fuel pressure should be checked. Often times on cars like this I isolate the systems if possible......IE set up a different fuel system to eliminate the one in the car. That just leaves ignition and carburetor tuning. Fact is, you need the exact Correct carburetors and linkage. Good enough is not good enough. Also adjusting for E10 while your at it. It takes time, and lots of hair to pull out of your head. Take your time, assume NOTHING, and check everything again.............good luck.

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You can do the "cylinder balance test" via the shadetree method, by making sure that each of the spark plug wire ends, either at the distributor or at the spark plug, can be easily disengaged from the cap/plug easily to check for the rpm drop with that wire unplugged.  Which ever one is easiest to do.  Just make sure to not be close to the car body when you pull the wire off!  Or with a heavy rubber fender cover between any part of your body and the car's metal.  As the spark can try to jump to ground, even through YOUR body.  Such things can give the operator some extra excitrement/jumps, by observation.  That's how it would have been done when the car was newer, when fancy diagnostic scopees were only in the fanciest shops.  Proceed at your own risk!

 

Or you can run the engine, then stop it, carefully remove one spark plug at a time, then restart it to check the rpm at idle.  More time consuming, but with equal results as with it running.  How recent are trhe spark plug wires?

  

Having one of trhe big exhaust analyzer mechanisms, with an igniton scope in it, can speed things along by just punching buttons, turning dials, and such, but equal results can be had without using one.  Just takes longer.  ONE advantage would be that you can sniff the fumes when you carefully remove the radiator cap to check for exhaust gases in the coolant, just don't let it suck in any of trhe liquid!!!  Keeping the probe just above the radiator filler neck.

 

BUT before resorting to such things, DO get the factory service literature on the care/feeding of the particular Cadillac's carburetors and such.  Invest in aome new carbv kits and ensure everythiung in the carbs is where it needs to be.  Including the jets and their correct position in the carb, as Old-Tank mentioned.  Plus that the carbs are accurate for the application!  Check for "play" in the throttle shaft-to-carb base plate, too.  Some is needed, but not too much.  Make sure the carb base gaskets are correct, too!  AND that any soldered ball-checks plugging fuel passage holes are not leaking/seeping from ethanol'd fuel!!!!!!!!!!!!!!

 

When you get everythung that bolts ONTO the intake manifold verified as correct, if there are still performance issues, then take the intake manifold off and replace the gasket with the correct one, just to ensure that that part is correct.  Plus looking at the manifold's imprint on the gasket removed, looking for thin spots and such, so that the new-installation does not have those possible issues.

 

ALSO ensure that the electronic ignition upgrade was done correctly, too!  NOT to forget that an operating vacuum advance is needed for best performance AND no vacuum leaks, too.  In that orientation, check the power brake booster and the HVAC system actuators for leaks, too.

 

NONE of this ia going to be easy, quick, or cheap to do . . . but sometimes you have to delved deeper into things that just what might be seen on the surface, from my experiences.  NO offense to whomever might have done the work, but sometimes things just need to be verified as "correct".  

 

IF a fuel pump pushrod is operative for that motor, remove the fuel pump and compare the length to a new pushrod.  In some engines that use them, the rods CAN wear and become shorter with time.  How old is the fuel pump, while you're there?

 

Please advise of your findings,

NTX5467

Edited by NTX5467 (see edit history)
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For good measure and accuracy, drive the vehicle, of possible, about 10-15 miles on the highway before starting the diasnostic/tear-down operations.  Check for off-idle hesiations/sags, lean surge at 50mph or so, how eager things tend to happen, etc.  Making sure the powertrain is at full operating temp BEFORE any adjustments are made can be important.  Just because the coolant temp gauge says they coolant is up to temperature, does not mean that EVERYTRHING in the engine is full to operating temperature, stabilized, too.

 

Just some additional thoughts,

NTX5467

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