I have 1972 Cutlass Supreme Convert with a 350 4bbl that is running very lean. When I got it I noticed that it showed signs of having run hot for a while. All the rubber lines were cooked, it has a slightly warped exhaust manifold, and the radiator had small cracks in the tanks. The plugs are running very white and if you put your hand over the carb too choke it out or close the choke butterfly it will increase in RPM. All the vacuum lines have been replaced and the carb rebuilt since I got it. I even swapped over another carb from another '72 350 to see if it was a bad rebuild but the lean problem was still there. The mixture screws are all the way out. I also noticed that the screws on the test carb were out at least 6 turns which had me wondering if this is related to the "emissions" changes that came about in '71. I've been told that not enough timing can cause a slight lean condition. I'm running the factory 12 degrees BTDC. I'm just about to pull the intake to look for leaks or cracks. I want to take this to the Iowa Quad States meet in a couple of weeks but don't want anymore long trip running this lean. Anyone else faced this problem?

 

At what RPM? Does it run OK, other than that?

Assuming at idle only, and it runs fine, you have a vacuum leak:

There are two on the carb, one on the modulator, and one on the power booster. Were they replaced? Are the diaphragms leaking?

Are they all routed correctly?

Is the vacuum advance diaphragm leaking? The plastic fitting between the hose and brake booster, is another potential leak.

Was it an attempt to fix this problem?

It probably came off an engine that had a vacuum leak.

Not related in any way.

I don't know that it does. If it did, it would not be enough to cause your problem, and it would not affect the idle, at all.
While you have it off, use a straightedge to check all mating surfaces for warping.

Norm

 

Thanks for the reply. It seems to run fine off idle but I'm not sure how to tell if it is still running lean at the higher RPM. All lines were replaced, diagrams checked, booster checked. Vacuum advance is actually new as I found it was full of tranny fluid from a previously leaking modulator that was apparently replaced before I got it. All lines are routed correctly as stock (all messed up when I got it). Went through and capped every thing off just to see if I had missed something but it still ran lean. I may be stuck pulling the manifold. I couldn't find a manifold leak at the top but maybe it is on the bottom. I'm hoping to find a huge crater like crack so don't feel like I wasted my time.

 

I do not do trannys http://www.urbandictionary.com/define.php?term=tranny

Having said that, I wonder how ATF could have found its way into the vacuum advance pot.

Norm

 

Check that intake manifold for leaks and cracks. If your exhaust manifold is warped then your intake might be warped as well. You can check for vacuum leaks with Starter Fluid. Or if you have access to an auto tech center you can use a Vacuu Tech machine.

 

The transmissions vacuum modulator supply is connected to the same source as the distributor via a "t" in the lines. If a hole forms in the transmissions modulator the engine sucks fluid up into the lines. The distributer vacuum advance is lower than the lines so gravity causes it to flow down into the can when the engine shuts off.

 

Remove the "T" and attach the modulator hose to a direct manifold source.

Then, hook the advance hose to the ported vacuum connection on the carb.

Norm

 

Heya! Like Norm said, shouldn't your vacuum advance be hooked up to ported vacuum? If it is hooked up to manifold vacuum then your car will run poorly (I kow this from experience)

Oldsguy and I were discussing your problem with ATF in your advance module. You would have one BIG leak if the tranny fluid ran up the line, into the manifold, and then back down to the advance module once you shut the engine off. Of course, you wouldn't get any fluid into advance module while the engine is running because all of the fluid would be drawn to the highest source of vacuum which is the manifold.

 

Hi all, I had a little car once that had a loose carb, ie it wasn't bolted tightly to the manifold. It exhibited normal behavior until hard acceleration and then it leaned out and coughed and sputtered. If your manifold has a warp on the carb mounting pad or some kind of imperfection that will not allow a good seal, regardless of the carb (you have had two on) you might find the same thing. I don't know if this would be enoough to cause the extreme condition you are experiencing or not but it is worth a try. When you inspect the manifold edges with a straightedge, check that pad as well. Hope this helps.

 

Hey, I am here with Olds64 (visiting my son). He wondered if your car is a California car originally and if it had an egr valve it might be possible that a faulty valve could cause a lean condition. If your engine is old with worn rings you might have excessive blowby in the exhaust, since it would be recirculating HOT inert gasses leaning out the mixture and cause it to run even hotter, hence an endless cycle. Does this make sense? I am sort of typing as he recites this so it is filtered through my dull brain. Anyway...