1970 Cadillac Lean Idle

[klinebau]

I have a 1970 Cadillac Deville with a stock engine and 97k miles.  The engine is a 472 CID with 10 to 1 compression and a 7040230 Quadrajet.  The motor is not modified and the carburetor was unmolested.  I am able to get a good idle at 600 in gear, but the problem is that the mixture screws are out about 7 turns.  Cranking compression is good with 170-180 in all cylinders.

I actually have two carbuertors and both of them are about 7 turns out.  I rebuilt one a couple of years ago and I recently built one last winter with parts I purchased from Cliff.  I would like to understand why the adjustment is so far out when the service manual says I should be 3 - 6 turns.  I can kill the engine by turning in the mixture screws, but I am not able to make the idle over-rich.

I have run the carburetor with all of the vacuum ports plugged (except PCV) and have made sure the intake manifold does not leak so I am sure there are no vacuum leaks.  The ignition is good and timing is set at 10.5* BTDC (stock is 7.5*).  I am using ported vacuum.

The idle tubes are drilled to .040 (factory spec) and the DCR is .052.  I have not measured the idle bleeds, but they are stock.  In an attempt to fix this, I have drilled out the idle tubes to .043, but it did not affect the adjustment of the mixture screws.

Here are my questions:

• Why is the stock calibration not working?
• Why did drilling out the idle tubes not affect the idle mixture?
• What should be my next step?  I can drill out the DCR some or I can drill out the idle discharge ports.  Since I am working with a stock carburetor, I didn't really want to mess with the castings, but I will if it is the right move.

[Cliff Ruggles]

Take a look at the mixture screws themselves.  Early Caddy Q-jets used a pretty "odd" mixture screw with a really long tapered tip.  It takes 6-7 turns out with them just to get a decent amount of opening area past the small holes in the baseplate.

I typically replace them with mixture screws with shorter tips and open up the holes to .093" or so in the baseplate at the same time......Cliff

[klinebau]

Thanks for the reply.  It never occurred to me to look closely at the mixture screw and it does indeed have a long tip with a very slight taper as compared to a "normal" mixture screw.

With this information, I am inclined to leave it as it is since the engine is getting enough fuel for idle and there is enough adjustment to richen it more.  Since the final adjustment is only a little outside the specs from the factory, I am inclined to believe that the new fuel combined with 100k engine is the cause.

[Cliff Ruggles]

Good plan......