Carb Tuning question

[Paul]

I have a Rochester 4 barrel carb on my 59 Cadillac, and the carb was recently rebuilt and cleaned at Uremco in Illinois. They used a flow bench and set it. The work was done properly. I went back and set the idle speed to 600 rpm using the idle bypass screw and adjusting the air/fuel mixture screws, per the shop manual, and its fairly smooth in Park and Neutral. When in Drive, it drops to about 480-500 rpm, which is correct, but its NOT smooth. In fact, it feels rough, the steering wheel slightly vibrates and it sounds like a truck clunking. Do I need to adjust the air/fuel mixture needles in Drive and smooth it out while keeping the RPM at about 500? I thought if I got the idle smooth, it would adjust properly when in drive? Is there something I'm missing or doing wrong? HELP!!

[Joe V]

I too would think that a carburetor that set up and running fine at 600 RPM would run fine at a slightly lower RPM. Wish I could speak from having experience with the 59 but in similar situations I would say it's important  to check timing and all vacuum lines  and vacuum advance before monkeying too much with a professionally rebuilt and set up Carb.

Might also want to verify that the air filter is new or clean, that there are no air flow blockages and all final testing/adjustment  is done with the air filter  attached and secure if possible.

[Paul]

I will try doing this in drive. One question, should I use the idle-bypass screw to set the RPM in Drive? Or just turn 2.5 times out and leave it alone? Not sure what affect the idle-bypass screw will have once the car is in Drive?

[Scot Minesinger]

Running a car in drive with emergency brake on and unable to release on it's own does not sound very safe in adjusting the carburator or any engine work (setting timing in 472/500 Caddys for example).  If by accident the engine speeds up the brake may not hold.  I always get a helper to have a foot on the brake while the engine is running.  However, the FSM does endorse the e-brake engaged while in D with engine running.

[Paul]

No worries, I have that covered. I wear roller blades when I work on the car in drive. Safety is no accident!

Jk, a helper will be used.

[Caddy Wizard]

Check the timing before you do any further tweaking.  With the engine idling in Drive, the timing should be 5 degrees  (the "A" mark on the pulley) BTDC with the vacuum advance disconnected.  Then reconnect the vacuum advance -- it should advance the timing a good bit at idle.  If it doesn't, check for proper vacuum.  If good vacuum and still not advancing at idle, send the vacuum advance unit out for a rebuild (Terrell Machine in TX) or buy a new one if available for your year. 


If the timing is not right, you are spinning your wheels trying to tune the carb for smoothness.  As the Johnny knows, too often we think carb issues when really it is something else...

[cadillacmike68]

I'm not sure about  way back in 59, but from the mid 60s on always all Cadillacs specify that the idle speed should be 60RPM (or whatever that year states) in DRIVE, vacuum advance disconnected and unplugged, AC off, etc. Idle speed and mixture screws adjusted to 1 1/2 turns out from a 20RPM lean drop each side and the idle at 600... Dwell 30deg, Timing 4-5-6 DBTDC etc. Look in the shop manual or radiator core cover label.

The idle speed, dwell and timing (no vacuum advance) should all be adjusted while in DRIVE.