Topic: Idle question

[TheJ]

I have a 1973 351 Cleveland and when I put it in gear it drops about 400rpm and the idle becomes rough and erratic.  Is this normal?

Thanks

[Soaring]

Normal idle RPMs should be around 800, and when you drop that automatic into gear it should drop to about 600 or so.  If it idles rough, then you have some tuning issues with the engine.  Sounds like you need a general tune-up on the engine.   

[Jeff73Mach1]

Not normal

check for proper tune, that is to say proper timing good clean and properly gapped plugs, a cap and rotor and wires all in good condition and proper carburetor adjustment.  I'd bet money that the problem will go away at this point.  If not . . . I would check for vacuum leaks (keep in mind that your transmission has a vacuum line run to it and if the diaphragm in the vacuum modulator fails, you can have a leak there)

Before adjusting your carb, make sure you know what you are doing-I have seen lots of misadjusted carbs (more than a few of my own way back when)

[TheJ]

The idle is better turns out the choke wasn't adjusted right making a really rich condition but it still drops 400rpm when I put it into gear.  What could cause this?

[Jeff73Mach1]

What is your base idle speed?

What is the current timing set at?

Is the vacuum advance attached to ported or manifold vacuum?

Do you have a vacuum gauge?

Have you adjusted the idle speed screws?

Is your transmission slipping or shifting early or late?

Does it still run rough after warming up?

Do you have A/C

Is this occurring with the A/C on or off or both?

Is your car equipped with a factory or aftermarket carburetor?

If aftermarket, what kind and what has been done to tune it to the car?

If it is a Holley have you checked for a blown power valve?

[TheJ]

The idle speed is at about 1000rpm in park and 600 to 700rpm in drive.  As for the timing its around 6 or 7 btc, although its hard to see the timing marks because of the paint and crust on the dampener.  The vacuum advance is from the manifold.  Vacuum is around 16 to 18in/hg.  Both the idle and fast idle are adjusted to spec.  The transmission works fine and was recently rebuilt.  No A/C and factory 2v motorcraft carb.

[66GTKFB]

Vacuum for the distributor should come from the carburetor. It is a function of the main butterfly. When the butterfly is closed (like at idle) no vacuum. When opened a bit, more vacuum is applied to the distributor and when WOT is achieved you get max vacuum. Manifold vacuum is max all the time.
Jim

[TheJ]

I'll have to switch that tomorrow.

Thanks

[Jeff73Mach1]

Make sure it is ported, there should be no vacuum at idle.  Also that engine should be timed at a few degrees more advance, try 10 to 12 degrees for  best performance and better idle.  Those two things dhould really help a bunch.

If you have a vacuum gauge, you can hook it to manifold vacuum and your vacuum should come up to its high point as you advance a few degrees past where you want to actually set it.  We set timing with a vacuum gauge a lot in the old days, turn to max vacuum and then back off about 2 degrees.