Big drop in idle when fans kick on???

[Pman1961]

Got back to the car this evening. Took battery to have tested Sunday and it was fine. Gave the battery a full charge before putting back in the car. While the engine is running the ammeter shows positive even with fans and lights on. Honk the horn with these devices on and the ammeter will drop in the negative zone. Crazy thing the fans do not drop the idle all the time but does the majority of the time. Checked initial timing at 1100rpm and it was at 18degrees with total set at 34. Let the fans drop the idle or just lowered the idle myself to 800rpm and the initial timing fell to 10degrees advanced. Tried lowering the idle a little more and it still seems to stay around 10 degrees advance. Sounds to me that the car is idling in advance as one has suggested and when the fans kick on the timing retards and drops the idle. Does the msd dizzy need to be re-curved or what suggestion does anyone have?

[Dana E.]

Originally Posted by Pman1961

Checked initial timing at 1100rpm and it was at 18degrees with total set at 34. Let the fans drop the idle or just lowered the idle myself to 800rpm and the initial timing fell to 10degrees advanced. Tried lowering the idle a little more and it still seems to stay around 10 degrees advance. Sounds to me that the car is idling in advance as one has suggested and when the fans kick on the timing retards and drops the idle. Does the msd dizzy need to be re-curved or what suggestion does anyone have?

I think what you find is happening is that as the added electrical load from the fans is applied to the motor at 1100 rpm, the idle speed starts to decrease, and since the advance starts at 800 rpm, as the engine speed decreases, the less the advance, and the worse the rpm drop will be with the fans on. You are correct, the car is into the mechanical advance curve at the 1100 rpm idle speed.

I'm going to presume there was no vacuum advance connected to this distributor when you checked the timing. The mechanical advance is starting below 1100 rpm since the initial timing is 10 degrees at 800 rpm and 18 degrees at 1100 rpm. That means you have 24 degrees of mechanical advance in the distributor. You need to limit the mechanical advance to 16 degrees so you can have 18 degrees initial and 34 degrees total. Depending on what distributor you have, either by installing a larger bushing on the advance stop pin, welding up the slot, adjusting the mechanical advance stop, or by using a laptop if you're using something like a Mallory Maxfire.

Once you have the mechanical advance part fixed, you can work on the advance curve with the springs. The curve I use starts at 1200 rpm and is all in at 2600 rpm. Once you have the timing set, then you can play with the carb.

I think once you get this advance curve setup in your distributor, the rpm drop when the fans come on will be negligible.