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?

[Jerry Clayton]

Most of your problems can be cured within the base plate of your carb

[Pman1961]

Jerry...pardon my ignorance but what are you suggesting? Carb is a Quick Fuel 750dp.

[Jerry Clayton]

I have went thru it before on here but was probably 2 or 3 years ago now-- and guess what--was a quick fuel carb on a KC 638 hp engine

[Pman1961]

I wonder if it is the gentlemen that owned I believe is called 'Radical Rides' and the motor was in a BackDraft. I was referred to him by KCR to get his thoughts on his cam selection since we had a somewhat similar motor.

[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.

[Pman1961]

Dana I have the common MSD 85805 I believe mechanical advance. I checked initial timing at 1100 rpm and was 18 degrees with 34 total advance. When fans kicked on or I lowered idle to around 800rpm the initial timing was 10 degrees. So it appears it is idling with initial advance and makes since when rpm drops the timing retards and continues to drop rpm. Called and talked with an employee with KCR. I did a test this evening as he suggested to see if the car idled better. I was asked to set the initial up to 26 degrees advance then lower idle, turn on fans and see what happens. I set initial at 26 degrees, lowered idle to 1100 rpm, turned on fans and the rpm dropped maybe 25 rpm and held. So I took the bushing out at the mechanical advance and it appears to have blue coloring and measured .340. I've sent him an email this evening of my findings to see what I should do next. Until tomorrow!

[Dana E.]

Perry,

it looks like the blue bushing is the standard one that comes with that distributor and is supposed to be good for 21 degrees mechanical advance. They make a black colored bushing (the largest one they list in the instructions) which is good for 18 degrees advance. Theoretically you should have 16 degrees initial and 34 total. 18-20 degrees initial timing would be better, but you can see what happens with 16 degrees. You may still end up having to make a bushing, or welding up the slot where the bushing rides.

It also sounds like someone installed the two lightest (silver) advance springs since the advance starts below 1000 rpm. You need to slow down the start of the advance curve-something like 1200 rpm. I would install the largest (black) advance bushing, replace one of the light silver springs with one blue one and see how that works. This should be the kit you need:MSD Ignition 8464 - MSD Advance Kits - Overview - SummitRacing.com

Dana

This scenerio is one reason why I like the Mallory Max Fire distributor: plug in the laptop (or remove the distributor cap), set the rev limiter, chose one of the factory preset curves, or design your own mechanical and vacuum advance curves, and you're done. Best of all: no MSD box to leave you stranded....knock on wood.

[Pman1961]

Dana...I just found a website called 4secondsflat.com that sells 10 and 14 degree bushings for the MSD distributor. My car likes a lot of initial timing at idle with my cam. Currently I'm 16 initial and 34 total. Been considering locking the distributor and using the msd starter saver but am going to try the 10 degree bushing first.

[Dana E.]

How big is the cam in your motor (duration at .050")? 20-22 degrees initial timing should be enough to make it idle good. Don't waste your money trying to lock out the distributor and using that starter saver.

[Gaz64]

How much vacuum does the engine produce at idle?

What is the idle A/F ratio doing as the engine slows down?

With a big cam, you'll find any large electrical load (fans, headlights etc) will cause idle rpm drop. The better the idle emulsification and manifold distribution is, the less the drop will be.

The different loads the engine sees at idle is one of the reasons why GM ran an ILC on their '80s carbed engines.

Can also be fixed if the car has programmable ignition maps.

[Pman1961]

Gaz64 motor at idle is around 10" of vacuum at 1100rpm. Dana E. the cam is 254 262 @ .050, 609 609 with 104 intake centerline and 108 lobe seperation. Putting in the black bushing was an improvement over the blue bushing. I would think the larger bushings from 4secondflat would put me where my motor wants to be. Not going to lock out the dizzy....I think I can make this work. I'll order the bushings tomorrow and keep you posted.