Idle decreases when headlights are turned on

[70spider]

Hello, I went to take him out for an evening drive and when I turned the lights on there was a noticeable drop in idle speed, 800rpms to 300rpms. I have an electric fuel pump installed, a 95 amp alternator and the headlight relay kit. I went through all the major grounds and even put a new one from the block to the frame with no effect. Puzzled?

[Nut124]

One would expect a little drop in RPM when turning on the head lights. But that large of a drop seems not normal.

What is your idle timing? Inadequate idle ignition timing can make for "weak" idle. Do you use ignition vac advance? Strongest idle is with about 10 deg static advance plus 15-20 deg vacuum advance.

[70spider]

Strongest idle is with about 10 deg static advance plus 15-20 deg vacuum advance.

The drop might not be that much after I cleaned up the grounds, but it still drops. As for the timing I set the static at 10 deg. and then fiddled with advancing the distributor 'til it smoothed out. Then monkeyed around with the carburetor to get it as smooth as possible then dialed in the distributor for the smoothest idle when nice and hot. Hot start is perfect and cold starts are good, but I haven't used a timing light cuz I don't have one and the 1438 has no timing marks on the crank pulley.
It is not really a big deal, just never experienced the idle drop with the lights on.
Thanks for the input.

[Nut124]

Setting ign timing w/o light is risky.

If you set idle timing for best idle you will end up with way too much timing at top end and burn a piston.

Best idle is somewhere around 20-30 deg advance. The centrifugal advance in the dizzy advances timing some 25 deg as rpm increases to 3500 or so. 36-38 deg is the ideal max advance at top end, 3500 and up. Too much and the engine will detonate and run poorly.

So, 10 at idle pus 25 by 3500 = 35 total. The vac advance can be used to give it another 15 deg at idle for 25 at idle, but it goes away as the throttle opens to avoid ping.

[18Fiatsandcounting]

Although it seems counterintuitive, the 95 amp alternator may be contributing to the problem. One reason that people switch to the 95 amp version is that it produces significant charging current even at idle, whereas the original 45 or 55 amp alternator didn't do much at idle. So, when you turn on your headlights, your 95 amp alternator starts charging to compensate for the drain on the system, and this can draw up to one horsepower or so. Since your engine is producing only a few HP at idle, the alternator load causes the engine to slow.

With the original alternator, there wasn't much charging current at idle speed, so not much effect on idle. The downside was that your headlights dimmed.

The issue might also be a sign of a weak battery. The quickest fix is to turn up your idle speed a bit, perhaps 900 or so.

-Bryan

[spider2081]

One reason that people switch to the 95 amp version is that it produces significant charging current even at idle,

Yes I believe this is true.

I believe the alternators shaft speed determines when the alternator is putting out. The car manufacturer usually designs the alternator to safely operate at the engines normal (cruise) RPM. This usually means the alternator will not have much if any output when the engine is at idle. The contributing reason for this is the alternator could be rotating above it's safe RPM when the engine is at cruise RPM. This can cause premature wear of brushes, slip rings and bearings. The pulley diameter on the alternator compared to the crank shaft pulley determines the alternators shaft RPM both at idle and cruise engine RPM. I believe if the alternator is producing current at idle most likely the alternator has the wrong size pulley for the engine it is operating on. While many people like the alternator to have an output at idle they do not consider most alternators have a maximum shaft rotation RPM that could be exceeded at normal engine cruise RPM. In addition the alternator's physical load on the engine can have a measurable effect on the cars gasoline consumption and overall engine performance.

As a side note much is written about alternators will work when rotated in either direction. This is true however the alternators cooling fan is designed for one direction only. So an alternator assembly as purchased usually includes the fan behind the pulley or part of the pulley. This determines the alternator's direction of operation .

[70spider]

Setting ign timing w/o light is risky.

Do you have any suggestions on timing the 1438 engine with a timing light? There is no reference pointer with degree marks nor any way of putting one on the timing cover. This is what it looks like:

Another note no vacuum advance on the early models just mechanical.

[18Fiatsandcounting]

There is no reference pointer with degree marks nor any way of putting one on the timing cover.

You do actually have timing marks on that timing cover. See those three little raised nibs towards the bottom, about a quarter inch apart, next to the pulley? The bottom one (usually slightly longer) is TDC, the one next to it is 5 degrees BTDC, and the top one is 10 degrees BTDC. There should be a notch on the outer edge of crankshaft pulley, although it is usually very hard to see. It helps to put a dab of white-out or white paint on the pulley notch.

Take out the #1 sparkplug, put a long screwdriver in the hole, and turn the engine by hand until the #1 piston is at the very top of its stroke. You should see a mark on the crankshaft pulley, and it should be opposite the longer mark for TDC on the yellow timing cover.

Quite frankly, the (hard to see) timing marks are a weak point of the overall design. You can also time by the marks on the camshaft pulleys, but it's not very accurate.

And yes, early spiders did not have a vacuum advance. That came later, mid 1970s as I recall.

-Bryan

[18Fiatsandcounting]

This usually means the alternator will not have much if any output when the engine is at idle.

True. In fact, some older cars (1950s, 60s and 70s) would have the battery eventually go dead if the engine was idling and the headlights were on. Not enough alternator output at idle as spider2081 mentioned. It was even worse for 6 or 8 cylinder cars where the idle speed was only 600 rpm or so.

-Bryan