RPM idle drop when headlights on

[KlassicBus79]

I had a big scare today. I drove the bus into work today with the lights on the whole way and the rpm was very steady when I stopped at red lights.
On my way home it started fine and drove fine, but I had to go into another place so I shut it off for a couple of minutes.
When I started it back up it sounded awful. It was the same type of noise as the loose valve seat I had last year (expensive). So I took it home and changed the oil and filter and prayed. I took it for a short drive no change.

So with the stethoscope I determined it was number 1 cylinder so I let it sit for a bit and then adjusted the valves on number 1.
Started it back up and noise was gone. I suspect it was dirt in the lifter.

Also took it for an alignment today.
So busdaddy it will be a couple of days before I can get back to the hooking up another vehicle to my bus because I have to fly for work a couple times this week and I'm gonna need my beauty sleep.
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79 Cal Transporter 2L

[Wildthings]

Don't know what oil your are running, but I strongly recommend synthetic oils if you have hydraulic lifters. Synthetics don't have the large additive packages that breakdown over time causing the buildup of all kinds of scuzz inside the engine.

[Timewizardbutterfingerkri]

Have the same thing happen on my 74 super.
Will run at 950to 1000 rpm no lights then goes down to 850 to 800 with the lights on. I have added an extra set of hella fog lights wich could be resulting in the added load to the alternator. Havent had a problem tho! Might be normal?
Car stays on and everything lights up great.
I have heard of the use of a smaller pulley on the alternator to spin it faster but have not looked into it. Maybe give that a search good luck!

[mikedjames]

Changing the alternator pulley for a smaller one will spin the alternator faster and allow the alternator to draw even more power from the engine.

If it works properly it should make the effect of turning on the lights even more noticeable as a bigger drop in engine RPM...

Fitting a bigger pulley will increase the cut in speed, so the warning light is more likely to be on at idle, but the RPM will not drop as much.

Your choice.

Fitting a higher capacity alternator will also increase the possible power drawn from the engine at a given RPM .

Alternatively, tweak the idle RPM up a little.
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Ancient vehicles and vessels

1974 VW T2 : Devon Eurovette camper with 1641 DP T1 engine, Progressive carb, full flow oil cooler, EDIS crank timed ignition.
Engine 1: 40k miles (rocker shaft clip fell off), Engine 2: 30k miles (rebuild, dropped valve). Engine 3: a JK Preservation Parts "new" engine, aluminium case: 26k miles: new top end.
Gearbox rebuild 2021 by Bears.

1979 Westerly GK24 24 foot racer/cruiser yacht Forethought of Gosport.
1973 wooden Pacer sailing dinghy

[telford dorr]

A little math:

Assume the alternator is supplying its full 70 amps, and the output voltage is 14 volts.

70 amp x 14 volts = 980 watts

980 watts x (1 hp / 746 watts) = 1.31 hp

Assume the alternator is 80% efficient:

1.31 hp / 0.80 = 1.64 hp

So the fully loaded alternator should add a 1.64 horsepower load to the engine (the fan load won't change, loaded or unloaded).

The question is: how much should this load slow down an idling engine?
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'71 panel, now with FI
'Experience' is the ability to recognize a mistake when you're making it again - Franklin P. Jones
In theory, theory works in practice; in practice, it doesn't - William T. Harbaugh
When you're dead, you don't know you're dead. The pain is only felt by others.
Same thing happens when you're stupid. - Philippe Geluck
More VW electrical at http://telforddorr.com/ (available 9am to 9pm PST)

[airschooled]

Richen the idle a tiny bit and try again. Sometimes the "power" gain from a slight increase in idle fuel can mask the drop. Vanagons use an idle timing stbilizer, you could hook one of those systems up if it really bothers you.

That should buy you time to fix the charing issue

Robbie
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