Have found a problem that doesn't make much sense, have not seen before (to this degree anyway)... Good battery (apparently - starts ok), good alternator (I think) -- no warning lights on, or any other symptoms (battery is charging, regulator not making noises). Engine warm (or hot) and idling at 900 - 1000rpm. Turn the headlights on, engine slows down. Put on high beam (90W each), engine slows more. Turn on anything else, e.g. wipers, then brake lights, and engine almost is stalling! Is there something obvious that could cause this?

Fairly standard 1600 late beetle engine. Alternator and fan spin freely when fan belt is removed. Earth connection near battery looks secure. Battery terminals removed and replaced to ensure good contact. Lights etc. all work brightly/normally.

whats the output voltage, load and no load.

OK, measured at battery terminals, +/-0.1V, after stable reading:

A. Engine Off:
1. Battery fully charged: 13.4V
2. Ignition & all available accessories, flashers, wipers, & lights on (abt. 30s): 11.7V
3. All items turned off again: 12.4V
4. During extended cranking of starter with points shorted (at least abt. 20s; cranking didn't slow noticeably): 10.7V

B. Engine at idle, warm:
1. No accessories/lights on: 14.1V
2. ALL available light/accessories on: 13.0V (And engine speed dropping as previously... maybe it's just 'normal' for this engine, or this alternator?)

Check your voltage at the coil.

The output from the gen is keeping the battery charged fine but all that load on an ageing electrical system is probably dropping power at the coil = weaker spark slowing idle speed.

One of the bugs I had was only at 10.4 volts at the coil with the lights and wipers on due to a shitty ignition switch with badly pitted contacts.

normally the alternator will drag the engine a little. Given the drop there, which is too much, perhaps the engine isn't making enough torque at idle for some reason to even overcome the drag from the alternator?

is it EFI or carb?

Try loading the electrical system again, measure voltage with increased revs (give if some gas grandpa). You should see at least 13.8-14v with load and higher revs.

Thanks young Vlad, will try that again with revs. (Yes has mild-cam, so slightly rough idle; but have run engine previously and don't remember this problem. It's standard late carb., 34 PICT-4).
Thanks Joel (I take it from your comment that the Voltages measured are normal then?), will check Volts at coil also, tomorrow#.

#(don't want to wake the neighbours; leave that to the grand-kids)...

repl h/light switch in my oval as it was arking out due to loose terminals and slowen down lots

Joel is right, it could be a crappy coil or something like that where it voltage dependent and causing a sort of feel back loop. Lowering engine performance as the voltage drops, which then drops the voltage more.


that, or you could bump the idle just slightly to stop it dropping below the alternator charge threshold.

I'd be looking at a high resistance in the ignition switch. Or around that area.

Thanks chasps for the suggestions... some things to check and as expected some I wouldn't have thought of.
To continue the readings:-

A. Engine OFF:
1. Battery Fully charged: 13.4V
2. Ignition & all available accessories, flashers, wipers, & lights on (abt. 30s): 11.7V
3. All items turned off again: 12.4V
4. During extended cranking of starter with points shorted (at least abt. 20s; cranking didn't slow noticeably): 10.7V
.............next day.........
5. At battery again (still well charged): 12.8V
6. At COIL (Ign. on, points open, no auto choke): 12.5V
7. At COIL (as above but with auto choke connected): 12.2V

B. Engine at idle, warm:
1. No accessories/lights on: 14.1V
2. ALL available light/accessories on: 13.0V (And engine speed dropping as previously... maybe it's just 'normal' for this engine, or this alternator?) No sign of gen. light coming on even when slowing down.
..............next day...........
3. At COIL, no lights etc. on: 13.4V
4. At COIL, all lights etc. on: 11.0V
5. Forgot to do this then at higher revs, memory fading, will try next time.

So presumably there could be a bad earth between engine and body?, or high resistance in ign. switch, or some other item robbing power, as suggested.
The coil is pretty new (standard Bosch) and works well. (-) connected to points (that's right isn't it??); body earthed onto fan housing.
Thanks again for all the comments.

Hi

Not sure if its relevant, I had a Chinese alternator on a 1300 Beetle, after fitting it I had to raise the idle speed, I took it off when I sold the car and I had to drop the idle speed, it always made that sound that alternators made when it was on like it was always charging.

Steve

A good double check to ensure its not poor motor earth is to disconnect the black coil 12 volt + wire, hotwire the fat red 12volt alternator lead to your +'ve coi terminal (just wrap some copper wire), turn it over and see if you get the same idle drop with electrical load.

You'll be bypassing the whole fuse / ignition barrel and thus any current drop due to dodgy fuse/ignition barrels should be eliminated for the purpose of testing.

What shape is the battery in?
Try a new battery!