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Old 11-11-2021, 08:54 PM   #1
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Default B20 idle slows and almost dies when headlights are turned on

It would seem there is something wrong with the charging system on my B20F (SU carbs) and could use some input because I am unsure where to look next for the problem.

Today I was out for a drive and got stuck in traffic in the woods where it was dark enough to turn on the headlights. I notice as the car sat there idling that it would slowly get slower. Eventually I had to feather the throttle and engage the fast idle through the choke to keep the motor running. I turned off the headlights and the problem fixed itself. It wasn't all that hot both under the hood and outside, which is generally what causes issues with my SUs.

It is running a GM 63 amp alternator, the lights are Hella H4s, otherwise stock wiring. My volt gauge reads about 14 volts when nothing is running in the car. It drops to 13 initially when idling with the lights on and eventually drops to 12 and keeps going down. Turning on the high beams or auxiliary lights on accelerates this. Parking lamps don't draw enough to cause problems. The battery isn't even a year old and never been drained enough that the car wouldn't start promptly. The alternator is outputting 14.35 volts at idle and seems to be working ok.

Normally it idles in the 750-850 range depending on how hot it is out. It'll drop below 700 and then hover just above the 500 mark on my tachometer while trying to die.

I've got the car sitting so I can get a voltage reading on the battery after it has sat.
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Old 11-11-2021, 09:11 PM   #2
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My initial thought was that you have an alternator problem. Something that affects the current output of it like a bad diode. Might want to have it load tested. it should output within 10% of it's rated current.

Then I also thought this might be going on. Take a look at the plugs. The engine could be loading up with fuel from a incorrect float bowl setting or needs a jet adjustment. What is the fuel pressure? The slower, and slower idle will cause the charging to stop and will run on the battery.
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Old 11-11-2021, 09:25 PM   #3
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My initial thought was that you have an alternator problem. Something that affects the current output of it like a bad diode. Might want to have it load tested. it should output within 10% of it's rated current.

Then I also thought this might be going on. Take a look at the plugs. The engine could be loading up with fuel from a incorrect float bowl setting or needs a jet adjustment. What is the fuel pressure? The slower, and slower idle will cause the charging to stop and will run on the battery.
I yanked the plugs just the other day to check my tune after a highway drive. It seems to be doing ok. Light gray and tan. Fuel pressure is sitting at 2.5 psi.

I'll take the alternator down to the parts store to get it tested tomorrow. The thing isn't even a year old. So a problem with that would be quite annoying.
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Old 11-12-2021, 12:53 PM   #4
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The last online rebuilt alternator I bought lasted about a year and a half. So you're right in the zone. Sad to say.
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Old 11-12-2021, 01:09 PM   #5
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The last online rebuilt alternator I bought lasted about a year and a half. So you're right in the zone. Sad to say.
Took it down to the auto parts store. It tested just fine.
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Old 11-12-2021, 01:17 PM   #6
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Glad it tested good. Then it's time to look for something sneaky going on. Something like a worn distributor or timing changing while running?
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Old 11-12-2021, 01:21 PM   #7
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Glad it tested good. Then it's time to look for something sneaky going on. Something like a worn distributor or timing changing while running?
Last I checked the timing it was OK. I'll get a timing light on it while the lights are on and it is trying to die. If all is good it should still be set at 10 degrees BTDC.

Maybe the ground strap for the motor?
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Old 11-12-2021, 01:26 PM   #8
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Make sure the timing moving on the crank pulley when looking at it with the timing light. Just seems like it would be loading up with fuel. Maybe a vacuum leak? Like at the intake/exhaust gasket?

It is probably a good idea to add another ground to the system. Is there ia good ground path from the block to the body of the car?
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Old 11-12-2021, 03:56 PM   #9
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Make sure the timing moving on the crank pulley when looking at it with the timing light. Just seems like it would be loading up with fuel. Maybe a vacuum leak? Like at the intake/exhaust gasket?

It is probably a good idea to add another ground to the system. Is there ia good ground path from the block to the body of the car?
I cleaned the ground strap from the bell housing bolt to the chassis, the ground for the alternator, and the ground from the battery to the chassis. The grounding strap was a bit loose where it attaches to the frame rail. It improved a bit. Wouldn't die but idled quite a bit lower when the lights were on. I expect a bit of a drop due to added strain on the engine at idle but not 250-300 rpm.

Then I hooked up the timing light and there was some strange behavior on cylinder 1. The light wouldn't pulse quite right. It was intermittent. I put the inductive pickup on the other three wires and it would strobe evenly. I tried swapping the cap and rotor. Nothing there. Moved some wires around too. I then just pulled out my spare set of spark plugs. I swapped and gaped them all at .029" and the issue fixed itself. After a quick test drive and resetting the idle It now sits at 800-850 rpm without the lights and other extras turned on and about 750 with the high beams, stereo, and fan on.

So you were right about it being ignition, but spark plug #1 seemed to be bad. Also cleaning the grounds solved my radio noise issue.

Thanks for the help.
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Old 11-12-2021, 05:11 PM   #10
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Glad you got things working well. I just recently was tinkering on my friends fathers Morgan and changing the plugs sorted that car out, too.
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Old 11-12-2021, 06:02 PM   #11
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Glad you got things working well. I just recently was tinkering on my friends fathers Morgan and changing the plugs sorted that car out, too.
This is why I carry an extra set of plugs and a whole distributor in the back of the car (the spare is a bit worn, but good enough to get it rolling again). This one was definitely a bit sneaky. The car ran fine otherwise, but at idle the plug had issues firing right.
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Old 11-12-2021, 11:02 PM   #12
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Glad you got things working well. I just recently was tinkering on my friends fathers Morgan and changing the plugs sorted that car out, too.
Apparently it was also a bit too light on ignition timing. It was a few degrees short of the often quoted 32-34 degrees at full advance. So I dialed up the timing at idle to 14 degrees. The motor is a good deal happier and I now get 32 degrees at full advance.
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Old 11-13-2021, 12:06 PM   #13
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Yeah the spares really come in handy. Sounds like it's back to fun pushrod driving.
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Old 11-17-2021, 08:29 PM   #14
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What would you consider a normal voltage drop with the high beams on? On my car it drops to almost 12 from 14 when I turn on the high beams. The problem is still there, albeit lessened. It won't die anymore. Just drop the idle significantly. Which makes me think I am just slapping band aids on it.
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Old 11-17-2021, 09:08 PM   #15
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Apparently it was also a bit too light on ignition timing. It was a few degrees short of the often quoted 32-34 degrees at full advance. So I dialed up the timing at idle to 14 degrees. The motor is a good deal happier and I now get 32 degrees at full advance.
Are you running the original B20F distributor with the carbs? If so, are you also running the B20F vacuum retard system? If so, with the distributor correctly set (10 BTDC at idle with the vacuum line plugged) the typical B20F idle ignition timing would be around 5 - 6 BTDC with the vacuum connected.

The mechanical advance function on the B20F distributor is around 12 degrees (distributor) 24 - 25 degrees (crankshaft). If you block off the vacuum retard port and set the idle timing to 10 BTDC you should end up around 34 BTDC (crank) at full advance. The B20F should run without the vacuum retard function just fine.
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Old 11-17-2021, 09:43 PM   #16
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What would you consider a normal voltage drop with the high beams on? On my car it drops to almost 12 from 14 when I turn on the high beams. The problem is still there, albeit lessened. It won't die anymore. Just drop the idle significantly. Which makes me think I am just slapping band aids on it.
I assume this is your Amazon that you are talking about and that the wiring is still original? If you check the wiring diagram, you will see that the headlights get there current supply through the same wire that supplies the ignition switch. Current flow for the headlights contributes to voltage drop in the rest of the car. A partial fix for this is to take the low and high output from the dipper switch and have it switch separate low and high beam relays. The relays would switch fused DC power directly from the battery which eliminates headlight current flowing through the cars ignition switch supply and eliminates the continuous voltage drop associated with that.

Having relay switched headlights probably will not completely eliminate voltage drop associated with switching your headlights on. On my 1971 142E, I still have the original Bosch 35 amp alternator. I have one of Dave Barton's separate adjustable voltage regulators and I have it adjusted so that my idle voltage with the headlights off is about 14.3 - 14.4 volts. When I switch on my lights (H4s) the voltage on the distribution block on the inner fender drops to about 14.1 - 14.2 volts. If you have a more modern / larger alternator than my 50 year old Bosch, I would expect that the voltage drop AT THE ALTERNTOR B+ TERMINAL when you switch the headlights on should be less than that. Any additional voltage drop above that will be due to the wiring in the car.

My B20E idles at around 850 RPM and when I switch on the headlights and the heater fan the idle speed does drop ever so slightly because of the increased load on the engine. I do run MSExtra and I use the idle advance function (but not closed loop idle control) which helps to reduce the idle drop when I load up the alternator. If your engine is dying when you switch the headlights on at idle I am thinking that the idle set up on the carbs might be a little suspect.
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Old 11-17-2021, 09:58 PM   #17
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I assume this is your Amazon that you are talking about and that the wiring is still original? If you check the wiring diagram, you will see that the headlights get there current supply through the same wire that supplies the ignition switch. Current flow for the headlights contributes to voltage drop in the rest of the car. A partial fix for this is to take the low and high output from the dipper switch and have it switch separate low and high beam relays. The relays would switch fused DC power directly from the battery which eliminates headlight current flowing through the cars ignition switch supply and eliminates the continuous voltage drop associated with that.

Having relay switched headlights probably will not completely eliminate voltage drop associated with switching your headlights on. On my 1971 142E, I still have the original Bosch 35 amp alternator. I have one of Dave Barton's separate adjustable voltage regulators and I have it adjusted so that my idle voltage with the headlights off is about 14.3 - 14.4 volts. When I switch on my lights (H4s) the voltage on the distribution block on the inner fender drops to about 14.1 - 14.2 volts. If you have a more modern / larger alternator than my 50 year old Bosch, I would expect that the voltage drop AT THE ALTERNTOR B+ TERMINAL when you switch the headlights on should be less than that. Any additional voltage drop above that will be due to the wiring in the car.

My B20E idles at around 850 RPM and when I switch on the headlights and the heater fan the idle speed does drop ever so slightly because of the increased load on the engine. I do run MSExtra and I use the idle advance function (but not closed loop idle control) which helps to reduce the idle drop when I load up the alternator. If your engine is dying when you switch the headlights on at idle I am thinking that the idle set up on the carbs might be a little suspect.
I think that may just be the solution to my problem. I'll start looking into a headlight relay. I just did the math on what I am pulling through that ignition switch. With 55w low beams on my H4s and 55w H3 bulbs on either fog lamp I am pulling 220 watts with those on. So 18 amps through that ignition switch plus everything else that relies on switched power (radio, fan, interior lighting, wipers, etc). Then look at 10 amps with the high beams on and 19 amps if I have driving lights fitted to work with the high beams. That is a good amount of current. The stock wiring diagrams have 45w high beams listed, so 7.5 amps through the ignition switch to the lights. I can pull more than double that with how the car is set up now. I am running a 63 amp Delco alternator but that much draw on a circuit originally spec'd for probably 10-ish amps is not good once you add in the fact that the wiring and switches are 53 years old.

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Old 11-18-2021, 12:12 PM   #18
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No need for anything too fancy. I visited my local pick and pull and lifted a nice little relay box from a 2000 - 2005 ish vintage Corolla. It has four relays in it and four fuse positions and is about 5" x 1.5" with a convenient bracket on the side to mount on the inner fender. I use it on my 142 to provide a high beam relay, low beam relay, electric fan relay and DRL control for my Hella Citi lights. Cost me $5 -$10 and came with 3 Siemens and 1 Denso high quality relays.

The reduced current flow through the vehicle wiring will improve everything. On my 142, when I still had conventional coil ignition the idling voltage on the + terminal of the coil was around 11.5 volts with the headlights and heater fan on even though the alternator was around 14.2 volts. Reducing current flow through the ignition switch will also extend its life which is important because I think replacements are NLA. I now switch my coils(s) through a dedicated relay which separates that load from the ignition switch. I have been thinking that I may put the heater fan on its own relay as the next step.

Being from CA, this is probably not an issue for you; but, in really cold weather the voltage at the ignition coil + terminal can be very low when the engine is cranking making for a weak spark. Having the coil on a dedicated switched / fused supply will improve the coil voltage which helps with cold weather starting.
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Old 11-18-2021, 01:06 PM   #19
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No need for anything too fancy. I visited my local pick and pull and lifted a nice little relay box from a 2000 - 2005 ish vintage Corolla. It has four relays in it and four fuse positions and is about 5" x 1.5" with a convenient bracket on the side to mount on the inner fender. I use it on my 142 to provide a high beam relay, low beam relay, electric fan relay and DRL control for my Hella Citi lights. Cost me $5 -$10 and came with 3 Siemens and 1 Denso high quality relays.

The reduced current flow through the vehicle wiring will improve everything. On my 142, when I still had conventional coil ignition the idling voltage on the + terminal of the coil was around 11.5 volts with the headlights and heater fan on even though the alternator was around 14.2 volts. Reducing current flow through the ignition switch will also extend its life which is important because I think replacements are NLA. I now switch my coils(s) through a dedicated relay which separates that load from the ignition switch. I have been thinking that I may put the heater fan on its own relay as the next step.

Being from CA, this is probably not an issue for you; but, in really cold weather the voltage at the ignition coil + terminal can be very low when the engine is cranking making for a weak spark. Having the coil on a dedicated switched / fused supply will improve the coil voltage which helps with cold weather starting.
I've already got a pile of metal Bosch and Hella relays I pulled from my 240 when it went to the scrap yard, along with other spares I accumulated. I'll probably just repurpose the headlight relays from that car. So the challenge is just making a small bracket for them. Kind of fitting that I put parts from the totaled car onto the one I bought to replace it.

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Old 11-18-2021, 08:28 PM   #20
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Whenever I install a Pertronix in the old Volvos. I add an ignition relay to help the coil get more voltage.
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Old 11-19-2021, 11:42 AM   #21
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Whenever I install a Pertronix in the old Volvos. I add an ignition relay to help the coil get more voltage.
I've already got the starter on a relay. Might as well put the big current draws on relays. Since the car used to be an automatic I decided why not keep the one that used to work as part of the neutral safety switch.

I should just make a relay mounting bracket for the inner fender at this point.
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Old 11-19-2021, 12:47 PM   #22
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Just use the inner fender bracket from a 240. I also add the relay because those fuse boxes don't like the extra current from the pertronix system going though it.
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Old 11-19-2021, 01:31 PM   #23
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Just use the inner fender bracket from a 240. I also add the relay because those fuse boxes don't like the extra current from the pertronix system going though it.
I've got a bunch of scrap aluminum lying around. So I'll probably just make a bracket from that.
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Old 11-23-2021, 08:49 PM   #24
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At this point I am rather puzzled. I have gone through the ignition system, valve lash, re-tuned the carbs (I was apparently running rich at idle), put high current draw systems (headlights, ignition, fog lights) on relays, the alternator tested ok at the local parts store, and I cleaned each ground I could find.

Voltage measured at the ignition switch doesn't drop much with the low beams on. A few tenths of a volt. But the idle drops considerably, at least 200rpm. Once I add the high beams, fog lights, or blower motor the voltage can drop below 13 and the engine starts struggling to stay idling.

Would my old belt squeal issues have anything to do with this? I tightened up the belt a little and cleaned the pulleys. That went away. It is tensioned on the tighter side though.

The battery seems healthy and stores a charge. Stays right at 12.7 volts every morning. Maybe the light switch? The foot dipper switch?
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Old 11-24-2021, 12:22 PM   #25
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I would suggest checking the voltage drop on the fuse box. I've had these fuse boxes cause major ignition issues when the pertronix current draw goes through the fuse box. If I recall correctly zvolv wrote that the drop shouldn't be more than 200 millivolts.
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