Vintage B20 idle slows and almost dies when headlights are turned on

dl242gt said:

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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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.

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.

dl242gt said:

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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I've got a bunch of scrap aluminum lying around. So I'll probably just make a bracket from that.

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?

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.

When you switch the lights and other accessories on, you increase the load on the alternator which increases the load on the B20. Like driving up a hill which increase the load, if you don't open the throttle a little bit the engine slows down.

When the engine is running all electrical load is supplied by the alternator (in a correctly functioning system). The battery is there just for starting and is generally along for the ride to get recharged following starting. The battery will not supply any of the electrical load when the alternator is operating unless the voltage drips below about 12.2 volts (which means you have a problem with your alternator).

I would be inclined to experiment with raising your hot idle speed a bit to see if that alleviates the problem. Also, what cam are you running in your B20F? My understanding of SU carbs is that they operate best with a higher manifold vacuum. If you have a cam with higher overlap the manifold vacuum deteriorates. The 'received wisdom' seems to be that the D grind is the upper limit for reasonable idle with SUs and you may need to nudge the idle speed up a bit to get a good idle. If you are running a cam with even more overlap you may have to live with your idle problem unless you crank up the idle speed.

dl242gt said:

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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That is one thing I haven't touched. I'll check that. I've been eyeing Ron Kwas' rebuilt fuse boxes. So far it is has been on the "nice to have" list. Or I could just clean it up and rebuild it myself. I should probably also check the light switch drop while I am at it.

142 guy said:

When you switch the lights and other accessories on, you increase the load on the alternator which increases the load on the B20. Like driving up a hill which increase the load, if you don't open the throttle a little bit the engine slows down.

When the engine is running all electrical load is supplied by the alternator (in a correctly functioning system). The battery is there just for starting and is generally along for the ride to get recharged following starting. The battery will not supply any of the electrical load when the alternator is operating unless the voltage drips below about 12.2 volts (which means you have a problem with your alternator).

I would be inclined to experiment with raising your hot idle speed a bit to see if that alleviates the problem. Also, what cam are you running in your B20F? My understanding of SU carbs is that they operate best with a higher manifold vacuum. If you have a cam with higher overlap the manifold vacuum deteriorates. The 'received wisdom' seems to be that the D grind is the upper limit for reasonable idle with SUs and you may need to nudge the idle speed up a bit to get a good idle. If you are running a cam with even more overlap you may have to live with your idle problem unless you crank up the idle speed.

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It is the stock D cam as far as I know. SUs definitely like strong vacuum. For now I've been keeping the idle around 850-900rpm instead of the stock 800rpm. I may turn it up to 900-950 and see how that does. It idles ok at the stock 800. A bit like a B230 with an A or B cam. But you can definitely tell the SUs aren't happy about it and you get into the danger zone between 600 and 700 once that alternator starts putting strain on the motor.

I do have the choke set up so that it'll have a bit more pull for just fast idle before it starts enriching the mixture (in mild California I don't have to really enrich it for a cold start, really just fast idle). So I could just use that as a hand throttle to adjust idle when more than just the low beams are on.

On a B20F (at least on the later B20F - not sure about 1972) , the stock cam would have been the K cam which has the same lift as the D cam; but, a nudge less overlap and as a result idles better. Also, a stock B20F would have been equipped with D jet and later K jet. D jet didn't have problems with operating with low idle vacuum. I am not aware that a B20F engine with SU carbs and a D cam grind was ever a stock configuration.

My B20E with the D cam does not suffer from dying when I switch on lights and accessories at idle, either with the original D jet or now with MS2 Extra. However, the D cam does give it a rather erratic idle.

142 guy said:

On a B20F (at least on the later B20F - not sure about 1972) , the stock cam would have been the K cam which has the same lift as the D cam; but, a nudge less overlap and as a result idles better. Also, a stock B20F would have been equipped with D jet and later K jet. D jet didn't have problems with operating with low idle vacuum. I am not aware that a B20F engine with SU carbs and a D cam grind was ever a stock configuration.

My B20E with the D cam does not suffer from dying when I switch on lights and accessories at idle, either with the original D jet or now with MS2 Extra. However, the D cam does give it a rather erratic idle.

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You never got SU carbs with a D cam in a stock motor. The switch from the D to the K cam for K-jet was in '74.

Although the D cam was sold by the Competition Service as a fast road and mild race cam for the B18 and B20. Although the competition service catalog I could find seems to imply using Solex side drafts with them. By the time you get the R sport catalog in '79 it seems like it was supplanted by the K cam. The catalog says it'll do fine with SU, ZS, and Solex downdraft (40mm two barrel) carbs. Although the GT kit it came in included a Solex downdraft carburetor (a Weber 38/38 seems to be the modern equivalent) and a higher compression head.

The old "Performance Tuning B18 Series Volvos" booklet shows performance tuning kit 419398 which included a D cam, higher compression cylinder head, different springs for the dashpots, and richer needles. They definitely intended for the D cam to be used with a higher static compression setup.

The dashpot spring part has me interested. A different spring would change how the carb reacts to vacuum. The question is which way to take the spring rate for lower idle vacuum. Relax or stiffen? The book also seems to recommend much lighter oil for the dashpots. SAE 5, whereas the normal recommendation I see is SAE 10 (ATF) or SAE 20 (what I've been using).

Well the hand throttle approach works ok. I rechecked the mixture again. With both the "fastest idle" and lift pin tests. The two points are within a flat or two of each other. So the mixture seems pretty good. Plugs look good. Insulator is a bit white for my liking but the plugs are also relatively new and the ground strap looks ok. Checked for vacuum leaks. No issues with the ignition. No issues with the fuse block either.