I'd like to sum up because there's good info in the posts already written but here's all of it wrapped up in one. "In the beginning..." When the 544, 122 and 140 models were sold Volvo used a copper wire wrapped in a red covering. Those ignition wires used the brown plastic spark plug connector as did VW in their Beetles and Buses. There was a resistor built into those plastic connectors to compensate for the copper wire and radio interference that ensued. With introduction of the 240 models the resistors were moved to those metal shielded connectors most newer (I'm talking relatively speaking) Volvo owners would be familiar with. The Bougicord wires-and others like it- do not have the resistor so "resistor plugs" should be used. I think the resistance for the metal shielded wire connectors is something like 5-7K.
While Volvo recommends Bosch plugs I've used NGK ever since working at a Honda dealership--that was early in the production of the CVCC motors in Civics and Accords. They had horrible problems with the complicated 3 barrel carburetors used for the CVCC system. Cleaning carbs and resetting float levels was a big part of any Honda tech's day. Float level was set by an external adjustment screw. The procedure was to warm the engine, then, in stages, raise the float level till the motor floods - then back off a specified amount (depending on the model). With the motor flooded and the adjustment made you'd now restart the motor to check for an even idle -- that done, job finished. And while completely flooded, start they did. They used NGK plugs and I was sold - been using them ever since. Try that with a Bosch plug and you had better have a fresh dry set available. Anyone who has ever owned an SU HS6 carb equipped Volvo can testify too the annoyance of a flooded motor due to either sticky needle valves or the overuse of the "choke" (actually an enrichment device to add fuel rather than restrict air like a normal thing we call a choke). That's the whole scoop. Resistance wires or resistor plugs--one but not both. -- Dave