RESISTOR PLUGS versus NORMAL PLUGS  

There has been a great debate over the use of resistor plugs in Vespa scoots. Generally, they are not a good idea since the NGK spark plug cap fitted in most p200s is already resistive, and the ignition on Vespas is pretty underpowered when compared to modern motorcycles. Let's have a look at the theory behind a resistor spark plug.

The reason to use a resistor spark plug is mainly to reduce radio interference into the environment and to protect the High Voltage electronics from the damaging side effects of reflections of a high frequency, high-power situation. The most common place we observe the effects of line reflections is in every day cable TV splitters. If we have a television set connected to one side, and an unused split connection on the other (open-circuit), under worst case circumstances, the television set will display a garbled picture. The reason for this is analagous to throwing rock in a perfectly still pond. When the rock hits the water, waves ripple in a spherical pattern; throw another rock in the pond and you will get overlapping waves. In an electronics sense, you have ruined the information being sent and risk sending a more powerful wave back to the transmitter and other receivers in the chain. This phenomenon is called an impedance mismatch. If a resistance, 220 ohms in the case of televisions, is inserted in parallel to all the other circuits, it will channel all the unused energy at a split into useful work instead of wasting power in the environment. Thus, the resistor will prevent the picture from degrading.

 

In our previous example, we were shown the effect of reflections on a television, now we look at the spark plug. Using a normal spark plug without a parallel resistance, we render all personal communications equipment useless. The reflections from the spark plug produce unwanted harmonics that interfere with any radio equipment you may have. Okay so what do we do? The magic number is 5000 ohms (or 5k) as 5k ohms is roughly matched to the characteristic impedance of the ignition transformer. If we put a 5kohms resistor inside the spark plug or the plug cap, the resistance will balance the transmission circuit and provide maximum energy to the spark plug's anode. Since there will be much less unused energy, the circuit will release much less electromagnetic and radio radiation. The end result is that we get a more efficient spark and less radio interference. The problem with the integration of a resistor is that it is designed for newer ignition transformers. The P200's ignition system's characteristic impedance may not be as high as 5kohms so the resistor may not balance the circuit like it should, which makes the system less efficient. Some find that their bike will just die once enough carbon collects on the center electrode. NGK even warns people on their site about the complications associated with older ignition systems, but so far I haven't experienced any igniton problems due to failure of a resistive plug.

Selecting a Spark Plug

Whew, an easy topic. Buy a B8ES. Done. No? Wait. So you're in a pinch and you need a spark plug. There's a B8HS and a B8ES R and a B8EGV and etc. Maybe one of these will do? Careful now, lets not do anything rash. What's the difference here? Okay, I see a problem developing here and it comes up again and again. Here's a table for what to buy. This table is for stock bikes.

B9HS, B8HS, B7HS, B6HS - These are short ended spark plugs and will not work with a P200, so ignore them.

B9EGV, B8EGV, B7EGV, B9EGV - These plugs have a tiny center conductor in them and may not always be made of copper. I have had no experience with these plugs, but from NGK's site, they appear to be for high end motorcyles, not Vespas.

B9ES - This plug may cause erratic running at city speeds, and will foul easily because it runs so cold. if you are running 'lean' because of your carb, this plug may be a temporary fix.

B8ES - Utopia. this plug is perfect for slow city riding and runs fairly cold for highway rides.

B7ES - A bit hotter than the B8ES. This plug is reccomended for quick city rides because it will not foul easily. If you're going on the highway go a little colder.

B6ES - This plug runs 'hot' this plug should only be used for special circumstances where the engine may not be getting to full temperature or the carhb is running rich (too much oil/gas mix).

'R' - R means resistor plug, there is a 5K resitor in parallel with the spark plug electrically to provide less EMI (Electromagnetic Interference) and RFI (Radio Frequency Interference). See my lengthy discussion of the pro's and cons of this plug above. These plugs aren't a bad idea because they generally make the system work better.

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