Plug caps.

Treating the old girl to new coils/points/ condensers/HT leads/NGK spark plug caps and plugs. What I’m wondering is, for donkeys years we’ve been fitting plug caps with 5K ohm resistors to suppress interference to analogue TV and AM radio. Now everyone is on digital TV and most radio is FM, what is there to stop me fitting plug caps with no resistors fitted? I see NGK sell them. Would they give a better spark? It’s always seemed daft to me making sure all HT connections are clean and tight, then putting a big resistor in the way…

The resistance of the air gap between the plug electrodes is very much greater than any suppressor resistor so no difference. Also I could be wrong but I thought there were some regulations about generating RF interference, or maybe not.

Not just TV but radios, mobile phones and engine management systems can suffer from interference from spark plugs without supression
According to NGK it is a legal requirement but I am unable to find any reference to the law they refer to.

Good question :bulb:
I used unsuppressed jetski caps for a few years.
This was as much for weatherproofing as anything else.
No discernible performance benefit but did last well and no ill effects with electronic equipment in the immediate area AFAIK. No complaints from neighbours or MoT tester either.
All the best

That’s all very well until someones radio control plane falls out of the sky onto your head.

Might do me some good.
The latest Guzzi’s have non resister caps but resister plugs.
Could this be the answer?

This Wikepedia page gives a lot of information about applicable standards in Europe

When there is a spark, there is an acceleration of charged particles (electrons in this case). Acceleration of charged particles always gives rise to electromagnetic radiation.
The harder the acceleration the shorter the wavelength of the emission. I don’t know for sure, but suspect that the resister has the effect of altering the frequency of the emission, which maybe takes it out of the radio band. I think it is in parallel so maybe doesn’t impact on the energy of the spark too much ? It might also be altering the phase so that you get some signal interference/ cancellation (bit like coax cable)

It’s not in parallel but in series. If it were in parallel there’d be no spark because all the energy will go to ground through the resistor. Electricity always favours the easiest path to complete a circuit. I’m not absolutely sure but I think how it works is that the resistor forms one part of a resistor + capacitor low pass filter. The spark will use the HT lead as a radio antenna, however this also has some capacitance so inserting a resistor between it and the spark plug reduces the RF signal to negligible proportions.

Alternatively a small RF choke aka coil may be used, the filter then becomes an LC type. But does same job, ie prevent a large RF signal coming out onto the HT lead.

I was following this thread, now I am just confused… :confused:
I realise that an unsuppressed HT circuit will cause disruption as it generates an electro-magnetic field, hence the need for suppression, however I do not know what the legislation is that would make unsuppressed plugs/caps/leads illegal, which I think was Al’s original question. The actual way the suppression works is of no consequence, what Al asked was would it make any difference to the performance and if there was anything to stop him doing it.

The more I delve into this, the more confused I am. It seems non-resistor caps/plugs are essential for magneto ignition engines. Also resistor caps are not necessary if you have spark plugs with built in resistor, ie, any plugs that have an ‘R’ in the numbers and letters.
Looking at my plugs, NGK BPR6ES, I guess I’ve been over-resisted…

I recently bought a 1974 Honda CB350F, the little 4 cylinder we never got here.

I got it cheapish as it needed some sorting.

I rebuilt the carbs, fitted new point and condensers but still had an intermittent misfire.

When I pulled the plugs to change them I found resistor plugs and resistor caps.

Once I fitted standard plugs the misfire was gone.

I would think modern ignition systems will cope with both resistor caps and plugs but my recommendation would be to use one or the other but not both.

My feelings exactly. I like to keep the original set up on the Convert. Fine when spinning, but starting (with modern petrol that won’t burn even when you put a match to it) it needs every volt it can get to fire the plugs. Non-resistive plugs going in…