Hitchcocks Forum

Any comments on changing the spark plug on my continental 535 which has the standard bosch wqr8dc which seems to do the job perfectly well.but will the iridium be better or even a standard ngk which I cannot seem to find the equivalent ,as I need to carry a spare it seems a good time for any advice.?

The only difference you will notice with an Iridium is it will last a lot longer. Read the instructions on the box which usually advise not to clean and not to alter the gap. Fit and forget.

I bought an NGK standard plug and cap for my GT to replace the original after reading problems with the bikes cutting out.

NGK BP6ES is the standard replacement. I've never liked metal suppressor caps so bought an NGK cap at the same time - bought at my local independent bike shop. I wouldn't waste your money on an iridium - the standard plug is perfectly adequate.

John

If it ain't broke.......

The NGK iridium is not suitable for a slow revving aircooled engine of relatively low compression - it fouls up very easily as fuelling is difficult to control on a big single. If you don't believe me then do a series of plug chops on the standard plug and you'll see how much variation there is through the rev range in 'wetness'/colour/soot etc. Sometimes puzzles me as to how the engine ever started in the first place! There are no NGK heat range equivalents for the OE Bosch plug and if its working as Pete F says then why waste yer hard earned???

Hi, I've tried both and found no difference to be honest, so went back to the standard bosh and just carry the iridium as a spare.

I concur with Johnl re what to do and with Nigelphoto re the unsuitability of iridium plugs in these engines. Whilst I would normally agree with the motto if it ain't broke don't fix it, in this case the supplied original plug is an Indian Bosch clone, not a genuine Bosch and they are known to cause poor running. Usual advice is to bin the original and replace with an NGK BPR6ES. These can be bought cheaply in packs of four at Halfrauds. As for the iridium plug, these are IMHO snake oil as far as the EFI engine of s concerned. It is true that they are supposed to be fit and forget and certainly on a modern multi for which one needs a degree in mining to remove a set of plugs they are a good idea. However, not only is the single spark plug a doddle to remove on an EFI, in my experience the iridium plug fouled up badly. For these plugs to operate correctly the fuelling/gnition system needs to be spot on. The EFI engine is prone to run rich when there is a fault; this kills the iridium plug in quick time because the electrode is so fine. There is no discernible improvement in the running of the engine either, so why waste money? I've never understood the concept behind metal cased plug caps. Chuck it in the bin with the Bosch clone plug before it starts shorting out - preventative maintenance, the antidote to the if it ain't bust argument.....

Or just leave it as it is and call the AA when you break down...

I stuck with the non-resistor BP6ES rather than the resistor BPR6ES plug because the NGK plug cap has a resistor built, in so a resistor plug isn't necessary. Whether having two resistors makes any difference to the spark I've no idea, but Hitchcocks list the non-resistor plug on this website. If buying car plugs from Halfords, just check the top cap can be unscrewed to leave just the thread to make contact with the suppressor cap.
John

JohnL,thanks for input on ngk plug cap,been out and checked mine and after looking the up codes it is the resistor type
So I am going for the BP6ES, as you say having a resistor plug and resistor cap which is fitted as standard seems odd ,I would have thought doubling up on resistors would reduce the strength of the spark ?

You're right in thinking that you shouldn't have a resistor plug and cap. FWIW I went the other way and fitted a Champion non-resistor cap and the BPR6ES plug. Come to think of it the iridium plugs are resistor types so probably don't perform as well with a resistor cap either.