Hitchcocks Forum

Help please
I have a 2017 EFI with an upswept aftermarket silencer from Mr H
The problem I'm having is constantly sooting up up of the spark plug and then cutting out I've replaced the spark plug cap with a resistor type NGK and have a silicone carbon fibre HT lead I've tried NGK r bpr5es plugs and boch wr7dc it's still soots up also tried an iridium plug
Also tried running with red x also disconnected the lambda sensor seem to run slightly better but still sooting up the plug now I've reconnected it and it's backfiring

Does the problem go away if you put the original silencer back on again?

With a resistor cap, a resistor lead and a resistor spark plug, I would say you have two too many resistors. There only needs to be one. Maybe a hotter plug might help, or as stinkwheel suggests, refit the original silencer and see what happens.

The primary reason for a plug sooting-up is that the plug is running too ‘cold’ – running at less than approx 500 degrees C at the plug tip and so not able to ‘self-clean’. This may be due to the grade of plug being too ‘hard’ for the engine and the work the engine is asked to perform – ‘classic’ riders often give their bikes a very easy life and the recommended plugs in many cases may be too ‘hard’. Plug fouling may also be due to too rich a fuel mixture.



A plug with heavy carbon build-up will enter a vicious circle of ever-decreasing temperature at the tip. The current will tend to take the path of least resistance and not produce the spark at the electrodes but within the body of the plug, so reducing the heat at the plug tip.



On the question of resistors it may be worth quoting NGK: ‘All motorcycle manufacturers now comply with current legislation which demands a standard of noise suppression on every vehicle. This is achieved usually by fitting NGK resistor covers though in many cases a combination of NGK resistor covers and NGK resistor spark plugs (R type) is necessary to satisfy the ministry tests. It is emphasized that despite the use of both resistor covers and spark plugs in such engines, due to the unique and superior construction of NGK products, there is no loss of performance or shortening of service life.’

The ‘backfiring’ complicates the problem. Backfiring often is linked with weak fuel mixture – the opposite of what normally produces heavy carbon on the plug, because it normally makes an engine run hot. The other main cause of backfiring is retarded ignition timing, and this could affect the temperature of the plug at the firing tip. Retarded ignition may occur with the exhaust valve still open at combustion producing a backfire. Ignition that is too retarded won’t allow the engine to generate a normal or sufficient amount of heat (the opposite, too far advanced ignition, will cause the engine to over-heat). Not achieving proper running temperature will cause the plug tip to soot-up. I’m going to suggest the ECU is at fault with this engine. (But maybe not!!)

Maybe I should add (!!!) that a fuel mixture that is too rich may also cause a misfire/backfire. Rich fuel burns more slowly than lean fuel and may still be burning as the exhaust valve opens. With the exhaust valve open unburnt fuel will escape into the exhaust port and is heard as a backfire. So maybe the problem is the ECU affecting fuel mixture (not ignition timing).

Certainly you have to many resistors in the IGN system, you only need one and new bikes have two as per the EEC Regs for new machines, a 5 number plug is on the hot side and the danger with a hotter plug is over time you may hole the piston, a seven should be OK, I would be tempted to put all back as standard and do a blink test to see if any faults show up in the ECU, it sounds like an 02 sensor problem or another sensor at fault.

If an NGK ‘5’ plug soots up what’s going to happen to a much harder ‘7’ grade? An additional resistor isn't by itself going to create the problem Colin describes. Anyway, everyone to their own. Maybe in time Colin will let us know what the remedy was.

I had the same symptoms from a leak in the exhaust header joint...

In fitting the aftermarket silencer, you might have misaligned the exhaust in the head?

The solution for me was to replace the rubbish cement biscuit 'gasket' with the copper ring versions supplied by our hosts. No issues at all since doing so.

I experienced a similar problem with my 2015 B5. It ran rich (exhaust analysis showed this) and there was no way to adjust it. The original twin earth plug sooted up and caused misfiring and the plug required cleaning about every 500 kms. In the end I installed the Hitchcocks Amal carburettor set up where I could adjust the mixture and choose what jets I wanted. The bike ran much better and in fact superior to others and the same generation on trips - uphill the bike would outrun them. I used an NGK iridium BPR6EIX plug (5KOhm )I think) with an NGK cap (also 5KOhm) and a K & N filter with pod filter. If I had my time again I think I would pay more attention to the sensors, particularly the oxygen sensor on the exhaust, and may have measured it's output to check it was within spec before changing to the carby as it would have been a cheaper fix. Nevertheless I was very happy with the carby system.