Motorcycle Owners Club

Honda CG125 but also some universal motorcycle information

Honda CG125 Spark Plug

< Servicing

Very important spark plug information, please read all of it even if you think you do not need to.

The older engines are meant to use NGK DR8ES-L for normal riding and have the gap set to 0.7mm
This appears to be for the old fashioned point's engines.
The chances are you do not need to check the gap when brand new, but I cannot guarantee this.

The more modern engines, probably 1997 onwards in the UK have Electronic Ignition / CDI instead of points.
They are meant to have NGK DPR8EA-9 for normal riding and the gap set to 0.9mm.
The gap is pre-set at the factory to 0.9mm so it does not need to be checked when brand new,
that's what the -9 means at the end.

A very experienced mechanic, who has owned many CG125 bikes over the years,
tells me you can use ether spark plug in ether engine.
He has done so and had no problems;
he said the main difference is the later one has a resistor to reduce air born interference, which does not affect the bike.

NGK is the name of the manufacture and the rest is their part number.
Different spark plug manufacturers make equivalent spark plugs but have different part numbers.
The spark plug manufacturers do have conversion charts so you can convert from one make to another.

NGK is regarded by most people as the best and Honda fit them as standard to the Honda CG125.

The following information should be relevant to all Honda CG125 engines and owners.

My Honda CG125 Front Disc Brake model uses the NGK DPR8EA-9

The book tells you to clean and check the gap of the plug every 2500 miles and replace every 5000.

I could not be bothered to clean and check the plug gap at 2500 miles
since a new plug only costs £2.99 to £5 dependant on shop.
That means at 2500 miles the plug is only worth £1.50 to £2.50 so not worth the work of cleaning and checking the gap,
so I put a new one in.
I have been told that cleaning spark plugs can go wrong, you ether do not clean it well enough or may even damage it.
It's also probably a good idea to change a spark plug every year even if you do not do the mileage.

You need a NGK DPR8EA-9 spark plug for normal riding and a NGK DPR9EA-9 for extended high speed riding,
The extended high speed riding spark plug is stated in the Honda CG125 Front Disc brake models Owner's Manual,
but cannot be totally sure for other models,
but it makes sense since it's the same spark plug but gets rid of more heat out of the engine
(that's what the 9 means compared to the 8 in the model number).

The following spark plug information is for the front disc brake model,
I cannot be sure if the same would apply to other models, but it should be relevant.

I found that 54mph in 5th gear is high speed for the plug and results in power dropping off if left at that speed for 1/2 mile.
The power drop off (plug overheating), results in the bike dropping to 50mph against a very strong head wind,
if you slow down a lot the plug will recover.

I fitted the DPR9EA-9 plug and end of problem = no power drop off and the bikes about 5 mph faster.

If you use the DPR9EA-9 for slow speed (especially idling a lot in traffic jams),
there's a chance the plug will not be hot enough
to burn off petrol burning deposits on the electrode and the bike will stutter or stall.
I have not had any problems, but would not use it if I was in city traffic all the time with massive amounts of idling,
of course as soon as you get some speed going the plug will heat up and burn off the deposits.
The petrol quality could also affect this; I use Shell V Power since it has extra cleaners in it.

You can also buy a NGK Iridium spark plug, I got the one that's equivalent to the DPR9EA-9,
it has made a slight improvement in smoothness, power, pulling power and less vibration.
Iridium must not be cleaned or the gap changed.
Iridium plugs often cost 2 to 3 times more than a standard plug, but will last longer,
I recommend replacing an Iridium every 7,500 miles (due to the gap getting to large),
that means per mile it works out at the same cost as a standard plug.
The Iridium plug also claims to burn off deposits more than a normal plug at slow speeds,
so could help the high speed plug potential problem.
It also claims to require less power from the battery and ignites the petrol better on cold days.
In the UK, Halfords sell the NGK Iridium plug in their motorcycle section for £9.49

NGK Iridium Spark Plug Conversion chart
DPR8EA-9 (NGK Normal Plug) = DPR8EIX-9 (NGK Iridium Plug)
DPR9EA-9 (NGK Normal Plug) = DPR9EIX-9 (NGK Iridium Plug)

Older engine, points.
DR8ES-L (NGK Normal Plug) = DR8EIX (NGK Iridium Plug)
High speed = DR9EIX (NGK Iridium Plug)

How to Change the Spark Plug

Remove the spark plug cap on top of the spark plug
(just pull it off with a slight twist).

Remove the spark plug with the tools in the bike
(see How to find your bikes Tool Kit and Owners Manual in the Servicing page).

If you have no tools,
you need an 18 mm spark plug tool
that's long enough to go over the top of the plug and reach the nut
(an 18 mm spark plug tool for cars are often to short).
If it's too tight to turn,
take the bike for a ride to get it up to heat or if it's already hot, try it stone cold.

Screw a new plug in hand tight (with no tools), then put the spark plug tool on and tighten it 1/2 a turn.
If you fit a used spark plug,
hand tight (with no tools) then put the spark plug tool on and tighten 1/8 to 1/4 turn (standard plug) or 1/16 turn (Iridium plug),
a turn is 360 degrees.
Put spark plug cap back on (just push on with a slight twist).

< Servicing