Ariel square 4 misfire

[simon.holyfield]

Well well well,
the plot thickens!
After more investigation of the ignition system I have worked out the actual problem. Fake NGK spark plugs. I had replaced my last set when I serviced the bike less than 100 miles ago and all looked/worked fine. The problem gradually became apparent with engine misfires within that mileage but assumed it couldnt be the plugs. After stripping out leads for continuity and looking for damaged outer lead covers I eventually replaced my older plugs. Problem gone!!
I just checked the NKG web site which covers counterfeit copies, and the plugs I removed showed two distinct giveaways.
1 - no batch number on the hexagon flat of the plug and 2, - loose sealing rings on the threads.
Last time I purchase plugs on ebay.
- The seller has now been informed
Just took the bike for a 12 mile test ride - back to its normal smoothness and acceleration with no misfire. (phew!)

Good news Mark, and a cautionary tale for us all.

I've been on Bosch for a couple of years now, after some good advice from a well known member!

[Colinbrindley]

Apologies for a thread hi-jack could I ask what Bosch plugs Rob and Simon are using.
Cheers Colin

[paul.jameson]

I am a little cautious about giving Bosch plug recommendations as I find I am working from a 1996/97 Bosch brochure and they appear to have changed their codes/labels a bit since then. That said, in my 1936 (37 model year) 1000cc Square Four I was running W9AC plugs until I went to electronic ignition when I changed to WR8BC plugs. You need to understand the coding system to appreciate the differences. So: W is all about the plug thread size and the type of plug seat. For Ariels with 14mm spark plugs, the first character will always be W. (For early Ariels with 18mm spark plugs, the first character would be M). Next comes R, or no character, as the case may be. The R stands for Resistor. If you are running magneto ignition, you should not use a resistor plug or, alternatively, a suppressed plug cap. I ran with a brass tag on the end of a copper cored HT lead so it was metal from magneto to plug which is the correct set up for a magneto. No,m I didn't have problems in the wet because I have a set of period insulators which go between brass tag and plug to keep the water off. (They are bakelite and best described as like a miniature upturned plant pot with the drain hole going over the screw terminal of the plug.) I have never seen another set so the alternative is an unsupressed waterproof plug cap. But for coil or especially electronic ignition, a supressor of some kind is essential, hence the use of resistor spark plugs. Next comes the number. For a pretty oily Ariel engine you can get away with using a 9 plug. A good engine would use a 7. I am using 8 because I have just rebored the engine so there will be more oil about until the rings seat properly as they run in. The next character is A or B. On an A plug, the centre electrode protrudes 1mm below the bottom of the plug thread. On a B plug the centre electrode protrudes 3mm, taking the spark point further towards the centre of the combustion chamber. The late Bob Brassington thought that having the spark as close to the centre of the combustion chamber as you could get it was a good thing and he was very probably right in this, although I can't tell the difference. So A or B is interchangeable to me and I choose what is available. The final character, C, is the electrode material which in this case is copper. Copper works well enough on Ariels and so will anything else, although alternative materials might make the plug more expensive.

I hope that helps Colin. If not, come back with another post telling me what model and year the bike is, what the ignition system is and what condition the engine is in and I will tell you what I would use.

[Colinbrindley]

Paul its a 4G 1000cc 1948 Ironhead, magneto. Full rebuild 1000 miles ago.
Cheers.

[simon.holyfield]

Apologies for a thread hi-jack could I ask what Bosch plugs Rob and Simon are using.
Cheers Colin

Colin, I'm using W8AC.

[paul.jameson]

So Simon's W8AC would be fine for your 1948 4G Colin, as would W8BC.

I forgot to add in my posting yesterday that the information I outlined covers short reach plugs only. For long reach plugs it is the third character which changes. So the equivalent in long reach plugs to Simon's W8AC would be W8CC and if you want the 3mm protrusion into the combustion chamber it would be W8DC.

If I could post the explanatory chart I would do but my scanner is on the blink at present.

[paul.jameson]

Miraculously, I have persuaded the scanner to work:

[simon.holyfield]

Interesting that you can get multiple electrodes. I wonder what they are specifically for. Anyone know?

[robjameson]

I’m running WR7BC on my 1948 4G. Bike runs perfectly when hot but misfires cold, which I need to get to the bottom of, I think it is carburation, never considered plug temperature setting, is it worth going up to an 8 or 9?

[paul.jameson]

Knowing your engine Rob, and knowing that it now smokes a little, I would go straight to WR9BC or WR9AC.