spark plug question

tpeever · Aug 3, 2016

Finished restoring a '68 Moto Guzzi V700 and a '76 BMW R75/6 over the past few years and have been comparing the spark plugs recommended for those bikes versus my Commando. Recommended modern plug for both bikes is NGK BP6ES. In my Commando, the consensus recommended plug seems to be BP7ES and I have always run this grade of plug with supreme, no ethanol fuel and plugs look fine. It seems to me like the cylinder heads are not fundamentally different on all these bikes so why the difference in plug recommendation? Is it just randomness in the translation from whatever plug was originally recommended (ie. Bosch to NGK versus Champion to NGK) or should the Commando really have a cooler plug? I think we had a similar discussion previously and some folks here were running BP6ES in their Commandos without ill effects so why BP7ES and not BP6ES?

comnoz · Aug 3, 2016

tpeever said:
Finished restoring a '68 Moto Guzzi V700 and a '76 BMW R75/6 over the past few years and have been comparing the spark plugs recommended for those bikes versus my Commando. Recommended modern plug for both bikes is NGK BP6ES. In my Commando, the consensus recommended plug seems to be BP7ES and I have always run this grade of plug with supreme, no ethanol fuel and plugs look fine. It seems to me like the cylinder heads are not fundamentally different on all these bikes so why the difference in plug recommendation? Is it just randomness in the translation from whatever plug was originally recommended (ie. Bosch to NGK versus Champion to NGK) or should the Commando really have a cooler plug? I think we had a similar discussion previously and some folks here were running BP6ES in their Commandos without ill effects so why BP7ES and not BP6ES?

I just depends on the cooling ability of the particular cylinder head. Norton heads tend to run hot.

If you run a BP6ES in a Norton then you must jet very rich to keep from overheating the plug. Jim

Rohan · Aug 3, 2016

comnoz said:
I just depends on the cooling ability of the particular cylinder head. Norton heads tend to run hot.

Its more really that BM and Guzzi heads in particular run a little cooler than average,
their heads sticking out into the breeze and all that.
Although this is the ole glass half full or empty viewpoint again.

If Commando heads were similarly out in the breeze, they also could probably run a grade cooler plugs.
Vertically stacked V4 anyone ?

P.S. If you read the manuals carefully, many makers once suggested warmer plugs around town,
and cooler plugs out on the highway.
Once-apon-a-time, owners would carry such sets of plugs, and even swap them as required.
Its a sign of development that one size fits all these days, and for quite some decades past...

o0norton0o · Aug 3, 2016

Rohan said:
Its more really that BM and Guzzi heads in particular run a little cooler than average,
their heads sticking out into the breeze and all that.
Although this is the ole glass half full or empty viewpoint again.

If Commando heads were similarly out in the breeze, they also could probably run a grade cooler plugs.
Vertically stacked V4 anyone ?

P.S. If you read the manuals carefully, many makers once suggested warmer plugs around town,
and cooler plugs out on the highway.
Once-apon-a-time, owners would carry such sets of plugs, and even swap them as required.
Its a sign of development that one size fits all these days, and for quite some decades past...

I believe you meant to write "hotter" where I underlined your quoted post.

I've run BPR6ES when I was trying to run a larger main jet. I had a cockamaimie idea that I could run a hotter plug and a richer mixture with a little more advance to get more power, while also getting some cooling from the evaporative effect of a richer mixture. What I got was a bike that stopped accellerating just above 80mph. I switched back to the spec'd plugs and jets, retimed the bike and it zoomed past 80mph.... If you have a stock bike, the stock settings generally work the best...

I do carry a pair of spare spark plugs, and I carry the hotter BPR6ES, figuring if I'm having some fouling issue, I have a better chance of limping home on a hotter plug...

spark plug question

tpeever

comnoz

Rohan

o0norton0o

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