What is your prefered spark plug gap?

[huntingdog]

Lets scratch our tongues.. um fingers on the keyboard a bit .
In the owners manual it is stated that the spark plug gap range is 0.8-0.9mm ( 0.031-0.035), while in the service manual it is stated 0.7-0.9mm ( 0.028-0.035), as I recently discovered.
As spark plug gaps tend to widen over time I always have put the the fresh set of plugs at the lower end of the given range ( so 0.8mm for the ST1100 as per the owners manual).
As far as I know, bigger gap gives a bigger spark, but can cause misfires at high rpms and puts more stress on the ignition coils.
Smaller gap gives a smaller spark, and maybe not as good of a burn to some small % , but the above mentioned problems aren't present. With the ST1100 I tend to ride around 3-3.5k rpm like 95% of the time. So maybe a bigger gap will give a bit better torque?
Has any one experimented with this, any thoughts? Do you follow the service or owners manual gap specs?

 

[jfheath]

I've never adjusted mine. I check them when I out them in new and when they are out and they are always the same - 0.9mm But I do change them periodically, so 'appen I don't give them chance to burn away.

Anyway, it's not more torque that I need. Less weight would be a much better result. I'm feeling quite noble at the moment cooling down after a hot Zwift session around Harrogate in the garage. So I think I'll say 'get thou behind me mr spark plug gap adjuster', and stop trying to tempt me with a different way of getting an increased power to weight ratio. It isn't going to work. I'm not going to give it another thought.


Hmmmm.....

So does it work the other way ? Does more torque make you feel lighter ?

 

[W0QNX]

Well mine didn't change gap any with 66,000 miles. So I check them out of the box and don't really "set them". Per NGK a -9 plug should have a factory set gap of .0354" (.036").

My spark plugs at 66,246 miles | ST1300 Related

I changed my NGK CR9EH-9 out with the same new plugs. These were in the bike the last 66,246 miles. The bike has 364,598 miles total now. It looks like #4 is getting some carbon in there. I also looked down the cylinders with a cheap bore scope type thing. The #4 does have some carbon around the...

www.st-owners.com

https://tro.bike/wp-content/uploads/2015/06/ngk-spark-plug-chart.png

 

[SMSW]

I've never read that race mechanics tune their engines by adjusting the spark plug gap a few thousandths. This would make a great dyno project. Strap your bike to one, install brand new plugs gapped to the minimum spec and run a few tests. Repeat with progressively larger gaps. I'd think several runs at each gap, averaged would be necessary. Let us know what you find out.

 

[rwthomas1]

Old school hot rodders used to index spark plugs so that the gap faced the combustion chamber in wedge shaped chambers or the intake valve for the incoming charge. Used soft copper washers, and swapped plugs around to achieve this. Not sure it made any difference at all, but it is amazing the lengths some will go for the slightest gain in power.

 

[huntingdog]

I always tend to check plugs when new before installation. Sometimes they are out of spec or from a set of 4 plugs ( the same) 1 or 2 don't have the same gap as the other 2. Maybe that is just my luck.
I have seen plugs with a gap that is way bigger than factory on neglectet cars. If you keep up with regular maintenance this shouldn't be a problem.

This was interesting.
Maybe on newer cars the tolerances aren't as critical as on older vehicles.

 

[Erdoc48]

I switched out my plugs on both STs for the Iridium type (NGK) and those specifically indicate not to change the gap (I guess because you could damage the plug) so I just swapped them without measuring (I think they were to be set at the factory)- bikes run fine.

 

[motornut]

Comparing racing engines with production engines can be very misleading. For instance the engines at the 24 hours of Daytona probably last about 25 to 26 hours before being torn down. The other end of the spectrum is dragster engines which life is described in seconds. Even Nascar engine are expected to only last to the end of the race, in past times (the 70s and into the 90s) when they raced the Charlotte 600 engines began to come apart shortly after 500 miles.

 

[huntingdog]

Today I experimented a little bit. Pulled the plugs out and regaped them from 0.8 to 0.9mm. Put them back in and went for a short ride when the rain stoped. It maybe my imagination but I think the bike runs a bit smoother.. I have to ride some more. In the end no harm done and I spend a little time in the garage and that is always good.
Cheers!

 

[Igofar]

I switched out my plugs on both STs for the Iridium type (NGK) and those specifically indicate not to change the gap (I guess because you could damage the plug) so I just swapped them without measuring (I think they were to be set at the factory)- bikes run fine.

Just keep you fingers crossed that you didn't install fake knock of NGK plugs...

Buyer Beware *NGK Plugs* | ST1300 Related

I've had three bikes come through the garage in the last month that had either the wrong plugs, or fake NGK plugs that were purchased on Amazon. Be careful what you put in your bike.

www.st-owners.com

 

[Erdoc48]

Good call…no, mine were the real deal- I’ve had those in several years already…when I put them in the Silverwing, I recall I think on the plug that it said Made in Japan as well as I had the suspicion about knock offs as well.

 

[motornut]

imagine how much of a pain in the butt to make phony plugs. There is a manufacturer somewhere making the damn things. And investors with shipping and office workers while making a profit. what the hell, why not just make good ones?