What spark plugs should I be using

[Oldfrt]

I have a 1978 GS750 with an 850 10.5/1 big bore kit and Wiseco pistons. What spark plugs should I be using?

[rphillips]

My opinion, Orig. NGKB8ES will be fine, If anything, maybe, one step cooler B9ES. Just thinking higher compression could possibly create a little more heat and get the plug slightly hotter, 1 step cooler may get the plug temp back to orig. temp.

[Oldfrt]

The previous owner fitted the original 850 kit and rode the bike as a daily commute until it's first life expired with a hole the size of 5 cents melted into the number 2 piston. My eldest son advised me of an opportunity to purchase the bike which was in boxes and in bits! He and I did the rebuild some years ago. My memory is not that reliable, but I believe we fitted B9ES plugs originally and that at a subsequent servicing B8ES were fitted. On a recent servicing I had a little difficulty getting B8ES plugs and in a discussion with my son, he suggested that I should probably be running a step cooler plug!! Thanks for your confirmation. In the interim I did manage to source some B8ES and bought 2 sets!!

[rphillips]

Oh, I didn't notice a new member... Welcome, and congrats on getting that thing back together. How'd you do it without our help?

[Oldfrt]

Thanks for the welcome. This site has a lot to offer, great resource!
Bought my first bike, a Honda 90 scrambler in 1968 followed by a Honda CB350 the following year and did my first engine rebuild the following year....had quite a few bikes since then...My son has aquired the same love of bikes and cars, he is also one of my best mates... so that build together was something special

[Nessism]

The hole in the piston was most likely caused by detonation. Be sure to use premium fuel with that big bore kit.

[LAB3]

Thanks for your confirmation. In the interim I did manage to source some B8ES and bought 2 sets!!

It sounds like NGK isn't making non-resistor plugs anymore, if you eliminate the resistors in the spark plug caps you can run the BR8ES plug in it's place, it has a 5k ohm resistor built right in.

[rphillips]

Spark plugs have always been kind'f complicated for me. Always wondered if you swap the B8ES fore BR8ES or the BR8ES for the B8ES, or even mix them in the same eng. would there be any noticeable difference in how the eng. runs?