Which plug

Do they make that spark plug in different heat ranges? With different compression ratios,, you might want to try them. My thinking is; the higher the compression the colder the plug needed.
 
This the NGK 7 plug you use for plate motors ? ThanksView attachment 8713
ND W16 FS-U plug is a higher heat range plug that we use in our small plate flatheads.
Denso offers a similar plug in that heat range for the clones that we've used successfully, but if I run them back to back on the dyno, the 3910X plug outperforms it every time. Granted it isn't as susceptible to fouling as the colder 3910X plug.
Now, I don't build many clone engines at all, and my carb set-up may be a big reason that my results differ from some others.
FWIW, I like to trim the ground strap back and bend it over to make it a side ground to the electrode. Gap around .040." I can't tell you if this is worth your effort or not - it's just something that we did on the old flatheads for many years so I incorporated into the very few plate clones that we have built before just plugging the gapless plugs in.
 
Try an NGK B9EG. They are marginally shorter than a standard NGK plug so clear the piston well if you are running a heavily skimmed head (80 thou or so). I recently used them on a 14+ hp GX160 motor that went to 8500rpm and they worked a treat.
 
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