Generator question - Spark-plug

[D_Ville_TX]

I have modified my 13HP generator to natural gas fuel and have a question regarding spark plug selection.

Engine is a GX390 clone that used NGK BPR6ES plug on gasoline fuel.
I understand that combustion temperature is higher with natural gas since it will not provide any cooling like gasoline does. The compression has also been increased from 8.5:1 to approx 10:1 to take advantage of higher octane fuel.
Spark gap has been decreased to 0.025" to more reliable ignite the fuel.

Should I use a cooler spark-plug after these modifications?

The plug stays clean, so I have no color indication to go by.

 

[kcfx4]

my thoughts would be, youll be ok

 

[Slick17601]

You could go to a BPR7ES if it would make you feel better. The 7 is one stop colder than the 6.

 

[BRIGUY]

How did you change the compression ratio? Did the Kit come with a new piston or head?

 

[D_Ville_TX]

I did not buy a kit. Just a zero governor regulator, and fabricated a load block and modified existing carburetor.

Total cost in parts about $70.00

Compression was increased by milling down the head 0.10" (2.5mm).

 

[BRIGUY]

O/K that makes sense. You must be a machinist or know a good one. $70 is cheaper than a kit from US Carb. But your fab work is still worth quite a bit. Thanks.

 

[TedT]

I don,t know if you need to go cooler, but I think that no matter how cool you go, the plug won't foul because the gas burns so cleanly.