Correct spark plug for Roatx 399 - Just to document. - Snowmobile Forum: Your #1 Snowmobile Forum

steve03052 · 11-14-2005, 06:58 AM

I'm not looking for responses, I just want to document the problem I was having along with the solution so maybe I can save someone else some pain.

The sled in question is a 1972 399 Olympic. The machine constantly fouled plugs, hated to idle and was generally a reluctant runner - always took lots of pulls to start, very sensitive to over oiling its gas. A real pain in the butt.

It had been professionally rebuilt after a carb fire with new wiring harness, coils, plug wires and plugs. It had B7HS plugs in it (according to the manual). It has a Tillotson carb, not sure if it is original. I had tried cleaning and adjusting the carb with no real difference in the way it ran.

I found another one that is a real runner. Always starts, never complains, even with a Tillotson carb and year old gas. The difference - the plugs! They were B8HS.

Changed the complainer over to B8HS, it started on the first pull! What a difference 1 heat range made. So for reference, use B8HS plugs in a 399 Rotax!

**smallengineguy** · 11-14-2005, 07:13 AM

The odd thing is that you said it always fouled plugs....B8HS is a COLDRER plug than a B7HS. In theory, you should be fouling them more than ever......

steve03052 · 11-14-2005, 08:07 AM

Beats the crap out of me. Maybe its something to do with the length of the electrode. It might have something to do with the factory gap. I didn't check the gap because I don't have a spec for it.

Here's NGK's take on numbers and heat rating http://www.ngk.de/Heat_rating_and_he..._nu.657.0.html

I don't understand why, I just know it worked.

HiLLbEllYcustom · 11-17-2005, 03:52 PM

yea my 73 399 ran b8hs, never a problem