Re: 1974 Evinrude 9.9 won’t idle at low RPMs, need help

[Casey Lynn]

quote FrankR:

The UL-81J is a booster gap plug. That is, the spark jumps a gap within the plug before getting to and jumping the gap at the tip. That forces the voltage up, which helps it to fire a fouled plug. I’m not familiar with the NKG’s, so not going to make a comment or choice there. Matter of fact, I don’t have up-to-date experience on those engines at all. Other guys here do. But an awful lot of these grey hairs were gotten while trying to make them run back in “the day”.

Okay….a couple of you seem to be stuck on NGK plugs. Quit drinking the Kool Aid. The engine was designed around a Champion plug. Read Frank’s post again and this time absorb it.
An auxilliary or booster gap as Frank calls it is the key word in this sentence. Does the NGK have this feature? Have you checked?
This aux gap FORCES the ignition system to produce more voltage to jump the AUX gap than is needed to jump the plug gap. This means you will have an extremely hot spark at the plug gap…..which is exactly what this engine needs as far as the ignition system goes.
Still believe your issue is fuel related and not ignition but at least this will put the ignition system to bed.