What’s considered "Sustained High Speed?"

[johnyrude200]

Looking at the service manuals for the 40-70hp motors, there is a section with recommendations on spark plugs, alternatives, and then ones used for ‘sustained high speed.’

Well, what would be considered sustained high speed….WOT for more than 5 mins?

[fleetwin]

sustained high speed running might be considered for using the engine for racing, a lot of water skiing, etc…
Evidence of an engine that is used extensively at high speeds might be excessive electrode wear.
Don’t know about the 5 minute time, that alone would not be considered sustained high speeds. It is more a function of the percentage of total run time that is spent at WOT, so I am thinking a percentage of at least 60% might be considered sustained high speed use….

[outbdnut2]

If you idle along trolling a lot, don’t go to the colder high speed spark plugs, or they may foul after awhile while trolling. I agree with Fleetwin’s percentage comment. I’ve had to go one or two heat ranges colder on the plugs on 6 to 15 HP motors run on my grandson’s 8 foot hydroplane,but there, the motors run at RPMs they were never designed to go to.
Dave

[chris-p]

When I restore a motor for someone, that is one of the things I ask, to install the correct plugs.

Some guys only idle up and down rivers fishing.

Others only use it at full WOT operation, say to go from point A to point B all the time.

Different plugs would apply to boater A and boater B obviously.

[johnyrude200]

All good info to have and points brought up I hadn’t really taken too much consideration for, other than deciding between a JC4 or JC6 or JC8 depending on the motor (i.e., the ’71-76 22 c.i. 18-20-25’s that blubber at the low end — hotter plugs for the trolling types).

My personal stake in this question is a ’93 70hp; I have been running fixed gap plugs because I am usually at WOT going several miles when on the ocean or big lakes. If it’s OK to switch back to the QL77JC4’s, I’m fine with going back to those. This particular motor is one that Fleetwin and I have had several discussions about and I’ve found when warm idles just fine, when cold, has to be trimmed almost out of the water to prevent a stall at low RPM’s.

Wondering if switching back to L77JC4’s might help the situation.

Ran into a ’97 50hp motor recently with the same issue — having difficulty getting it to run above 100°.

[chris-p]

Interesting problem!

Trying to think of how the trim angle would affect anything…..

1. Possibly an exhaust leak of some kind?
2. Fuel/Float issue rectified by the angle?

The plugs may help, but not sure how that would solve a problem that you are having….strange.

[johnyrude200]

The loopers sufocate if trimmed too low and the problem is amplified by a cold running motor. In my fairnly limited experience working on these ’92-thru motors, the newer you get the worst the issue. The 40-50hp motors tolerate it better, the 3-cylinder 60-70hp motors not so much.

The pre ’89 motors had fixed jet carbs that just seem to be so simple and good running. Post ’88 things just get more ridiculous the ‘newer’ you get.

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