Idle and Carbon buildup question

[hoeser]

You may have seen my posts relating to the recent rebuild of my 1984 Evinrude 140 Crossflow v4. One of the rings let go due to carbon buildup. The more I read about carbon buildup the more I am concerned about my WOT RPM. Pre-rebuild we could get the engine around 5400-5500 with two people in the boat. It is on a 1970 Searay SRV190. With 4 or more people in the boat it was more like 5200-5300rpm.

I am not sure of the prop specs. Is this high enough or should I be closer to 6000rpm wide open? I have not run WOT yet post rebuild.

Hypothetically speaking if you were making a 30 minute trip across open water... would you build up more carbon at wide open than say, 75% throttle? I'm confused. I always thought I was being "easier" on the engine running it at 75% but it sounds like I may be doing more damage that way.

 

[LubeDude]

Re: Idle and Carbon buildup question

It's pretty well agreed upon here that becaus of todays fuel quality, or lack there of, that it's better to run between 5,500 and 6,000 RPM. You might want to lower your pitch to the next lower size.

You will get less carbon build up at higher RPM, but you shouldn't be getting much at 75% either.

If you are worried about carbon buildup, use a good quality synthetic oil, and run some Seafoam in your fuel.

 

[Chinewalker]

Re: Idle and Carbon buildup question

It's not so much that you actually run it at 5500-6000 all the time, but rather that it be CAPABLE of doing so. If the absolute most your motor will reach is 5200-5400 or so, then it is lugging and working harder to get there as it WANTS to be able to reach higher revs but can't.

Think of it this way, your car or truck is geared to keep the engine in its power band where it is most efficient at highway speeds. Add a trailer and a hill and keep it in 5th gear and what happens - ping city! It's working its guts out trying to keep up. Drop it down to 3rd and the motor is much happier. It's turning more RPM but isn't working as hard to do it. No more pinging, either...

- Scott