2003 Optimax 135 - low rpm at WOT

[BartmanMN]

My motor should run at 5000-5500 RPM at WOT. It currently is only running at ~4100 at WOT. While running at WOT (4100 RPM) it does not run steady, sounds like it is going up and down rrrr, rrrr, rrrr.

This motor only has ~68 hours on it and I assume the original prop (but not positive). Any ideas where to start?

Other than this issue there is no hesitation or indication of problems any other place in the power band.

[dudewheresmyboat]

How old is the gas? I had the same issue with my 2003 efi. I just replaced my fuel line and bulb, rebuilt the pulse fuel pump and changed the plugs. My fuel line was deteriorated, I had a fuel leak under load and I got a bad batch of fuel.

Good Luck.

[BartmanMN]

The fuel is Non Oxy from the fall when the previous owner put the boat away. It's a 38 gallon tank, so it's taken some time to work down. I think I am just under half now.

[dudewheresmyboat]

I would start with the basics fuel, spark and compression test. It does sound like a fuel issue to me.

[BartmanMN]

Yeah, I am thinking something to do with the Fuel Pump not reaching the full PSI needed for WOT.

TB

[dudewheresmyboat]

Also check your fuel filter? I'd take it out and dump it into a clear container and look for any contaminants in your filter. I have a feeling something is clogged up very well. Something i've learned in the past few weeks of addressing my problem I don't fill up my tank any more like I use to. I pump fresh gas on my way to the lake and try to make sure I burn it up in a week or less on my outings. These days with all this ethanol I add startron and make sure my gas doesn't sit for weeks on end.

Please post up your results as I am curious to what the issue was.

Good Luck!

[BartmanMN]

Also check your fuel filter? I'd take it out and dump it into a clear container and look for any contaminants in your filter. I have a feeling something is clogged up very well. Something i've learned in the past few weeks of addressing my problem I don't fill up my tank any more like I use to. I pump fresh gas on my way to the lake and try to make sure I burn it up in a week or less on my outings. These days with all this ethanol I add startron and make sure my gas doesn't sit for weeks on end.

Please post up your results as I am curious to what the issue was.

Good Luck!

Will do. Fuel filter is likely something I can take a look at. This is my first boat, so I don't have any experience with outboards. I have printed off the engine diagram, so I can take a look.

[BartmanMN]

Found out my neighbors brother is a certified Mercury mechanic. I am taking him out on the boat tomorrow to diagnose. He said he can hook his laptop up to the motor as well.

[leech~~]

Found out my neighbors brother is a certified Mercury mechanic. I am taking him out on the boat tomorrow to diagnose. He said he can hook his laptop up to the motor as well.

He may suggest dropping down in prop pitch a number or two which will bring up your WOT rpm's.

[BartmanMN]

Prop is an aluminum 19 pitch right now, which he thought should be fine to get to the 5000-5500 rpm. I'll let you know what we come up with.

[BartmanMN]

Took the boat out last night with the computer and found cyclinder #6 not firing. He swapped the coil from another cylinder to 6 to confirm the coil and leads are good. We did not bring the tool box out, so he is coming back tonight to check the plug. I hope it is just a fouled plug. Motor has 69 hours on it and the computer showed that the majority of those hours are at idle.

[BartmanMN]

Plug was fine. Appears to be a stuck oxygen injector on the number 6 cylinder. No air = no spark. Pricing one out today. Shouldn't be a big deal.

[BartmanMN]

Air injector did have quite a bit of carbon build up on it. Soaked it in de-carbonizer for a couple days, which did clean it up, but still is not ticking at all when tested with the DDT. Have to bite the bullet and order a new injector. Everything will be good after that.

[BartmanMN]

I just wanted to close this out in case anyone else with an optimax has a similar issue. It was indeed a bad Air injector on the #6 cylinder. The part is 402.50 from Mercury, but I found a new one on an auction site for $200.

This corrected my issue and the boat now runs at the correct rpms. I was able to hit 47mph last night.

[marine_man]

Thanks for the follow up!

marine_man