Still 500 RPMs short, Merc 2+2, VIDEO

[Mader12345]

What engine I have and how it performs:

I have a year 2000 125HP mercury outboard. Serial #:0t088785. Since last season, she won't run more than 4500-4600 RPM (engine rated for 5250 RPM, a few years back I would get to 5000). She runs just fine at the speed, great at idle and slow speed (top 2 cylinders). I did rebuild the carbs 2 seasons ago. Rebuild meaning soaked them, pushed a small poker through every orifice, new caskets. There was zero gunk in them.

What I've done to engine in last week:

Replaced all fuel lines and bulb from tank to top carburator. Redid all fittings/barbs. Replaced fuel pump (the entire pump), replaced accelerator pump (the whole pump). Did link and sync per factory manual. Carb plates are wide open at full throttle. Confirmed spark on all spark cables (1/2 gap, blue spark). Reset low speed timing and max timing per manual. Verified compression at 115 PSI for all cylinders (cold start).

Today's lake test:

Started up fine. Idle and low speed fine. bottom carbs kick in and get me to the same 4500-4600 RPM. In other words no change. Then I pulled plugs and sprayed seafoam right into cylinders and also into the carbs. Ran her for a while, no change in performance.

Then I pulled the spark wire on the bottom cylinder and grounded the boot. Then ran her and she topped at 3000 RPM. Replaced spark wire to bottom cylinder and removed spark wire on the 3rd cylinder. Restarted, she topped at 3600 RPM, a 600 RPM difference. As a reminder this engine runs on the top two cylinders till the RPM gets to ~1800 RPM, then the other two join in.

Here's another fact: I ran the engine with the plate that covers the carbs off. I didn't realize you should not do this above 3000 RPM. Too late. But i did find that the engine would get to 5000 RPM that way. I wouldn't call it a stable 5000 RPM. It sometimes dropped to 4700 RPM or so.

Here is a link to a video I took starting at idle and slowly adding throttle until it tops out. Point of clarification in that the throttle can still be pushed forward a few more degrees when it tops at 4500-4600 RPM, but the engine does nothing.

IMG 0911 - YouTube

I haven't checked any electrical except for spark gap test when cranking.

Would love some input.

MArk

 

[TD_Maker]

Re: Still 500 RPMs short, Merc 2+2, VIDEO

Hmm. I have a few questions. You say a few years ago you got it to turn 5000 RPMs. Is the boat heavier? Do you keep it outside and perhaps it could have absorbed some water? Did you install new batteries or equipment to add weight? What does the boat bottom look like? How about the prop? Good shape?

The motor sounds good. I would try to borrow a prop (with smaller pitch) and see if that helps. If you are turning a 19 pitch, try to get a 17 pitch.

 

[Mader12345]

Re: Still 500 RPMs short, Merc 2+2, VIDEO

Yes the boat is heavier than the 5000 rpm days. I put a larger rear deck in. The stern does lay a half inch more under the water. I don't know what sized prop I have put I will check. Could it be that "simple"?

Forgot to mention above that priming the bulb at wot has no effect.

 

[Mi duckdown]

Re: Still 500 RPMs short, Merc 2+2, VIDEO

Pics of rear deck would help. SO you were never at max WOT.???

 

[Mader12345]

Re: Still 500 RPMs short, Merc 2+2, VIDEO

Before I rebuilt the deck (the whole boat actually) I don't remember exactly the rpms I maxed at. It was at least 5k. I can tell you that the engine would lose power at wot at that time after several mins of running. I assumed fuel starvation so I replaced fuel lines, hoses, etc at that time. I did it again, and replaced the pumps, this time.

 

[TD_Maker]

Re: Still 500 RPMs short, Merc 2+2, VIDEO

Yes, it could be that simple. Somewhere on your prop there is a size stamped on it. May be stamped on the prop body, or it may be stamped on the hub. You should be turning AROUND 5500 RPMs with your motor. Rule of thumb is for every size in pitch you drop, you will gain 200 RPM's.

 

[Mader12345]

Re: Still 500 RPMs short, Merc 2+2, VIDEO

Thanks. I have a 19" prop and I just ordered a 17". Note that my engine is rated up to 5250 RPMs, so I shouldn't ever be hitting 5500, as you say.

Wouldn't mind entertaining alternative theories. I've been down this road before and I'm not convinced the prop is it.

 

[TD_Maker]

Re: Still 500 RPMs short, Merc 2+2, VIDEO

You are right. The 2+2 version of the Mercury 125 is rated at 5250. The Optimax 125 is rated at 5750 . Nevertheless, the 17" Prop will put you in the ballpark.