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Mercury Optimax help quick please!

Old 06-30-2020, 08:02 PM
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Default Mercury Optimax help quick please!

I've posted this on 2 other forums, so please excuse if you see this again.. just trying to get some quick help.

2008 Optimax Pro XS 225 with 480hrs.

I just got a prop out of the shop and took the boat to test it out today. Started right up like usual, ran it for a few hours most of the time at WOT since speed is what this prop is supposed to accomplish. Not a hiccup, turned full RPM, ran perfect.

Anyway, go to power load on the trailer. Hammer down, and motor bogs down and wont go over about 1500rpm. Literally doesnt have the power to push the boat on the trailer. So I take it back off the trailer.... Go to run it again, and sure enough, jumps right out of the hole and once again runs wide open no problem. Go to get back on the trailer, same problem. Wont go over 1500rpm. Take back off trailer, run it even longer, screams like a new motor, then same thing happens when trying to power on the trailer. Weirdest thing I've ever seen!

Only thing I've done diagnostic wise (and I BELEIVE this might be the issue, but want to double check): I put my coil tester on and tested all 6 coils.. The #5 coil doesnt light up as bright as all the others, so I assume this may be my issue. However, the reason I ask is because I've already replaced 2 coils, but when those went bad, they didnt light up at all and the boat wouldn't even get on plane. #5 coil still lights up, but is much dimmer than the others.

Fuel filter is only a few weeks old.

Taking the boat out of town this Thursday which is why I'm trying to get some quick input before I buy a coil.

Any other ideas throw em at me. Thanks everyone
Old 06-30-2020, 09:42 PM
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Maybe it went into docking or trolling mode?
Old 07-01-2020, 04:42 AM
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You keep saying runs Wide open, what is your wide open throttle RPM?

What pitch is your propeller?

it’s a whole different load on the engine when increasing RPM’s while the boat is moving through the water VERSUS using the engine and prop having to move water with the boat stationary (as in loading onto a trailer)
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Old 07-01-2020, 04:45 AM
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A lot of time an outboard won’t spool up power loading on a trailer. Trim the motor up until it can build RPM
Old 07-01-2020, 05:24 AM
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The OP knows his own boat. Something isn't right.
Old 07-01-2020, 05:43 AM
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Thanks for all replies.

WOT is 6000rpm, 25p 4 blade prop.

I'm also beginning to wonder if its prop related, because everything ran fine for a long time with my back up 3 blade prop. I'm going to throw the other prop on and see what happens then go from there. It just wouldn't make sense because I ran that 4 blade since I got the boat until I messed it up

On the other hand, anyone on here think that my coil diagnosis sounds right?
Old 07-01-2020, 05:44 AM
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Maybe something is getting kinked when you tilt motor up to power load on trailer. Could be fuel line or throttle cable.
Old 07-01-2020, 05:47 AM
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Originally Posted by leonreno View Post
Maybe something is getting kinked when you tilt motor up to power load on trailer. Could be fuel line or throttle cable.
This was also suggested on another forum I'm on pretty often.

I'm going to give that a check, but I dont think that's the issue because when I'm running wide open down the water, I'm trimmed up to the same level that I am when loading up.

Old 07-01-2020, 05:48 AM
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Originally Posted by fritz423 View Post
The OP knows his own boat. Something isn't right.
I’m sure he does, which is why he stated “Not a hiccup, turned full RPM, ran perfect.”

If the engine “turned full RPM, ran perfect.” And the prop was the only change made, which he said was adjusted for Speed (assuming higher pitch) I would start looking at the prop.

Look at a High Performance boat like a Fountain Lightning, they will use 700+ HP engines to turn a 32 pitch 6-blade cleaver BUT they have to introduce air to the prop at low RPM’s in order to slip the prop so the engine RPM’s can increase. Picture Below.




Old 07-01-2020, 06:05 AM
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Originally Posted by kranch07 View Post
I’m sure he does, which is why he stated “Not a hiccup, turned full RPM, ran perfect.”

If the engine “turned full RPM, ran perfect.” And the prop was the only change made, which he said was adjusted for Speed (assuming higher pitch) I would start looking at the prop.

Look at a High Performance boat like a Fountain Lightning, they will use 700+ HP engines to turn a 32 pitch 6-blade cleaver BUT they have to introduce air to the prop at low RPM’s in order to slip the prop so the engine RPM’s can increase. Picture Below.


I'll try my other prop before anything else. Thanks for the reply
Old 07-01-2020, 08:10 AM
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Alright well figured out the problem

Anyone who said it's a propping issue, beers for you.

Went down to my local ramp... backed it in with the 4 blade (the one I had fixed) and it was doing the same thing I mentioned. Wouldn't go over 1700rpm on the trailer.

Pulled out, threw my old 3 blade on, and I'll be god danged if it screamed like a new motor.

I repeated this process 6 times just to make sure it was the issue. Sure enough, everything runs perfectly fine with my 3 blade.

So now that we know this, heres the only I can think of.. is it possible that the prop is slipping on the shaft? My guess is when the motor feels that it's not under a load and not pushing, it wont rev over 1500rpm.. so the only thing I could guess is that the inside of the 4 blade is worn out and not getting a grip on the shaft
Old 07-01-2020, 08:19 AM
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It is possible for a “spun hub” in the prop. But you would notice that if you were out on the water running the boat.

That engine doesn’t have any way to sense load and reduce RPMs. If it neutral it will limit RPMs but if in gear it will allow max rpm (If the load on the prop is not higher than the engine can overpower)

What pitch is the 3 blade? What pitch is the 4 blade?

Last edited by kranch07; 07-01-2020 at 08:26 AM.
Old 07-01-2020, 08:23 AM
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Prop is getting too much grip and not letting the prop slip and allow the engine to get higher up in rpm's where it makes more power to push the boat on the trailer.
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Old 07-01-2020, 08:29 AM
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Originally Posted by xlr8ngn View Post
Prop is getting too much grip and not letting the prop slip and allow the engine to get higher up in rpm's where it makes more power to push the boat on the trailer.
Nailed It!

introduce some air to slip the prop and then the engine will be able to turn more RPMs in turn making more power.

I don’t remember exactly what the RPM range it is but Optimax seems to have a power band around the 2,500 rpm to 3k rpm area
Old 07-01-2020, 12:14 PM
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4 blade is a 23p, 3 blade is a 25p
Old 07-02-2020, 05:46 AM
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Vent holes in the 3 blade prop?
Old 07-05-2020, 06:16 PM
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Originally Posted by waytooslow View Post
Vent holes in the 3 blade prop?
Yes sir
Old 07-05-2020, 06:28 PM
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Does the 4 blade have holes?
Old 07-05-2020, 08:01 PM
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Originally Posted by leonreno View Post
Does the 4 blade have holes?
no sir it does not. If I remember tomorrow I'll post some good pics of each prop so everyone can see them


I ran the 3 blade all weekend with no issue, just loss of top end like the 4 blade has

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