Lower unit problems or is it normal behavior?

[Radarcontact]

Hi.

I have a 1986 “Tower of Power” in-line 6, 90hp Mercury (Model number 1090726).

Been having issues with the prop “blowing out” at about 75% throttle, so my friend John, a Charter boat Captain, dropped by today to take a look at the motor height, prop, etc. for me.

In looking, feeling, etc., he discovered something and thought it might possibly indicate a lower unit issue:

• When you put the throttle in full forward (motor off) and (standing facing the motor) turn the prop to the right CW it “clicks” like a ratchet wrench. Turn it to the left CCW it holds tight.
• See pic

He wasn’t sure, and didn’t want to scare me, but thought it might be something bad. , he said it could be a gear issue, or maybe something wasn’t put back correctly after a water pump changeout, etc.

The prop hub seemed to be ok since it would lock up turning CCW and also in reverse both directions.

Before I do anything else or take it anywhere I wanted to see if this is normal for this motor, or if this indicates any one thing in particular that someone might know of.

Any help will be greatly appreciated!

Thanks

// Radar //

 

[racerone]

A charter boat captain ??-------That is absolutely normal for it to click one way in forward and to lock up the other way !!

 

[QBhoy]

What you describe with the prop is totally normal. I’m amazed that your friend didn’t know such a thing.
your issue is likely ventilation or similar. Caused by lots of things from trim position,engine height etc to prop suitability

 

[Radarcontact]

Thanks for the quick feedback!

He’s an everything Captain, really, lol. He’s running the Destin Florida dredge operation, among other things right now. I’m guessing that driving the big boats is his forte... keeping a 42’ rig 2 feet from the dock against the wind and swells feeding through the Destin Pass while guys are loading up for 20-30 minutes is something I wouldn’t want to do. I don’t expect him to know everything, but he’s really helped me out tons with buying the boat and I appreciate him doing so. That said, that’s why I checked here to see what you all had to say... go to the guys who have the specific knowledge you are needing! Forums are a life (and wallet) saver.
So, thank you very much! Much respect.

Reference the ventilating problem... that’s exactly what we thought too, wrong motor height. I think it’s too low. I.e., from everything I’ve been able to read about pontoons and motor height, the cav plate should be right even with the rear of the transom tunnel.
Am I correct with that, or no? It’s a two-toon not a three, btw. I think that maybe makes a difference as well.
Mine sits about 4” or more below that. Pic attached.

Again, thank you for the knowledge and information. I surely appreciate it all!

// Radar //

 

[Radarcontact]

What you describe with the prop is totally normal. I’m amazed that your friend didn’t know such a thing.
your issue is likely ventilation or similar. Caused by lots of things from trim position,engine height etc to prop suitability

A charter boat captain ??-------That is absolutely normal for it to click one way in forward and to lock up the other way !!

John deals more with the big diesels. He’s gonna swap out one of his for this one after hurricane season.

 

[Faztbullet]

Motors way to high..needs to be flat on transom and bolt holed redrilled...

 

[racerone]

Extension kit is another option.

 

[Radarcontact]

Motors way to high..needs to be flat on transom and bolt holed redrilled...

Can you elaborate a little on that conclusion, please. Spec, or reference height to use? It’s about 4”+ lower than bottom of transom tunnel right now... how much lower are you thinking?

 

[Radarcontact]

Extension kit is another option.

I’ll research those. Would a simple jack plate be a good option as well?

 

[racerone]

Look for ----bay manufacturing ----For a kit.----There are many things to look at on these installations.-----My concern would be the design of the structure / quality of the welding.----Aluminum is known to crack from flexing / poor welding.-----Motors have been know to fall off pontoon boats.

 

[Radarcontact]

Look for ----bay manufacturing ----For a kit.----There are many things to look at on these installations.-----My concern would be the design of the structure / quality of the welding.----Aluminum is known to crack from flexing / poor welding.-----Motors have been know to fall off pontoon boats.

My transom itself is bulletproof. I paid a boat builder to fab it for me out of aluminum to replace an old wooden one that was really sketchy. However, that said, the transom tunnel is what came with the boat so yes I totally understand that. The builder put the motor exactly where is was on the wooden transom — I hadn’t run the boat so there was no reason to think it was in the wrong place/height.

 

[racerone]

Sorry I could not tell from post #4 that it was built that way.-----I say the motor is mounted too high.----Posting a video of boat / motor while the issue occurs might help.

 

[Radarcontact]

My son actually took one while they were here on vacation last week... I’ll see if I can get it posted!

 

[Radarcontact]

Sorry I could not tell from post #4 that it was built that way.-----I say the motor is mounted too high.----Posting a video of boat / motor while the issue occurs might help.

Here’s the video. If you wait till the very end at around the 16-second mark you’ll here it ventilate then I immediately throttle down.

https://youtu.be/VPiEWQ_irK4

HOWEVER...
We took the boat out on Saturday I took my buddy John’s advice. He said, “Once you get away from the dock, trim the motor all the way down and underneath as far as possible, and then hit it full throttle. See what happens and let me know.”

I did, and it ran like a champ! Never got the cavitation/ventilation problem at all. In the past I never tried to throttle past the “problem”, I always backed off. But also, this time there were 3 people vs 6, and we were all sitting in the back, vs 3 up front and 3 in back last time.

prob got up to 25 mph? It was choppy so I didn’t hold it at top speed for too long, maybe 4-5 minutes then slowed down due to the water roughness.

I’m still perplexed, but I’m NOT COMPLAINING!

 

[flyingscott]

Here’s the video. If you wait till the very end at around the 16-second mark you’ll here it ventilate then I immediately throttle down.

https://youtu.be/VPiEWQ_irK4

HOWEVER...
We took the boat out on Saturday I took my buddy John’s advice. He said, “Once you get away from the dock, trim the motor all the way down and underneath as far as possible, and then hit it full throttle. See what happens and let me know.”

I did, and it ran like a champ! Never got the cavitation/ventilation problem at all. In the past I never tried to throttle past the “problem”, I always backed off. But also, this time there were 3 people vs 6, and we were all sitting in the back, vs 3 up front and 3 in back last time.

prob got up to 25 mph? It was choppy so I didn’t hold it at top speed for too long, maybe 4-5 minutes then slowed down due to the water roughness.

I’m still perplexed, but I’m NOT COMPLAINING!

Perplexed about what?? Your motor is mounted to high!.

 

[asdasc]

Easy fix is to only go out with 6-10 people, mostly standing or sitting in the back!

 

[Texasmark]

In F gear you have to have some kind of mechanism to allow the engine to slow down when the boat is at speed and the water pressure of the prop moving past/through it causes it to spin faster than the proportional engine rpms. This is similar to a transmission Power Take Off on a tractor where you have a mower running and decide to reduce the rpms on the tractor. The mower will continue to turn (kinetic energy stored in the blades) the PTO and drive the tractor and won't let you shift out of gear....dangerous situation.

On the boat it would act like a brake and, having had an engine freeze on me at WOT once, it could give you a mouth full of windshield....but I didn't have one and that saved the day.

So, an "overrun clutch" is installed on the rear of F gear and on the front of the shifting "Clutch Dog" that locks F or R gear to the prop shaft. It's saw toothed shape so that in the reverse direction it will spin, moving in and out about ⅛", making a clicking noise, and locking hard to the crankshaft in the opposite direction. Since the engine is usually doing the drive and in testing you are using the prop as the drive mechanism, CCW locks in and CW ratchets.

R isn't used at any speed to amount to anything so Merc. uses a vertical cog and slot mechanism which has to be lined up exactly to engage and once engaged is locked in both directions. For this reason you should never attempt to shift into R unless someone is rotating the prop as you move the shift lever, or the engine is running. The service manual says you can damage you shifting mechanism if you try to force it otherwise.

Go boat and have fun.

 

[Radarcontact]

Perplexed about what?? Your motor is mounted to high!.

I touched base with my boat builder guy to fab up some nice motor mounting brackets for the transom he built. Then I can move the motor (up or) down with minimal effort. He’s gonna give me a quote. It’ll be less than store-bought, strong as hell, and totally custom for my boat.

 

[Radarcontact]

I touched base with my boat builder guy to fab up some nice motor mounting brackets for the transom he built. Then I can move the motor (up or) down with minimal effort. He’s gonna give me a quote. It’ll be less than store-bought, strong as hell, and totally custom for my boat.

$250. I’ll drop it off next week after I get back in town.