Johnson OB RPM increase with loss of thrust?

[jimn]

A friend called to ask about their boat, which made a strange sound after running for about ten minutes, so they went back to the slip. I went out with them yesterday and noticed it not long after we left- it started easily, never stalled and every time it was started during that trip, it fired right up.

The issue- at about 3200 RPM, it intermittently jumps to around 4600 RPM and seems to lose some amount of thrust but returns to normal after reducing the RPM and increasing it again. The first time, I noticed that the motor was trimmed up a bit, so I had the driver trim it down farther, but not fully. It didn't repeat the problem until we passed through the wake from another boat and as before, it returned to normal after reducing the RPM.

We never needed to squeeze the primer bulb after I checked it before we rant it the first time, so I can't see how it would be a fuel issue, especially since the RPM increased. We took a fuel sample last Fall and at the beginning of the season- smelled normal and was clear, with no evidence of water or phase separation.

This isn't the original motor, but I was told that it never did this before. It wasn't run last year and it was used little during the previous season but judging by the condition of the spark plugs and general appearance, it was maintained fairly well. No water in the gear oil, either.

My thoughts-

Since the RPM will increase normally, I don't think it's an internal motor or gear problem. I didn't look at the prop before we went out, other than to see that it's stainless and looks to be in good condition but that doesn't mean it hasn't hit anything. That possibility leads me to my next idea- maybe it DID hit something and the hub has been damaged. I don't know the brand, but if it has a plastic insert, I have usually seen them fail to propel a boat normally, so maybe it's rubber.

Maybe the prop nut had backed off in the past and damaged the spline or the bronze washer- if it's barely digging into the spline, maybe it slips and then jams itself against good metal after the RPM reduction.

I haven't seen the model or serial numbers, so I haven't looked for a manual, yet. I should be getting this info tomorrow.

Does this sound familiar?

Thanks for any info

 

[racerone]

Prop hub lost grip.----Try another prop.

 

[jimn]

Thanks- will tell them to change it.

 

[jimn]

I looked at the prop- one blade is bent and is missing some material from the tip. Two of the other blades have very minor scrapes at the tips, but aren't bent. Someone hit a rock and didn't say anything- it wasn't this year, though.

It's stainless, so the cost to repair is far more than a new aluminum prop.

 

[jimn]

Well, they got a new prop with the correct diameter & pitch- did the same thing. He said it improved in some ways, but the symptoms remained.

Does anyone have the mounting height for this motor? IIRC, the anit-cavitation plate is supposed to be about 1"-2" above the V of the hull, but this boat is a 26' Regal LeizureCat, which is basically a fiberglass pontoon boat.

https://www.smartmarineguide.com/L49829843

 

[fdrgator]

It's ventilating. Some would call it cavitating. Whatever you call it, it is getting air pockets around the prop.

 

[jimn]

It's ventilating. Some would call it cavitating. Whatever you call it, it is getting air pockets around the prop.

That's our working theory- what is your opinion on the mounting height I posted (plate should be 1"-2" above the lowest point of the hull)?

Thanks

 

[fdrgator]

Sorry, I don't have enough experience with pontoon boats to give an opinion on that.

 

[jimn]

Yeah- pontoons are a different animal. I'm waiting for a photo with the motor trimmed so it's vertical- that will show where the anti-cavitation plate is WRT to the hull.