1991 Johnson VRO - 70 hp -- RPM issue

[tmljunkmail]

Ok.....this forum had never failed me before so here goes.

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Specs:

1991 Johnson VRO 70hp
1991 Sea Nymph Fish-n-Ski SS175
Old Prop (explained below) 13.25 x 17 OEM
New Prop 13.25 x 19 Hustler
Johnson spec WOT RPM range 5000-6000
Normal RPM at WOT 5400
Normal speed at WOT 33 mph

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Been running this boat for a few years with no problems. Same prop no changes to boat or motor.

The other day I was running some WOT speed tests to replace the OEM prop which was pretty beat up.

Got almost the exact same reading each time (5400 RPM @ 33 mph) with no issues at all. Which, is dead center in the range for specs and that prop.

So far so good........

Later that day I was again at WOT and noticed that the tach had pegged at 7000 RPM (or higher). However, I didn't notice any change in engine sound or performance. The boat was running pretty much dead on to its top speed of 33 MPH.

I figured my tach must be going bad or something.

A few days later I was running it and noticed an issue at lower speeds. For example, at 1/3 throttle I would normally get about 12-15 mph at 3300 RPM. But that day it was sometimes normal and sometimes running up as high as about 5500 RPM at the same speed. Again, no apparant change in engine noise or the 12-15 mph range at the throttle.

If I pushed the throttle over it would either coninue on its high RPM course and eventually peg out above 7000 RPM, but with normal WOT speed. OR it would suddenly kick the RPM back down to the expected 3300 level and rise at its normal pace up to the expected 5400 RPM. Again no apparant change in engine noise or performance.

Today I changed out the prop to the 13.25 x 19 (not because of this issue, but because I had already planned on it).

Same issues as with the old prop. The only difference was that at WOT when things were running "normally" I was getting about 5000 RPM (which I expected given the 2" pitch increase).

In an event, my point is that it does not appear to be the prob.

I spent more time tinkering today and I did notice what I think sounds like a bit oven "over-rev" at the moment the tach jumps from normal RPM to being pegged, but am still getting same performance.

I am NOT an outboard guy as far as knowing all the mechanics of the engine. But I grew up on boats and am pretty handy. As a total NON-outboard guy.....my amature guess would be that I've got a gear or clutch slipping in the gearcase, but I have no way of knowing if that its just a stupid guess.

I'm really hoping that someone doesn't come back and tell me that my lower unit is fried.....but I have an odd feeling that is gonna be the case here.

Does anyone have any ideas for me?

 

[guy74]

Re: 1991 Johnson VRO - 70 hp -- RPM issue

I would suspect your tach is giving you false readings. You would notice a change in rpm that big if it really happened. The tach on my 1988 60hp Johnson acted like you describe when the stator was going out. I also think a rectifier going bad can cause tach problems. The prop I run on my 60 hp gives me a WOT of 6400 rpm if I push it. Beleive me, it sounds like it's turning fast at that speed. You would notice it if your motor was turning that fast.

 

[tmljunkmail]

Re: 1991 Johnson VRO - 70 hp -- RPM issue

I figured I would notice that big of a RPM jump too. But.....it doesn't seem to be JUST the tach (or the tach at all) because I do FEEL something in power when it pegs like that.

So I am guessing that SOMETHING is going on.....

 

[guy74]

Re: 1991 Johnson VRO - 70 hp -- RPM issue

Do you have a voltmeter on your boat? Do you know if the chaging system is working? If the clutch dog slipped in the gearcase it would feel like hitting a log. If it does it with both props can probably rule out a spun prop. Look under the hood, smell for anything that smells like burning wires, or look for goo dripping from under the flywheel.

 

[tmljunkmail]

Re: 1991 Johnson VRO - 70 hp -- RPM issue

The system is charging properly with no apparent issues there.

I don't think I recall anytime when we have had an "unusual" incident with the drive.

I mean.....there have been the occasional bumps with underwater objects.....but nothing around the time it started acting up.

 

[jtexas]

Re: 1991 Johnson VRO - 70 hp -- RPM issue

engine revving but the boat slows, or at least, doesn't speed up.....couple things that can cause it -

1. spun prop - but what's the probability of 2 spun props? you can scribe a mark across the end inner hub & outer, run it then check if your marks line up.
2. cavitation - something interfering with the flow of water over the prop blades - irregularity on the lower unit housing, vegitation, had a little stick caught in the diffuser ring once made me cavitate like crazy.
3. ventilation - air from the surface gets introduced into the prop blades.

You and everyone within earshot would hear a change from 5500 to 7000 RPM - folks standing on the dock half a mile away would hear it.

Troubleshoot your rectifier/regulator - stator/timing base issues - could be the slightest change in sound from some electrical funny business going on, giving you the feeling of a change in power. Heck, could even be a slight change in power, throwing gremlins into the tach.

guy might be on to something with the dripping goo.

 

[tmljunkmail]

Re: 1991 Johnson VRO - 70 hp -- RPM issue

Ya know......I was kinda thinking the same thing.

I had a buddy at the helm when it was doing it so I could look at the lower unit in the water at power.

It ALMOST seemed to me like there was too much water displacement near the lower unit. Then again.....i never looked at it before to see what it was like before I began having problems.

However, just to the totally INEXPERIENCED eye, it looks like the transom/lower unit/hull is creating a heck of a "hole" in the water when it is running at WOT.

Doesn't really explain why such a thing would happen all of the sudden though.

 

[Cricket Too]

Re: 1991 Johnson VRO - 70 hp -- RPM issue

If you didn't change your set up (engine height) and have nothing stuck around the lower unit or have anything bent on the lower unit, since the last time it ran fine, I'd bet you are running exactly the same as you were before.

If you were ventilating you would know it, it is loud as heck and it goes in and out as the prop bites again.

Look at the stator and see if you see any "dripping", also check the stator with a volt/ohmeter and see what shows up. If that checks out fine, and it might since you say your charging voltage is OK, but check that out too to make sure your reg/rec is good, then I'd say your tach is shot.

I does not sound like you are actually going 7,000RPM, like was said you would hear it screaming, plus you would never get to 7,000 RPM without cutting out, I doubt your stator would handle that RPM, and you may even have a rev limiter in your power pack that would prevent that.

Your ears are probably playing tricks on you also, it happens when you listen to an engine, you start to hear all kinds of stuff.

I'm curious as to why you went up 2" in prop pitch to get a 5,000 WOT RPM, that's is lugging your engine pretty hard and is only going to cause further problems. Your 17" pitch prop at 5,400 was Ok, but you could have stood to go a little higher, as close to 6K as you could get. I would drop back to the 17" if I was you, and do a good decarb on it to be safe.

 

[tmljunkmail]

Re: 1991 Johnson VRO - 70 hp -- RPM issue

I was trying to pick up a little top end speed b/c I've got plenty of power for what I need.

Do you think I should have stayed at 17" or maybe even gone DOWN to 15" (which in THEORY would have put me at 5800 at WOT)??

How would I deal with the prop vent if that is the issue?

And no.....I didn't change any setup.

 

[guy74]

Re: 1991 Johnson VRO - 70 hp -- RPM issue

I get 35mph (gps) out of my 60hp with a 15" pitch hustler prop @ 5900 rpm, 28mph @ 6250 with a 13' pitch hustler. This is on a Galstron v 156 tri-hull. I think your tach is giving you a bad reading on the rpm if everything else stays the same.

 

[tmljunkmail]

Re: 1991 Johnson VRO - 70 hp -- RPM issue

I would tend to agree.......unless of course I really am getting venting and its WORSE with the new prop. That might explain the expected drop in RPM but no real increase in top speed.

Do you think I should switch back to the 17" (or even maybe a 15")?

I was really trying to speed it up a bit.....but I don't want to be doing damage to the engine.

 

[jtexas]

Re: 1991 Johnson VRO - 70 hp -- RPM issue

At RPM = 5400, pitch = 17, and speed = 33, you have slip = 15% (assuming your gear ratio is 2.23)
If you go up to a 19" prop, drop 200 RPMs, at the same amount of slip you would expect a top speed of 36, all else being equal.

A 15" prop spinning at 5600 RPM only gets you to 30 MPH. Your 15" prop would have to be twice as efficient (8% slip) just to maintain your present top speed.

Even if you're ventilating, you're not hitting 7000 RPMs - I'm thinking you'll need to chase down your electrical gremlins to have enough confidence in your instrumentation to select a prop.

Do you have power trim? if not, have you experimented with moving the thrust rod out?
Engine height: where is the anti-cav plate in relation to the keel?
Consider weight & balance: to increase speed you need to get the boat out of the water - shifting weight aft might help.

 

[tmljunkmail]

Re: 1991 Johnson VRO - 70 hp -- RPM issue

Ok.....so it doesn't sound like a 15" is the way to go.

Is it only 200 RPM for 2" of pitch change? I thought it was 400 RPM for 2".

If its 400......then the new prop is running at about the correct RPM but speed didn't increase.

If its 200.....the the RPMs dropped about 200 more than expected.....again no speed increase.

Took it out yesterday for a fishing tournament and.......the more I test it and read on here......the more I am thinking that it IS over-reving and power is decreasing.

If I am reading everyones great suggestions correctly, it sounds like either a spun prop or venting.

I assume a spun prop means the splines on the prop shaft are stripped or something. However.....mine are good as new and the turningpoint Hustler prop came with a new hub (#33 for those who care). Plus I had this BEFORE the prop change.

As for venting.....I guess I am starting to think more and more that this is the issue.......but again......had this prob with the old and new prop. I'm pretty inexperienced, but that doesn't seem to make sense.

I guess I don't really know what to think at this point.

I guess I am thinking it is venting. If that's true.....what can I do about it?

I am hoping no one is gonna say I have to scrap the prop (which I just paid $150 for).

 

[tmljunkmail]

Re: 1991 Johnson VRO - 70 hp -- RPM issue

Oh....and to answer a few of the questions....and add some more details that may or may not help.

I do have power trim.....and it APPEARS to be an issue at all trims. Though its possible that it was better trimmed all the way down.

Also, not sure if it matters....but my boat porpoises....BAD on plane. If I trim way down it is better.....but it is a serious bouncer. I thought this was normal for alum boats (see above for boat specs), but I am wondering now if my case is extreme.

Also.......MAYBE this helps as to the issues arising with the old prop....it was pretty beat up (which is why I replaced). I did file it down a bit.....but did NOT reduce the diameter of it. I just knocked off some of the gouges.

 

[jtexas]

Re: 1991 Johnson VRO - 70 hp -- RPM issue

sorry my bad, 400 RPM for 2". changes my estimate of 19" prop to 34 MPH and 15" to 31 MPH. Your gear ratio is probably closer to 2.4 which changes my estimate of slip to 9% but doesn't affect the speed.

The porpoising is a problem....I wouldn't think you were underpowered with 70hp on a 17.5' aluminum - what's the max HP rating on the boat? And compression readings on the engine? Do you have batteries, fuel tanks, livewells or something in the bow? You have to get this under control.

To check the prop for spun hub, scribe a mark across the end inner & outer hubs, run it then see if your marks still line up.

Do you get the same result in calm water as rough?

Engine height: compare the height of the anti-cav plate (motor trimmed vertical) to the keel.

Max RPM spec is 5000-6000 but a lot of folks say she'll be happiest at the upper end of the range. Try to max out your RPMs by adjusting weight & balance and engine height, then prop up to gain speed.

p.s. - theoretical MPH = (RPM x PITCH) ? (RATIO x 1056)
slip = 1 - (actual MPH ? theoretical MPH)

- to estimate speed of new prop:
MPH = theoretical MPH x (1 - slip)

 

[tmljunkmail]

Re: 1991 Johnson VRO - 70 hp -- RPM issue

Ok....that sounds better.

So my RPMs should be dropping to 5000 with the 19" prop. That might be making sense then b/c I am getting only about 34-35 mph (gps).

You are right.....I think it is 2.4

I don't know what my compression is.....that's a project for this weekend.

Boat is actually 16.5 (despite its model #). It is max HP of 90 hp (1600 max pounds).

Don't really have any weight up front.

Have 3 batts in the rear and fishing gear......most if which is aft of midline of boat.

Fuel cell is aft to about midline of boat under the center......about 15 gal.

I do have two livewells up front (under the bow seats) for a total of 20-30 gal.

I tend to fill them up when having this problem and the extra weight helps a little.

However......trimming the motor down (so it pushes the nose down) seems to help the most......but I obviously lose speed and I have to trim down quite a bit.

Either way......my nose bounces like crazy at speed and its not really tolerable.

And yes I get the same result in calm as rough water.

I just figured it was because it is an alum boat and is really light....but its too much if you ask me.

Where should my anti-cav plate relative to keel?

What combo of weight and engine height are gonna help me move up in RPMs?

I assume less weight.....but what about engine height will help increase RPM (up/down)?

 

[jtexas]

Re: 1991 Johnson VRO - 70 hp -- RPM issue

Rule of thumb, anti-cav plate should be even with, up to a couple inches higher than the keel.

Reduce drag by minimizing contact with the water to gain RPMs. Typically trimming up raises the boat - less suface area of hull in contact with the water. Obviously any given hull can only come up so much. Raising the engine gets more of L/U out the water.

Porpoising happens when the engine can't lift the boat up onto the bow wave. Typical of underpowered or overloaded boats, or weight distribution to far aft. I'm assuming you're starting with the trim all the way in to get the best hole shot, then trimming out after you're on plane.

Aluminum boats seem to get blown around more & ride bumpier in waves than fiberglass - I don't know about your bouncing bow. Does filling the livewells improve that aspect?

 

[tmljunkmail]

Re: 1991 Johnson VRO - 70 hp -- RPM issue

Ok......I just thought of something that might affect the analysis.

My boat came with a hydrofoil on it.

From what I am hearing......not so good. I'm thinking about taking it off. What do you guys think?

As for the bow bouncing......I thought that was porpoising.

Yes.....adding water to the livewells does help....a little.

Putting someone up front does too......but I don't always have the extra body for that.

 

[tmljunkmail]

Re: 1991 Johnson VRO - 70 hp -- RPM issue

UPDATE**

Ok.....so to address a few things that I checked out this weekend.

First, it is not a spun prop. Scribe marks were ok after a run on Sunday.

Also, I did remove the dolphin fin and it didn't seem to do anything.

One point of note from other threads that I saw the other day......

......my anti-cav plate was under water at WOT when I ran it this weekend. Am I reading the threads wrong or shouldn't this be out of the water at WOT??

Its submerged about 2-3" when running wide open.