WOT 4500RPM Bad?

[apaulos]

Earlier this year, I purchased a used 17ft center console with a 88HP 1993 Johnson.

The motor always starts easily and has a decent idle. The throttle response is good and the boat planes with 2 adults, 1 child and all our fishing gear at about 2500RPM. We cruise at about 3500RPM. WOT is exactly 4500RPM.

The WOT rpm seems low. I don't know the history of the motor and if it was propped this low or if something is out of tune.

Question: since the current performance is acceptable should I just forget about the current WOT RPM or should I go with a less aggressive prop?

 

[rickdb1boat]

Re: WOT 4500RPM Bad?

What kind of speed are you getting? It may be that the Tach. is wrong or set on the wrong setting. I would check that out first. Take a peek at the back of the Tach and see if the dial is set to 6 pulse. If so, I would then try to check it with a known good Tach. If all that checks out, then taking a look at how the motor is set up and the prop pitch would be a good idea. You would want that motor to run around 5800 at WOT with a normal load on. If it doesn't, the engine will be short lived due to lugging. Very hard on those engines when they are run below optimum RPMs...

 

[reeldutch]

Re: WOT 4500RPM Bad?

check compression first before you do anything..

should be above 100 psi and within 15% between lowest and highest cylinder pressure.]
100 psi isnt realy that importand as long as they are even.
if you have 3 of 100 and 1 of 85 i would be concerned.

 

[DJ]

Re: WOT 4500RPM Bad?

What kind of speed are you getting? It may be that the Tach. is wrong or set on the wrong setting. I would check that out first. Take a peek at the back of the Tach and see if the dial is set to 6 pulse. If so, I would then try to check it with a known good Tach. If all that checks out, then taking a look at how the motor is set up and the prop pitch would be a good idea. You would want that motor to run around 5800 at WOT with a normal load on. If it doesn't, the engine will be short lived due to lugging. Very hard on those engines when they are run below optimum RPMs...

If the tach. is correct, you need to look at the set up and possibly re-prop it to turn 5800. What prop. do you have now?

Where is the cavitation plate in relation to the bottom of the hull?

 

[Silvertip]

Re: WOT 4500RPM Bad?

If this is a saltwater boat that is moored in a slip, check the bottom for fowling. That will slow you down a bunch and limit speed. Then look at setup, state of tune, and finally prop.

 

[apaulos]

Re: WOT 4500RPM Bad?

rickdb1boat - I'm not sure. I don't have a gauge or a GPS. At WOT, I estimate about 40+ MPH. I'll check the tach and try running the boat with my external timing/tuning tach. I haven't a clue how to determine the pitch. The prop diameter is 13". I just measured it. I'll try to pull the prop off as soon as I get a chance. I haven't yet pulled the prop yet and I need to check the prop seals. Are props usually labeled on the inner hub?

reeldutch - I performed a couple of compression tests using a cheap gauge I picked up at Harbour Freight. All cylinders were within a couple of PSI and were in the 110LBS range. The readings were the same before and after decarbing using the aux. tank method with SeaFoam. The engine performance is and has been the same since I purchased the boat.

dwj - The current prop has some minor rash I caused by running it a couple of times in sand. Nothing really nasty or something that would cause the WOT RPM to drop 1000RPM+. But I'd like to replace the current prop at some point and 'fix' the WOT issue at the same time. The cav. plate is in line with the hull. I've got another hole to move it up and a few holes to move the engine down. I've given some thought to moving the engine up to the last hole to gain one less inch of draft. I don't believe that the engine is too far down in the water. If anything it is to far up. If I hit full throttle from a stand still, I get some caviation and the WOT shoots up to 5 to 6000RPM. This only happened recently when I was testing the performance, hull shot, plane time, sort of nonsense stuff.

silvertip - The boat is currently trailered but it does have a partial coat of anti-fouling paint. I suppose at one point it was moored full time. The paint is flaking off. So the hull is less than optimally clean but it shouldn't be hurting performance much.

 

[DJ]

Re: WOT 4500RPM Bad?

Good info. Let's find out what prop is on it.

 

[apaulos]

Re: WOT 4500RPM Bad?

I just realized that shortly after getting the boat on the water I pulled off a StingRay Hydrofoil that the previous owner had mounted on the engine. I never performed a WOT run before pulling off the fin.

This just got me to thinking... that perhaps the boat might have been propped to having the fin mounted. The assumption here is that the fin would bring the rpm up. Thinking... the fin might have added 750RPM, a new clean prop another 250RPM, and another 250RPM for tuning/timing being a tad off.

4500RPM(Current WOT) + 750RPM(HydroFoil) + 250RPM(Prop) + 250RPM(Tuning/Timing) = 5750RPM

Wishfull thinking...

 

[Cricket Too]

Re: WOT 4500RPM Bad?

All the fin would do would be to plane you a little faster and give you a little less torque steer, but it would not increase WOT RPM, and definitely not by 750, it may in fact decrease it, as there is more drag.

But it may be an indication that it's not propped correctly. The person before may not have been satisfied with it for whatever reason and stuck the fin on, rather than figure out the what the problem was.

You can take the prop to any prop shop and they can tell you what the pitch on it is. General rule of thunmb is that every pitch size that you drop, you will increase WOT RPM by 200, so I doubt your gonna gain 1300 RPM by just dropping pitch, you'd have to be severely over propped.

What kind (brand) tach do you have right now? I'd try and see if that tach is giving you an accurate reading before doing anything, especially since you say you plane off nice and easy.

 

[baxter66]

Re: WOT 4500RPM Bad?

Flaking bottom paint will make a bigger difference than you think I gained 2 miles an hour by sanding mine down to the gelcote again.d

 

[apaulos]

Re: WOT 4500RPM Bad?

Got to thinking and reading...

The OMC manual for the motor lists WOT 4500 to 5500RPM for the 85,88,90,100 and 115.

Last season my top speed with fishing gear, half a tank of gas with 2.5 passenger was 37mph measured w/ a GPS. I just did the WOT timing via the "Reeves" method and it was 4 degrees (20 vs 24) off. The actual WOT timing for the motor is 28 degrees.

After a link and sync also noticed that the WOT stop screw needed to be reset since the carb roll pins were past vertical. I'm going to try a few WOT passes to check on top speed and RPM's after the work I just completed next time I'm out. Hopefully, I don't blow the motor