WOT RPM's

[vranasaurus]

The motor is a 1989 90 HP Evinrude.<br /><br />I finally solved my problem with it running higher RPM's(would only pull 4200 with hood on) with hood off(hood latch grounding out the coil). But now I have a new problem.<br /><br />I am only able to get 5400 RPM's no matter which prop I use. I have a 13 3/8" X 17 Stainless prop that I am able to get 5400 RPM's and 35-36 MPH(GPS). I bought a new 13 1/4" X 15 Stainless prop and now I get the same 5400 RPM's at 29-30 MPH. It makes no difference what load I have in the boat I can go by myself or have five guys in it. I should be able to get a lot more out of this set up. The boat itself is a 22' flat bottom aluminum jon boat that weighs about 700 lbs. <br /><br />Here's some info on the motor:<br /><br />I rebuilt the motor.<br /><br />Carbs rebuilt.<br /><br />Spark is excellent on all 4 cylinders.<br /><br />I had a mechanic link and sync the motor(After four days I decided $60 was a fair price).<br /><br />Fuel pump rebuilt.<br /><br />Fresh 87 Octane gas.<br /><br />Aside from this the motor has plenty of hole shot power and idles great. <br /><br />Per the experts on this board I am trying to get into the 5800-6000 range. But something has to be up. <br /><br />I am willing to try anything.

 

[CATransplant]

Re: WOT RPM's

Well, it's clearly not the prop, so I'd put that 17 back on.<br /><br />Does this outboard have a rev limiter of some kind on it? If so, check that out.<br /><br />What kind of tach are you using? That's a long shot that it would be off or refuse to indicate any speed higher than the one you're seeing. Does the engine speed sound the same with both props to your ear?

 

[vranasaurus]

Re: WOT RPM's

After researching this forum I don't think it has a rev limiter, but I could be wrong.<br /><br />I have not verified the tach(Teleflex set to 6p). I don't think it's getting anymore RPM's because the speed dropping like it does with the 15 would indicate it is turning the same rpm's. The engine speed sounds the same.

 

[ddbyrd3]

Re: WOT RPM's

Is this the same tach you had in the boat prior to the rebuild, or did you change it after the rebuild????<br /><br />I am chasing the same problem you are, and I am at the point of verifying that my tach readings are accurate...<br /><br />I go from a 15 pitch to a 19 pitch prop and only gain 100 to 200 RPM... Something isnt right... but I can hear the difference in the motor....<br /><br />If you solve the problem please post the results...

 

[vranasaurus]

Re: WOT RPM's

I installed the tach after my rebuild. I do not want to take my boat to a shop around here because I don't trust these guys. And the mechanic back home says 5400 is close enough because he says 5500 is the target. But something is definately wrong if the R's don't change. <br /><br />What have you tried?

 

[CATransplant]

Re: WOT RPM's

If the RPMs are actually the same with the two props...I mean exactly the same...then the problem lies somewhere in the engine's ignition system, assuming there is a problem at all.<br /><br />There may not be a separate rev limiter on that engine, but there may be a built in limiter in the ignition module. I don't know this for sure, but your constant top RPM of 5400 with both props sounds exactly like a rev limiter kicking in.<br /><br />Since you're hitting a higher speed with the 17 and at the same RPM cutoff as the lower speed with the 15, that's about all it can be.<br /><br />If I were you, I'd check this RPM with a different tachometer. Yours may not be accurate, in the first place, and may be indicating a lower RPM than you're actually achieving.<br /><br />One thing's certain, when you hit this fixed RPM, nothing you do will make the engine run faster, so start looking at the ignition and find out for sure whether there's some sort of built-in rev limiter.<br /><br />All that said, you're doing 35-36mph on a 22' boat at this RPM. Perhaps that's a good speed. You're not going to damage your engine that close to the rated RPM for that engine. <br /><br />Try another tach. If it reads the same, go enjoy your boat and forget all this.

 

[Goodoleboy]

Re: WOT RPM's

Ive got a 87 40hp and it turns 5400 just like your 90. I also have a telflex tach. set on 6 as per inst. sheet that came with tach.<br /><br />we all strugle with good fire and plenty of good fuel. <br /><br />But the other element thats needed to turn more R's is plenty of good air.<br /><br />I wonder if the reason we dont get more RPM's is because we are already pulling all the air into carb's thats easyly avaliable.<br /><br />Perhaps thats why as we drop prop pitch notheing changes. Just because we cant get anymore air into carbs.<br /><br />Mabey we need to work on air restriction problems.<br /><br />Good luck!

 

[vranasaurus]

Re: WOT RPM's

Air is not a problem because mine does the same thing with the hood off.<br /><br />BTW, where in Iowa are you from and where do you boat.

 

[ddbyrd3]

Re: WOT RPM's

Since you changed the tach AFTER the rebuild, hopefully you are a pack rat and you kept your old one. Hook it back up and take it out again and see what you run... I am beginning to think that may be where the problem is...<br /><br />I have the EXACT same problem with my 140... I do not have a rev limiter, all of my ignition componants are new, (literally everything) and I am still having the same result..<br /><br />Check the tach..... I, like a knucklehead, threw out my old OMC tach, talk about learning the hard way!!!!!!

 

[ddbyrd3]

Re: WOT RPM's

 

[vranasaurus]

Re: WOT RPM's

dd,<br /><br />I installed the tach as in I didn't have one before. <br /><br />When I got the motor it had a blown cylinder.

 

[CATransplant]

Re: WOT RPM's

Interesting. Two almost identical outboards with the same 5400 rpm limitation. Same tachometer, too.<br /><br />I think the very first step should be to try another tach on these outboards, just to make certain that they're indicating actual RPM.<br /><br />There's one other thing I found, but it doesn't apply to your exact models, but to the later ones with optical ignition triggers. Apparently, stray RFI signals at a specific RPM, in that case 4900, can cause the internal rev limiter in the powerpack to trigger at that RPM. Wierd, I know, but it's a specific symptom. The solution is to replace the powerpack.<br /><br />I haven't been able to find anything similar for your engines, but it's something to think about. Trouble is: new powerpacks are expensive, so swapping them out is an expensive test.<br /><br />Where's Rodbolt? He's the expert on these engines, and may have some input. I'm just going on the logic of the symptoms. You both have the same limitation on your outboard, which is independent of prop pitch.<br /><br />It's not air flow. The lower pitch prop will require less horsepower to reach that 5400 rpm figure, so the outboard absolutely should rev higher with it on there, all other things being equal.<br /><br />What is fascinating is that you both report the same maximum rpm. I'm betting that this is something with that particular ignition system, something similar to what happened at 4900 rpm on later models.

 

[ddbyrd3]

Re: WOT RPM's

CAT, my motor is a 1981 and my max RPM is 4800 to 5000.... <br /><br />Whats even more interesting is that most of the people reporting low RPM's at WOT are all using Teleflex tachs.<br /><br />I AM NOT insinuating they are bad.... just a coincidence..<br /><br />Also CAT, I replaced my power packs, and I still have the problem...<br /><br />VRAN, you should consider having your RPM verified by a shop tach... Or hook up an OMC tach, thats my next step, but all shops are 3 weeks out....

 

[Goodoleboy]

Re: WOT RPM's

You mite be right CAT....It could be something about the ign. on these motors.<br /><br />But the service manual sure dosent say uh word about it<br /><br />As for me Iam satisfied with mine. I decided awhile back that the tach. was off by 100 and that puts the engine at 5500.<br /><br />Idd rather fish than mess around with it. Iam to old to worry about a couple hundred more revs..Grin!<br /><br />Rus! I live in north east Iowa right on the MN. border and the Mississippi river. Pool nine.<br /><br />Good luck!

 

[CATransplant]

Re: WOT RPM's

There ya go, Goodoleboy! The outboard runs fine...pushes your boat along fine. Go fishing!<br /><br />I understand folks wanting to get the max out of their outboards, but there's a limit to my patience with such things. If the boat planes and goes over about 22 mph, I'm a happy guy. With my ancient Johnson, I don't even have any idea what the RPMs are at WOT. The engine sounds very happy, and I rarely run WOT anyhow. It gets much better mileage at 75%-80% throttle, and I only lose a couple of MPH, so I don't care.<br /><br />Gotta fish, not wrench. I hate grease under my fingernails unless it's absolutely necessary.

 

[Mark42]

Re: WOT RPM's

The rev limiter is built into the power pack on later model evinrude/johnsons. The OMC service manual models lists the rpm range and limit. My 93 40hp won't do over 5800 rpm or so because of the limiter. The suggested operation rpm is 4500 to 5500. So a 5800 cutoff makes sense. What is the listed WOT operation range? <br /><br />Regarding this statement.... "Per the experts on this board I am trying to get into the 5800-6000 range. But something has to be up." Johnson/Evinrude does not agree, hence the RPM limiters on their late model motors.

 

[vranasaurus]

Re: WOT RPM's

Mark,<br /><br />The later models are loopers if I'm not mistaken. It is different technology than my X-flow v-4.<br /><br />It's all about making sure it is running right. I don't need to go any faster I just don't want something to be wrong that will cause the motor to go out prematurely.

 

[CATransplant]

Re: WOT RPM's

That's not going to happen, vranasaurus. You're running at 5400 rpm WOT. That's OK, despite what folks here have said to you. I doubt that your tach is accurate to better than 10%.<br /><br />I'd forget about it and go use your boat with the 17" pitch prop so you can go 34-39 mph, as you stated.<br /><br />Your outboard will be fine. Now, if it were running at only 4400 rpm, you could worry about it, but not at 5400. Just enjoy. Heck, my old 35hp is rated at only 4500 rpm.

 

[CATransplant]

Re: WOT RPM's

Oh, yeah. If your last name is Vrana, I knew a Ralph Vrana when I lived in Los Osos, California. Small chance you're related, but I thought I'd mention it.

 

[vranasaurus]

Re: WOT RPM's

Yes that is my last name. Probably not related. It's a slavic name that means crow. Fairly common in eastern Europe I'm told. <br /><br />I will continue to run the motor but with the 17 back on it. Maybe somebody will come up with a solution and everybody will say Duh why didn't I think of that. Because it is probably something so simple and stupid that nobody is thinking about it.