# Motor Won't Reach Operating RPMS



## danthemanx07 (Oct 7, 2007)

I have a 1991 140hp Johnson that will not make it to its top end of 5500-6000 RPMS. Here are the things I have checked:

1. Fuel lines between tank an engine are fairly new. I replaced them 2 years ago, and all fittings are good and tight.

2. High speed jets are clear.

3. Good spark. I checked this when the motor was cold before putting it in the water if that matters.

4. Compression is an even 90 on all 4 cylinders. Not sure if that is where it should be though. I used two different gauges, one read all at 60psi and the other all at 90psi, so I am assuming 90 is correct.

When I throttle up, the boat will power up and plane fine, but it tops out at 30mph and 4000 rpms.

Any thoughts? 

Thanks!


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## kanaka (Oct 3, 2007)

Did it ever reach those rpms before?
Have you decarboned it? 90psi is kinda low. http://www.wmi.org/www/boating/boatboard/t14563.htm
Are the throttle plates opening up all the way at WFO?


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## cbigcarl (May 28, 2009)

The 90 psi sounds like its worn but not worn out. The 60 sounds bad.


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## danthemanx07 (Oct 7, 2007)

kanaka said:


> Did it ever reach those rpms before?
> Have you decarboned it? 90psi is kinda low. http://www.wmi.org/www/boating/boatboard/t14563.htm
> Are the throttle plates opening up all the way at WFO?


Truthfully, I bought the motor used and hung it myself about a year and a half ago, and have only been out with it on there a handful of times since then. I don't think it has ever reached the proper RPMS. Sadly, I have not had the time to really dig in on the problem. I have been just using it as is because it operates just fine for my purposes. I know this isnt good long term though...

I did pull the carb air silencer off and spray Seafoam directly into the carbs while running. I will check the throttle plates to make sure they are opening all the way, I have not done that yet. I'll check out the link too.



cbigcarl said:


> The 90 psi sounds like its worn but not worn out. The 60 sounds bad.


I am not sure I trust either of the readings really. The motor was hung on an old Aquasport that a fella was rebuilding that never reached completion. It was near spotless under the cowling when I bought it. I don't think there are many hours on the motor.


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## kanaka (Oct 3, 2007)

If you're real lucky, the prop is the wrong pitch for your application then. Too much, can't reach full rpms.


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## danthemanx07 (Oct 7, 2007)

kanaka said:


> If you're real lucky, the prop is the wrong pitch for your application then. Too much, can't reach full rpms.


Prop is 17 pitch.


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## Ocean Master (May 20, 2008)

Are you running around a 14x17 prop. That should be about right. The compression should be around 150 psi but yours being equal at 90 should be OK.


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## danthemanx07 (Oct 7, 2007)

Ocean Master said:


> Are you running around a 14x17 prop. That should be about right. The compression should be around 150 psi but yours being equal at 90 should be OK.


Close to that. 13.25" diameter.


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## Ocean Master (May 20, 2008)

After the work you have done it may very well be the prop. If you can find one to use try a prop with 15" pitch. I believe the stock prop size for that motor is around 13 5/8" x 17" pitch. But it all depends on the boat and weight.

I'm assuming that it runs very well since you have neen using it. What kind of boat do you have? Is the old Aquasport still around?


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## danthemanx07 (Oct 7, 2007)

Ocean Master said:


> After the work you have done it may very well be the prop. If you can find one to use try a prop with 15" pitch. I believe the stock prop size for that motor is around 13 5/8" x 17" pitch. But it all depends on the boat and weight.
> 
> I'm assuming that it runs very well since you have neen using it. What kind of boat do you have? Is the old Aquasport still around?


Luckily I have a 15 pitch aluminum prop that I can try out. I would be surprised though if the change brought me 1500-2000 more rpms though. I thougy ht the general rule was 200 rpms per point of pitch. I'll give it a try though.

It runs amazingly well at all the rpms that it will reach. Starts easily for a 21 year old motor. It is on a 1972 Mako 19. The 19 is a shallow dead rise design. Not sure about the fate of the Aquasport. It was a nice 20 foot model. If I had been in the market at the time, I would have considered the whole package.

I am likely going to pull off the carburetors tomorrow and order rebuild kits unless someone thinks this would be a bad idea/waste of time.


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## cbigcarl (May 28, 2009)

If a motor is down on power, a prop change will make a bigger change than 200 rpms


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## kanaka (Oct 3, 2007)

More food for thought.
Aluminum props tend to loose shape therefore lose pitch/cup after extended use.
Are you sure your tach is correct? If so check this out.....

http://www.mako-boats.com/past-model-specs/view-specs.cfm?modelyear=1992

FYI, I have a 18' bay boat that SHOULD weigh about 1500 with all the crap I put in it with a Yami 90 2 smoke powering it. Max speed measured with GPS is 37. Boat speedo says 47 but sitting in the driveway, speedo right now says its doing 20 mph.......


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## danthemanx07 (Oct 7, 2007)

cbigcarl said:


> If a motor is down on power, a prop change will make a bigger change than 200 rpms


Lower pitch in theory should provide more thrust, but it can only increase the RPMS so much (based on the physics of the motor and propeller relationship). The motor operates fine at all rpms as said above. It should just be capable of 1500-2000 more RPMS. I really don't think a 15 pitch propeller is going to get me there, but if I try it and it does, y'all can say I told you so...


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## danthemanx07 (Oct 7, 2007)

kanaka said:


> More food for thought.
> Aluminum props tend to loose shape therefore lose pitch/cup after extended use.
> Are you sure your tach is correct? If so check this out.....
> 
> ...


Prop is stainless. The same hull making 5100rpms from a 1977 Johnson 140hp did 36-37mph before it died. Same prop. 

Mine is 19*72* hull. Weight listed at 1400lbs.

I think I am going to buy a new tach to cover that base, just in case, but I think that it is correct.


One symptom that I forgot to mention because I was not sure if it was related - when I removed the air silencer, it had fuel in the bottom of it like fuel had leaked out of the carbs or something.


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## cbigcarl (May 28, 2009)

danthemanx07 said:


> Lower pitch in theory should provide more thrust, but it can only increase the RPMS so much (based on the physics of the motor and propeller relationship). The motor operates fine at all rpms as said above. It should just be capable of 1500-2000 more RPMS. I really don't think a 15 pitch propeller is going to get me there, but if I try it and it does, y'all can say I told you so...


If your motor is down on power(say 50 hp for example) and you make it easier for the motor to turn, the motor will turn more rpms.


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## danthemanx07 (Oct 7, 2007)

Gotcha. I'll try the lower pitch prop and see what happens.


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## murfpcola (Aug 28, 2012)

If you can borrow another compression gauge I would do that just to see if it reads the same as one of yours. Some tachs have different ?pole? settings that are for different motors depending on the signal that particular motor sends out. Maybe yours is on the wrong setting?


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## kanaka (Oct 3, 2007)

Think the fuel in the silencer is just from the motor "burping", common in 2 strokes. At WFO, have you tried to give it a little choke to see if it speeds up?
And just to muddy up the water more... http://forums.iboats.com/showthread.php?t=283363
Look in the section " Testing fuel system...... for vacuum". Maybe :confused1:


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## Ocean Master (May 20, 2008)

I have a good compression tester you can borrow.


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## danthemanx07 (Oct 7, 2007)

murfpcola said:


> If you can borrow another compression gauge I would do that just to see if it reads the same as one of yours. Some tachs have different ?pole? settings that are for different motors depending on the signal that particular motor sends out. Maybe yours is on the wrong setting?


Pole setting is correct per the factor service manual at 6.



kanaka said:


> Think the fuel in the silencer is just from the motor "burping", common in 2 strokes. At WFO, have you tried to give it a little choke to see if it speeds up?
> And just to muddy up the water more... http://forums.iboats.com/showthread.php?t=283363
> Look in the section " Testing fuel system...... for vacuum". Maybe :confused1:


I have not tried the choke at full throttle. Wish I had read this before I went out to it this morning... Thanks for the link to the article. 



Ocean Master said:


> I have a good compression tester you can borrow.


Thanks for the offer. I am over in New Orleans though. Guess I should change my profile.

UPDATE - Rebuilt the carbs with Evinrude kits. Popped them back on. Ran the boat this morning. Boat runs even better all the way up to 4100RPMs. It is exceptionally smooth all the way through the RPM band. Ran from an external tank with ETHANOL FREE gas.

BUT I was still only able to get up to 4600RPMS this time by trimming the motor up at WOT.

I also changed the prop - put on a 15 pitch to see if anything improved. NO change whatsoever. I at least expected to see a 400 or so RPM increase, but I didnt even get that. This seems to further suggest to me that the problem is not prop related.

Any other ideas besides vacuum? I am running short. Think I will start looking for a mechanic...


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## murfpcola (Aug 28, 2012)

I am just grabbing for straws here but I had a 48 Special Evinerude several years ago. It began running bad and when I took it to my mechanic he found that the little roler on the carb linkage that advanced the timiing had dry rotted. The new roler had a kind of clear looking "tire" around the black plastic center. When the "tire" fell off the timing was not being advanced as much as it should be. From what I remember though, it did not run good up to a certain rpm like yours, it ran bad any time you hit the gas. As I said, I am grabbing for straws here, probably not offering helpful advice. Good luck and tell us how it turns out in the end.


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## Patriot (Jun 4, 2009)

With motor in neutral, will engine rev up above 4600RPM? If so, then you are getting good fuel, air and spark. Your throttle linkage may be out of adjustment

If not, it can be a few things still.

Could be you are dropping a cylinder. Each cylinder can account for approx 1000 rpm. the top two cylinders kick in at WOT. if they are not firing, can only be spark, fuel or air.

Weak coils, bad spark plug wires and usual suspect. You can check coils with a multi meter or go buy a new coils, plugs and wires if they are old. engine vibration can cause weak electrical components to wig out. Can also be a problem in your power pack. Check all wires coming out of powerpack for good connections. Look at back side of powerpack for signs of overheating. 

There is a screen in the engine fuel pump that can get clogged up. Because the carbs needed cleaning indicates the motor sat for a while and gummed up, you need to clean out the lines and replace the engine fuel filter also. 

Can also be reeds are dirty and not allowing proper air flow. That can be remedies by spraying a full can of motor decarb through the engine. let it sit for 15 minutes and then run it again until the smoke stops. 

Try these things. If it doesn't improve, time to go see your local engine mechanic.


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## danthemanx07 (Oct 7, 2007)

murfpcola said:


> I am just grabbing for straws here but I had a 48 Special Evinerude several years ago. It began running bad and when I took it to my mechanic he found that the little roler on the carb linkage that advanced the timiing had dry rotted. The new roler had a kind of clear looking "tire" around the black plastic center. When the "tire" fell off the timing was not being advanced as much as it should be. From what I remember though, it did not run good up to a certain rpm like yours, it ran bad any time you hit the gas. As I said, I am grabbing for straws here, probably not offering helpful advice. Good luck and tell us how it turns out in the end.





Patriot said:


> With motor in neutral, will engine rev up above 4600RPM? If so, then you are getting good fuel, air and spark. Your throttle linkage may be out of adjustment
> 
> If not, it can be a few things still.
> 
> ...


Thanks for the tips guys.

Tom - I'll get back over to the boat and try those things. Thanks for the detailed post. I had been thinking about ignition bugs and the fact that 1000rpms would be about on for one cylinder not running. Everything on the motor is original I think.  I'll check the coils, but I may go ahead and order new wires. Have new plugs ready to go in too.

When you say motor decarb, what kind of product are you talking about? Good ole spray carb cleaner, like Gumout? Or something different, like Seafoam in a spray form?


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## DOUBLE "D" & LV (Oct 2, 2007)

DO NOT use carb cleaner or such on a two stroke spraying it through the carbs. You will wash away all the oil and could blow up the engine. Sea foam is limited on how it will decarb the engine. There are sprays specifically for decarbing an engine. CRC, Sierra, Mercury, and the other engine manufacturers have the spray. It is called carbon free, engine tuner, power tune, engine decarb and such. It has lubricant and will not harm the engine. It is best to let it set for a minimum of an hour after spraying it into the engine through the carbs. You can let it set overnight if you want to also. Please do not rev the engine to high rpm's in neutral. It does not diagnose anything because there is no load on the engine and it plays hell on the rods and bearings when you do it. It is not designed to do so with no load on it.


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## Patriot (Jun 4, 2009)

Double is correct on decarb. West Marine sells engine decarb spray designed for outboard engines. Follow the directions on the can....i.e warm up engine, spray full can into engine while running until it chokes and dies, let sit for specified period of time and the then start up and run until smoke clears. 


A momentary and immediate relaxing of throttle to determine is shift to throttle linkage is causing the engine to not rise above 4600 rpm will not harm the engine. I mention this to direct attention to linkages. You can do this without engine running and watch how the linkages interact when in neutral and in gear. If you have a large tank/plastic barrel to put the lower unit in water you can run it up in gear momentarily also. Shops take off prop and put on a weighted test wheel to load engine. 

There are stops screws that allow the linkages to be adjusted. You also need to look at how the throttle body linkages are manipulating the plates. They need to be all the same and in the dorrect positions per the throttle linkage position. 

The adjustment of these linkages is refered to as a link and sync. Unless you are familiar with outboard mechanical work and have service manual, you probably need to have a outboard technician do this procedure.

Go through all the fuel, decarb and electrical stuff first before messing with linkages. The electrical system on the older evinrudes have always been their weakness. The powerpack sits on top of the block. Take it off and check the backside of the unit. If it is bulging, then it has overheated and is most likely your culprit. Powerpack, coils, wires are your primary focus.


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## danthemanx07 (Oct 7, 2007)

DOUBLE "D" & LV said:


> DO NOT use carb cleaner or such on a two stroke spraying it through the carbs. You will wash away all the oil and could blow up the engine. Sea foam is limited on how it will decarb the engine. There are sprays specifically for decarbing an engine. CRC, Sierra, Mercury, and the other engine manufacturers have the spray. It is called carbon free, engine tuner, power tune, engine decarb and such. It has lubricant and will not harm the engine. It is best to let it set for a minimum of an hour after spraying it into the engine through the carbs. You can let it set overnight if you want to also. Please do not rev the engine to high rpm's in neutral. It does not diagnose anything because there is no load on the engine and it plays hell on the rods and bearings when you do it. It is not designed to do so with no load on it.


Thanks for chiming in Dave. Glad to hear from you. If the boat were in Pensacola, I would have already called you on it :thumbsup:.

I am not revving the motor in neutral. The 4600RPMs is the highest the motor will reach while running. I'll look for some of the specific spray you mentioned.


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## danthemanx07 (Oct 7, 2007)

Patriot said:


> Double is correct on decarb. West Marine sells engine decarb spray designed for outboard engines. Follow the directions on the can....i.e warm up engine, spray full can into engine while running until it chokes and dies, let sit for specified period of time and the then start up and run until smoke clears.
> 
> 
> A momentary and immediate relaxing of throttle to determine is shift to throttle linkage is causing the engine to not rise above 4600 rpm will not harm the engine. I mention this to direct attention to linkages. You can do this without engine running and watch how the linkages interact when in neutral and in gear. If you have a large tank/plastic barrel to put the lower unit in water you can run it up in gear momentarily also. Shops take off prop and put on a weighted test wheel to load engine.
> ...


Thanks for the info Tom. Link and Sync had crossed my mind as well. I have a FSM, but I would probably pay to have it done. It looks like something that would benefit from a professional touch.

I'll run down the other things though first as you suggest.


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## danthemanx07 (Oct 7, 2007)

Patriot said:


> Double is correct on decarb. West Marine sells engine decarb spray designed for outboard engines. Follow the directions on the can....i.e warm up engine, spray full can into engine while running until it chokes and dies, let sit for specified period of time and the then start up and run until smoke clears.
> 
> 
> A momentary and immediate relaxing of throttle to determine is shift to throttle linkage is causing the engine to not rise above 4600 rpm will not harm the engine. I mention this to direct attention to linkages. You can do this without engine running and watch how the linkages interact when in neutral and in gear. If you have a large tank/plastic barrel to put the lower unit in water you can run it up in gear momentarily also. Shops take off prop and put on a weighted test wheel to load engine.
> ...


Alright. Got out and did some things as suggested today.

Used Mercury Decarb spray as directed.
New plugs.
2 new coils (all I could get locally).
Pulled the fuel pump apart, but there was not screen in there anywhere. FSM doesnt show one either. Must not have had it on this engine.

Spark plug wires are lost in the mail. Will have to try again on that one. I did not check the power pack as directed. I forgot my to-do list, so I was trying to go by memory.

Put the boat in the water and ran it - NO CHANGE.

So, my last hopes (before linkages) are replace the plug wires, replace last two coils, and check power pack.


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## Patriot (Jun 4, 2009)

You are following pretty standard troubleshooting techniques. Patience and attention to details will get you there.


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## danthemanx07 (Oct 7, 2007)

Patriot said:


> You are following pretty standard troubleshooting techniques. Patience and attention to details will get you there.


I know. Patience is not my virtue...

It wouldn't be so bad if the boat were not 40 minutes away. I can really only get one trip over there a week, so it is a slow process. 

I'll cross my fingers for the wires, remaining two coils, or the power pack (not so much here).


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## danthemanx07 (Oct 7, 2007)

kanaka said:


> Did it ever reach those rpms before??


I wanted to respond more thoroughly to this question. Maybe it will help maybe it won't. I thought it was a fluke, so I didnt think too much of it. Let me see if I can explain, and y'all can tell me if it is relevant.

A few months ago I bought a prop off craigslist. Guy said it was for a v4 Evinrude, and I took his word for it (my mistake). I did not inspect it very thoroughly either (another mistake). Anyway, I got to the boat to put it on and noticed the funny configuration of the back side of it would not accommodate the thrust washer. Believing this to be maybe just a newer design, I put the prop on with the thrust washer on anyway. I figured the thrust washer would prevent any lower unit damage, and if the prop was incorrect, I would know it pretty quickly. Pictures are worth a thousand words so here are a few.

Typical Evinrude v4 prop:









Evinrude prop and thrust washer (disregard toasted hub):









New prop and thrust washer:









No workie (or so it would seem):









Anyway. When I used this poor combination (the first time), the results were remarkable - RPMS up to 5500-5700, boat speed up to 36-39. I was ecstatic.

The second time I used this combination - COMPLETE FAIL. Boat wound up like I had spun a hub. High RPMs but going no where. The symptoms were exactly those of a prop sitting too far out on the prop shaft and allowing exhaust to escape around the prop. With this combo, the prop did not quite sit down into the lower unit like my Evinrude props did. I pulled the prop off and switched back to a known good Evinrude to the dismay that is this thread.

SO - TWO QUESTIONS
1. Is this relevant, helpful, or useful information at all?
2. Does anyone need a Suzuki/Johnson 4-stroke propeller 19x13, 13-spline hub? (what the propeller turned out to be)

Thanks!


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## DOUBLE "D" & LV (Oct 2, 2007)

That hub looks like it did spin. If you changed the thrust washer and the splines are the same, then it should fit your engine. From what you're saying, the engine did get up well with the different prop (until it spun). I would pursue the prop issue. You could take it to a prop shop and they could see if they think the hub is spun and press in a new one. They should also be able to get you a thrust washer that would fit.


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## danthemanx07 (Oct 7, 2007)

DOUBLE "D" & LV said:


> That hub looks like it did spin. If you changed the thrust washer and the splines are the same, then it should fit your engine. From what you're saying, the engine did get up well with the different prop (until it spun). I would pursue the prop issue. You could take it to a prop shop and they could see if they think the hub is spun and press in a new one. They should also be able to get you a thrust washer that would fit.


The prop scrapes the lower unit when seated properly. It is too big to sit where it should without making contact with the lower unit.

It is also a mystery to me how a higher pitch prop would spin greater RPMS than a lower pitch. Hence why I called it a fluke. And it didnt work at all the second go around with no changes.

I didnt want to shift this issue back to the prop. That is why I didnt mention this before. I don't think what happened was related to prop pitch.

I just meant to say that the motor is capable of those RPMs because I have seen them before, on a strange occasion.


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