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Discussion Starter #1
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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Discussion Starter #4
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.

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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Jaded Old Phart
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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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OCD Reel Service/Repair
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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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OCD Reel Service/Repair
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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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Discussion Starter #10 (Edited)
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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Jaded Old Phart
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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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Discussion Starter #13
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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Discussion Starter #14
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.......
Prop is stainless. The same hull making 5100rpms from a 1977 Johnson 140hp did 36-37mph before it died. Same prop.

Mine is 1972 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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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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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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Jaded Old Phart
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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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Discussion Starter #20
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.

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.

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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