Starts and idles fine, but cannot get full speed (1985 Johnson 140 hp - J140TLCO)

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Liberty60

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Jul 24, 2023
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First time poster here...
I re-motored an old Bonito 18.5 with an '85 Johnson 140 hp (J140TLCO). I've had it running well for a while, but then it lost the ability to get any kind of speed at WOT. It starts and idles just fine. What I've tried to date:
  • Removed the VRO, and pre-mix my own gas (this was done way before the current issues)
  • Replaced fuel pump with after-market to bypass the oil reserve
  • Replaced plugs (old plugs were for a lawnmower?) and got a strong blue spark on all cylinders.
  • Check engine compression (all cylinders within a pound or two of 119 psi)
  • Removed and thoroughly cleaned all carbs (they were not even mildly dirty)
  • Checked the timing and it is set at factory spec
  • Tested and ruled out spun prop
  • The boat has an internal gas tank (small leak somewhere below the deck - another story). So I ran with an external tank, properly vented, and still no joy.
The boat starts and idles well, as stated. However, when you open it up, it seems to max out at less than 2k RPMs (a guess as I do not have a working tachometer). The boat does not plane out.

I am truly at wits end here and would greatly appreciate any advice for additional troubleshooting steps.
 
L

Liberty60

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Jul 24, 2023
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Hull is spotless.

How would I go about checking the power pack or rev limiter without simply replacing them?
 
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saltchuckmatt

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Jul 19, 2019
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Sorry, platform looked up for some reason. Try unhooking the temp sensor at the head. Unhook the wire and see if that gives you some joy. I.Boat system still won't allow to post pictures but it's part 28.
 
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saltchuckmatt

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Jul 19, 2019
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You could also try eliminating the control box as a problem by starting it, disconnecting the red plug at the motor and see if it's still a problem.

Are your butterfly's opening all the way? I'm thinking they are.
 
R

racerone

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Dec 28, 2013
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Checked flywheel key ?-----Does spark jump a gap of at least 3/8" on all leads ?
 
C

cyclops222

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Mar 21, 2024
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Next time out move the throttle to something higher than the limiting speed position. Apply choke enrichment by pushing in the ignition key. Do that at different throttle lever positions. Any speed increase for a second ?
Check that the throttle cable is still fully connected, correctly, at the engine at WOT.
 
Faztbullet

Faztbullet

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Mar 2, 2008
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15,902
Try unhooking the temp sensor at the head. Unhook the wire and see if that gives you some joy.
Click to expand...
All this will do is bypass horn as engine does not have SLOW
 
Faztbullet

Faztbullet

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Mar 2, 2008
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  • Check engine compression (all cylinders within a pound or two of 119 psi)
Click to expand...
This is a small bore looper and compression is 10-15 psi low. Pull head or scope cylinders as this is a bridgeport block and bad about scuffing exhaust side of piston when overheated. Bridge and exhaust port will be scuffed
 
L

Liberty60

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Jul 24, 2023
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No modification for me to the engine. It is stock as it comes. I haven’t got the spark tester as of yet. I will post when I’ve seen the results.

By the way, what is the length of spark I should be looking for. I’ve seen 3/4” and 7/8”. Any clarification is appreciated.
 
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