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  1. M

    2005 Johnson 90hp 4-Stroke J90PL4SO Lower Unit Knock at Idle Forward/Reverse

    For anyone that stumbles upon this, dropping to a lower pitch and lighter weight 3-blade Rubex prop completely solved this. I also gained 4mph.
  2. M

    2005 Johnson 90hp 4-Stroke J90PL4SO Lower Unit Knock at Idle Forward/Reverse

    Well some learning occurred during this little troubleshooting adventure. One important thing to note is that I installed a tach when I purchased this motor not long ago. I thought I had the 10 pole/12 pole switch on the back set correctly for this motor but during this idle test I noticed the...
  3. M

    2005 Johnson 90hp 4-Stroke J90PL4SO Lower Unit Knock at Idle Forward/Reverse

    Well, after spending the day boat tinkering I did some more reading on this subject. I see it often referred to as "prop knock". Some say it's pretty normal, some say it could be certain prop combos, some call it clutch dog rattle. One explanation was similar to what I described as the "lash"...
  4. M

    2005 Johnson 90hp 4-Stroke J90PL4SO Lower Unit Knock at Idle Forward/Reverse

    Ok so confirmed a few things. Reverse is worse sounding than forward, and the whole issue isn’t present with the prop removed. Tried a lighter weight aluminum prop and same sound. The sound is coming from right behind the prop, and not deep in the hub. Trying to include some shirt videos. Am I...
  5. M

    2005 Johnson 90hp 4-Stroke J90PL4SO Lower Unit Knock at Idle Forward/Reverse

    Running without the prop first this morning.
  6. M

    2005 Johnson 90hp 4-Stroke J90PL4SO Lower Unit Knock at Idle Forward/Reverse

    Both of these are on my to do list. I’ll report back.
  7. M

    2005 Johnson 90hp 4-Stroke J90PL4SO Lower Unit Knock at Idle Forward/Reverse

    Thanks for the replies.
  8. M

    2005 Johnson 90hp 4-Stroke J90PL4SO Lower Unit Knock at Idle Forward/Reverse

    Ok, looking at the exploded diagram in my Seloc manual I see the clutch dog and see what you mean about always being in mesh. A bad description on my part. I should have said something like "keeping the gear teeth in continuous contact in one direction". The play seems normal to me too, but for...
  9. M

    2005 Johnson 90hp 4-Stroke J90PL4SO Lower Unit Knock at Idle Forward/Reverse

    Thanks for the quick reply. On a hose and in the water. In gear and engine off I would say there is about 10-15 degrees of rotational play if you turn the prop back and forth by hand. It's as if a constant forward or reverse pressure is not being maintained on the gears to keep them meshed...
  10. M

    2005 Johnson 90hp 4-Stroke J90PL4SO Lower Unit Knock at Idle Forward/Reverse

    Hi everyone, I'm running this engine on a 18' Starcraft Mariner center console, circa 1974. I tried numerous props before settling on a Turning Point 13x23 stainless 4-blade to get me in the right RPM range at WOT. I'm getting a knocking noise in the lower unit around the prop area only at...
  11. M

    Help with motor height adjustment

    Well, here is my final update. Took the boat out today with a 13x23 TP 4-blade prop just to see how it would run. The boat performed great. Still hitting 5300-5500 rpm depending on how I trim. Three people in the boat which eliminated the porpoise. Could trim pretty much wherever I wanted to. I...
  12. M

    Help with motor height adjustment

    I didn’t get lost! I have fresh info and videos. Took the boat out for a run. As a reminder, this was with the jackplate removed and AV plate about 1.5” above the keel. The boat handles really well! I lost maybe 1-2mph on the gps, but the porpoising was reduced. Trimming up to the sweet spot...
  13. M

    Help with motor height adjustment

    Lot of good info in here. I have to read it all a few times and do bits of side research to make sure I understand the various points being made. Sounds like I’m not far from where I need to be, and looking forward to the next test. Sticking with the 4-blade TP prop for now since is supposed to...
  14. M

    Help with motor height adjustment

    I’ll report back after the next trial. No planned changes until then, other than a new trolling motor 🥳. I did just realize something that might be relevant that is new to me. All my props have cavitation burns in the middle of the blade. Little quarter-size round spots that are apparently...
  15. M

    Help with motor height adjustment

    Well, I stared at the jack plate through several cups of coffee yesterday. Decided something about it was bugging me so I pulled it off and bolted the motor directly to transom as nature intended. Dropped the motor down to where the AV plate is 1”-1.5” above the lowest point of the hull at the...
  16. M

    Help with motor height adjustment

    I can do this, but that is about where I started with this motor if I’m not mistaken. That will put the AV plate submerged I’m pretty sure. I’d love to hear your thoughts on how things may change with this experiment. I was thinking of removing the jackplate all together. The motor is very...
  17. M

    Help with motor height adjustment

    Ok, so after a few setbacks I had a chance to go test the adjustment. First had to reseal the lower unit because I had some milky gear oil in the prop hub. Then, just as about to pull out of the driveway I noticed the angle of the the trailer tongue looked different. Turns out the welds on the...
  18. M

    Help with motor height adjustment

    Thanks Texasmark, always appreciate your input. Sound like I’ve got a direction to go from here and will go experiment on the water. I’m only running inland lakes, and so far nothing bigger than about 10,000 acres. One has a lot of submerged timber, so getting the motor up higher would be nice...
  19. M

    Help with motor height adjustment

    Thanks for the reply. That should be easy enough to do. I don’t know the name of the plate you can see in the video, but I know the anti ventilation plate is the next one down. I should be able to measure the distance between the two plates and raise the motor by that amount. I’ll have to...
  20. M

    Help with motor height adjustment

    1974 Starcraft 18’ center console 2004 Johnson 90hp 4-steoke Vance engineering 4” setback manual jackplate 13x21 4-blade turning point prop My previous motor was a 25” shaft and my transom is for a long shaft. I used the jackplate to get the motor to a more reasonable height. I recently...
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