Prop Slipping, RPM Jumping up?

I have a 1989 Johnson GT 100 on a 1982 18' Venture Bass boat. I have an aluminum prop 14Dx17p . I usually achieve @ 45 MPH. The last few times i have taken it out I have lost b/w 5-7 mph at full throtle. I also notice the RPM's going up when i lose the speed. When the rpm's go back down i begin to pick up the lost speed again. it has also seems to have started slipping when i go from nothing to full throtle, almost like a slipping transmition in a auto. I do not have a tach because i bought the boat used @ a year ago and it did not have one. If anyone has any ideas please post. Thank you in advance.

Re: Prop Slipping, RPM Jumping up?

bm258.Take a close look at the prop hub and see if it has spun.You may need to mark a line across the pressed in hub to the prop barrel and test run to see if the line still matches up.If not ,that means that under load the pressed in hub is breaking loose from the prop barrel and will need to be rehubbed.<br /><br />Worst case scenario is that something is amiss in your lower unit gearcase,but it sure sounds like a spun prop hub to me.Good luck.

Re: Prop Slipping, RPM Jumping up?

just like ob said the prop hub is slipping better get it rehubed.

Re: Prop Slipping, RPM Jumping up?

Thanks for the suggestion. I am going to do that today. Do i need to take it somewhere and run it, or can i do it at home hooked up to the flusher?

Re: Prop Slipping, RPM Jumping up?

Gotta be done on the lake to put the proper loading on it...

Re: Prop Slipping, RPM Jumping up?

Hi again. I marked the prop and just got back from the lake. all of the same problems happened(Felt like i was hitting a log when taking off, losing speed and gaining RPM at top end). I looked at the prop, and the marks still lined up. I lose most of the speed as I trim it up. Should I take the lower unit off and see if something is wrong there? If so, do i need to do anything other than draining the gear lube beford i remove the unit?

Re: Prop Slipping, RPM Jumping up?

If it feels as if you are hitting a log as engine loses forward momentum and rpms come up,it seems that the worst case I mentioned has shown its ugly side and there is a problem with respect to the gearcase.<br /><br />Either the shift linkage is not allowing a complete shift or the clutch dawg and or gear lobes are rounded off and losing mesh when under load.One other thing you could inspect is the shift linkage to assure that it is adjusted properly.Its a long shot but possible.With the engine controls in the neutral position(engine not running) remove the pin from the shift arm that connects it with the shift cable linkage.Once out ,use just the arm to shift gearcase from neutral to forward and back to neutral.Now see if the shift link and shift arm pin hole is perfectly aligned.If not adjust the shift cable trunion nut until it is and reinsert pin.You will have to manually spin the prop to prevent binding while shifting.If your handy you can use your foot to rotate prop slightly while shifting since you'll be standing by the engine anyway.Let us know what you find with respect to the shift linkage.<br /><br />If the gears and dawg are worn as I suspect they probably are,You may save some time and headache by just locating a complete rebuilt lower unit for your engine.Good luck whatever the case.

Re: Prop Slipping, RPM Jumping up?

i had to adjust the linkage forward a little so that the holes would line up. I also took the lower unit off and inspected the upper gears of the long rod(drive shaft?) and they did not seem to be rounded off. I did not know what else to look for because I am not very familiar with lower unit maintenance. I am going to put it on the flusher tomorrow to make sure it still shifts porperly before putting it in the water. thank you again for your help

Re: Prop Slipping, RPM Jumping up?

i had to adjust the linkage forward a little so that the holes would line up

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Please explain

Re: Prop Slipping, RPM Jumping up?

I had to adjust the shift link forward a little so that the pin hole on the shift link was properly aligned with the shift arm.

Re: Prop Slipping, RPM Jumping up?

i have just returned from the lake (again). The take off seems much better after adjusting the shift control arm. I am still loosing a little speed and gaining RPM's at WOT, only when i trim the motor out. When the trim is in it seems to be running ok. My prop is also slightly scuffed on the tips of the blade from hitting the sand on the bottom in shallow areas at idle speed. 1. Is there a way to visually inspect the clutch dawg and gear lobes? 2. Could the trim thing be the problem? 3. Would the prop cause anything like this? Thanks again for your help.

Re: Prop Slipping, RPM Jumping up?

Well,If you're trimmed so far out that the prop begins slipping it could very well cause your symptom.You mentioned that the prop has had several encounters with sand in shallows.If your aluminum prop has lost a measureable amount of its original diameter due to heavy use ,it could also impede its ability to bite at higher trim angles.Are you absolutely sure that when you marked the center hub and prop barrel that you didn't place the mark on two places that are common?I ask because it sure sounds like a spun hub. <br /><br />Question: Are you still feeling a thunk from the lower end when your issue happens?If the problem lies in the lower unit it will need to be removed and dismantled to inspect the gear condition.One thing you could do is to drain the gear case oil and closely inspect its contents.If there are any metal flakes or shavings present it could serve as an indicator of worn gear lobes.Good luck

Re: Prop Slipping, RPM Jumping up?

ob, thanks again for the help. I felt the thunk once today. The thunk only has happened on when i am at a stand still and then start forward(before the boat planes). I drained the lower unit yesterday and did not see any metal flakes. In reguards to the prop, I assume by "two places that are common" you are asking if i marked the center hub twice? I amrked the center hub, and then i marked the next rim out(on the outside of the rubber impact absorber). I have had the lower unit off before. once i remove the unit what steps do i need to take to check the gear lobes and clutch dawg?

Re: Prop Slipping, RPM Jumping up?

The disassembly procedure while not rocket science ,requires removal of several parts to include the bearing carrier assy.If you are going to perform this operation yourself ,it is my recommendation that you have an engine specific manual to use as a guide and for tolerance specs.<br /><br />Sounds like you marked the hub just fine and if the marks still align ,you'll need to look elsewhere for the problem.In an earlier post you stated that you dropped the lower unit.Was it the driveshaft splines that you looked at to see if they appeared stripped where they enter the crank?<br /><br />My guess is that you will find that the lobes of the gears in the lower unit are indeed rounded off causing the mesh to slip when under load.Don't know if this engine is new to you ,but a primary cause of gear lobe wear is shifting slowly into gear from neutral.This allows the lobes to grind before making up and eventually leads to enough metal being ground off that they no longer mesh fully.Always shift briskly from neutral to forward or reverse.

Re: Prop Slipping, RPM Jumping up?

ob, it was the drive shaft splines that i looked at. they seemed to be ok.

Re: Prop Slipping, RPM Jumping up?

See my above post.I added a few comments.

Re: Prop Slipping, RPM Jumping up?

ob, thanks for the advice. I will see if i cn get into the lower unit and check the lobes. If it is indeed the lobes, are thev expensive to replace, and where would i get them from? Do i need to drain the lower unit before opening the gear case?