Knocking in gear on trailer after lower unit service.

Serviced my lower unit this weekend. New water pump, shift boot, and new oil in lower unit. All went swimmingly, no issues that I detected.

I am running Amsoil 80w-90 sythetic. I assume this is good oil.

This was my first time doing this on any motor, but all in all it went very well. Reinstallation of the lower unit went well. Tightened the bolts and fired up the motor. Gears work, goes in and out of gear with no problem. However, I noticed a fairly loud banging noise coming from the motor when in gear at low rpm....about 1200rpm or less if memory serves. This is in gear, both forward and reverse. Once above about 1200rpm, smooth as a baby's *** in both directions. If out of gear, any rpm is great. Motor sounds great through all rpm ranges.

I dont remember this noise before, but I didnt put the motor in gear out of the water before.

Could it be some slap of not having a load on the prop?

This GD boat is cursed. I am fighting so hard to give this thing new life, but it fights me every step of the way!!!!

I am going to put it in the lake this weekend and see how it does. If there is no noise with some resistance on the prop, then Ill be happy. But, the noise does not appear to be coming from the lower unit, it sounds like within the motor itself. But motor runs with no abnormal noise in all conditions other than IN GEAR and low rpm.

Re: Knocking in gear on trailer after lower unit service.

Could be the shift cables needs adjustment or if its a plastic hub prop it may not be torqued correctly and rattling on the hub....

Re: Knocking in gear on trailer after lower unit service.

Try it in the water
buy a trashcan and fill it full of water if you have to.

I remember reading somewhere that you want to only idle while not in gear on the muffs.

I have a $11 yard trashcan I got from lowes that I use, it has a couple prop holes in it, taped over with cammo duct tape, but it still works great

As for the gear oil, sounds like automotive gear oil?

I'm sure it's fine but I have been sticking with the marine gear oil my self, mercury brand is easy to get at walmart.

Re: Knocking in gear on trailer after lower unit service.

That is an Mercruiser I/O with an Alpha 1 drive? It sounds like forward is not engaging fully.

When you install the lower gearcase onto the upper gearcase, the shift shaft can be aligned several ways. All but one way are wrong.

Re: Knocking in gear on trailer after lower unit service.

Chris1956 said:

That is an Mercruiser I/O with an Alpha 1 drive? It sounds like forward is not engaging fully.

When you install the lower gearcase onto the upper gearcase, the shift shaft can be aligned several ways. All but one way are wrong.

Click to expand...

No no...its just a 40hp outboard. The drive shaft is uniformly keyed, couldnt imagine the socket on the power head looks any different. It looks like the input shaft on a manual car tranny.

Re: Knocking in gear on trailer after lower unit service.

I would no use synthetic in the lower end..02 Plus I would NOT rev that motor on muffs or in a barrel. You keep reving it like that . Your New thread will be, Blown motor Now What.

Re: Knocking in gear on trailer after lower unit service.

Mi duckdown said:

I would no use synthetic in the lower end..02 Plus I would NOT rev that motor on muffs or in a barrel. You keep reving it like that . Your New thread will be, Blown motor Now What.

Click to expand...

Didnt rev it. I ran it, then put it in gear to test the gears after reinstalling lower unit. Hate to get all the way to lake only to then realize gears dont engage.

Re: Knocking in gear on trailer after lower unit service.

Bryan, When you said replaced shift boot, the only shift boot I could think of is on an I/O.

Rereading your thread, you say it bangs in both forward and reverse? I have no idea what that could be.

Re: Knocking in gear on trailer after lower unit service.

Chris1956 said:

Bryan, When you said replaced shift boot, the only shift boot I could think of is on an I/O.

Rereading your thread, you say it bangs in both forward and reverse? I have no idea what that could be.

Click to expand...

Yeah, its a little boot that covers the area where the shift shaft enters the lower unit.

Tonight, I topped off the lower unit fluid. It may have needed just a bit, I pulled the prop off, and pulled the midsection shrouds off so I could get a visual on the drive shaft and fired it up. NO NOISE now!!! Forward and reverse sounds normal. The driveshaft is perfectly straight, couldnt even tell it was turning without looking closely.

Seems to eliminate a lower unit problem, Im going with the no load theory for now. Itll hit the water this weekend and Ill see how it does.