Gears out of sync after impeller change?

Tims Osprey · 14 May 2025, 17:48

Hi all, I've just done a routine impeller change on my Yamaha 70 2 stroke. A fairly straight forward job right. Well yes and no, after reinstalling the lower unit, the motor won't shift properly into gear? it seems something has misaligned after refitting the lower unit? i'm pretty sure it's not a gear box issue, as the gears were fine before removing the lower unit. A little step in the right direction would be much appreciated on this annoying issue. Cheers.

Fenlander · 14 May 2025, 18:17

I'm sure someone will be along soon who knows this motor but what did you have to do to separate the gear linkage... something to unclip or a screwed collar etc?

Tims Osprey · 14 May 2025, 18:34

No, it's just basically a case of dropping the lower unit after taking out the outer casing bolts which fixes it to the upper unit. Basic impeller change procedure, standard for most outboard motors. Usually a peice of pisss. I didn't even remove the prop, so nothing internally was touched.

Tims Osprey · 14 May 2025, 18:37

I dropped the lower unit with the gears in neutral, should I have had them in reverse?

Fenlander · 14 May 2025, 18:53

Ahh yes I see it's a twist splined gear change rod so in theory should slide back on if both leg and gearbox ends have remained in the position they were pulled off?

Tims Osprey · 14 May 2025, 19:01

Some outboards have a gear shift and shift rod on an up and down system, while others work on a clockwise or counter clockwise system. This particular motor is the later. So I supsect Iv'e misaligned the shift rod on dropping the lower unit, or while reinstalling it? Surely someone here on this forum has experience with this same problem on these Yamaha's? I'll figure it out myself enventually I guess.

Tims Osprey · 14 May 2025, 19:07

Quote:

Originally Posted by Fenlander

Ahh yes I see it's a twist splined gear change rod so in theory should slide back on if both leg and gearbox ends have remained in the position they were pulled off?

Yes indeed, but I'm having trouble getting it back on with everything in sync. i.e the throttle lever is in reverse, but the remotes aren't in sync with the gearbox. Even when I manually shift the gearbox into reverse before reinstalling. really annoying issue. Never had this before after an impeller change.

spartacus · 14 May 2025, 21:58

Interesting video from Dangar Marine. Suspect the gear linkage isn’t mated properly.

On Tohatsu it’s a different design, and I usually work on the basis of having it in neutral. Then you connect a 10mm stainless bolt on the shift rod and Bob’s your uncle so to speak.

Stu from Dangar has this Yamaha in reverse, the idea being you can rotate the prop to line up the splined drive shaft.

https://youtu.be/srGVSl4_n8Q?si=0AJl0fDtp58zwW7g

Tims Osprey · 16 May 2025, 09:23

Quote:

Originally Posted by spartacus

Interesting video from Dangar Marine. Suspect the gear linkage isn’t mated properly.

On Tohatsu it’s a different design, and I usually work on the basis of having it in neutral. Then you connect a 10mm stainless bolt on the shift rod and Bob’s your uncle so to speak.

Stu from Dangar has this Yamaha in reverse, the idea being you can rotate the prop to line up the splined drive shaft.

https://youtu.be/srGVSl4_n8Q?si=0AJl0fDtp58zwW7g

Yes, thanks. So it turns out this must have been the issue. As I was trying to refit the lower unit I must have been inadvertently shifting gear while twisting and shaking it to enable it to slide into place. If that makes sense? This is something for others to bear in mind with these types of motors and gear shift systems. Make sure you're not inadvertently shifting gear while refitting the lower unit. The best aproach I think is to have the motor in reverse gear when dropping the lower unit, then follow the steps in the video above, using a winch strap to take the weight and to hold the lower unit in place. Then slightly twist the prop to align the drive shaft so it slides back in place without too much fuss.

Tims Osprey · 16 May 2025, 09:29

I've always had a motor in neutral before dropping the lower unit for an impeller change. Usually a straight forward task. But not with this particular motor. According to the above video you should have the motor in reverse gear before dropping the lower unit.

beamishken · 16 May 2025, 23:31

It realy doesnt matter what gear it's in when you drop a box as long as top & bottom are in the same gear when you reassemble the box to the engine. Some engines sit better in certain gears & all you need to do is select the same gear at the control as you reassemble the box. It's quite a course spline so usually relatively easy to determine where you need the control set

Tims Osprey · 18 May 2025, 15:06

Quote:

Originally Posted by beamishken

It realy doesnt matter what gear it's in when you drop a box as long as top & bottom are in the same gear when you reassemble the box to the engine. Some engines sit better in certain gears & all you need to do is select the same gear at the control as you reassemble the box. It's quite a course spline so usually relatively easy to determine where you need the control set

Yes you are right, in theory it shouldn't matter which gear it's in when dropping a lower unit for service or repair.

Anyhow, the plot thickens, this time I was extra careful not to misalign anything or to inadvertently shift gear on reassembly! However, I still have the same problem with forward gear not engaging properly. The prop slips and clicks while rotating it clockwise by hand in the forward position? I've checked the remotes and control box, no issues there at all. I also took the cable and link off the shifter from the remotes to engage it manually and to push it forward to the maximum, but forward gear still doesn't engage properly.

Everything was working just fine before dropping the box for a simple impeller change. It's highly unlikely the gears or dog clutch have failed while the boat was not in use, and in that short space of time. Too much of a coincidence for me. The motor only has around 400 hours on it too. However I guess it's not impossible, so I'm not ruling out premature gear box failure.

I've been working on the motor again this afternoon. Just done a gear box oil change to check for any metal bits or bad oil. Nothing, no metal fragments whatsoever, and the oil looks fine.

Worse case scenario I guess is a dog clutch or gear box issue right? How involved or dificult is this job for a reasonably experienced and failrly capable home mechanic? How much roughly would a boat repair shop charge for a gearbox service or repair? Cheers.

Tims Osprey · 18 May 2025, 15:17

I guess it's a question of how easy or dificult the inner casing or gear box case comes out. Can anyone recommend the right tool for a Yamaha gearcase removal. I'm going to tackle the job myself, that is when I'm 100% sure it is a gearbox issue?

Tims Osprey · 18 May 2025, 15:40

Real shame about this issue, as I was looking forward to taking the girls out this weekend on their first RIB ride.

Fenlander · 18 May 2025, 16:11

Bear in mind I don'y know this specific motor so thinking of mechanisms in general.

Is there any chance with the gearbox off the selector spline was turned beyond its normal range causing something internally to become misaligned... something that is not irrecoverable and could be re-aligned with the gearbox off with a bit of jiggling about.

Tims Osprey · 18 May 2025, 16:20

Quote:

Originally Posted by Fenlander

Bear in mind I don'y know this specific motor so thinking of mechanisms in general.

Is there any chance with the gearbox off the selector spline was turned beyond its normal range causing something internally to become misaligned... something that is not irrecoverable and could be re-aligned with the gearbox off with a bit of jiggling about.

It's possible I guess, but I don't want to force anything on the slpine shift rod with the lower unit removed. Thanks for your help. I'll get back to you in a day or two after trying one or two more things. I just find it inconceivable that the gear box or dog clutch would have failed the exact same time I removed and refitt the lower unit! Nothing was forced or jammed. I'm extremely careful with my work.

Tims Osprey · 18 May 2025, 16:39

Gearbox failure usually shows clear signs before complete failure, right? Clunking or grinding noises, especially while shifting, and general rough running sounds. Bad gearbox oil, metal fragments on the drain screws or in the oil.. etc. None of that, I've just lost forward gear all of a sudden after dropping the box. wierd. It has to be some sort of inadvertent spline shifter misalignment like you said fenlander, rather than mechanical failure per se. I'm 90% sure the gears are fine.

Tims Osprey · 18 May 2025, 16:47

There's always the chance the previous owner serviced the gear box, or had it serviced by someone who forgot to replace one of the shims, washers or springs correctly on reassembly? The thought had occured to me earlier today.

beamishken · 18 May 2025, 17:41

Are you saying it skips when you turn the prop forward when in forward gear? If you are its supposed to do that & will skip the opposite way in reverse, check it before you strip anything

Tims Osprey · 18 May 2025, 17:51

Quote:

Originally Posted by beamishken

Are you saying it skips when you turn the prop forward when in forward gear? If you are its supposed to do that & will skip the opposite way in reverse, check it before you strip anything

Yes it does, but it rotates differently in reverse. It grips both ways in reverse while trying to turn it by hand in either direction. Wierd. I ran it in my tank earlier in the week on first reassembly after the impeller change, and it wouldn't engage into first after shifting and opening up the trottle, which quite surpised me. I dropped and refitted the lower unit again yesterday, as described earlier in the thread, but noticed the prop skips/slips in forward gear while turning it clockwise by hand.. I'll run it again in the tank tomorrow, see how it goes. Thanks for your help.