Merc 4.5hp barely pissing....

Fired up my new-to-me 80/81 4.5hp Mercury earlier and discovered it barely/doesn't pee. It barely drips. I looked at the screen near the prop and I can see a chunk of black plastic that I'm assuming is a chunk of the impeller.

Obviously I need to replace it, anything else I should do while I'm at it? Does anybody know the part# for a water pump/impeller kit and somewhere to buy it? I looked it up in the Mercury parts catalog but it lists several and mentions shaft size.... Is there a way to figure out shaft size without tearing it apart?

Serial # 5809982 if that makes a difference.

Any help would be greatly appreciated, thanks in advance.

Anybody? I really don't want to order the wrong parts....

[url=https://www.tinboats.net/forum/viewtopic.php?p=325618#p325618 said:

MiPikeGuy » Today, 13:32[/url]"]Anybody? I really don't want to order the wrong parts....

Click to expand...

I imagine Mercury is like most OB motors and used the same impeller for several years in a row. I would go to marineengine.com or westmarine and look up the impeller for 2 tears each side 80/81 for a 4.5 OB. I suspect you will find the same part number listed.

There are plenty of them out there going back a lot farther than than.

Go on an take it apart and make sure that the impeller is the only problem. You may need more parts than just the impeller and the only way to know if taking it apart. And if you need to know the shaft size you'll have it apart where you can measure it. Although I'm not sure why you'd need to know that. I've replaced many merc impellers and never needed to measure the shaft.

Well I found this it lists 2 different impellers with different inside diameters.
https://www.boats.net/parts/search/Merc2/Mercury/4.5%20%281%20CYL.%29/5595532%20AND%20UP/GEAR%20HOUSING%20%28DRIVE%20SHAFT%29/parts.html

But it also lists this water pump kit mentioned in the link below
https://www.boats.net/parts/detail/merc/M-70941A3.html

From what I read here if I'm reading it correctly, it seems like the impeller matches to the water pump, so if I order a water pump/impeller kit they would come matched and it wouldn't matter?

"Replacement impellers can be a pain trying to find the right one as some look alike but may not be compatible. The outside diameter is the same, but the thickness may be different. If you use the thinner one in place of the thicker one, the pump will not be efficient, but if the new impeller is too thick, it will be so tight that on the initial start up, you will have a ruined impeller plus possibly both the top and lower pump housing. Also if the water pump unit was changed to a newer style, it can use a different impeller than the original. And if that is the case you have no way of knowing. There are at least 3 look alikes for this model. So on this motor, if you are not sure, order a new water pump assembly complete. "

https://www.leeroysramblings.com/mercury_4.5.htm


I'm pretty sure I'm going to order that water pump kit. Still debating on whether or not I'm going to attempt to tackle this problem myself or just bring it to a shop. I like to think I have decent/good mechanical ability, but this would be my first time with an outboard and I'm just not sure.

A water pump kit isn't going to solve your problem if you don't get the right one. And based on what I'm seeing, they made 2 different impellers for that motor and each one had a different ID. Which probably means that they started using a larger diameter drive shaft at some point in the production run. So even if you go with a new pump, you'll still need to know what the diameter of the drive shaft is.

Well the more I think about it with winter rapidly approaching here, I'm just going to winterize it and wait till spring, then take it to a shop and just have them go over/freshen up the whole thing. Thanks for the help.

You are probably going to spend more than what the motor is worth by taking it to have a shop do it. It sounds and looks a lot harder that it really is. Just dive in and give it a go.

I was kinda thinking the same thing, and I'm poor so if I could do it myself that would be the better choice without a doubt. Maybe I'll try to do it this winter with the abundant amounts of free time I'll have.

Check youtube for a video of the replacement. It's not hard to do if you have some hands on experience.

The replacement does look super simple, I guess I'll order the parts, that is if I can figure out the right one!