Merc 400 45hp questions

Does anyone know the correct gas/oil mixture for this motor? I assumed it was 50:1 but I'm not sure. Also, it has a 2 bladed prop and I'm thinking if I can find a 3 blade it would be much more efficient. Thoughts?? Thanks,max

Re: Merc 400 45hp questions

Hi Max,
The Merc 400 would be a 1960 or 1961 model year (depending on serial number). Although it could probably run quite well at 50:1, and many folks do run their old Mercs on that mix, the recommended mix is 24:1. Personally, I run mine somewhere in the middle at about 32:1. Mercs of that era are pretty advanced as far as bearings and scavenging goes and they handle the leaner mixes quite well. BUT, go leaner at your own risk...
As for the propeller, it is entirely dependent upon what you are going to do with the motor. And blade count isn't nearly as important as pitch. Light runabouts respond well to 2-blade, high pitch props. Heavy cruisers and pontoons need more blades and lower pitch. You want your RPM to be able to hit the sweet spot in the 5000-5500 range, regardless of what the application is.

Re: Merc 400 45hp questions

Thanks for the help. Any other thoughts on this motor? max

Re: Merc 400 45hp questions

Very solid motors. I've had a few of the older 44-cubers over the years and like them very much. Mercury built similar inline 4s for over 40 years, so the basic design is one of the better ones out there. Smooth runners, too! Things to check - driveshafts were prone to rusting, particularly at the waterpump base seal area. Check that area when you change out the impeller (still readily available at Merc dealers!) You will need to be a little creative in making special tools to remove waterpump cover, etc.

Do a compression check before digging deeper. Ideally, better than 100 psi in all four jugs, and no more than 10% variance between high and low readings.

Re: Merc 400 45hp questions

24:1

yup, i can second the rust problem around the water pump area. i got stuck out in the lake twice because it overheated slightly. my solution was that i got a bigger key and dremmeled the impeller out a little. it fit in there nice and snug and now it's pumping great. CAUTION: get the right size key and do not dremmel to much or the impeller might crack from weakened base (made of brass)

corroded wires are a big deal, so be careful with them.

hopefully your magneto is in great shape because they're expensive and will cause a miss at lower speeds (figured out why my engine is so hard to idle) it will cut 1 or 2 cylinder out and i wouldn't even notice. i think my rotor and cap need replacing.

i used a punch of the right size to take the water pump cover off (previously punched before me) careful! don't ruin it though.

CHECK GEAR OIL. almost always they have water or will leak later down the road. solution: replace seals or just change the oil a lot earlier depending on the severity of the leak.

they're good screaming engines though. use 24:! tcw-3 oil.