6.0 briggs motor on top of 4hp LU

[drewmitch44]

i am going to put a 6 hp briggs motor on top of a 1972 yahtwin lower unit. anyone done this before? I got the motor from a brand new yard man push mower which had a bent up deck right out of the box. Its got electric start and all. I see how i can join the two together. I was searching a lot and i havent seen a lot i just hear a lot of people who do this. I have a perfectly running 15 gamefisher but i want to do this for fun and who knows might work good.

 

[DJ]

Re: 6.0 briggs motor on top of 4hp LU

Your going to basically end up with a similar unit to the current Briggs outboards. Theirs sit atop a Merc. lower.

Should be a fun project.

 

[steelespike]

Re: 6.0 briggs motor on top of 4hp LU

Other than the fun of creating an unusual motor you will end up with a vibrating,noisy motor with a very hot exhaust.The Briggs outboard has a rev limiter as these motor are sensitive to overreving.
Your 6 will no doubt have a gouvener not sure how this will effect the performance.Briggs must have removed the gouvener on their motor for some reason.

 

[Mark42]

Re: 6.0 briggs motor on top of 4hp LU

Sounds like somthing I would do. LOL! If you can, get a larger muffler to help quiet it down because it will be a lot closer to your head when you are operating it as an outboard than lawn mower. Also, some extra sheet metal flashing around the side of the motor facing forward will help to deflect some sound.

The Briggs yard service motors are usually governed to about 3000 - 3500 rpm. My guess is that the origional 2 cycle motor probably ran to a higher RPM - probably 5000 plus. See if you can get a larger pitch prop to adjust for lower rpms.

Good luck, and post some pictures when your done.

 

[drewmitch44]

Re: 6.0 briggs motor on top of 4hp LU

I think that the motor will bolt up to the lower unit pretty well. The only thing im having a problem with is mating up the shaft's. The mower has a screw type for the blade and the L/U has like the star thing that fit into the old power head. Any ideas?? I think im going to leave the water pump installed in the lower unit and make it shoot up water behind the boat as it is moving in the water. I think that will look cool.

 

[itstippy]

Re: 6.0 briggs motor on top of 4hp LU

It sounds like a fun project. Keep safety in mind. The 6HP B&S is much heavier and vibrates much more than the original 4HP OMC powerhead. This may stress the bolt-up beyond designed specs. A loose, running, 6HP motor flopping around in the back of the boat would be exciting. Inspect the mount-up and transon bracket for cracks etc. often.

I agree with Mark42 - post pics!

 

[AlanR]

Re: 6.0 briggs motor on top of 4hp LU

I agree with Mark42, Prop it so it dosnt have to Rev very high, But i disagree with Mark42 when he mentions the exhaust, it wouldnt be that loud... Just like mowing your lawn

 

[Bob_VT]

Re: 6.0 briggs motor on top of 4hp LU

Leaving the water pump connected will steal horsepower (not that you will miss much).

Do you have your old crank from the upper end? Have the crank to driveshaft piece cut off. You will and to use a rubber type coupler to absorb shock.

Have the drive shaft piece mated to a rubber coupler and have that with a mounting plate to fit the B & S output shaft.

I have always wanted to convert an old 125 or 250 dirt bike 2 stroke motor to a lower unit. They put out some real horsepower to weight! Good luck with it.

 

[i386]

Re: 6.0 briggs motor on top of 4hp LU

Maybe pipe the exhaust into the water. Something like a lovejoy spider coupler for the shafts?

 

[Scaaty]

Re: 6.0 briggs motor on top of 4hp LU

Governor a zero problem.....disconect and run a minibike twist grip throttle and cable right to the throttle shaft. On the shaft, get some dimensions and have a Machine shop make a steel collar/coupler so both ends fit in a good inch or so, grind a flat on both shafts, and drill and tap for double set screws on both ends. Blue loctite them in good and snug. Or if ya can, just weld it together. You don't need any rubber cushioning for shock either...no other outboard has anything like that

 

[Paul Moir]

Re: 6.0 briggs motor on top of 4hp LU

The mis-match between the prop & lower unit and the engine is going to be a problem. You don't have a choice in props to take the new found torque (and I really doubt that particular lower unit would take it anyway even if you did set up a custom prop) . You're restricted to much less than 4500 rpm. So the briggs will only be able to put out about 3hp at full rpms. Good for engine life though!

Technically speaking, an underwater exhaust would require water cooling to meet USCG standards. This is why briggs uses a muffler on theirs, and the old '70s air cooled outboards had water cooled exhausts. If you want to do it, don't forget you'll need to relief the exhaust back pressure for idling by drilling a couple small holes in the pipe.

Let us know how you make out. I hope I'm not coming across as down on the project. I just want to make sure you know all the issues.

 

[Mark42]

Re: 6.0 briggs motor on top of 4hp LU

If there is not enough resistance with the stock prop, and you can't find a bigger prop, maybe you could double up the stock prop to double the resistance.

 

[Chinewalker]

Re: 6.0 briggs motor on top of 4hp LU

My Dad built two of these home-brew motors back when I was a kid. He had so much fun with them he went out and bought a can of Wheel Horse red and a can of John Deere green for each one, plus appropriate decals. Neither ran worth a hoot, as they vibrated like crazy, but the sure got the looks at shows and at the ramp. With the pair of them side-by-each we had more folks scratching their heads, saying "I never knew they made outboards..."

He made angle-iron brackets to mount the B&S 3.5 powerheads to Eska style lower units. He made couplings out of square steel stock, boring either end to size and then pinning them with set-screws or roll pins (I can't remember which). I'm sure he wasn't absolutely perfect in his alignment, which may have played into the vibration issue, but it worked.

- Scott

 

[Paul Moir]

Re: 6.0 briggs motor on top of 4hp LU

I didn't mention a bigger prop for two reasons. 1 there isn't one for that l/u and 2 because 6 full real hp would blow that weeny l/u to doomsday.