11 HP briggs outboard conversion

[Dave T]

This is my briggs outboard that I have just completed. This is an old 12.9 chrysler outboard that I got for free at our local junkyard. I had access to three 11 horse briggs motors that were also free. The conversion was fairly simple I first removed the old power head and then the water pump impeller and plugged the water inlet and outlet. I stripped down the old two cycle motor and cut off the bottom of the crankshaft to get the spline drive so I wouldn't have to modify the outboards drive shaft. I threaded the end of the shifter shaft with a 1/4 28 die to fit a 1/4 inch ball rod end. I then reassembled the lower and mid section. To make sure that the crankshaft and drive shaft would be aligned I first cut a template from 1/8" clear plastic I drilled a 1/2" hole that would just fit over the outboards drive I made some pointed studs and screwed them into the mounting holes in the center section placed the plastic template over the drive shaft and tapped it with a hammer to mark where the mounting holes would be I drilled the mounting holes and then clamped the plastic template to a piece of 3/8" aluminum plate and transfer punched the holes. [More Later]

[Dave T]

I then drilled the aluminum plate from the transfer marks and drilled the 1/2" hole out to 1" to fit the briggs crankshaft clamped the plate on the motor and transfer punched the two mounting holes. I believe the pictures will pretty much explain the rest of the process but if anyone wants additional info please ask. After cleaning the carb and other stuff I fired up the motor and it ran good but oil was running all over the floor I determined it was the lower crank case gasket. I removed the motor made a new gasket reinstalled the motor and started it again after about the fourth start all the plastic teeth were gone from the starter pinion gear and oil was leaking from the bottom crankshaft seal. I should have replaced when I had the motor apart oh well. I went to our local parts store picked up two new starter drives, head gasket for one of the other motors and a new oil seal. Removed the motor again and installed the new seal cleaned all the ring gear teeth and installed the new starter drive. I will reinstall the motor tomorrow and let
run for awhile.
Will have some questions about props later.

Dave T.

[cor]

Looks great, I can't wait to see how it works out.

C.O.
http://whatsintheshop.blogspot.com/

[tom kane]

Great to see folk having fun doing things for them selves.

[Dave T]

This is the finished project mounted on my homemade test stand. The motor runs OK now I may need to change props but I'll wait until I test it on my boat next summer.

[tom kane]

It looks great I am sure you will have as much fun as I did with the outboard like the one in the image.
It was a local manufacture and you got a roto lawn mower and a outboard leg that you could fit the motor mower motor onto to go fishing.
I fitted a flywheel to the output shaft to help starting,it was a good worker and being a four stroke ran well and good fuel consumption compared to two strokes of the day. The motor was a 2.5HP Clinton.

[hoytedow]

I have a couple of old stock Chrysler props that would fit that if it was a 15 hp.

I'll dig them up and find the diam/pitch.

[Dave T]

Thanks for the reply I sure would like to have those props. I figure a prop from a motor about 25% bigger than the 12.9 would probably be about right because the Briggs will run at about 75% of the RPM. Let me know what you would want for them I will pay shipping or you can charge it to my shipping account. Don't worry about if they will fit the 12.9 I have a machine shop and as long as they will clear the cavitation plate I can make them fit.

Thanks
Dave T

[pistnbroke]

you done real good with this ...using the end of the old crankshaft is the key to a good installation ..done this myself with 3.5h and 4hp leg...pitty you did not shorten the drive shaft so you could drop the coupling down out of view....maybe a cover over that area is the way to go ...the noise is the biggest problem.

[Dave T]

With the 11 horse Briggs and the design of this outboard I couldn't lower it more than 3/4" and clear the tiller or transom mount so it wasn't worth shortening the crank for that and I did not want to make any modifications to the outboards drive shaft. I probably will add an aluminum cover around the adapter so that if water splashes on it won't throw it on the operator. I wanted the coupler to be easily accessible I can change motors in less than a half hour if needed. As far as noise I will experiment with running the exhaust under water after I find out how the motor performs.

Dave T

[Dave T]

Got the Briggs outboard mounted on the boat couldn't run it on maiden voyage because we were on an electric only lake. I'm going to get the steering wheel assembly throttle and shift cable made and connected next and then I'll see how it works on the river.

[masalai]

Good stuff, Neat work and quite inspirational...

[HelicopterJohn]

Hi Dave,

You have done a really nice conversion. Many thanks for sharing your design. I would like to do something along the same line. I need to look around and find an old outboard and lawnmower engine.

Do you have any idea what the finished outboard motor weighs?

John

[Dave T]

John

I just weighed one of my spare 11 hp Briggs without the carb air cleaner and muffler it weighs 60#. These motors are extra heavy because they are synchro balanced which means they have a heavy counter weight inside but they were free. The 12.9 horse Chrysler center and bottom section with the extra parts used for adapting weighs right at 40#. My best guess would be 110# total. You could use a lighter motor a 10 hp Tecumseh like I used for my steam engine weighs 42# and you could go lighter if you don't need as much hp.

Dave T

[HelicopterJohn]

Hi Dave,

Thanks for the information on the weight.

I like the idea of being able to replace the motor with minimal effort should that ever be the case. These motors are easy to find as they are found in low end riding lawnmowers.

Have you had a chance to try the motor in the river yet?

John