B&S 12.5 twin II opposing cylinder questions

[Jayoutdoors 82]

I am picking up a B&S  12.5 twin II, the plan is to put in a project (rat rod tractor go kart hybrid).  Has anyone run one?  It is the opposing cylinder style, not a v twin.  What's the good, bad and the ugly of these engines?  Do you have any idea of the cc displacement and a suggested clutch to run with it?

model 404437

type 1210-01

code 89120612

Edited December 5, 2023 by Jayoutdoors

[ebinmaine 67,438]

I've had good luck with Briggs over the years.  

That I/C might tell you it has steel cylinders.  

 

You'll want to remove the heads and see if the bores are outta round. 

Some opposed twin engines tend to wear the bottom of the cylinders. 

 

 

[Handy Don 12,214]

[Achto 27,565]

Only real issue that I know of is the fuel pump. The diaphragm is prone to drying out causing it to leak and or not work. Inexpensive B&S rebuild kit usually gets them going again. 

[WHX?? 48,815]

30 minutes ago, ebinmaine said:

That I/C might tell you it has steel cylinders

Fer sure... I had a couple of those in the 18 HP flavors in Murrays that were power houses I keep the mice out of them... rest of the mower was junk but still. 

Clutching is whatever you can rig up..

Edited December 5, 2023 by WHX??

[RED-Z06 2,221]

The 12.5 flavor was the smaller displacement version, 40ci still though, they were good engines, starters were a problem..nylon gears, nylon bendix..and it mounted on the bottom.

[Achto 27,565]

8 minutes ago, RED-Z06 said:

starters were a problem..nylon gears, nylon bendix..and it mounted on the bottom.

 

Never liked the plastic starter crap on the B&S engine, but again low dollar repair. Even with genuine B&S parts.

[Jayoutdoors 82]

1 hour ago, ebinmaine said:

I've had good luck with Briggs over the years.  

That I/C might tell you it has steel cylinders.  

 

You'll want to remove the heads and see if the bores are outta round. 

Some opposed twin engines tend to wear the bottom of the cylinders. 

 

 

I will definitely check that out before i go to far with it! thank you

[Jayoutdoors 82]

54 minutes ago, Achto said:

Only real issue that I know of is the fuel pump. The diaphragm is prone to drying out causing it to leak and or not work. Inexpensive B&S rebuild kit usually gets them going again. 

I have a carb kit and pump kit in the cart to order after it gets here

[Jayoutdoors 82]

thank you everyone for the advice!  I am excited for this build over the winter!

 

[WHX?? 48,815]

25 minutes ago, RED-Z06 said:

starters were a problem..nylon gears, nylon bendix..

Been there done that ... nice thing replacements in the jungle cheap. 

[Achto 27,565]

2 minutes ago, Jayoutdoors said:

I have a carb kit and pump kit in the cart

 

They made 2 pump kits, a 3 screw and a 4 screw kit. I believe the 3 screw kit was the lower priced one. You will need to know which one you have before ordering the kit.

[Jayoutdoors 82]

20 minutes ago, Achto said:

 

They made 2 pump kits, a 3 screw and a 4 screw kit. I believe the 3 screw kit was the lower priced one. You will need to know which one you have before ordering the kit.

good heads up!!

 

[J854D 137]

@Jayoutdoors, the first two digits in the Briggs model number indicate the cubic inch displacement. And as @RED-Z06 posted, it is 40ci. That is equivalent to 655cc. And for what it’s worth, the first six digits in the code number indicate the date of manufacture. In your case, Wednesday December 6, 1989. Soon to be 34 years old. -JD-

Edited December 5, 2023 by J854D

[gwest_ca-(File Mod) 11,041]

 

[Jon Paulsen 290]

 

Check this channel, many tips on the B&S twin https://www.youtube.com/results?search_query=captain+315

 

He's got a build thread in here somewhere. 

 

I've got one of those twins. It's looking for a chassis 

[Jon Paulsen 290]

Found it: 

Look familiar?

 

Mine dresses in all black. Rebuild first winter I am gainfully unemployed. I love my job, but I'd rather be home rebuilding the old twin. 

[kpinnc 12,007]

On 12/4/2023 at 8:28 PM, Achto said:

Only real issue that I know of is the fuel pump.

 

I have two of these opposed twins (12.5 and 18), and I've never even looked at this BUT- would an electric pump work? 

 

Just wondering if it could be a work around. Since the pump is integrated to the carb, not sure you could get fuel in there without it...

[Achto 27,565]

37 minutes ago, kpinnc said:

I've never even looked at this BUT- would an electric pump work? 

 

I have never tried, don't think it would be a simple bolt on. Maybe with a carb from a Vanguard?

[ebinmaine 67,438]

45 minutes ago, kpinnc said:

 

I have two of these opposed twins (12.5 and 18), and I've never even looked at this BUT- would an electric pump work? 

 

Just wondering if it could be a work around. Since the pump is integrated to the carb, not sure you could get fuel in there without it...

 

6 minutes ago, Achto said:

 

I have never tried, don't think it would be a simple bolt on. Maybe with a carb from a Vanguard?

 

 

On the 1984 GT 1800 that we have, it has a Craftsman 18 horse opposed twin flat. 

 

The fuel pump is part of the side of the carburetor but it went bad sometime in the past. A previous owner retro fitted a later model vacuum type fuel pump.

The old original fuel pump is empty.

 

[WHX?? 48,815]

On 12/5/2023 at 6:18 AM, Jon Paulsen said:

 

 Rebuild first winter I am gainfully unemployed. 

😆... how is it I am gainfully unemployed but the projects are getting done as fast as @ebinmaine 's...

[Jon Paulsen 290]

3 hours ago, WHX?? said:

😆... how is it I am gainfully unemployed but the projects are getting done as fast as @ebinmaine 's...

ask me in two years 

[WHX?? 48,815]

1 hour ago, Jon Paulsen said:

ask me in two years 

Gotta tell ya Jon it's everything it's cracked up to be. 

Jay I usually only do it to EB!

Edited December 6, 2023 by WHX??

[bds1984 1,429]

I saw this one for sale but didn't think I needed another project (no space).  They are good engines.  Many of them were installed in cheap mowers, maintained by cheap people, which gave them a bad reputation.  I grew up with a 12.5 and 18 HP variant that mowed tons of lawn for close to two decades.  The big concern I would have is using that engine with a WH style clutch.  Many of the opposed twins do NOT have the necessary bearing for the load of a WH clutch.  This one does according to your model 404437, (having a 3 in the fifth character indicates ball bearings) and I find that quite rare for a 12.5 HP model and is mostly common on the 18s as found in the GT1800.  Change the oil often and RUN TO THE HILLS!



 

[Greentored 3,201]

On 12/5/2023 at 7:04 AM, Jon Paulsen said:

 

Check this channel, many tips on the B&S twin https://www.youtube.com/results?search_query=captain+315

 

He's got a build thread in here somewhere. 

 

I've got one of those twins. It's looking for a chassis 

I am he!

Love a good running opposed twin!  Gobs of torque, smoothe as glass, and if you open up the ports, shave the heads a touch, bump the timing a bit and put a bigger carb on, they will flat out fly!

Never messed with a 12.5, but whats interesting is its 40ci, which is what the 16s are. My guess is its just a smaller carb to 'detune' to 12.5? They did that for years with outboards- take a 15hp engine, put a baby carb on it, boom, 9.9hp for the smaller lakes with restrictions.

18hp Onans are identical to a 20hp, with a smaller carb