B&S 18 HP Oppy Questions

Alright, I got handful questions and issues that I've found of over the past couple weeks messing around with reviving my tractors after about a year and a half of sitting patiently. Both of which have 18 hp Briggs Opposed Twins, one being on a Companion tractor and the other being a Mont. Wards.

1. How are the charging systems wired, both of the tractors wiring systems got messed up on these when previous owners thought they new what they were doing, so neither have functioning charging systems.

2. What can I do to help the carburetors function better, both have older 3 bolt pumps and a mixture adjuster on them. Neither will change tune much at all whether I have the mixture screw all the way in or almost completely taken out. The other problem with the carbs is they seem to "sneeze" gas out of the top of the venturi when given more throttle. These don't have accel. pumps like car carbs, so I don't understand how the excessive amount of fuel ends up spraying out.

3. How hard would it be to run dual carbs on these tractors, and what size would I want? I don't have a welder, so I will either buy intakes or go to a buddies house and build them if needed. My other idea was to keep with the single set up and run a VW Pict 30/31. The mounting flange looks similar, so if figure if I can figure out linkages or cables, that that would do good for it as well. I'd obviously have to run an electric fuel pump or an external pulse pump if it can keep up.

4. Both tractors run kinda rough, no matter what I do. I gave one of them a complete tune up the other day and it kinda helped, but it seems to run rough, like it has a misfire that comes and goes all the time. They both smoke, so they may need valve jobs or new rings to help, so that could definitely contribute to the issue.

I know that was a lot of stuff to read through, but I figure some of y'all should be able to help me get some of this stuff sorted out. Thanks for any help you guys can give me

1. Coming out from under the flywheel is a plug with two wires running into it. One will have a diode on it with heat shrink over it, that's the charging wire. The other will be headlights. The other common setup I've seen is a single wire coming out from under the flywheel with a diode after, yours could be either or even a different one. Either way, they're not complicated.
2. I've noticed the mixture doesn't change much too. Best thing you can do is just clean the carbs a lot. Focus on getting the jets clean, poke wires through them all but be careful not to hog them out any. Stock settings on mixture screws is 1-½ turns out from lightly seated btw.
3. It's been done. I think the costs outweigh the benefits, as unless you have a tank above the carbs you'll need to set up a fuel pump, as well as a way to synchronize the throttles. I think mr.modified here did it with a pair of single cylinder Tecumseh carbs on his GTR Four Zero build.
4. I'd need a video or have to hear it to tell. At idle these engines sometimes seem to skip firing here and there, kind of inconsistent, but as soon as you throttle them up and give them a load, they clean right up. At least that's my experience.

Pull either of the spark plug wires when it's running, if the rpm doesn't drop as much on that side it's a good indication that there's something wrong with that cylinder.

did you clean the carb? If it runs the same adj screw all the way in or out something else is going on....

Either the carb/fuel pump needs rebuilt, or cleaned.

if it was mine, in priority order I would
1: rebuilt carb/fuel pump
2: remove intake check gaskets
3: remove heads check gasket and valve clearances, clean the carbon
4: while the cover is off check the ignition coil, clearance etc

with all that apart you should pretty much be able to see anything that is easy to mess with....but start simple.

Well, I'm not exactly an expert on oppies, and you've been getting some pretty good advice so far, but one thing I'm wondering.... When you bought the tractor that has fuel spray problem, did it do it then? We had one that would constantly spray fuel out the carb. Come to find out it was missing the compression ring. Also, two slightly different pistons could cause that. It's not likely if you haven't had them apart before, and impossible if they ran right before. But just thought I'd ask.

You guys have given me some great info, thank you. Any other input is always appreciated  

Murraymountain wrote:

Well, I'm not exactly an expert on oppies, and you've been getting some pretty good advice so far, but one thing I'm wondering.... When you bought the tractor that has fuel spray problem, did it do it then? We had one that would constantly spray fuel out the carb. Come to find out it was missing the compression ring. Also, two slightly different pistons could cause that. It's not likely if you haven't had them apart before, and impossible if they ran right before. But just thought I'd ask.

I believe it did. I hadn't had this one running for about a year and a half cause the starter died and I didn't have money to buy a new one at the time. Now that I'm thinking about it, I remember both tractors doing it, and it would even pool up in the bottom of the intake after turning the tractor off and letting it sit for a bit. I have yet to take one apart, but it seems fairly simple compared to the v8s and opposed 4 cyls I've taken apart and rebuilt. I will see if I can clear up a space and maybe tear into one at some point to see what's goin on.

mrshaft696 wrote:

did you clean the carb? If it runs the same adj screw all the way in or out something else is going on....

Either the carb/fuel pump needs rebuilt, or cleaned.

if it was mine, in priority order I would
1: rebuilt carb/fuel pump
2: remove intake check gaskets
3: remove heads check gasket and valve clearances, clean the carbon
4: while the cover is off check the ignition coil, clearance etc

with all that apart you should pretty much be able to see anything that is easy to mess with....but start simple.

I have not done a great cleaning of the carb, just cleaned a bunch of green and white corrosion from ethanol gas. My only other concern is that I've broken head bolts on one of my parts tractors with the same engine. Is there a way to help combat this and not break them as easily, or am I just gonna have to deal with it? Thank you for the advice btw

To set the coil gap, I use a paper business card and slide it in between the flywheel magnet and the coil. That sets just the right gap and is the easiest way I've found to do it

CraftsmanQuad19 wrote:

To set the coil gap, I use a paper business card and slide it in between the flywheel magnet and the coil. That sets just the right gap and is the easiest way I've found to do it

I will check that to make sure everything is happy and functioning correctly. Thank you

So here's a pic i found of the bottom half of an oppy carb. If you can make it out there, the brass insert the bread tie is sticking out of is the overflow jet for the main jet. It lets excess fuel out of the main jet system if need be. If this small jet were plugged partially or fully, excess pressure wouldn't be able to escape the system, and you'd likely have excess fuel enter the carb on acceleration.


As i recall, every oppy I've had blow fuel out of the intake on acceleration was also surging at the time, and a carb clean fixed both issues.

RichieRichOverdrive wrote:

As i recall, every oppy I've had blow fuel out of the intake on acceleration was also surging at the time, and a carb clean fixed both issues.

That sounds like exactly the problem I'm having with the carbs on these tractors. Sounds like I will have to get out there and take the carbs apart and clean 'em out real good later once I'm done doing school crap. I guess I'll take a bread tie out with me to help clear out the jets and other orifices haha

Yeah, if you have a set of torch tip cleaners or even some old guitar strings those tend to be a bit easier to work with, but really any piece of wire small enough to feed through the jets should work.

I'll look around for stuff like that. Thank you for the ideas.

Also make sure the vent hole for the float chamber isn't partially blocked (when you put the aircleaner base back on). Things will run stupid when you do that. Ask me how I know....

Sounds like a plan. Hopefully the carb isn't too gummed up and is relatively simple to revive.

RichieRichOverdrive wrote:

So here's a pic i found of the bottom half of an oppy carb. If you can make it out there, the brass insert the bread tie is sticking out of is the overflow jet for the main jet. It lets excess fuel out of the main jet system if need be. If this small jet were plugged partially or fully, excess pressure wouldn't be able to escape the system, and you'd likely have excess fuel enter the carb on acceleration.


As i recall, every oppy I've had blow fuel out of the intake on acceleration was also surging at the time, and a carb clean fixed both issues.

that is pretty gross, and pretty typical of ethanol fuel... I cleaned these so many times that I ended up buying the ultrasonic cleaner from HF.... So depending on how long it was neglected you might be able to get it going...

I have had a few of those carbs so gunked up with gross ethanol crust that i threw them away, but you look like you got the right idea with the wire etc... My method now is spray with carb cleaner, ultrasonic if needed, then carb cleaner again, then blow out with compressed air....

Sounds like I might need to invest in an ultrasonic if my carbs are that bad. What cleaner do you put in then mrshaft696?

maxwellvis3 wrote:

Sounds like I might need to invest in an ultrasonic if my carbs are that bad. What cleaner do you put in then mrshaft696?

this is the one i have https://www.harborfreight.com/25-Liter-Ultrasonic-Cleaner-63256.html
no complaints other than you can set it to go for any longer than a few mins.....


I use hot water from the tap and dish soap....water has to be hot... I only use it when I am doing a full rebuild and taking out all the gaskets etc.

I would finish your manual cleaning first & see what you get, the oppy fuel pumps are kind of a pain but after you have done it 2 or 3 times it makes sense where the little parts go etc.

mrshaft696 wrote:

the oppy fuel pumps are kind of a pain but after you have done it 2 or 3 times it makes sense where the little parts go etc.

Thank you for the link. Yeah, I've taken the pump part on mine since or twice just to make sure no nasty grime had made it's way in, it shouldn't be too bad.