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 My Experience & Question With B&S Opposed Twin Carburetors

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Brianator
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PostSubject: My Experience & Question With B&S Opposed Twin Carburetors   My Experience & Question With B&S Opposed Twin Carburetors Icon_minitimeMon Nov 22, 2021 3:10 pm

Hi (first forum post Smile)

So I wanted to chime in an give my few years of experience with both the Walbro and Nikki carburetors for opposed twins after having worked on opposed twins for over 5 years now.

------------------------------------Walbro--------------------------------------

Since day one of getting my first opposed twin in 2015 on an old MTD vari-drive, I have mostly fought many different Walbro's start to finish trying to get them to run right, but I haven't really succeeded. Over that time I have collected a total of 3 opposed twins being a 19.5 HP, 18 HP (my first engine), and a 16 HP all of which are 1990's models that came stock with a Walbro, commonly referred to as the "3 screw" carburetor.

I have at least 6 Walbros lying around (lost track) some of which came on my 3 engines, others I've bought hoping they would run better than the last after receiving a full rebuild, including buying some Zenith / Gravely 3/8 adjustment needles, drilling out the main jet, and replacing the bowl plug with the adjustment needle for high speed mixture adjustment on the later model Walbros that didn't come with it from the factory.

I also have a couple early model carburetors that came with both idle and high speed mixture adjustments from the factory, 1 of which I got running but it wanted to stall at idle if I let off the gas and came to a stop for 10 seconds or so, which I wasn't ever able to figure out despite cleaning the carburetor more than once.

My 3 biggest frustrations with these carburetors include:

Acceleration

The Walbros are notorious for having a massive flatspot when you step on it, some of which can be helped by richening mixtures but only to an extent and your engine will most likely run rough. I can't tell you how many times I've stepped on my gas pedal (especially in cold weather) and the engine just dies, even when fully warmed up and richened.

Power

In my experience, you are guaranteed to loose at least a couple HP by running a Walbro over a Nikki. I have two tractors that I have noticed this on, both are pulley swapped, both still governed (which I believe makes this issue more apparent than if I were to go ungoverned). A perfect example of this is a little incline that I have in my backyard where going up it in a mid to high gear causes the Walbro to bog ever-so-slightly (which has the governor compensate) whereas the Nikki acts like the hill isn't even there and goes right up it without loosing power - I have reproduced this on all three of my opposed twins.

Adjustability

For some reason that I haven't managed to figure out, the Walbros, even when fully adjustable seem to be very difficult to adjust sometimes, especially in colder weather. Sometimes I'll fire up one of my tractors to go out and drive around, or do yard work with them and yet they aren't running good, being too rich, or too lean. So I'll stop and adjust the high speed mixture or idle depending on what needs adjusting, but sometimes the screws just do little to nothing at all, commonly found in cold weather. Just a few weeks ago, I took my 16 HP out to move some gravel with a trailer in 40 degree weather and even though it hadn't cooled off all that much outside (around 10 degrees) the tune was miles off, causing the governor to surge on the high speed circuit despite it running great the last time I had it out, and no matter how much I turned the mixture screw, it made little difference so ultimately I had to finish the day with the choke half on, otherwise it was very difficult to drive.

Lastly, to address the elephant in the room, yes all these troublesome Walbros were cleaned thoroughly (some more than once) and I always run clean, fresh, ethanol free gas in them, with fuel filters so they are in the best possible conditions they can be in.

------------------------------------Nikki----------------------------------------

Now onto the Nikki, or the "4 screw" carburetor. My first Nikki was in the summer of 2019 as after fighting the Walbros for so long I wanted something different and I ended up coming across some cheap, chinese Nikki carburetors on eBay (I know, not the best) to which I bought one and put it on my 18 HP at the time.

Initially, I felt like the hunt for a good carburetor was over as it actually got off the line with no flat spots and had a noticeable more amount of power, but there was just one big problem - no mixture adjustments (thanks EPA). Where I live, it's 90 degrees in the summer (when I first got the carburetor) and below 30 and snowy in the winter, so as soon as the cold hit a few months later, that carburetor was now too lean and surging unless I ran with the choke on halfway, which isn't ideal.

Now came seeing if I could somehow at least make a main jet adjustment but that fell short quickly as Briggs thought it would be a good idea to make the bowl plug a metric size so that if you wanted to swap it out with an adjustment screw, you would have to get something custom made. Additionally, the location of the idle mixture screw on the Walbros was now capped of with a brass plug, though I believe that could be removed, tapped, and fitted with an adjustment screw which I'll get to later.

So that's where I am now. As you probably gathered from this piece, I am not interested in completely transforming my opposed twins at this time, which is why I still run them with their original style carburetors, air box, and governor with the biggest change I've made being a better, louder exhaust.

In the end, I know a lot of people would say that these engines and carburetors were just designed for mowing grass, and not driving at high speeds with varying throttle positions, and yes, I get that, but I know Briggs And Stratton could have done better with these (namely the carburetors) because they *have* done way better as evidenced by them having fixed the massive flat spot in the Walbro with the Nikki design, so in my opinion it's a matter of them not wanting to do the right thing on day one with the Walbros but due to so many people experiencing frustrating flat spots with their engines, B&S did eventually fix the issue. And from the Quantum to the old all cast iron engines from back in the day, I do not know of a carburetor B&S put out that have as many problems as these carburetors (especially the Walbro) do.

--------------------------------End Question------------------------------------

Now, my question after all of this is do you know if a shop of some kind (e.g. machine shop, carburetor shop) would be willing to modify and fit a Nikki carburetor with custom idle and high speed mixture adjustments, and if so, would you recommend a specific place in the US? I doubt it would be very cheap (especially since I would have it done on a OEM Nikki) but if it could be pulled off, I believe it would create the ultimate stock style carburetor for these engines.

---------------------------------Conclusion-------------------------------------

In conclusion though, I've really come to enjoy running and working on these opposed twins for the last 5+ years and I couldn't see any other kind of engine in my tractors at this point, I just want them to run properly, not perfect, but properly at least. Let me know what you think about this.

I apologize for the long read Rolling Eyes

Thanks!


Last edited by Turbo360 on Mon Nov 22, 2021 5:12 pm; edited 1 time in total

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Brianator
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PostSubject: Re: My Experience & Question With B&S Opposed Twin Carburetors   My Experience & Question With B&S Opposed Twin Carburetors Icon_minitimeMon Nov 22, 2021 4:38 pm

Nice write up and welcome to the ATLTF! I'd like to encourage you to quickly introduce yourself in the Introduce Yourself Here! area so everyone can get a chance to say hey and get to know you a little.

I personally don't have experience with those specific carbs but if they're anything like the older B&S 2 piece updraft carbs, the carb body halves love to warp and wreak all sorts of havoc and cause tunability issues. If they weren't near impossible to plane it wouldn't be so bad but the one half is a real bugger.

Again, nice write up. I like that you thought it out, have it organized and easy to read, well done!

That's all i got, see ya around! Lol

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PostSubject: Re: My Experience & Question With B&S Opposed Twin Carburetors   My Experience & Question With B&S Opposed Twin Carburetors Icon_minitimeMon Nov 22, 2021 4:40 pm

Welcome to the forum!
I would like to find a twin cylinder engine some day. I love the sound of them.

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PostSubject: Re: My Experience & Question With B&S Opposed Twin Carburetors   My Experience & Question With B&S Opposed Twin Carburetors Icon_minitimeMon Nov 22, 2021 5:10 pm

Thanks for the welcome and kind words!

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PostSubject: Re: My Experience & Question With B&S Opposed Twin Carburetors   My Experience & Question With B&S Opposed Twin Carburetors Icon_minitimeMon Nov 22, 2021 6:59 pm

Welcome!  Glad to have ya here!

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Have a question? Ask me! Type in "@MightyRaze" in your post!

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PostSubject: Re: My Experience & Question With B&S Opposed Twin Carburetors   My Experience & Question With B&S Opposed Twin Carburetors Icon_minitimeTue Nov 23, 2021 11:16 am

i have an older twin carb from a older style and ive also got a complete newer 1990s 20.5hp twin i just havent use it on a tractor yet and i plan to one day! i had to refresh my starter and thoroughly clean it. WELCOME!

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PostSubject: Re: My Experience & Question With B&S Opposed Twin Carburetors   My Experience & Question With B&S Opposed Twin Carburetors Icon_minitimeSat Nov 27, 2021 8:27 pm

You got alot of knowins on carb workins, it seems. I would suggest to check the float level in the carbs. To low of a fuel level in the bowl makes it hard to pull fuel out of the bowl and into the venturi. The lower the fuel level, the harder the vacuum has to work to pull the fuel up, explains your hesitation maybe. If that seems good, I would look into the ignition system. Bad spark plugs would cause the issues you mention. A bog or dead spot, motor wanting more fuel when it's warmed up. I would suspect bad spark plugs or going bad. Next, would be a bad coil losing spark when it is heat soaked or getting hot. Next, I would look for a vacuum leak on the intake.

I know alot about carbs, any kind. They all work the same. I believe there is no such thing as a bad carburetor design since they all work the same. You should be able to get the performance you want out of the 3 bolt Walbro carbs, as you would out of the Nikki carbs. I think there are other issues going on.

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PostSubject: Re: My Experience & Question With B&S Opposed Twin Carburetors   My Experience & Question With B&S Opposed Twin Carburetors Icon_minitimeSat Nov 27, 2021 8:28 pm

Oh, welcome to the forum

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PostSubject: Re: My Experience & Question With B&S Opposed Twin Carburetors   My Experience & Question With B&S Opposed Twin Carburetors Icon_minitimeToday at 3:02 pm

Thank you for the info and tips. Unfortunately however, all of the possible issues and effects you mentioned are not present on any of my Opposed Twins / Walbro carburetors, which I have verified.

Float Level

The float level in all of my running Walbro's is set correctly - as in when the floats are in their highest position they are 1/4 of an inch from touching the top half of the carburetor, and when looking at them in this position, they appear "level" or parallel with the top half, if that makes sense. Additionally, I have a genuine B&S Opposed Twin repair book which does state that this is the correct float level for the Walbros. Something else I would like to note relating to vacuum is that all 3 of my engines are in good condition, with a moderate to low amount of hours on them, good compression and all have had frequent oil changes under my ownership.

Ignition

I can confidently say that all of my engines have good, strong spark. In fact, my 18 horse has a newer coil on it that I put on just a couple years ago. The coils in the other engines are still in great shape, including the wires and ends that snap onto spark plugs.

Spark Plugs

For the past few years now, I've always ran E3.10 spark plugs in all of my Opposed Twins (and all other flathead small engines I own) which I've had great success with. I have ran Champions and NGK's in the past, though they don't help or worsen these issues with Walbro carburetors.

Thanks again.

Edit: I completely forgot to mention this Laughing but I have checked for vacuum leaks as well, but came up empty on both the carburetors themselves and intake manifolds. Both my 16 HP and 19.5 HP twins have brand new gaskets on the intake manifold to intake port surface as well.
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PostSubject: Re: My Experience & Question With B&S Opposed Twin Carburetors   My Experience & Question With B&S Opposed Twin Carburetors Icon_minitime

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