Hi (first forum post
)
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
Thanks!
Edit: I would like to add that all of my Opposed Twins have the correct float levels applied (when it comes to the Walbro carburetor, Nikki isn't adjustable), no apparent vacuum leaks, and strong spark.
Subject: 
November 22nd 2021, 5:10 pm
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.
