1200S: service time, part 10

You'd be forgiven for not remembering about the Sportster's ongoing overhaul. Since the last time I was able to work on it, almost FOUR MONTHS have passed as I waited for parts to arrive. These delays in the supply chain are infuriating and unjustifiable. Better to crack on with it:

CARBURETOR

The reason I'm getting into the carburetor is to improve what's already there and tune it keeping in mind this motorcycle now has a pretty unrestricted air filter and the wonderful Vance & Hines SS2R exhaust (fun fact: that exhaust system was developed for the AMA Harley-Davidson Super Twins class around the mid 90s, and debuted at Laguna Seca).

Before even getting started on this, it's a good idea to have a rebuild kit available, as you'll probably find that certain items will need replacement:

(Yes, if you look closely you'll notice that that brand-new-still-in-the-package gasket is torn. Great.)

It was during the Stealth Trip 2 that I had noticed the engine felt like it was running a bit lean, and the spark plugs would seem to indicate as much, as the electrodes look pretty white.

Nowadays with electronic fuel injection being ubiquitous, and general owner disinterest being prevalent, the concept of "tuning your machine for best performance" is somewhat of a moot topic, but those like me, who appreciate a carburetor and strong, simple air-cooled engines, still know what time it is, if you know what I mean.

I became a staunch Dynojet enthusiast after fitting one of their "stage 1" kits to my old Honda;
I was completely surprised by just what a big difference it made for such a (seemingly) simple modification, to the point that I can't help but wonder how much better my other bikes would run if they each had one fitted.
For example, I thought about the BMWs and how that would work; or would the little Yamaha gain some much needed horsepower and torque? In both cases I don't think a kit is available, though you could probably make your own if you reached out to Dynojet and asked for advice.
Now, what is available is a kit for the Sportster, and for pretty much any variant of the Sportster in every state of tune. Since I'm trying to make this bike be as good as it can be without compromising tractability and real-world usability, this is something I had been mulling over from the start.
First of all I checked what was actually in the carburetor (just in case it already had Dynojet components fitted) and what I found was a pretty stock set configuration, albeit without the welch plug over the idle fuel mixture screw. The latter had been turned way out, at just over two and a half turns from seated, which is extreme for a stock motor -if compared to the factory setting of 1&⅛ turns to comply with emissions regulations- and must have been an attempt at enriching the mixture after fitting the exhaust*. Obviously this does absolutely nothing where it counts, and only affects the idle. This is confirmed by the overall lean mixture I noticed.
The kit that suits my motorcycle is the E8103 and after an unacceptable delay of over three months, it finally came with definitely more stuff than what the Honda got: there is a comprehensive selection of main jets, the all-important needle, as well as a drill bit for the slide (how daring!), a spring, assorted clips and washers, and an emulsion tube.

Above: as per Dynojet's instructions, the 195 is appropriate for my bike, with an aftermarket exhaust and air filter. That said, I can go up to a 200 if needed, and I can raise or lower the needle as well, meaning I will get to optimal carburation no matter what.

Installation is very simple, you just have to be careful and use gentle hands; this is something that calls for finesse, not "Hulk smash!"...

I must admit that drilling out the slide made me a bit queasy, as it's obviously irreversible, but what the heck, no guts no glory!

A 1200S in stock/factory condition should make about 65bhp. With a good air filter, Dynojet stage 1 and the Vance & Hines, we should be looking at 72/73 real bhp (I am aware that this puts me squarely in the "danger zone" for the dreaded oil pump drive gear failure, I will discuss this with a competent mechanic and make a plan.).

Any work on the carburetor must be approached with extra caution, as you are dealing with small, delicate parts that can be very easily damaged. And damage here means lots of cash, so, you've been warned. I see one overall pitfall one could fall into when working on a carburetor, an involuntary misconception that "this is a small, self-contained component, therefore it's simple"; obviously it is precisely the opposite, this is complicated and delicate, and requires attention to minute details.

It is especially important to use properly fitting tools (such as the screwdrivers to remove the jets and float bowl screws, and the drift to remove the float spindle, although I wouldn't do that unless absolutely necessary) and to be very careful if using compressed air: it may seem like a good idea to blow 90psi down an air or fuel passage, but you might inadvertently blow some debris into it, with predictably bad results. It's better to carefully clean the outside before opening anything up, and always keep parts accounted for and safe from damage while on the workbench. The next photo serves to illustrate two things, the first being why you should clean the outside of the carburetor before opening it, the other is the serial number, which you'll need when ordering parts:

Make a note of all settings as found (air/fuel mixture screw, the idle position, needle position, and of course jet sizes) and don't assume that what you find is necessarily correct: some idiot might have been in there before you and forgotten a washer or reversed the order of some parts, inserted a needle seat upside down... you name it.

In my case this is what I found:

• air/fuel mixture screw set at 2.5 turns from seated, without original welch plug
• idle stop with 8.8mm of threads exposed
• stock non adjustable needle (pretty grimy)
• stock pilot jet (this is pressed in and not meant to be removable. It is also what meters fuel to the "enrichener" circuit and works in tandem with the small tapered needle that you actuate when starting the engine from cold)
• 42 slow (or intermediate) jet
• 195 main jet
• standard emulsion tube (filthy and crusty)
• standard float height 
• some dirt and impurities in the float bowl and related passages

The smallest, imperceivable defect could lead to an engine that's not running quite right, and you might have everything installed correctly, yet be unaware that a little bit of dirt left in a float bowl passage is causing that problem. Be thorough, be methodical and go over everything. 

While you're here, probably not a bad idea to thoroughly lubricate the throttle cables...

With the exception of checking that the accelerator pump nozzle is correctly oriented to shoot down the center of the bore, I wouldn't do any of this with the carburetor on the motorcycle, way too uncomfortable and too great a chance of damaging something. Bring this to the workbench and get comfy.

Above: stock needle at the top, Dynojet item below. Notice grooves to adjust mixture (with an 'e' clip, shown in the next photo).

Next, a look at the emulsion tubes, stock on the right, Dynojet on the left. It's easy to see there is a big difference and it results in two different types of acceleration. While the stock item probably delivers a smoother transition as the throttle is opened, the Dynojet delivers snappier response. Amal did something similar for the police model of the Triumph (nicknamed the "Saint"), in order to help officers quickly catch up to all sorts of troublemakers, agitators, communists and run of the mill lowlifes; it was a needle jet without the cross drillings, which supposedly gave brisker acceleration. Who knows.

Assorted notes:

You can check float height (after tilting the carburetor to the correct angle, see workshop manual), using a vernier caliper as a depth gauge.

If you need to remove the float pin, pay attention to the arrow cast in the carburetor body, as one end of the pin is larger than the other, and knurled.

Once reassembled, tip the carburetor to simulate position on parked motorcycle (kickstand test) and check for leaks. You need fuel in the bowl for that...

Obviously check the diaphragm and accelerator pump membranes: they must be supple, free of punctures or tears, and they should not be deformed in any way.

Above: properly clean and properly assembled. The two large bolts on the saddle are the breathers for the cylinder heads.

A note about marketing ploys, fools and their money (they're easily parted!): there are many products on the market that don't really do much of anything. Some call them "the answer to no question", others claim that something like a Dynojet kit is just expensive and unnecessary, and that you can get even better results with just a larger main jet that will only set you back a couple of bucks. Let me be clear: Dynojet kits really work as advertised, and if they seem expensive "because it's only a few small bits" then I must assume those people don't truly understand how carburetors work. I appreciate that I won't know for sure until I try it, but I am confident enough to bet on a noticeable gain in overall performance and throttle response. Watch this space, I'll write back to either confirm or eat humble pie.

Now, obviously any extra performance comes at a cost, and yes, gas mileage will suffer as a result... but not by much. And trust me when I say that when you're on the road enjoying the bike as it was meant to be, it really won't matter at all.

*remember, this screw meters fuel, not air.