Carburettors - more than you wanted to know (using the example of Mikuni TM38s)

Now there's plenty of good and some really dreadful advice out there on how to setup carburettors and yet I plan to add one more to the list, mainly because I haven't found some good info on how to set needles correctly, because that's usually where most people tend to go "a bit funky" and just mess around to the best of their knowledge.

At first, let's flog a dead horse by having a look at the following schematic:

Source: Mikuni Corp, 2012

As you can see, this is a slightly newer version of this well-known graph that contains diamond-shaped indicators to highlight the fact that a carburettor will NOT digitally switch over from one jet to the next, but that multiple jets (within reason) affect the performance at various points in the slide lift. (And that's completely omitting the fact that the engine has rather varying air- and fuel demands depending on the terrain you're driving on!) But there's more that's missing in the graph, because it assumes that the setup is correct. Not to worry, we'll get to that point.

Let's just simply follow the carb slide through a usual day at work and identify the problems I encountered. (and most likely you did too, because quite frankly otherwise you probably wouldn't be reading this!)

For coldstart we obviously need to pull the choke knob and enrich the mixture and then run the bike (mainly) on the pilot jet and the additional choke jet. There's mainly two things aside from the choke affecting the mixture at this point and that's the pilot jet and the slide cutout. (called throttle valve in the schematic above) Now it should be obvious that the smaller the slide cutout the less air goes through at the same height of the slide, also the slide cutout basically determines how steep the curve rises that determines how much air gets to the engine at very low slide opening. This (within the limits of the engine) determines how "snappy" an engine will feel to the user. Additionally a smaller slide cutout will help to keep things more moderate and make jetting the pilot easier.
Real world example: a VM38-9 has got 2.5mm slide cut-out and a TM38-86 has got 4mm. The jetting on a Yamaha XV1000/1100 engine is pretty close in both instances, but trust me, a TM38 with the bigger slide cutout will feel a lot snappier.
Pitfalls (or also why you shouldn't start jetting from the bottom up): For rather obvious reasons it would be very tempting to start jetting from the bottom up, because then even if everything else fails, you at least would end up with a decent idle. And here comes the catch: as I pointed out before the schematic above only applies, if everything is working perfectly and even then the whole system is rather dynamic, meaning if your jetting is off, it simply doesn't add up. Assuming that for example your mainjet is dramatically oversized, it will overlay and obscure incorrect (lean) jetting in other areas. The art of jetting carbs is knowing some recipes, which usually work and then apply some common sense, e.g. if you run a #300 main-jet and the smallest pilot jet IIRC a #12.5 and your bike is still running so rich it will fire up without using the choke, there is a strong reason to believe that maybe your pilot jet is not the reason. Let's assume you get past this point and your jetting is somewhere in the vicinity of #20 pilot and a #185 main-jet. (Seems to be roughly the golden numbers for a Yamaha XV with a TM38, when sucking through the frame with a correctly sized airfilter and a free flowing exhaust.) Yet still it seems to idle weirdly when cold and once hot it will race to stupidly high rpm or even worse, when really hot will fall on its face again and not idle properly. The next thing is to look at the slide height: One of the standard pitfalls to compensate an overrich pilot is to raise the slide until it will idle just fine. This will then result in climbing idle for a bit only to have the bike lose idle once it has warmed through.

Next let's look at mid throttle openings. As stated before when the slide opens, the slide cutout is the main determining factor for how quickly the amount of air increases that gets into the engine. In parallel, as can be seen in the picture above, the carb needle isn't a straight control rod either, but is tapered. And now this is where things get tricky, because there's a lot of things that can be adjusted at the same time and even though on paper the results in AFR will be comparable the consistency of maintaining said target AFR. This brings us to the resulting question: Which options are there and what does what?

1. The needle taper controls the steepness of the curve (progression). A thinner needle will mean a richer initial mixture and depending on how steep the needle is, it will mean also affect how rich the mixture is at max. slide opening and thus affect the main-jet.
2. The clip position will set up where the starting point on the taper is. This can be very helpful, if you run a needle with a relatively flat taper, but almost impossible to use with a needle with a steep taper as even small increases dramatically change the effective diameter of the needle in the needle jet. (And that's completely ignoring the effects of worn carburettor needles, which may have a ridge worn into them at one point...) It has to be noted that some carbs come with needle shims to somewhat compensate for this, when quite literally the setting changes that can be achieved with the clips are too coarse.
3. Lastly the needle jet, this is the only linear modifier in this equation. A bigger needle jet will quite simply increase the amount of fuel all the way from bottom to top. 

So in order to choose the right needle, needle-jet and slide combination a bit of seat-of-the-pants tuning is mandatory. The easiest case would be that for some reason your bike idles like a champ and once the revs have climbed sufficiently full throttle makes your bike go like a scalded cat. What you would do next is to lower the clip a bit and see if that improves the situation. For reasons outlined above that will improve pickup at  low to mid throttle openings, but will maintain a large gap at 3/4 throttle until it picks up and it runs nicely again on mostly the mainjet. So you lower the clip once more and now it's better at the last bit, but all of a sudden it falls on its face up to about half throttle. What you established now is that you need a bigger needle jet and then have to re-check, if the needle taper matches your desired engine characteristics. You could try to off-set the situation by running a smaller pilot jet, but this will lead to a much inferior idle, in the sense that it will become very temperature-dependent. And this is where the varying tapers also come into play. The exact tapers for a certain are simply not published, so you have to get your own micrometer out and determine what you have in your carb and then get a needle that matches the data as close as possible.

Source: Mikuni Corp, 2012

As you can see this list contains the recommended needles for use in a Mikuni TM38. All of them are from the #6 family and the leanest three are of the 6F-type, meaning their initial diameter is 2.522mm. The subsequent letters and numbers outline where the taper starts and how steep it is. Also just to confuse you: the code overall doesn't say anything about the length as these are all around 68.x mm long, whereas a 6DP01 is 62.2mm long. After doing quite a bit of extensive research on the difference of the two-stroke (snowmobile) and four-stroke (TM38-85 and -86) variants of the TM38 carb, it turned out that there literally only where two main differences aside from main- and pilot-jets that catered to vastly different fuelling requirements. Basically what Mikuni did to turn a TM38 into a two-stroke carb was to add a slightly richer needle and install a restrictor into the air-port in the back and thus making the mixture a lot richer at part throttle. (Which is where most seizures on two-strokes happen during the setting up of carbs...)

The last stop in our journey is full throttle. Even though this is the point, where the most can go wrong, it's also the easiest to jet. Following some gutt-feeling and experience one usually starts this "a tad" richer than is expected to be right and once the bike has warmed up, literally just hammers the proverbial snot out of the engine by doing full throttle pulls. Installing smaller mainjets until the performance can be felt to go down, pinging occurs or the oil-temp goes through the roof. (Usually it''s a combination of all three, but with a bit of careful testing one can keep this within safe limits at the very first point of the list.) With the orifice of a main jet being so vastly bigger than all of the other jets that's also why you usuall start there and try to get the mains right for a start as mistakes there will trickle down all the way and even affect pilot jetting.

So where does this post come from: Basically I was trying to organize my thoughts and trying to work out, what exactly needs to be tackled to get the TM38 carbs on my Everyday TR1 working perfectly. The situation is as follows: Bike idles fine when cold and up to a certain temperature, even though the RPMs vary a bit, once really hot (as in above 110 degrees C of oiltemperature) it completely falls on its face and will only idle when the slide is lifted a bit, clearly indicating an over-rich pilot. At the same time when up to temperature, there will be a certain amount of pinging at low (1/4 to 1/2) throttle opening indicating a relatively lean mixture at that and the mainjet is quite substantially larger (#185) than on my old VM38-9 carbs (#165), even though the effect is less felt at wider throttle openings.
Following the long winded text from above everything is pointing towards a somewhat too lean needle and as the problem isn't really linear (especially as the main-jet is actually about right, keeping the engine oil-cool, no detonation, power seems about right and the plugs aren't fouling), it all points towards the needle I am using isn't perfectly right for the application. As I actually ordered the new needles before finding the chart above, I now have a set of 6FJ40 needles to play around with. (Once the snow and salt are gone, some serious testing will commence!) I expect to finally be able to decrease the size of the pilot by one size and go back to a #20 and overall experience lower engine-oil-temps as the bike isn't running on a lean midrange.