I just completed a carburetor modification on my 2008 S-40. The bike has close to 800 miles on it and while the backfiring was not a big issue I did have a little at times. I used Lancer's kit and for what it's worth here are some suggestions. I first bought one of Harbor Freights small impact drivers. It was very inexpensive and had several bits with it. I NEVER EVEN ATTEMPTED TO USE A PHILLIPS SCREWDRIVER TO LOOSEN ANY OF THE BOLTS, I broke every one loose with the impact driver. It worked flawlessly. The bolts were as follows: The diaphram cover had 4 M5 bolts 9.5 mm long from the head down. The bowl cover had 4 M5 bolts 16 mm long at the thread portion. The clip bolt was an M4 and 5.5 mm long. Because I used the impact screwdriver, those 9 bolts were removed in perfect condition. I will locate some socket head, stainless steel bolts before I take the carb off next time and will replace the Phillips heads at that time.
My carburetor had the factory 145 main jet and the white nylon spacer. I replaced the 145 with the 155 from Lancer's kit and replaced the white nylon spacer with 2 of the 4 washers in Lancer's kit. I replaced the pilot jet just because it was included in the kit. The bowl gasket was included but I didn't need it. After completion the bike started immediately but I had to increase the idle stop slightly to keep it from dying. I had removed the brass plug previously and turned the needle valve out approximately 1 turn. I now screwed it back in until I noticed a slight increase in RPM. I now notice a remarkable change while running in 1st gear, the engine no longer "shudders" along in first.

I will let you know if the mods affected the gas milage. I really don't care as I have been filling at about 100 mile intervals and the bike runs a lot better than it did before. My butt can't take long rides anymore so fuel stops are a welcome relief. I do understand your concern. I toured Spain in my younger days so I appreciate getting every mile out of a bike between fill ups.

Many years ago I bought a Triumph Bonneville 650 while in Spain and took out on a couple months tour of southern Spain and Morocco. It was really critical to know how far you could get between fill ups because many of the smaller towns and villages didn't have gas stations. That being said, the only time I got into any trouble was one time when I forgot to switch back from reserve and ran out of gas. I manage to get a Spaniard to drain a wine bottle of gas from his cooking stove and made it to the next town about 10 km down the road. A big road cruiser is great for the highways here in the states but the roads I traveled in those days, a big bike would have been a liability. If you ever get a chance to take a long tour don't pass it up. I would recommend doing it with a good friend.

Roosky: Gibraltar is attached to Spain with a road. It has all the same earmarks of going through any border checkpoint. Back then the Spanish hated the fact that Gibraltar was a British "country" so they imposed a 20 minute time delay on every vehicle going into Gibraltar. The beer on Gib. kinda sucked but the fish and chips were great. My Spanish was somewhat limited but it was surprising to find a lot of Spaniards who spoke English. When the bike arrived it had places for license plates on the back and along the front fender. While waiting in line to get into Gibraltar one of the  La Guardia Civil" (Franco's special police) came over and asked me if the "M" that prefixed my license number was Madrid. I perked up and started listening to their conversation. One of the officers said the color of the numbers was wrong. It dawned on me then that I had simply stenciled my numbers in white paint to the black metal plate area. It should have been black numbers on a white background. Some things were comical but others could have had serious consequences for me. I only had one close call while on the ride and since I was diving for the ditch and my bike escaped damage I probably would have escaped too. The "But, if..."is a whole different story. Sometimes an inch is as good as a mile. By the way, the La Guardia just shrugged their shoulders and let me pass.

Roosky:
I guess I got caught about my age. Yes, it was a long time ago. The Bonneville was a new 1965. When you applied for license plates you were issued the numbers and it was up to you how you displayed them.There was a ferry that went to Morocco from a small town near Gibraltar. Talk about a spooky place, Morocco was scary even in those days. Morocco had just recently gotten their independence from France so there two curencies being used. French francs and the Morocco franc. The French coins were worth 10 times as much as the Moroccian coins. The French silver coins were bronze, the bronze, aluminum and so on. I didn't stay long in Morocco as it wasn't a very safe place for an American. As a side note, I am probably one of a very few people who ever got bitten by a rock ape on Gibraltar. I didn't think it was much of a big deal but a German lady tourist almost went nuts trying to find their first aid kit. Golly! those were great days. I've kind of gotten a ways off track from the orginal post.

rooksky: I didn't venture very far into Morocco. The hotel I was staying in had me park my bike in the lobby (which was huge) for safety reasons. I was allowed to get off the ferry because the customs people mistook some green card I had in my billfold as the required green insurance card. I don't think you would want to travel without insurance overseas, especially in Morocco. ( I had been told I could buy insurance after I got to Morocco but you had to have it to embark) I went downtown to the market one evening and was followed back to the hotel by several very unsavory people while ducking from building to building to "escape".
I was anxious to get back to Spain where the people were friendly and helpful. I had the Triumph shipped back to the states and rode it from Philladelphia home to Kansas. This was in about February. I have never been so cold and miserable in my life. I barely had enough money left for gas and boy, I sure could have used some motel money. I hit an oil slick in Ohio, laid the bike down and slid through an intersection. My right foot flew up and hit the handlebars so hard I was forced to get a motel and soak my foot all night. Aother guest kickstarted it for me the next morning and I made it home in one piece. I guess something good can be said about the vibration of a Triumph because by evening I could walk on it again. I hope I am not boring you with my chatter. I could go on and on. Thanks for reading.
Before I forget, I rode that Triumph for several more years and then feeling sorry for my brother, sold it to him for a couple hundred dollars. It had a useful life until sometime in about the 80's when he sold it.

Royal:
I wouldn't call it chatter. It's your story. I'm a teacher and I've come to think that the most important things we tell our students are not directly related to the subjects we teach, but the stories we tell of our lives. I've noticed that the students listen to the stories much better than the stuff we tell them about the subjects we teach. We learned from the experiences that created our stories and since they want to learn (not necessarily in a school way) they do listen to our stories.

This discussion should probably be moved to the "Cafe", no?

Well, I was wondering today, while riding my S40 to work for the upteenth time, what made you decide on the S40? There must be a reason.

BTW, I'm going out of town to do some turkey hunting, so I may not reply until Tuesday to anything that comes my way.