I can honestly say now that the reason the original airbox removal wasn’t that impressive is because there is a lot more to this story than I initially understood. Let me explain.
Yesterday, on the 4th of July, I did some additional work on my 2008 Bonnie. I removed the seat, gas tank and side covers, then removed the carbs.
I did this partly to install TPUSA Billet Manifolds (black) purchased from New Bonneville (see next photo).
So I removed the carbs and the original OEM manifolds, and installed the billets.
But alsoI also wanted to dive deeper into the carb adjustment details that I had been reading and studying in a PDF file I found on the web. The PDF is published by Jenks Bolts (JB) out of the UK, and is the most complete document I have yet found on carb tuning of Hinckley Bonneville’s: Tuning jets, idle mixture, needles, shims, slides and other information for Hinckley Triumph Bonneville Keihin CVK Carburetors (updated 2007 version).
In this document I learned that with my particular setup with: airbox removed, air injection removed and free flow exhausts, they recommend: adding 1 shim to the needle valves, and using a 140 or larger main jet and a 40 to 42 pilot jet, with 3 turns out on the idle mixture screw. They also mentioned that using main jets this size one should consider drilling out the slides to 3 mm up from the stock 2.5 mm. This permits more air to flow into the combustion chamber with the gas.
So I was prepared to do these things on the 4th of July when I removed the carbs. However, I discovered that there was already 1 shim on the needles. Then I realized that when I had my previous exhausts installed by the dealer (Triumph Off Road or “TOR” pipes) the dealer had already made some carb adjustments – which is where the 1 shim came from. So, I decided to try 145 Main Jets, 42 Pilots, drill the slides to 2.95mm, add one more shim (2 shims on each needle) and see how it ran.
All I can say is WOW! What a difference this made! The bike starts up and idles very nicely, had plenty of torque on the low end to get going, even with a 19 tooth front sprocket, and feels like it is just settling in at 80 MPH. The extra shim improved power response in the lower RPM range from 2500 to 3500. The drilled slides made throttle response much quicker, almost instantaneous at all RPM’s most noticeable from 3500 up.
I now feel like all the modifications are coming together and working in sync with each other. The current settings are close enough, but I want to put the bike on a dyno within the next month to fine tune it.
So, that’s what I’ve earned so far about the “airbox removal” modification.
Let me know if I can answer any additional questions about my experience.
Added July 16, 2012
An exploded view diagram of the Keihin carburetor parts used on my Bonneville, with notes about my modifications.
Added July 19, 2012
Last night I removed 1 of the 2 shims that were on each stock carb needle, and changed the idle mixture screws to exactly 2.5 turns out. This has actually improved the overall performance of the Bonneville. My current carb settings as shown above: 145 main jets, 42 pilot jets, slides drilled to 2.95 mm, stock Bonneville needles with one shim each. Idle mixture screw is 2.5 turns out from closed. My next modification will be to try Thruxton needles instead of stock Bonnie needles.Added July 20, 2012