YB 10 Dieci Jet Kit Installation and Needle Jet Upgrade

  After struggling with the rich running conditions in my Dieci, I finally broke down and bought a jet kit with Titanium needles ($150) from Factory Pro  (www.factorypro.com). I opted for the Ti needles in hope that they would alleviate the wear issues in the needle jets that caused the rich running in my bike.  I also broke down and bought replacement jet needles ($120) from a local Yamaha dealer, since none of the Mikuni importers such as Sudco sell parts for the Dieci’s 38mm CV carbs.

  Installation is pretty straightforward for the jet kit and atomizers. Make sure that you place a clean shop rag in the intake ports of the engine so that none of the small carb parts end up falling inside your engine!!!

   The Mikuni carbs are fairly easy to work on. The only pain in the ass is the fact that the idle mixture adjustment screw is inside the float bowl, so there’s no means for easy adjustment. You definitely don’t want to go through a bunch of jetting iterations on this bike, hence the need for a well-tested jet kit.

The jetting as I received the bike was as follows:
Needle: (stock) 5CEW9, 3rd clip from the top
#40 pilots
3.5-5 turns out on idle screws
128 mains inside/outside
stock (?) slide springs
stock (worn) Y-O needle jet

This uses larger mains than the stock FZR engine, so someone had obviously swapped mains when they added the open pipe.

I decided to use the following combination of jets:
Needle: 1175h-90q-70r-45 2nd clip from top
#40 pilots
2.5 turns out on the idle mixture screws
125 mains inside/outside
FactoryPro slide springs
Updated Y-O needle jet

I replaced the #40 idle jets while I was in the carbs, since they came with the kit anyway.

My bike retains the stock EXUP valve. It has a gutted carbon fiber pipe and a modified airbox with K+N filter. Ports and engine are stock. I live at near sea level in Los Angeles.

   The difference between the new and old needle jets was subtle, but noticeable. The new jets had a little bit different chamfer on the orifice, perhaps to address the wear issue.  Did Yamaha/Mikuni update the design of these items? I have been told to look on the engine side of the atomizers for wear, as the slide and needle get sucked toward the engine and wear a groove into the needle jet. The old jets had very little visible wear. I hardly noticed any.  It doesn’t take much to screw up your jetting, though…Actually, the needles themselves showed more visible wear than the slides- the needles had visible scuff marks along their length.

   After starting the bike, and having it immediately scream up to 6000rpm, I realized that my throttle linkages were getting hung up on the hose clamps for the rubber manifolds! Check to see that your throttle will return all the way before you re-assemble all of the bodywork on the bike!

   I took the bike for a blast up in the canyons, and everything seemed to be hunky-dory. There’s a slight bit of popping on trailing throttle, so I may adjust the idle mixture screws slightly the next time I have the bodywork off the bike. The next step is to take the bike to Evoluzione to see what the dyno and lambda sensor have to say about the new jetting…TO BE CONTINUED

Ed Milich