Dieci Jetting/Rich Running Issues

   I spent the good portion of a Saturday pulling apart my Dieciís 38mm Mikuni carbs in order to cure a rich running problem. This seems to be a low and midrange issue where the bike bogs and doesnít want to run. It always occurs when the bike is hot. I saw reference to this issue on a number of FZR1000EXUP pages on the web. Supposedly this is caused by the needle jet wearing the emulsion tube, and allowing more gas to pass. This effectively richens the jetting in the low and medium throttle openings. The recommended fix is to replace the emulsion tubes. I want to play around with the jetting a bit to see if I can fix this running issue through jetting, though. After that, Iíll probably take the bike to Ken Zellerís Evoluzione Cyclesport for dyno runs and lambda sensor readings.

   SoÖ I tore into my carbs. Getting them off the bike wasnít as hard as I expected. I was under the mistaken assumption that one had to drop the engine to get the carbs off (a la the Dieci sparkplug change), but this is not the case. There are four little knobs that extend to the rubber inlet tube ring clamps. These allowed easy removal of the ring clamps. I actually left these intact and chose to remove the upper ring clamp with a 3 mm ball-headed Allen head socket and a looooooong extension. You can also use a regular 3 mm Allen socket and universal joint. Be careful not to drop any parts in the wide-open intake ports! I stuffed a rag in each inlet to prevent this from happening. The carbs came right off and I pulled the bowls and tops off to have a look-see. After realizing how expensive carb gasket kits and jets were from a dealer, I decided to only replace parts if they were obviously damaged or worn. I mean, emulsion tubes ALONE were $22 each. They wanted $20 for each needle. A gasket kit was another $100. Heck, the bike is only 10 years old, with 17k miles, so nothing should be in that bad of shapeÖ All of the diaphragms looked good. Nice and pliant and no pinholes, so I put them back in. Gaskets all looked good. No sign of any leakage. The steel needles showed a little wear where they contact the tubes. Strangely enough, the softer brass emulsion tubes showed no visible wear. It may have been the case that I couldnít see the wear, as the tube orifices are very small. OK, so no visible signs of carb trouble. The first step I took was to drop (lean) the needles in the slides one notch. Instead of the 3rd notch from the bottom, where they were, I used the 4th notch from the bottom. Weíll see what that does to the performance.

   I also noticed that the main jets were all 128ís. Stock FZR1000 jetting (from the Haynes manual) is 122.5 for the outside cylinders and 125ís for the inside cylinders. My bike has a fairly free flowing pipe on it, so whoever jetted the thing probably just bumped up the mains. All of the other jetting looked stock, although I couldnít find a part number on the carb needles, and I couldnít find a specification for the stock needle clip position.

  I also checked the fuel level in the bowls using a piece of clear tubing. The fuel level was where it was supposed to be, as indicated by the level line on the carb bowl, so no problem there.

   I also was reminded why I love Italian bikes and the enthusiasts that they attract. I called 4 local Yamaha dealers looking for carb parts, and not one of the parts guys that I talked to was very helpful, or even familiar with the FZR1000 carbs.They were all clueless parts droids. Iíll bet that none of them have ever even had a carburetor apart. This is never the case when dealing with any longstanding Guzzi, Ducati or similar dealer.

   Update: I took the bike to Evoluzione Cycles in Simi Valley for dyno testing. It was a little bit rich up top (12:1 A/F), so Iíll drop main jet sizes to 125ís. The lambda sensor also provided some illumination about the low range boggingÖit turns out that the full throttle runs had a terribly rich (<10:1 A/F) mixture in the low rpm rangeÖit looks like itís time to replace those needle jets and needles before I change anything else with the jetting. Ken at Evoluzione can get these parts from Tucker Rocky, and I can also get parts from Sudco. As soon as I get the Mikuni carb parts in, Iíll swap them out and see where I stand with jetting. I should also mention that the 38mm constant velocity Mikuni carbs in the YZF1000 are oddball items, and not listed in any of the Mikuni catalogs that Iíve seen. 

   Even with the jetting not quite right, peak horsepower was still in the 125hp range at about 11k RPM. Amazing. When I had the carbs off, I noticed that the inlet manifolds are very poorly matched to the ports. A simple stage I port job could turn the Dieci into a 140hp machine!

...to be continued

Ed Milich
bimotatech@yahoo.com