Have you tried running it on the manual prime position, perhaps the diaphragm could be going bad in the vacuum petcock? Mikuni BS34 problem 06 Apr 2013 12:40.

I have an '81 Yamaha XS400 with dual Mikuni BS34 carbs. I cleaned the carbs once before, but then could never get it to idle right.

Had to play with the choke to get it to idle normally, which I didn't want to try to fiddle with while riding. So I'm at it again: On the top of the carb where the diaphragm is, there is one hole for the choke, and another hole for.? Mine were plugged up with crap, and I got some of it out to the point where I can see shiny brass a little ways down. Are these holes supposed to flow air or fuel, meaning I should continue my cleaning attempts, or is this just a hole leftover from the casting/assembly process?

If it's supposed to flow something, why do I see brass a short ways down the hole? FOG 17/3/2007, 16:50 น. On 17 Mar 2007 14:46:35 -0700, Suggest you do some searching on the Mikuni USA website. There's a.pdf manual there for their normal carbs, may also be one for CVs. -- Ivan Reid, School of Engineering & Design, _____________ CMS Collaboration, Brunel University.

Ivan.Reid@[ ] Room 40-1-B12, CERN GSX600F, RG250WD 'You Porsche. DoD #484 JKLO#003, 005 WP7# 3000 LC Unit #2368 (tinlc) UKMC#00009 BOTAFOT#16 UKRMMA#7 (Hon) KotPT -- 'for stupidity above and beyond the call of duty'. Germaine 18/3/2007, 8:22 น. On Mar 17, 1:46?pm, wrote: > On the top of the carb where the diaphragm is, there is one hole for > the choke, and another hole for.? Mine were plugged up with > crap, and I got some of it out to the point where I can see shiny > brass a little ways down.

Are these holes supposed to flow air or > fuel, meaning I should continue my cleaning attempts, or is this just > a hole leftover from the casting/assembly process? If it's supposed to > flow something, why do I see brass a short ways down the hole? Because it's the non-removable PILOT AIR JET. Prison break. When you have a CV carburetor that has the idle mixture screws on top, you're usually going to find the pilot air jet under the diaphragm.

It regulates the flow of air through a fixed orifice. The carb sucks gasoline up through the pilot or idle jet in the float bowl and the fuel and air mix high up in the carburetor. The idle mixture screws just control one of four idle mixture ports, the other three ports are fixed transition ports that allow the engine to get extra gasoline while vacuum drops off slightly as you just barely open the throttle. Transition ports are the CV carburetor's equivalent of the accelerator pump that a car carburetor has. You need to have all the idle passages open and all four ports clean or you'll have to run on the 'choke'. Why can't you get the engine to idle right without using the 'choke'?

Your carbs don't have a 'choke', they have a starting enrichener device, which is like a tiny carburetor built into the side of each main carburetor. When you pull on the 'choke' knob, you're opening an air valve and the vacuum downstream of the throttle butterfly sucks gasoline up a small diameter passage like a kid sucking milk through a straw.

It's a very rich mixture, but that's what your engine needs to start when it's cold. So, your idle jets, idle ports, and idle passages are all plugged up. Tighten the idle mixture screws all the way until they seat lightly. Count the number of full turns and fractions of turns and write the information down. Then remove the screws and keep each screw, spring, washer and o-ring in a separate container so you can get each screw back into the carb it came out of. If you don't find a tiny rubber o-ring, look into the hole the idle screw came out of. Should you remove the idle jets for cleaning?