Hitchcocks Forum

Hi, does anybody have any tips or links for setting up Mikuni carbs?

My 500 Bullet was running lovely on the standard square-boxed air filter so naturally I decided to fiddle with it.

I fitted a Bellmouth with gauze purely for aesthetic reasons, and unsurprisingly it didn't fuel very well at all - trouble starting and ran rough. I refitted the air filter box - back to running like a dream and ordered a bigger main jet and pilot jet. More air needs more fuel right?

Fitted these bigger jets when the bike was warm and it ran, but rough. Much more trouble starting from cold, some improvement when warmed up.

Next plan is to order the next size up for both main and pilot and try those... just using trial and error at the moment. Should I be considering changing the needle jet and/or throttle slide too?

Hi - do you mean Mikuni or Mikcarb? If its Mikcarb there is a useful item in the Technical Notes section on this site.

Sorry! yes a Mikarb.

I worked with the notes as per the linked file - but that seems to be more about setting the idle speed, balancing the throttle stop screw and the pilot air screw. Did that and achieved a satisfactory idle (once warm) but it's not fuelling at all well under load...

...I think I may just have to experiment with jet sizes.

Cheers
Chris

driverchris,

The most critical part of the carb is the needle jet and the air slide, as this works between 1/4 open and 3/4 open throttle (in other words the normal driving range). You will need to change to the maximum sizes that Hitchcoks offer for your carb, particularly if you have changed both the air filter and the exhaust. No point on changinmg one without the other as you won't get the full benefit. Remember the more air that you put in the more fuel you need to give it. What you will find with a bell mouth is that the maximum needle jet from Hitchcocks still wont be enough even with the needle on the richest groove setting. So what you also have to do is to raise the level of petrol in the float chamber to give the jet a higher head to get more fuel through it. You do this by bending the tang which touches the float fuel cut off valve (needle). Having done all of the above and still finding that the carb still spits back as the mixute is still too lean, the only option then is to pack up the needle jet with small washers under the needle circlip to effectievly raise the height of the needle in the air slider. I have managed to put up to 4 small 2.6 mm - 3 mm diameter washers under the circlip, which then gets contained by the cable end block when that is replaced with the two 3 mm screws. The washers I am talking about are mild steel and are pennies from B&Q.

You wont be dissapointed with the performance either, but obvoiusly fitting a bell mouth and removing the filter does use bit more juice. Doing this you won't any longer get 80 MPG -but then that is the sacrifice.