Potentially a daft question, so apologies in advance!

Would there be any change in carb performance if the main jet was moved from the banjo bolt in the bottom of the float bowl, to being screwed into the main nozzle? I appreciate that it would mean more of a faff to access it, but it would eradicate any 'O' ring related running issues.

Thanks.

On some models (mostly bigger bikes) the jet setter threads in the needle jet and main jet threads are different.  The Mikuni round main has the different thread for a bike with different threads.  Or you could possibly change the needle jet to accept a hex main. 

On the later AT/CT the needle jet has the jet setter built in, so definitely not easy there.

Finally, the common hex main jet flows different amounts depending on direction of flow.  The "correct" calibrated direction is the way they are installed in the jet setter.  Installing a hex main into a needle jet is "backwards" flow...  yet on later bikes that became common.  There has been some speculation that Mikuni has changed the design and/or flow calibration over the years but I've never seen proof of that.

But that's not what you asked...  If o-ring is good I can't see any performance difference.  Back in the day we ran AT and CT's (early) with the main in the needle jet when the o-ring got messed up...  didn't notice any difference. 





 

"But that's not what you asked...  If o-ring is good I can't see any performance difference.  Back in the day we ran AT and CT's (early) with the main in the needle jet when the o-ring got messed up...  didn't notice any difference. "

Thanks Mark.

I've got some carbs in various states of (dis)repair, so my question was partly to decide what I should do with them and partly idle curiosity.

I don't think you can do it in a 250/360 carb unless you swap to a round main jet.

Yep, there are two different threaded jet setters. There's is a thread(no pun intended ) here somewhere I posted about this. After witnessing a friend burning a hole in the piston. The maim jet in the banjo bolt was a really bad idea (just my opinion ) the jet in the very bottom of the float bowl, where any crud could would settle and clog main jet. Yes you can thread main jet (I highly recommend) into jet setter.

Thanks chaps.

Oops, sorry for the confusion. What I meant to say is that I eliminated the jet setter and screwed main jet directly into the main nozzel / needle jet. I've done this on my 250A and RT2. There are two different styles of the main nozzel. There's the 188 version that the jet setter screws into and there's the 159 version that the main jet will fit. The jet setter and main jet have different threads. And if I remember correctly??? there are two different jet setters. One version has the same thread as the main jet.
 

And yes! There are two versions of the jet setter, one with same thread as main jet and one with different thread. So depending on which main nozzel you have you may be able to simply remove the jet setter and screw main directly into main nozzel, along with the brass sealing washer of course.
 

Oh yeah, as far as performace, there's been no noticeable difference in performance with this change. When I first did this with my 250A I was interested to see if there would a be change. My thought was that with the main jet higher than it was at the bottom in the banjo bolt, the fuel had to be drawn slightly less distance vertical (less resistance) and would maybe be richer? But no noticeable change. I'm running buy the book stock jetting and there's been no noticeable change in the spark plug color or performance on both bikes.

Thanks Kenny, that's all good info.

In amongst my carbs is one for a DT360A (advertised as being for a RD350!  ), which has had the jet setter removed and replaced with a hex main jet (#200 for some reason, so who knows what it was used on).