View Full Version : over fuelling

simonollman - n/a
23-Apr-06, 09:57 PM
any help would be great and i would appreciate it even more so as i need to get this sorted by next friday.

which i thourght i was all ready for.

My carb is over fuelling at full revs and bogging down, which has lead me to assume that my fuel air mix screw may be the fault as

Itís shaped like a fullstrum and I would have expected a point? And appears short to me for some reason, its a mikuni carb on a 235 rotax.

If somebody has one and could measure its length and let me know, I would greatly appreciate it.

I will explain, as you look in to the mouth of the carb you can see the hole for the fule air mix at the bottom center just inside the lip of the carb mouth, the bore of this hole is controlled by the fuel air screw, so far so good, but the problem is when I look up this passage, I can see past the point that the screw should intersect it and up to the emulsion tube, I think thatís what its called. So the fuel airscrew appears pointless. No pun intended, I have heard of some people over tightening and shearing the end off which stays in there, maybe that has happened and now has come out after cleaning.

Coursing the over fuelling. that sound right? or am I up the wrong feild and up the wrong tree. Itís not a blocked jet or incorrect set tab on the float arm. Or the pin height. Or may be it is, still I need to eliminate the fuel air mix screw first.


No 12

team black - n/a
25-Apr-06, 09:41 PM
More likely that your float valve isn't working properly

Paul Fitz - n/a
26-Apr-06, 08:36 PM
Simon, I am not particularly familliar with your engine, but the fuel air screw has no effect on the mixture at high revs. On most carbs it controls mixture up to about 30%-50% throttle opening, after this the main controls are the needle jet, needle and main jet. At about 3/4 throttle the carb is usually just coming off the needle and running on the main jet. If you are sure it is *over* fuelling you could try dropping the needle a little to weaken the mid range and increase the power. If this does not work than the most likely problem is as Ewan suggests a leaking float valve or incorrect float height. If this is ok then the main jet is too large.