Wierd air/fuel curve

[CossiEst]

HI, i've been riding my 1216 Bandit with td04 for two years now, but i think my air/fuel ratios could be better. Look at the graph, and you'll see what i mean. Bike has never been to a dyno, i'm using dynojet wideband commander to log it. It has mikuni bst36ss carbs with 125 mains and #40 pilots, needles are somewhere in the middle. I tuned it first at full throttle, to get the best mains and afr under 12. Then fiddled with needles and mixture screws to run smoothly. It is running good, no smoke or coughing, comes on boost nicely. Works fine and pulls hard, but i'd like it to be spot-on.  By the time this ''pull'' was made, i was running 7psi, but now 10 and it's a bit richer everywhere (under 12 at full throttle), but the curve has the same shape. Any suggestions, to get that rich spike out? Is it the tune or system design? (bottom pic is pitot placing before the plenum)

[Fula28]

Pitot  I'd say, myn were twin set up at 45' to each other' worked a treat .

[MeanBean49]

Bigger mains, less base pressure might help. Will richen up the top end and lean out the bottom. What base pressure are you at?

[CossiEst]

Can't lower fuel pressure, it will starve on longer under-boost runs, it's about 2,5-3 psi.

So, i did a fresh ''pull'' and that's RUN1, not too bad (richest was 10,6). Then started to lower the needle, mains seem to be ok. In the center groove it was better (RUN2), no change in top end (richest was 10,9). But lowering it again did THIS (RUN3) made midrange leaner but wtf happened to the top end?

Also noticed that cruising ( 60mph) AFR was changing big time because the needle position: RUN1-12.5       RUN2-13,4         RUN3-14,3  What should be ok for cruising?

 

[nlovien]

i'd hassard a guess that run 3 = you've lowered the needle to the point where its restricting full flow from the main jet ( not able to lift clear)  - your running CV carbs ? - i.e. throttle position is not directly linked to needle position - i.e. you've found a nice picture of the limits of play with the needles you have and if this is the case, the picture also helps to sugest what needle shape your needing

[CossiEst]

I raised the needles back to the center groove, but this problem stayed, it starts leaning out after 7000 rpm. I'm going to strip the carbs and give them a good clean, and assemble with most care. There has to be something wrong, because this setting (RUN2) worked fine before

[Leblowski]

If it worked correct in the past you might want to check your fuel reg

[Tombola]

Like said above, if it worked before what's changed. Fuel filter ok? Diaphragm in wastegate ok? Can you move fuel pressure gauge so you can see it while ridding? 

[MeanBean49]

Have you got a dump valve, might not be sealing properly, more boost, more loss, richers fueling

[CossiEst]

Problem solved. The o-ring under the carb top was blocking the signal to dump valve, and it was probably leaking. For peace of mind i cleaned the lot, checked float heights and measured fuel pressure. Now its between 11 and 12 all over the rpm range (on full throttle) Cruising is 13.5 and idle 11.5

[CossiEst]

After messing about with the carbs in september i noticed that bike doese'nt like starting with choke anymore.

Usually when starting up cold engine i pulled the choke fully on and everything was like it should, the same when it was NA (revs rising and slowly backing off, til it was warm enough)

Now it starts with choke but seems to flood the engine (afr gauge drops under 10:1) in few seconds and stalls, if i back off the choke, then the revs are too low and it stalls again. Only way to get it warmed up is to start with half choke and play with the throttle, same time closing the choke circuit. When its warm, then all is good. Cleaned the carbs, checked for leaks in needle valves, plugs all look the same color. Tried with fuel pump turned off, but nothing.

I'm out of ideas. it can't be because i lowered the needle by two clips?

[FJD]

sounds like you need to adjust idle mix screws