Last couple of bugs to work out, mid-throttle sputtering

[ID_DUC_MON]

Here we go! 97 M900, carbed, Dyno-jet kit, K&N filters, Shorty Remus cans, coffee can removal, vac fuell cut-off replaced with a manual valve, in-line fuel filter, all fuel line replaced.

Bike starts and idles like a dream, when on the gas it runs up smooth and with lots of power. low RPM cruising is baby smooth, high RPM is as well. Mid RPM, say 5-7k, is choppy and sputtery. If I throttle on past that point if does so wonderfully, decel through it is smooth when engine braking.

I have re-gone over the plugs in vac ports of the intake several times, all seems good. I have clear blue fuel line and I can see lots of air. My thought is I am getting restricted fuel flow via the filter, cut-off, line, or all three. The pump pulls it though nice but if just never seems to fill all the way.

Thoughts, questions, comments?

[koko64]

Been thinking about your description of symptoms.

What needle position are you running? What main jets?

It's running better from 7k to redline (9k) which is top end. 5-7k is the upper midrange. Slow jet overlap helps a lean needle position down in 'lower midrange'. So your needle position may be too lean. Try raising the needle two notches since it's spluttering. If it was a bit soft on power I'd raise it one notch, so go two.
Consider your main jet size also, particularly if you have chopped the airbox lid (are you running pods)? The main jet will overlap on the "upper midrange".
I found that spluttering can also be caused by too rich a needle position if it is happening at cruising, small throttle positions, but accelerates well. In that case you may have to lower the needle a notch or two.
Chronic leanness can cause spluttering at cruise speeds and will absolutely die in the arse if you open it up in that range and through that range.
Also consider your needle jets if they are stockers with over 5k miles on them. They wear oval and cause a rich, rough running down low at small throttle positions. Buy stainless factory Pro needle jets which wear much better and are priced well.

If you are confident your jetting is correct try checking the fuel pump.  I think it is most likely to be jetting in the upper midrange, but the pump is an easy check. You might as well test it first just to be sure. The aerated fuel in the clear line is interesting, just a few bubbles or big gaps of air? To test the pump bypass the fuel pump to the carbs with a full tank of gas to get enough fuel pressure. Also check the vacuum line from the manifold to the pump. If your pump is failing buy an after market rectangular Mikuni one from Cal Cycleworks or another Forum sponsor. It's nearly the price of a rebuild kit for the stocker.

Let us know how you go.

I'm gonna run clear lines now.

[koko64]

Sorry if that's confusing. There is alot of circuit overlap in Mikuni CV carbs and similar feeling symptoms can have different causes.
Also (I forgot), try a plug chop in that problem rev range if you can do it in a safe place.
Feel free to PM me if you get bogged down.
The carbs are a compromise when tuning for the overlapping influence of the butterfly and the CV slide.
They are Mikuni CV's after all.

[ID_DUC_MON]

Easy answers first: K%N pods, The air is large gas in the fuel, easily 1-2 inches in length starting at the take to the filter, then from the filter to the manual petcock, then again from the pump to the carbs. when running or just cranking, fuel flows very well, but the air never seems to leave. I wonder if there is a non sloppy way to purge them...

It is supposed to have a dyno-jet kit, needles and jets. I couldn't tell you what sized/positions though as I didn't do the work. give me a couple weeks or so and I will pull them down and look.

Last little bit, this started after my total frame-up re-build I did this winter. I did so many things at once, I can't narrow it down to one area. Emissions removal, new hoses, new throttle cables, manual petcock/filter, total carb cleaning...yada. It might even be electrical as I had the entire harness out and cleaned up some past "bubba" jobs but I would think that would affect the entire RPM range.

[64duc]

  Easy things first. Sounds a lot like mine when the needles and tubes were worn and causing a rich condition. Check your plugs to see if you are running rich (black). Don't worry about air bubbles in the fuel line. It's normal and won't cause any problem, besides if there is a way to get rid of them I don't know about it.

[koko64]

So it didn't do it before the frame tear down? Did the pods go on with the rebuild?

[ID_DUC_MON]

The pods, jet kit, and exhaust were all done prior to the tear down I did this last winter and the bike ran butter smooth then. That said, since I didn't do the work to it and bought it with those mods, I am unsure as to the quality or completeness of the work inside the carbs. I do know it ran fine before I did the other stuff. I will pobably order up another kit with fresh needles and jets for I have an assortment to work with and fine tune it.