RVF: Engine hesitant at 3,000...3,5000 rpm

[gepe]

Hi.

Let me introduce myself. I have an RVF since autumn last year and I only put approx. 200 km on the clock before giving it the winter overhaul. The bike had 15,000 km.

I have been busy in a German RVF forum over the winter but that forum just does not have enough members with enough experience to get useful answers. So I am trying my luck here.

My goal was to get the engine/fuel system/exhaust system back to the original configuration and start with improvements from there.

Re. carburettor and engine I have done the following:
- checked/adjusted valve clearance to spec
- new air filter, new sub filter, new spark plugs, battery is new and new OEM rec/reg (from David Silver)
- dealer I bought it from said he had the carburettors ultrasonic cleaned just before I bought it
- float levels set to spec (they were too low)
- complete new gasket kit four all carbs (for fuel, carb housing, etc.)
- new vaccum line
- new fuel cock diaphragm set
- carbs synchronised with vacuum gauges
- idle mixture screws set to 2 turns out (position that dealer claims these carbs had when the bike was delivered)
- checked that original jets, springs, jet needles etc. are installed (I did not check the idle jets)
- replaced one jet needle (corroded)
- new intake rubbers, bands tightened just well enough to prevent air leaks
- had the tank professionally cleaned, anti-rust treated and coated with epoxy on the inside
- cleaned the fuel filter
- original exhaust, all new exhaust gaskets


But there is a nasty problem which I do not know whether this is typical for the RVF or just due to some issue specific for my bike.

Starting at around 2800 rpm to 3500 rpm the engine is very reluctant to take up speed. The bigger the load, the more hesitant it gets, it also starts to shake then. Once above 3500 rpm, it shots forwards and I need to roll the throttle back (e.g. in the city). I'm having problems finding the correct English terms, but the exhaust sound gets "flat" in this region and not really like a 4-stroke.

As the only variable I wasn't sure about was the idle mixture adjustment, I tinkered with that quite a while - with results I just don't understand.

I read a lot about idle mixture adjustment in this forum and others but the experts don't seem to agree on a few things. Mainly the setting of the mixture screws.

But the essence seems to be: according to the "idle drop" method, set the mixture so the engine runs at the highest rpm between too-rich idle drop and too-lean idle drop, then the a/f ratio is optimal and there is enough range to accomodate changes in air humidity, air pressure, air temperature.

Ok, I tried various approaches (I have a tachometer with 10 rpm resolution and an averaging period of 0.5 sec. and the readings are beyond doubt).
1. Started with 1 5/8 turns out on all 4 screws. Then turned one further out by 1/4 turn, waiting a few secs. between turning. I was trying to achieve a noticable increase ~50 rpm) , but nearly nothing happened, the increase was 20 rpm at best for 1 add. turn out. I tried this on all 4 screws.
2. I again started off with 1 5/8 turns, now increasing by 1/4 turns at a time on all 4 carbs. Same result.
3. I test-drove with 1 5/8 (when I didn't understand that was just the initial setting) and at 2 turns out("factory setting).
4. There is no noticable difference whether it is cold (12 °C) or warm (25°C).
5. Getting quite desperate, I set the screws to zero turns out. To my surprise, this is the setting that I can obtain the highest revs. The bike starts well and idle is fine but the "flat spot" at + 3,000 rpm is stil there.
6. I did the throttle blib test. At 2 turns out, rpm drops quickly until the last 2-300 rpms, then it takes a bit longer. But I wouldn't say revs hang. At zero turns out it drops right directly to idle rpm (at least on the built-in tacho).

I must admit I am stuck on this.

So here are my questions:
1. The mixture screws on the RVF carbs control fuel flow, not air flow. Right? So out is rich, in is lean. (I am confused because there are a few posts stating it's vice versa, e.g. viewtopic.php?f=24&t=40726#p298196)
2. Has anyone adjusting their RVF pilot screws got rpm changes in the 50 to 100 range?
3. As I don't have any comparison, maybe this is standard RVF 400 behaviour? (which would leave me pretty disappointed).
4. Highest idle at zero turns out seems so wrong. What could be the reason for this?
5. Any more ideas what I could try?

Sorry for the long story, but I wanted to give a complete picture.

Thanks for help!
gepe.

[EstebanCB1]

Hello, I'm from the future, I have exactly the same problem on an NC23, I tried 2 different carburetors and the fault is the same.