Misfire Driving Me Bonkers!

My cafe racer was built for display and I am not sure that it had ever previously run in its current config.
650 Elefant engine in a Pantah frame with custom alloy tank etc.
36mm Dell'Orto carbs with no filters.
Starts fine, ticks over fine, pottering around is fine.
But when you open up through the gears, goes like a rocket but when up to 60-70 it starts to misfire.
Sounds like one carb gets starved of fuel.
Shut the throttle and ride slowly and again all is fine.
The taps are standard "new style" Paioli. I stripped them and noticed that the internal passageways were not well formed so I carefully drilled and cleaned them for better fuel flow.
Fuel pipes are 7mm and carefully routed so there are no kinks.
The carbs have been rebuilt (by Vapourworx) but I stripped them anyway and all is correct in there.

Any thoughts and suggestions gratefully appreciated!

 

Fuel filter is the standard tube on top of the taps - both clean.
Float height checked - spot on.
Tank breather is an interesting one - after reading that post I stripped and checked the filler cap - breather venting to atmosphere OK though.

 

On the ignition side, it has a new Ignitech unit, new Ignitech matching coils, new HT leads, new plugs. Rebuilt the ignition pickups with high temp wiring, and the Ignitech unit says the signals are fine. The ignitech unit checks out OK (it has an interface to a laptop). Running the engine at various speeds shows the correct dwell angle. Because it runs fine when accelerating hard though the gears until you get to 4th or so, I thought the ignition must be OK. Seems to be one cylinder rather than both. No backfiring or spitting, just fires very intermittently on that cylinder. It's a bit tricky to work out which cylinder though.

 

I shall find a nice straight empty road (they do exist here in Cambs) and do some test runs. Many thanks for all the useful feedback.

 

Still trying to sort out this misfire. Bike starts and ticks over fine - runs fine to 1/4 throttle - above that it runs very badly. Bearing in mind that the bike may never have run in this configuration (650 Elefant engine in a Pantah frame) the problem might be more complex than a new set of plugs etc. Question on the (Dell'Orto PHF36) carbs positioning: the horizontal cylinder has the carb at the usual semi-downdraft angle, within the 0-40 degrees specified by Dell'Orto. But the vertical cylinder has the carb in a slightly UPDRAFT position, ie the inlet is slightly lower than the outlet on the engine side. That doesn't look right to me. Just wondering if the carb should be level or slightly downdraft - any thoughts please....

 

To answer my own question: yes it should be downdraft between 0 and 40 degrees. I had a 4am thought - why don't I just rotate the inlet stub 180 degrees... That worked and the bike is now running 100%. FYI, the symptoms were misfiring and running rough, basically fuel starvation into one carb. This bike having been built purely for display, whoever did it obviously thought that the vertical cylinder carb looked neater when tucked in. Now it has the standard "look down the throat" Ducati old school look. And boy does that 650 desmo engine sound good at 8500 rpm.

 

...and just to add a small item to the total sum of knowledge; as the symptoms were fuel starvation, I tested the flow out of the tank into the carbs. No figures seem to be published on this, but from my experience, you should get a flow of about 0.6 litres per minute from each tap. Might be useful if you are testing for a blocked tap or tank filter. I also found that the internal passageways in the Paioli taps were not well finished, so I drilled them out a bit.