Cuts out when cold

Hi, my XJ900 has just developed this fault. it will start cold and idle with choke normally, but when you take off it gets to ~2,000rpm and misses, the tacho drops to zero. You can ride slowly if you don't put it under load or try and go over 2,000. When it warms up a bit (~1Km) it comes good and rides normally. Any ideas?

 

Idles on enrichment circuit, so it's not the enrichment valves.

Runs fine below 2,000rpm, so probably not the pilot jet.

Above 2,000 you are starting to use the main jets for fuel delivery.

Could be clogged main jets.

Could also be a problem with the slide diaphragm or slide itself not operating properly allowing the proper air/fuel mixture.

I would start with the basics though and make sure it's getting petrol to the carbs, starting from the tank and going through each point. Tank, petcock, line, filter, fuel rail, etc.

I'm by no means an expert on the 900s, but it definitely sounds like a fuel delivery issue.

 

I don't know much about the XJ900, but if it has an electronic tach and its diving to 0 and the engine is still turning then I would look for some electrical gremlin starting to act up.
If the XJ900 has a mechanical tach, like my XJ650, please ignore this.

 

It has an electronic tach and you're absolutely right. If the tach is suddenly going to 0, then there's something going on with the ignition; and that symptom points it toward the pickup/coil circuitry which is where the tach draws its signal from, I do believe.