My type 3 1600 twin only wants to run on 2 cylinders. I checked the spark plugs and replaced the carby, put in new dissy and leads last week, so there is fuel and spark so it can't be a carby issue (the carby's off an old engine), also checked rocker clearance. If I let it rest or idle for a while it comes good, but only for a 500m run then 3 and 4 loose all power.
This problem started after all that rain (Sunday night), suspected water in the fuel but the other side runs fine (limped home on half an engine).
I'm on my 2nd tank of ethonal fuel (10%) and usually use premium unleaded, could this cause the valves to stick? It's not to long ago I had the heads done to rectify sticky valves so I didn't need to use fuel additives.
Should I fill with with standard and use additive and hope it will fix itself or am i looking at a bigger problem?
Oh, the oil was a bit low as well.

Being no expert I can't diagnose the exact problem but if I were you I would be checking the electrical side first.

Check the non running cylinder leads for a spark. No spark = plugs/leads (too short and not making proper contact - Arcing in dizzzy /dizzy problem.

If there is a spark, check to see if fuel is reaching the cylinders. (carby/fuel pump problem).

Failing that, it could be something more technical (heads/valves).

I don't think low oil would cause any of this and I would steer clear of ethanol fuel on an early vehicle. IMO.

I use a simple checklist for problem diagnosis.
Fuel.
Electrical.
Mechanical.
if it aint any of the above problems then call in the experts!
Good luck.
- Adam

have you checked the vacuum balance line and plastic elbows for cracks and sucking air

There is no problem on the electrical side or the fuel, I wouldn't drive 30km on 2 cylinders without doing a simple check of these things (it was 11pm on a bush road and I even pulled the plugs out) but thanks for the advice. What is the problem with ethanol fuel in older engines? I did hear it can retain water, anything else?

I will fire it up tomorrow and check the vacuum balance lines by spraying RP7, but if it is this thats the problem wouldn't it be more consistantly running bad.

I still suspect sticking valves, is ther any way (non mechanical) to get them working again?

how is the rotor cap for cracks damp air will find them out try another cap