However the bike does start so before going on what might be another expensive & time consuming goose chase lets get back to basics. Has the bike ever idled correctly? even at the dealers before you bought it?
Yes and no. It was ticking over at a high idle - probably around 1200rpm at a guess- in a video the dealer sent me before I went to view the bike. When I saw the bike, it started and ticked over and when the bike was delivered it started and ticked over. However, this tick-over was high, uneven and it was prone to cutting out. When I took a good look at the bike, I saw that the throttle was 'sticking' and this was due to the throttle barrel having broken up and the rubber grip 'dragging' on the bars as well as the top cable being badly kinked and frayed. So, I sourced a new throttle and a new top cable and fitted them both. It was at this point that I realised that the lower cables were too short for the carbs. The bike had obviously been fitted with 32mm carbs originally and whoever rebuilt it with 40mm carbs hadn't changed the cables. The upshot was that the slides were 'hanging' from the cables and wouldn't bottom out. This would explain the high idle. I ordered some longer, 40mm cables and whilst I was waiting I decided to give the carbs a good clean as they were pretty manky. So, I gave them 30 mins in the ultrasonic bath and rebuilt them using mostly the original seals etc. which were fine, although I did change a couple of the fibre gaskets just to be on the safe side. I then refitted the carbs and tried to set them up 'from scratch' having swapped the slides over as the ramps on them were worn - badly on one side. I could only get the bike to run evenly by lifting the slides using the cable adjusters. It wouldn't idle on the throttle stops at all and, having changed the idle jets and double checked everything was clear and correctly adjusted, came to the (probably incorrect) conclusion that the slides were worn to the extent that they needed replacing. Rather than do that I decided to bag a bargain set of new carbs - now fitted- and they don't seem to have helped at all. Aside from the fact that it will not idle, the bike runs well and pulls cleanly. It hadn't occurred to me before, but if the timing advance isn't stepping back correctly, that might well cause my problems whilst allowing the bike to run well between 2000 and 4000 revs which it does. Alternatively, it could be that the '1.5 turns out' starting point for setting up these carbs is nonsense and I'm going about it all the wrong way. I've been setting up carbs for 45 years and never had this much trouble before, but they are Italian.....
Oh... it sounds like the tickover has never been right then. Did the dealer give any reasons for this? Back to basics again. Have the fuel taps/tank being checked for rust and/or debris? Is the fuel level at the reserve point? And as for doubting1.5 turns (revolutions) you'd be doubting the accepted wisdom of many many, more knowledgable than me, bevel aficionados. And I'm going to say it one last time but I still think having the fuel lines on a gravity fed system higher than the tank fuel tap at some point in it's path is not ideal.
@Andy Bee Memories of my physics classes would have me think it’s not so much the relative position of the hose to the tap, that matters, than to the fuel level in the tank. Even with gravity only, fluids will climb in a hose, as long as it is below the tank level. The amount of pressure given by gravity will actually allow said fluid to climb as high as the tank level. So as long as the carb stays below that and the highest point in the fuel hose is not above the fuel level in the tank, everything should be fine. It’s the principle of an artesian well, isn’t it?
I was just about to write all that Pressure, flow, etc in the pipe is wholly dependant on the fuel level in the tank. However, any air 'bubble' in the pipe is an annoying distraction and you should be able to purge it...
Indeed. Fluid would still be able to flow around that bubble but probably at a reduced rate. And as you wrote @Keith_P , there has to be a way to bleed the fuel hose.
Whilst clutching at straws I fitted new plugs to the bike. It starts and idles perfectly now. I've had plugs go on me, but never like that before. Anybody need a set of 40mm PHMs?.....
Any reduced rate of flow, depending on the head of fuel in the (saddle) tank, an air bubble in the pipe or size of 'loop' perhaps can cause issues at idle. As mentioned in another thread mine has always showed air bubbles in the 40 years or so I have had the bike. My suggestion was purely to help eliminate simple solutions first, which yes, may be completely wrong but at least they don't cost any money or time. Plus when proven wrong at least I'll shut up...
