Now another story ....
Getting the jetting right with ulp... who else has had the privilege of setting up a pair of non standard carbies with the bloody stuff ?
After the second build I removed the 626 Amals (a mismatched pair) and fitted a brand new set of 900 series 28mm Wassels.
These came setup std with 180 mains, 3 slides, 106 needle jets and needles in the top groove., ran like crap, made black porridge on the piston tops and felt like the cylinders were full of slow burning wool.
I tried different main jets and every needle clip combination to no avail.
2 weeks later and I'm now finally cooking hard brown/grey/black carbon on the piston crowns and can run flat out for an hour in 100 degree temperatures with no issues on BP Ultimate 98 ron unleaded.
I gave up trying to get intelligent plug reads with ulp and started reading direct from the piston crowns with a cheap usb plug in boroscope.
What did I end up with?
Well a phone call to Preston at British Bike Spares in Brisbane to ask did they in fact stock 105 needle jets revealed a well known Norton Commando tuner in Cairns now routinely fits 105 needle jets to all the bikes that came with 106's so he had a good supply of them in stock so that was my first clue.
Further digging told me that BP Ultimate and Shell Vortex, the locally available high octane (98 ron) fuels here will both cause rich running right through the range on carburetter engines due their increased density over the old leaded and lead replacement fuels. (I found details of that on the BP and Shell websites)
Enlarged hi-res photos of the spark plugs also showed slight traces of alloy balls on the insulators caused by detonation, so I knocked the timing back to 37 degrees which completely cured that one.
I ended up with #3 slides, 105 needle jets, needles in the middle position, a 170 jet main on the right cylinder and a 160 main jet on the left cylinder.
Don't ask me why it needs different main jet sizes... I don't know.
Since the initial startup this has been the case with this engine, the valves all seal perfectly, the compressions are dead even, the tappets are set identical the timing is the same on both cylinders with the TriSpark ignition.. So, it is what it is although a half jet size smaller on the right cylinder would make it almost perfect.
Another minor detail that came to light when talking to Steve Kelly from TriSpark, he asked me what plugs I run (I run NGK Iridiums in all my bikes) and he told me all Iridiums are resistor plugs so as I was running the spark plug caps he supplied with the dual lead coil which are apparently the resistor type I in fact had double the required resistance in the ht cables (10k hot instead of 5k)
(I had no idea all Iridiums have internal resistors)
Now with a modern TriSpark setup it probably made no difference but I can imagine with an older 6 volt twin points set up or a Boyer with a wonky charging system it could well be an issue.
Anyway I changed the plug caps out for NGK non resistor ones today and it might be the placebo effect but the bike does seem to run stronger at 5500rpm plus in top gear.
I've attached a photo of the spark plugs after a hard 10 mile run this morning in 92 degree heat.
I'm also getting very little black soot at the ends of the Dunstall style mufflers now compared to what it used to make.