Engine fading coming down to idle

[Ageing Rotarian]

Has anyone experienced the bike fading when hot and going down to idle?
This is something which has got progressively worse but is still a little intermittent.
Started by pulling up to a junction and then petering out, press the button engine starts and you pull away. Sometimes it will idle ok but now more often than not it fades. Had one of the club members come over to look at carb settings, we reset the main jet heights with a manometer (wasn’t far out to be fair) balanced the carbs and it all looked good with the bike idling for an extended period. Took it for a test ride first junction the bike fades.
So, thought it might be fuel tap /fuel supply, changed tanks with another Classic and no difference. Had a quick word with Graham (who always seems to have time for the club) who suggested removing the wires from the idle temperature sensor. Wires removed and the bike idles albeit at around 1400 ish rpm, bike has a minimag fitted.
Question, has anyone experienced anything similar or any problems with minimag?
I have the old Boyer system which turned out to be ok in the end and I am looking to refit it to see if it makes any difference.
As always any help would be appreciated
Regards
Roy

[fogrider]

How far down did you set the jets ? MIne is happy at 3.5mm but the factory apparently started further down on first start up. A rich tickover seems to suit mine but of course with SU's it's essentially a full range effect. ( ignoring the idle by-pass screws ). I use an air/fuel ratio meter and set it just as normal turns to the bottom of rich.
I assume the SU pistons are free in the dashpots ? and the damper oil is not too thick. I prefer 3 in 1, any thicker can affect the response when the oil is cold.

Keep at it, good luck.

[Ageing Rotarian]

Jets are set to 3.2 as per the manual and Richards (bless him) old setting gauge which was acquired by a club member through eBay. Basically it is a slide with a plunger clock mounted in it which when placed on the carb body reads directly onto the top of the jet, basic but pretty accurate.
When adjusting the carbs and the air screws to get the thing to balance it seemed fine with no faltering of the tickover at all. It ran smooth and picked up cleanly, only when I took it out for a ride did the faltering come back. In answer to your question, the pistons were free and new return springs were fitted. Oil is engine oil with the level taken filled to the top, again just as per the manual. Before having a ‘fiddle’ the dashpots were filled with ATF but the faltering was still there. Can’t seem to get my head around why it ran so clean when tuning the carbs and then the tickover just disappearing!
Anyway maybe it’s these things that’s the reason why we have these quirky bikes to keeps us on our toes.
I’ll keep you posted

[fogrider]

Yes, for a basic carb like the SU, it's hard to see what can go adrift, usually the rest of the system or electronics. I find the idle by-pass screws on my rotary make a bit of a setting problem because they are pretty well independant of the main jets. It's best to set them separately with vacuum gauges but both are capped off on mine at the housing adaptors because the rubber 90 degree elbows were perished and leaking, that really buggered up tickover and did cause slow run-down. They're not available so I've had to set the by-pass screws the old way - feel and sound at the exhaust outlets. Been doing that 57 years on advice from an old hand mechanic at the time ( I had a twin SU MG Magnette ZB ).
Are the vacuum adaptors OK ? and one more thing to check, the throttle plates should be almost closed. Air entering there does effect tickover and revs run down.
I'm talking F.1. sport, if you have a Commander I'm not fully aware of the plumbing on them, always too heavy a bike for me, brilliant machines though.
Keep at it, regards, Terry.

[fogrider]

Just spotted yours' is a Classic, know nothing about them either !
Been out today on mine, dry empty B roads in East Yorkshire, running fantastic after an issue with fuel pumps. The original died years ago and I put a cheap Chinese job in thinking for the low mileage I do, it would be OK. It packed up,
( outside a fancy looking country club that turned out to be a swingers club ! ) I had another one in stock, that packed up after only 1 hour into the next ride.
Now has a Facet fitted. Mega first run with that in today, hope I did'nt pass any unmarked Police cars !