So another day, another road test. After the weekend, where I checked out the B5HS plugs (pale grey centres, all the same - as expected) and fixing a very dodgy crimp in the distributor circuit and filling the tank, I took an early evening bimble along the lovely A149 North Norfolk coast road, into the setting sun. As usual, the SQ4 was fine for the first few miles, though I thought a little more reluctant to rev from a closed throttle than normal. We stopped for a break and a picture at Blakeney:
There's a narrow lane down to the beach here, maybe 300 yards down a hill which you do at 20 mph or less - idling, essentially. At the T junction at the bottom, there was a reluctance to rev and I coaxed the bike to a parking place where I enriched the idle mixture half a turn thinking that it had been reluctant to come off the idle jet since leaving the house.
A few more miles, and once out on the open road again she was flying. A little stumble now and then but generally great, until I got to a little village called West Runton, which has a mile or so of descent down a narrow lane into the village centre - again, 20 mph max, throttle closed, followed by a short hop over a railway bridge. By the time I was at the bottom, opening the throttle would kill the engine and there was no way I was getting over that bridge until the engine had cleared its throat. Once on the open road - more or less OK again. I'm convinced it gets very rich on the overrun, or descending hills on closed throttle.
So, back in the shop I am thinking that I have the idle mixture screw set way too rich - but it's only two turns out. My second thought is that there is an internal leak - possible in the bi-starter, or possibly the bi-starter is not closing fully.
We'll have to pull the carburetter off again.