Thursday, 14 August 2025

SQ4 - a useful modification

 I've been fiddling about with the ignition timing this week as there had been some significant hesitation coming off-idle and some poor performance at low speed - what we used to call 'kangaroo petrol'.

I retarded the timing a bit and realised the bike had stopped kicking back on startup, and the slow running was better, and it would fly along like a bat out of hell - but coming off idle still wasn't as smooth as I'd like.

In the end, after fiddling with the idle mixture and getting nowhere, I stripped the carburettor.

I didn't find much. Some debris in the float chamber, the flange fixing to the head looser than it should have been, and one of the float chamber screws was loose. I blew out all the jets and ports with carb cleaner and found everything clear.

Putting it back together, I added this little tee to the idle mixture screw to make it easier to adjust (it's a bent bit of brazing rod, soft soldered to the brass screw), and used some Threebond on the flange.

We'll see what happens.

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