Tuesday, 5 December 2017

Carburetter Rebuild

Regular readers will know that there have been a few challenges around the carburetter recently, and that I've started poking around in it.

The first sign of trouble was on the overrun, heading downhill into town with what felt like fuel starvation. A couple of days before, I had run out of petrol on the SQ4, through my own fault (the SQ4 has no reserve tap, a long story) so I was congratulating myself for my inattention again. Pulling on the reserve tap made no difference and limping to my destination saw the tools out and a chunk of debris removed from the main jet. I went home with the jubilation that only a roadside repair can provide.

I started wondering why the bike was quite so slow. Investigating the carburetter slide revealed it wasn't opening fully as previously reported:

The investigation also revealed an uncomfortable tendency for the slide to stick caused by a non-standard throttle return spring, something that showed up in the shop but not thankfully on the road. The carburetter would need some investigation soon.

And remember that blocked jet? That came back to haunt me while coasting downhill into a village near here, when everything started popping and banging, spitting neat fuel back out the carburetter. I limped along for a while with a wet knee, eight-stroking (I thought only Bantams did that?) eventually realising that going home was not really an option, because after all what would I have done if I had been hundreds of miles away?

Pulling the tool roll out, I took the Amal carburetter spanner and removed the jet cover or 'Float Chamber Holding Bolt' to check the main jet - which wasn't there. It was sitting, loose, in a puddle in the bottom of the jet cover.

Perhaps someone didn't tighten it properly? And while you're at it, that fuel leak you have been complaining about, that you thought was the float valve not sealing? Perhaps you ought to tighten the big nut on the bottom of the mixing chamber?

So anyway, the carburetter is back together, not leaking and the bike is running happily with the needle in the right notch. However, we definitely need a new throttle spring, so we better get on to Burlen Services. Investigation of their website reveals there is a short spring and a long spring, but which one?

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