Author Topic: The old spitting back saga.... cured  (Read 446 times)

Online CheeserBeezer

  • Valued Contributor
  • ****
  • Join Date: Dec 2021
  • Posts: 409
  • Karma: 14
    • Priory Magnetos Ltd
The old spitting back saga.... cured
« on: 22.05. 2023 21:21 »
Here's a long post if you're bored,.... and I mean very bored! A few years ago I fitted a brand new Amal 389 to my trusty, bog-standard, A10. I checked the head before fitting and yes, it was 1.125 venturi, i.e. one of the later heads so a 389 was correct. All jetting as standard. Result? cr4p! I rode off up the road with the bike spitting and jerking worse than ever. When I fitted the carb I thought the slide was a bit sloppy for a new carb. So I wasn't entirely surprised that the bike didn't go very well. Anyway, by happy coincidence I had a brand new, original, 3.5 slide which I fitted and rode off up the road again. Amazingly (or not) the bike ran well so I attributed the previous poor running to a loose slide which had slipped through quality control at Amal (Burlen). Now, about four years later, the carb has started spitting back again, so much that it's embarrassing at traffic lights and junctions when all my mates pretend they've been shot! Surrey Cycles (very helpful, Michael) supplied me with a slide which is 0.003" oversize. Apparently he has these machined and stocked on the shelf so that we can give our old, worn out carbs, a few extra years of life. Interestingly, the slide fitted into the carb body and slid down about half-way before jamming. Michael had explained that the body would need the tight spots easing out with a bit of fine wet and dry. With the carb completely stripped it was quite easy to identify where the tight spots were so I did as required and fitted the carb. I haven't quite got it right yet as there's still a bit of drag and the slide doesn't shut off as reliably as it should. In the meantime, whilst waiting for the new slide to arrive I built a carb out of second hand bits, just as an experiment. I grabbed the best 3.5 slide I had and the best 1.125 body and throttle valve I had and, surprisingly, the slide was a bit stiff in the bore so I checked the bore and slide for roundness. They were OKish so I gave the inside of the body the slightest of rubs with 600 grade wet-and-dry until snug and smooth, fitted the carb. Remarkably, the bike runs perfectly. The slide is a really good fit which, I suppose, is the cure for the spitting back. I noticed that the carb body will distort if it is overtightened; I thought it was only the concentrics which were prone to this. Not sure what the moral of the story is but who cares any way? The bike runs great!

Online Colsbeeza

  • Resident Legend
  • *****
  • Join Date: Aug 2015
  • Posts: 709
  • Karma: 5
Re: The old spitting back saga.... cured
« Reply #1 on: 24.05. 2023 00:42 »
Very interesting Andrew. I fitted a new 389 to my standard A10 recently, tightened the nuts gently. It runs well once going with a rare spitback but is a pig to start on these cold mornings. When hot, I adjusted the idle screw but there is no sweet spot, so I left it at 1.5 turns outwards. I haven't taken it off to check yet but seems that the idle circuit may have some swarf in it or something. If I cannot get it right, I'll refit my old carb which has had the carb body ovality removed and a resleeved slide. Let you now later.
Col
1961 Golden Flash
Australia

Online CheeserBeezer

  • Valued Contributor
  • ****
  • Join Date: Dec 2021
  • Posts: 409
  • Karma: 14
    • Priory Magnetos Ltd
Re: The old spitting back saga.... cured
« Reply #2 on: 24.05. 2023 08:20 »
Very interesting Andrew. I fitted a new 389 to my standard A10 recently, tightened the nuts gently. It runs well once going with a rare spitback but is a pig to start on these cold mornings. When hot, I adjusted the idle screw but there is no sweet spot, so I left it at 1.5 turns outwards. I haven't taken it off to check yet but seems that the idle circuit may have some swarf in it or something. If I cannot get it right, I'll refit my old carb which has had the carb body ovality removed and a resleeved slide. Let you now later.
Col
Sorry if you already know this but it is possible to clear out the idle circuit with a piece of fine wire (I know you shouldn't do this!!). If you look closely, there is a very small hole near where the carb joins the head, in the inlet tract at the bottom. This is the fuel hole from the primary circuit. If you remove the idle screw and poke a piece of wire down the hole you should be able to see the wire by viewing into the idle screw orifice.. Provided the idle jet is clear there is no other way fuel can't pass into the venturi. I'm sure you know this already but I've posted it for the benefit of others who may not.