After finally getting my rebuilt engine run in, its time to concentrate on the fine tuning. For those of us that are familiar with the Amal concentric carb tuning notes that Jim Bushman wrote, I wonder if anyone can clarify his comment in point 8 of the pilot screw adjustment section. He writes..." When you hear the engine revs race, you want to turn it back a 1/8 to ¼ turn ‐ the sweet spot is not at the extreme" what does he mean by "turn it back" is he saying back it out, or back towards closed. More or less air? I can't figure it out. I wrote to him but no reply. Thanks
His text in point 8: "The pilot jet air screw may be adjusted at this time, but by very little, no more than ½ to 3/4 turn in either direction. The effective range of adjustment for the pilot jet is 1 turn to 2 turns. This is where a bit of skill & experience comes into play. By turning one screw a little at a time with the engine running (on both cylinders) you can gauge any difference in running and find the sweet spot. When you hear the engine revs race, you want to turn it back a 1/8 to ¼ turn ‐ the sweet spot is not at the extreme. The idle may need to be dropped equally again. "
The revs pick up as you…
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Thanks Jon. That's exactly…
Thanks Jon. That's exactly what I needed to know.
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Jon is bang on, and his…
Jon is bang on, and his advice overcomes the the short coming in the guidance that is out there. The best part of the setup should be done on a hot engine after it has been ridden for some miles. Lift a spark cap and run the engine on one cylinder - do not follow the Amal or workshop guidance. Adjust the throttle stop to give a sensible tickover on one cylinder, if you really need to adjust the pilot air screw then use a very small amount preferably in the rich direction. Repeat for the other cylinder. Start engine with both plug caps on and the tickover should be high, reduce it to a sensible level and not stupidly low like many set.
If you follow the Amal guides and the workshop manual then you will notice that you'll lean it out too far at this stage, yes, it will tickover but the handover off the pilot circuit is compromised which is not ideal and could lead to stalling at lights etc more so if set with a low tickover. You may also notice a falt spot as you start to open the throttle. The bike will also cut out when shutting the throttle from high revs ie, if you take a slip road off a motorway and change down through the gears though many don't notice this.
The Amal guidance and the workshop manual method is flawed in that is gets you weaken the mixture and drop the throttle slide to compensate which is far from ideal.
The pilot circuit has to do 4 things, enable the bike to start, enable the bike to tickover, deliver rich fuel on the handover to the slide as it starts to rise until it becoems the controlling item, keep the bike running when the throttle is shut from high revs, ie gear changes and slowing slip roads when needed.
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The revs pick up as you weaken the mixture, depending on carb this could be adjusting in or out. When you get to the point the motor starts to pop or run unevenly you have gone too far. Get it to the point of even high revs, then richen the mixture slightly (1/8 - 1/4 of a turn). This will help keep the piston temp down and prevent pre ignition.
The comment on 1 -2 turns I believe relates to the effective range of the adjustment screw.
J