Apologies if this is old hat to many of you. It was new to me:
The gent who recently overhauled the NA-S3A1 carb on my L-4 is an acknowledged expert in the field. He returned the carb to me with clear instructions to start the idle mixture adjustment with the mixture screw three turns out, and to richen the mixture further from that point as needed. I tried this more than once, but it yielded an engine that sooted the spark plugs and never really ran well at idle.
His advice is to make any adjustments only with the engine stopped, as adjusting with the engine running is "just too dangerous".
I'd noticed that before the rebuild (and the onset of another problem) that the engine had idled very nicely with the idle mixture screw out only about a turn and a half.
Yesterday, I uncowled the engine (again) and set the screw to three turns out. Then, I started the engine and adjusted the screw with the engine running. I stood on the left side of the engine, grasped an engine mount tube with my left hand, and turned that screw in (lean) slowly with the fingers of my right hand.
At first, no change. Then with continued clockwise turning the engine got smoother and smoother. Then the RPM picked up slightly. This had to have been the optimum idle mixture. More turning. RPM fell off slightly, and engine operation noticably roughened. I eventually had turned the adjusting screw all of the way in. The engine still ran.
I returned the screw to the best running/ highest rpm point, and then opened it about 1/4 turn more. Job done. The engine idles great and didn't foul the plugs on the test hop that followed.
Any time the idle mixture is adjusted, the idle speed will have to be addressed, too. These two items are "tied together with a real short rope".
I do not feel that adjusting the idle mixture with the engine running is dangerous at all. Just plan the job, move with deliberation, and know where your body parts are at all times. It's probably the only way to perform this little operation accurately.
Anyway, the bottom line is that three turns out on the idle mixture screw might be a great place to start, but it's too rich for good idle quality, and further enrichening will only make things worse.
I'm running the correct carb with proper part numbers throughout on a A-65-8F. I have all of the original manuals and later revisions for this carb. Nowhere in them is there any technique given for adjustment.
Your mileage may vary.
The gent who recently overhauled the NA-S3A1 carb on my L-4 is an acknowledged expert in the field. He returned the carb to me with clear instructions to start the idle mixture adjustment with the mixture screw three turns out, and to richen the mixture further from that point as needed. I tried this more than once, but it yielded an engine that sooted the spark plugs and never really ran well at idle.
His advice is to make any adjustments only with the engine stopped, as adjusting with the engine running is "just too dangerous".
I'd noticed that before the rebuild (and the onset of another problem) that the engine had idled very nicely with the idle mixture screw out only about a turn and a half.
Yesterday, I uncowled the engine (again) and set the screw to three turns out. Then, I started the engine and adjusted the screw with the engine running. I stood on the left side of the engine, grasped an engine mount tube with my left hand, and turned that screw in (lean) slowly with the fingers of my right hand.
At first, no change. Then with continued clockwise turning the engine got smoother and smoother. Then the RPM picked up slightly. This had to have been the optimum idle mixture. More turning. RPM fell off slightly, and engine operation noticably roughened. I eventually had turned the adjusting screw all of the way in. The engine still ran.
I returned the screw to the best running/ highest rpm point, and then opened it about 1/4 turn more. Job done. The engine idles great and didn't foul the plugs on the test hop that followed.
Any time the idle mixture is adjusted, the idle speed will have to be addressed, too. These two items are "tied together with a real short rope".
I do not feel that adjusting the idle mixture with the engine running is dangerous at all. Just plan the job, move with deliberation, and know where your body parts are at all times. It's probably the only way to perform this little operation accurately.
Anyway, the bottom line is that three turns out on the idle mixture screw might be a great place to start, but it's too rich for good idle quality, and further enrichening will only make things worse.
I'm running the correct carb with proper part numbers throughout on a A-65-8F. I have all of the original manuals and later revisions for this carb. Nowhere in them is there any technique given for adjustment.
Your mileage may vary.
Last edited:
