John Deere 1943 A Carb

Anybody know about what causes surges in a carb when idling? My buddy just got his A running, but it is clearly not adjusted right. We did the half hole thing on the linkage of the carb and did the proper starting adjustments on the idle and load screw for starting out. He does have bigger pistons in it.

He also broke the stop pin for the carb. I'm not sure if that will be a problem drilling out or what.

Thanks,

Robert

Pistons make no diff. Did you change the throttle shaft bushings and felt seal at top one? Can you wiggle the throttle shaft sideways at top when in idle position? They surge when sucking air through these. If this is the case you can smooth it out by slightly closing choke. If there is NO wiggle at shaft, check throttle plate itself. If it hasn't been replaced it probably need it. They are brass and they wear out on the bottom from their own weight and the tractor vibration. Once worn, they sit lower in the bore and leak air all along the top edge. Once you check/fix all this, then you can adjust idle/load mixture as follows. Open both 2 turns. Start and warm up for 5 min or so. Make sure you have new or burned clean plugs. Now throw throttle lever on steering post wide open. This will be fast idle. While at fast idle close LOAD needle all the way. It should stay running at fast idle with no difference. If it stops then you have plugged idle passages meaning it was NOT rebuilt. If it passes this test then start turning the idle mixture screw in a few clicks at a time (slowly) until such time you hear the slightest hesitation...quickly turn it back out 1/4 turn and leave it alone. If tractor stops before you get it opened back up then go back to step one and start all over. Once idle is set then throttle back down to slow idle and open load needle back up 1 1/2 turns. Now throw throttle open real fast and notice whether it spits/pops or accelerates with a lot of smoke at first. It should accelerate evenly with just 2 or 3 puffs of dark smoke and then cleanly run at fast idle . Do this 2 or 3 times . Adjust load down until it spits or pops doing this and then back open 1/4 turn increments until it snaps right up to fast idle with a few dark puffs and then clears right up. If you are actually going to put this under load somewhere then you may want to do some final adjustment on load screw then. DONE Now adjust your slow idle screw/spring for speed on throttle shaft. Long as your spring is good you probably won't need to replace the post. Most never hit the post anyway before they stop. It's just a press fit piece of dowel if you want to drill and replace.