John Deere A and B

Lyn Jones

Member
Hi Guy's. I have a 1940 JD A and a 1939 JD B which surge quite badly when not under load. I have totally dismantled and cleaned the carbs. Adjusting the needle's on the carbs make no difference. However, if I close the choke to about half way, the engines seem to run smoothly.
Can anyone point me in the right direction.
Thanks in advance.
Lyn.
 
Your carbs aren't clean. You need to remove all the brass plugs and drill out the passages...or send them out to be professionally rebuilt.
 
Also did you remove the Nozzle & clean there? There is NO other way to get every passage clean except the proper sized aircraft-length drill bits. Most every passage in the carb relates to idle. Also the bowl vent needs to be clear.
 
Thanks for your quick replies. I did remove all of the brass plugs and the main jet nozzles. I blew out all the passages with compressed air but I don't have suitable drills for the passages. I may have to look at getting some if they are available here in the UK.
 
So you have proved it is running lean. Two reasons for that,either too little fuel or too much air. Plugged idle passage would make for too little fuel but what if drilling out that doesn't help? Some will argue this but worn throttle shaft/bushings/plate and even carb throat wear from the plate will ALl accumulatively create enough air leakage to prevent it from pulling fuel up through passages, through those little pin puncture wound ports to let it idle properly. They will do exactly as you describe. Get the "secret passage" drilled first and let us know what you find. RB
 
Which one do you consider to be the "secret passage", Randy? I'll get the carbs off again tomorrow and have another look.
 
Take the bowl off and in the stem should be one or two brass screws depending on which carb you have. (that old they could have any of 5 or 6 different carbs by now). If it has two then it is behind the top one, and naturally if it only has one then it's behind it. Don't snap it off if it doesn't want to come out. Use a brazing tip or even butane or propane torch to heat that screw. Don't get it red hot, just 15-20 seconds will do it. Let it cool completely and should come out easy. There is a chamber in behind it and down 1/2" into that chamber will be a little hole about 1/16" diameter. If you see the hole then chances are it is open but could need sharpened up a little. If you don't see the hole then naturally it is rusted shut and will need drilled out. It only goes in about 1/4 inch so drill it by hand and not power. I assume you don't have the correct long aircraft bit so use whatever you have or can get that is approx. 1/16 " or about size of lead in pencil. Then insert a tiny piece of wire down through where the idle needle seats and you should be able to see the end of it through this secret passage. If so , it's open. Use a needle to make sure the little ports in throttle bore by plate are open. Just use needle nose pliers to bend a right angle 1/4" from end of needle. If you need anymore on this my e-mail is open and you should already have one from me. Thanks. RB
 

Randy. I have had those brass screws out but did not look for the small hole down there. I'll have a look again tomorrow and will see if I can find a suitable drill to clean out the hole. I'll also check out the little ports in the throttle bore by plate.
 
Too much air is impossible, & I have 2 A's with throttle shafts so worn they bounce back & forth 3/16" ( worn 1/2 way through, significant bore & plate wear also) & idle perfectly on both cylinders slow enough to count the fan blades- to prove it. If all passages are clear the idle needle adjusts to compensate. You can even drill 2 1/4" holes in throttle plate & it'll still work fine. Low float level setting also contributes to surging idle.
 
Randy. Thanks to your advise, I managed to find that important idle passage on the JD B carb this morning. Of course, it was blocked and I managed to clean it with a bit of wire. I checked all the other passages again and put the carb back together. I installed it on the tractor and it started up easily and ran smoothly. I’ll do the same to the JD A when I get a chance and will let you know how I get on.
Thanks to everyone who responded.
 
Anytime I've had worn throttle shaft bushings the engine did not run smoothly, I don't agree with your theory. .
 
Bottom line is , you can open the idle needle all you want, the fuel still has to flow uphill to get to the bore ports. It will not flow uphill unless there is a low pressure area (vacuum) on intake side of throttle plate sufficient to suck the fuel up there. It will not do that in the proper amount with air leaking all around the plate/bushings/bore wear. Unless one partially closes choke or has leaky needle seat causing very high fuel level or 50wt oil in air filter. Wish I could afford the travel expense to see/hear one idle with two 1/4" holes in throttle disc.
 
It's not a theory. It's a fact, as I said I have 2 of them in that condition that run perfectly. I also have a ford 300-6 with an aluminum 1-barrel carter (factory carb, '84 model) in the same condition that also runs perfect except sometimes the throttle plate hangs up & you gotta tap the pedal again to get idle speed. I'm not saying it's correct or even ok but it's not necessary to make it run right. My $ says there are other issues.
 
His issue was plugged idle passage. He has posted back on and verified that. I'm sure you've seen it by now.
 
Well it has coolant in both cylinders from a leaky head gasket & it's -5 here so it takes an hour of hand crank remove plugs torch dry repeat to get started or I would video it. At idle you still have air going around the throttle disk. With holes in it you simply close the disk further. The path of the air don't matter nearly as much as most people think. Each intake stroke on a 2 cyl is a huge gulp, not a steady flow. There's lotsa ways to modify these carbs & still have good operation. Another example is boring out the Venturi till it's almost gone for more cfm at WOT but still it idles & transitions fine. Drag racers do all manner of tinkering on 4 barrels etc. there's never just one way to make it run right.
 
I guess you haven't seen the one GTX mentioned above either , or the one with 2 -1/4" holes drilled in throttle plate.
 

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