1940 Deere Model A run/idle issues

[Grisse]

Hi all - new to forum and hoping to find some advice and suggestions. Bought a 1940 Model A tractor (SN 490470) last November - simply because I liked to tinker and thought it was cool. It sat outside for 20 years. Finally got it running about 1.5 months ago. Here's the list of work I did thus far - Changed oil and coolant, removed cleaned and reassembled brakes, cleaned/adjusted clutch and engagement mech, cleaned rust from both fuel tanks, rebuilt carb, new fuel sediment bowl and shutoff, removed and repaired missing governor fork, opened clogged oil feed hole for fan bevel gears, replaced oil slinger seal, balanced fan and rebuilt fan shaft, greased everything, rebuilt magneto, new plugs & wires, and a lot of other little things. Prior owner rebuilt cylinder head, installed new piston rings and connecting rod bearings in 2002. Once I rebuilt carb, it now starts fairly easy but I really don't have throttle control to speak of. When running it generally spins around 875 RPM. Any attempt to significantly lower the RPMs will kill it. When I rebuilt carb, I cleaned every orfice and airway with the proper sized drill bits. Float is set right. The idle screw has little effect, but load does. I am unable fully open choke and keep it running. Have to leave partially on. Should I take carb out and reclean it again? Fuel sed bowl has been pretty clean since I got it running. Any help would be appreciated. If it's possible, I'll post some pics once I better understand how forums work. Thanks in advance!

 

[Grisse]

A final forgotten note: When I readjusted the throttle linkage, I used the "half hole back" for aligning the front linkage to the throttle body. For the most part, the throttle arm really doesn't move much from the "idle" position. If you manually force it (overriding the governor), it will run way too fast. Seems like there really isn't anywhere for the throttle (and therefore idle) to go. How much movement should there be in that arm?

 

[Mark-Ia]

Yes, you need to clean and run drill bits through the passages again. Especially that hidden passage at the bottom of the stem.

 

[Grisse]

Thanks for the response. I must have found a dozen passages in that carb body, and cleaned every one of them. I also watched the farm tractor.com video for additional guidance. Thought I got them all. When you refer the hidden one on the bottom of "the stem" where are you referring to? There's always a chance I missed something. BTW, pulling that main nozzle out of the barb was quite a booger. Glad I had a good tap set. Every part that could be replaced on the carb was (butterflys, nozzle, brass plugs, needle valve/seat, float, etc.).

 

[Mark-Ia]

There is a plug at the bottom of the stem. Directly across from that is your hidden passage which is commonly obstructed.

 

[Grisse]

I ll take it apart this weekend and give it a gander. Thanks. Any thoughts on how much the actual throttle arm should move when swinging between full throttle and idle? Even when my A is turning at the 875 rpm, the throttle is barely open. Kinda confusing.

 

[WTW]

Sir, Sounds like you have a good understanding of the tractor and have done all the things you should
of to make this tractor idle. The single missed step that most people miss when cleaning these
carburetors is the final idle circuit check to confirm idle circuit is truly clean / open. This check
can be made before and after cleaning to see not only it is open after cleaning but also to see if it
is actually plugged before cleaning?? You may or may not seen the idle circuit drawn in service manual
and cleaning instruction?? If not think of the idle circuit in the shape of the letter "U" The inlet
at the top of one end of U and the outlet at the other end. In the bottom of U passage is where the
idle metering needle meters raw fuel into the idle / U passage from bottom of bowel. This U shaped
idle circuit passage is mostly in the stem of carburetor. The secret passage some call it, is the
fixed / drilled orifice located at the bottom of the U and is the most restrictive part of the idle
passage. To check the idle passage that it is open from the air inlet to the exit idle jets at end of
circuit, first shut the idle adjusting needle all the way closed until it is seated. This will block
fuel inlet at bottom of U. With idle needle shut / closed check idle passage using an aerosol can of
carburetor cleaner or brake clean with a straw on spray nozzle. Place nozzle straw into / against
inlet of idle passage and spray cleaner product through idle circuit to see if it ex-spells out outlet
jets (two jets in most carburetors). The inlet hole is reached from the choke end of carburetor and is
drilled into the idle needle passage at venture step in throat of carburetor. The outlet jets are
located at throttle plate where edge of plate meets inside bore of carburetor bore when plate is shut.
If you can spray into the inlet of circuit and it sprays out outlet jets the idle circuit is open! If
you cannot push product through circuit it is still plugged. To check the fuel inlet passage open idle
needle and spray again and product should leak / run from bottom of stem where end of idle needle
protrudes through. Hopefully you can follow all this. Good luck, DW

 

[Grisse]

Wow! Thanks for the excellent guidance. I haven't cracked down and gotten a service manual for this tractor as of yet, but plan to do so. Much of it has been fairly straightforward as I tear into various areas for cleaning and/or service. What you describe makes perfect sense, and seems fairly easy to confirm by pulling the carb and doing a little checking. Nice project for this weekend! I'll give that a go, but from what you describe, I don't think I investigated that idle circuit to the degree necessary. If correct, that would explain why I need to leave the choke partially on to keep it running at low throttle. From what I can tell the throttle butterfly really doesn't move very much regardless of where the throttle handle is; UNLESS the tractor is under load. Under load, the governor seems to manage the butterfly to maintain rpms. One of the fork screws in my governor had fallen out and allowed the fork to drop and clash with the timing gear. This rendered the governor useless and gave me a bit of a runaway engine/fan when I first got it started. Prior owner simply shifted the governor arm on its shaft to allow the tractor to run anyway. That's all fixed now. Really just trying to better understand how much actual throttle (speed) control these old beasts have. Seems like not a lot. Thanks again and I'll post an update after doing some exploring this weekend.

 

[Grisse]

well, you were spot on with the diagnosis. I had originally cleared the entire passageway except the small orfice leading into the base of the needle seat. And yes, that is clearly the most pinched point in the system. I've got flow through the entire "U" now. Since its now dark and cold outside, I'll have to wait to test the setup. Pretty confident there will be a difference. Will save that for tomorrow. Here's hoping for a successful start and idle down. Thanks again!!

 

[WTW]

You will be as excited as a kid waiting for Christmas morning to hear this old tractor idle down and pick em off. Play / adjust with your idle adjustment you will be happy! 1 1/2 to 2 turns out on idle screw it should idle and tune from there. Enjoy!

 

[Grisse]

Update - started pretty easily. Choke came all the way off for the first time, and I was able to get it "somewhat" tuned with the idle adjust. I got it to idle around 500-525 rpm though it sounds like it wants to quit at that speed. That's a big improvement. It never really seemed to settle into a "comfortable" idle, however I realize that's often the way these poppers are supposed to run and sound. When all was said and done, the idle adjust was unscrewed a little over 3 full turns which also seems high but so be it. Will keep tuning to see how tweaked I can get it. Does that seem to be about where it should idle at the low setting? I'm adjusting the throttle stops now so pulling it fully back will bring it to idle rather than kill it. Also, I'm burning premium unleaded in it right now. Should I switch to regular or other? Lots to learn and thanks for the help!

 

[WTW]

Sir, Congratulations on your carburetor repair. You can say you did it yourself without having to send it to anybody!! Now your next accomplishment will be to super tune it. Do you know what your timing is? The old A will not idle as slow and smooth as a late A will with distributor ignition but you can do allot with this one to get it to idle slow and smooth. I can make a old B run / idle better than an A but I have a unstyled 37 A that will idle very slow and smooth. Besides timing, the carburetor can be improved more without altering it as they are pretty crude in their stock form even in good condition! This not only means to have them in shape including throttle shaft and bushings which you may have addressed, but also that there is no wear on throttle plate. To control fuel air mixture better I drive throttle bushings a bit deeper into bore just far enough to shape bushings ends to same shape as inside diameter of bore of carburetor to reduce air leaking at throttle bushings. This will force air flow to go by idle jets to give better fuel air consistency. Simply carefully shape bushing ends with round file and emery cloth to same shape as throttle bore without altering body bore until until throttle plate turns in between bushing ends without binding. This will make a better and slower idling carburetor because of better control of air fuel idle mixture. Another trick I do for better tuning is good chance the load nozzle bore in stem of carburetor is / was rusty and now pitted after cleaning? To seal the nozzle in stem of bore I fit an O-ring in bore between nozzle and retaining spring to seal a common loose fit. The next trick is wrap the stem thread with Teflon tape before final installation of bowel and nut, stem thread without any seal will allow fuel from bowel to leak into the nozzle circuit. This leak is uncontrolled fuel that load needle adjustment will not have control of. If you don't learn nothing else from this you will learn what a nit pick I am but before you laugh, come listen to my tractors run and idle. It worth the extra effort to me. DW

 

[Grisse]

With snow coming this weekend, I m going to try and fine tune this carb as you recommend. Will also check the timing though the magneto may well be the limiting factor in how it ultimately runs. Stay tuned for continued updates.

 

[Grisse]

This past weekend gave me a day of cold, so I elected to spend it in the shop working on the tractor. I followed your guidance by installing new throttle bushings, driving them until they were proud of the carb interior. Filed them down and then polished them until flush with the interior surfaces. Also did the O ring around the nozzle in the stem. Probably a few more things I can do, but went ahead and gave it a go again. Quite an improvement by eliminating short circuiting around the throttle shaft. Working to get the idle a bit slower, but all things considered, I'm much more satisfied with where it is now. I'm going to do a timing check this weekend and also make sure the impulse timing setting in the magneto is set correctly. It is all the little things. Ordering some petroleum-rated sealant for brass plugs and the bowl nut. Will do another round of fine tuning this weekend and then give it another go. Thanks again for the help!