Grateful for firsthand experience explained well. I bought this tractor on an online auction sight unseen. Night it was unloaded the tractor started up but not response from throttle. Yes prior owner replaced intake and put on aftermarket carburetor. Looks like it could stand to be rebuilt. Mind you tractor starts up runs well just poor throttle response for idle. Also when I got this jewel prior owner neglected hydraulic system it was whining horribly. Hydraulic oil change (no water In system) new filter and unit was quiet and full hydraulic function worked. I proceeded to convert oil filter to spin on filter. The dome was leaking like usual on every IH unit I have worked on leaked until conversion. My last fix is water pump and thermostat. The shaft on pump is loose wobbling 1/16-1/8” movement. Also seeing unit getting pass normal for temp at idle for 25 minutes. What knowledge do you have T/A it has one that will not remain engaged.From high rpms to idle should be nearly instantly if under no load. The 504 uses the C-153 engine, and governor is pretty much the same one used on the C-123's, so I'm a little familiar with them. Open this link to show an exploded view of that governor. CASE IH | Schematic, Manuals, Specifications and Diagrams for governor | MyCNHi US Store
The 2 most common problems that would cause what you are experiencing are #1, sticking fly weights (#27 in the exploded view) #2 the connection rod from the gov. rockshaft to carb. (#1 in the exploded view) Normally if spring # 10 breaks, it will allow the engine to runaway @ high rpms. If thrust bearing#31 has come apart, they most generally will only run at about 1/3 maximum rpms.
Before going much further into detail I have a few questions. #1, Is this a new to you tractor..?? Was it like this when you bought it..?? #2, If not a new to you tractor, did this seem to happen all at once, or come on gradual..?? #3 If this seemed to happen all at once, I have to ask did you work on it, and what did you do to it..??
I have to ask, because if the carburetor was removed to be cleaned, or for whatever reason when re-installed if the connection rod to the carb was installed from the engine block side, it will bind on the engine block and cause these symptoms. I know, because I did this very thing when I rebuilt the C-113 engine in my little BN Farmall (also pretty much the same Gov.) many moons ago.
If you look at the exploded view, you will notice the rod is turned to hook to the carb. through the hole from the carb body side towards the block. Small cotter pin goes through the rod to keep it in place on the engine block side. An easy enough mistake if you don't have a photographic mind, and apparently I don't. That was 40+ years ago, but a lesson I learned, and do remember to this day. Hopefully that is the problem, if not post back, and we'll go from there.
Thanks you got me one step closer will verify connecting arm. Tractor is not bad condition considering both it an I are same age. It needs TLC and some IH red paint. The Grill is in excellent condition rare find as these are tore up first. My father was red power man he had passion for 66 series and a hand me down 450 from his father. We had shared interest of A 1939 H farmall had shot through rear Case from a damaged bull gear. We put that unit back together used it drive an elevator. Most reliable tractor had not run for 20 years until we brought it back that was in 1978. Thanks for your help.I'd check that rod, at least that would be 1 easy fix. Only knowledge I have on TA's is my 656 has it and has never caused any problems in the 25 years I've owned it. I do know they will free wheel when in the low side going down hill. I've read in forums where many have been torn out, using it as a brake when free wheeling.
Hopefully it just needs adjusted. Maybe someone else with more knowledge on them here will have the answer for you.