Farmall H - Only runs on 3/4 choke

JMFarmallH · Mar 30, 2023

I am stuck with a running problem on my Farmall H - starting running rough and cutting out end of last year. Working on it this spring I have check what I think are the most likely issues -

Found rear manifold slightly loose - tightened - no change - replaced studs/gaskets/torqued - no change
Insptected tank - installed in tank screen/replaced screen and thoroughly cleaned fuel bowl - confirmed good flow to carb - no change
Removed/cleaned/rebuilt carburetor - the inlet screen on the carburetor was clean when removed - set idle and fuel jet to factory settings - no change

Fires right up with full choke - runs smooth at 75-80% choke - move choke to 50% and the engine will immediately start sputtering and die - move to 100% choke and the engine will sputter and die - the tractor will immediately restart at full choke (some fuel will drip out of the weep hole) - set to 75-80% choke and it will run.

Any guidance would be greatly appreciated

caterpillar guy · Mar 30, 2023

Did you check the flow out the bottom of the carb with it on the tractor? Then did you fill a pint jar in a few seconds? Could also be a plugged hole still in the carb. Sounds like it needs a better cleaning like a soak over night or for a few days depending on what it is in.

rustred · Mar 30, 2023

i always ask the important question... was the carb soaked in gunk overnight? you have a plugged orfice. was there junk in the carb when you took it apart?

used red MN · Mar 30, 2023

Hello JM welcome to YT! Is the main jet needle that angles up from the back into the bottom of the carb turned out 3 to 3 1/2 turns from screwed in and lightly seated? Is the venturi in the carb, see link? Slips into the carb throat when the two halves go together. The expectation of filling a pint jar in a couple seconds out of the carb drain is a little ambitious. Filling a pint jar in 2-3 minutes is adequate fuel rate for an H. This does not count the initial 5 seconds or so of what immediately drains out of the filled carb bowl.
Farmall H carb venturi

Charlie M · Mar 30, 2023

Sounds more like an air leak to me. You said you found the manifold loose. Maybe the gasket is bad or missing. I'd also look over the manifold for a hole that has rusted through on the intake section of the manifold. You had the carb off - do you have a good gasket there to the manifold connection.

used red MN · Mar 30, 2023

Charlie, read it again. He replaced the manifold gaskets on the second try. You may have a good point on the carb gasket. It takes one heck of a leak to make an engine with no load not rev up. Usually a bad vacuum leak will let an engine rev up but then hunt and that would be a hole you could stick your finger in. Possibly if it was drawing exhaust gases it might keep it from revving up. In my opinion by what he is describing fuel delivery is his problem.

Charlie M · Mar 30, 2023

Can't argue with possible fuel. I'm thinking on a carb issue the tractor would at least run without choke but run pretty crappy. Could be wrong. I missed replacing manifold gasket. I always look for the free or simple stuff first when diagnosing a problem. I've had a leaking manifold before and know partial choke makes it run better.

rustred · Mar 30, 2023

yes fuel delivery from inside the carb to the combustion chamber when off idle. its all good from tank to carb even if it is flowing out that is not a true test. just because the float bowl is full dont mean the vacuum will suck it up once off the idle circuit. too much of that ooolllddd brainwashing here. the experienced eye dont need to be measuring with a pint jar. pull the plug at the bottom gas will pour out for 2 seconds, then it will keep trickling at the same speed it is running past the needle and seat. i have never measured fuel flow with a jar in my life.

JMFarmallH · Mar 31, 2023

Thanks Charlie - I thought I had it when I found the manifold loose - after I replaced the gaskets and tightened it up I sprayed the manifold and gasket area with carb cleaner while the tractor was running - no RPM change at all - back to the drawing board

JMFarmallH · Mar 31, 2023

Thanks Used Red - I will check the flow out of the bottom of the carb after the initial flow - venturi is in

JMFarmallH · Mar 31, 2023

Thanks Rustred - the carb was pretty clean inside - no varnish build up or junk - I did not soak it but I did spray all passages with carb cleaner - might need a soak if I can verify that flow to the carb is sufficient

JMFarmallH · Mar 31, 2023

Thanks Catepillar guy - I will check the flow through the carb - I probably should remove and soak just to rule out the carb

JMFarmallH · Mar 31, 2023

Carb gasket looks good and the sealing pattern from the manifold and carb shows complete contact all of the way around the gasket after I removed it - I am going back to fuel supply - All of the ideas have been helpful

JMFarmallH · Mar 31, 2023

I am with you - I would think that it would run - just run poorly - When the choke is moved from 75% to 50% the tractor dies within 30 seconds after popping and sputtering - I am going to continue to chase down the fuel issue/flow and soak the carburetor for a couple of days and start over -

rustred · Mar 31, 2023

make sure you remove the pickup tube also. after soaking rinse with warm water and blow out all passages with air. then you can squirt some wd40 in the passages and blow them out and watch and see if they are open and free. plus use a wire and dig around in them passages. plus the pickup tube has little holes in the side and need these all open they can be plugged.

J.Wondergem · Mar 31, 2023

Why does nobody use a vacuum test gauge anymore?

JMFarmallH · Mar 31, 2023

I replaced that tube with the rebuild of the carburetor - I did not dig out the passages - a good soak and digging through the passages is next

JMFarmallH · Mar 31, 2023

Good thought - it has been a minute since I have used mine - where would you connect to test and do you have any
readings/ranges that you should expect

rustred · Mar 31, 2023

uumm because its an old tractor? dont see how you could tell a plugged pickup tube by vacuum.

JMFarmallH · Apr 10, 2023

Thanks again for your advice - I ran back through everything I had already done and before I took off the carb I pulled out the main jet/idle mix and idle passage screw and sprayed carb cleaner through each of the passages - I let it sit overnight - started it the next day and it runs great - it has not run better

Farmall H - Only runs on 3/4 choke

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