Farmall A stumbling

[rick pa]

Hello,I am back to working on the clubs Farmall A.It idles nice and runs good when under load,but when it is above 1/2 throttle without load it stumbles,unless some choke is applied.
I replaced points,condenser,cap,rotor,plugs and wires (magneto) checked rotor timing gears,but not governor end, rebuilt carb, but kit did not include main or idle jet,did have shafts.Every passage and jet is open.
Today while playing with it,I turned the idle mixture screw in to 3/8 turn out and it seems to run much better,I did not get a chance to run it under load.I thought the idle mixture screw would only affect idle not upper range.

 

[Dave41A]

It is difficult to say exactly what the cause is, but what you describe sounds like either a plugged main jet or possibly a fuel supply problem. Make sure the fuel bowl below the tank has a clean screen. Also, I think some carburetors had a fine screen just inside the fuel inlet on the carb itself. This screen is notorious for getting clogged up.

It is possible you are getting enough fuel through to idle but not run at full throttle. It is possible that the advance is set wrong causing it to miss at higher RPM's, but choke wouldn't fix that. So it really sounds like a fuel problem.

What kind of carburetor is it? Good luck, Dave

 

[ESVADuke]

2x on fuel supply, my Super AV would do that, rust flakes in the tank covering the hole to the outlet to the settling bowl. if you blow air backwards and it goes away for a bit, might be. Had to take the tank off, clean and seal the inside.

I hope yours is an easier solution!

 

[rick pa]

Thanks for the replies,the screen in the fuel bowl and fuel inlet elbow appear to be long gone, inside of tank is clean, there is an inline filter, but when I removed the main jet plug/cover, gas flowed well out of carb.
It has a zenith carb. It seems like a lean condition on main jet. I will pull the main jet again and see if I can see anything, is there a number on jet I should look for? maybe to small?
I removed the cap from air cleaner stack and placed my hand over stack and stalled the engine, so it can't have to much air leaks.

 

[Dave41A]

If the tractor ran well at one time with that jet, I would think you have the right one. Usually it is an adjustment of the main jet to tune the carburetor.

That carb should be the 161X7-9752 carb. It has been a while since I worked on one, but I am pretty sure the main and idle jets adjust independently, and both screws adjust out for "fuel." This is different from the IH carbs, where the idle screw was idle "air" (out means "leaner"). I only mention this if you're used to the other kind.

The only other thing I could think of would be float adjustment. I do not know what the float setting on a Zenith should be, but now that it is identified, hopefully you can find that or someone will chime in.

Also, when you say fuel flows from the carb, do you mean just at the start, or sustained flow? There should be a 1/4 NPT pipe plug (brass, usually) on the front end of the bowl facing the generator. If you unscrew this it will drain the bowl. Fuel should then continue to flow out after the initial "whoosh." It is this second flow that indicates how much you are getting from the tank. You probably knew that, but I wanted to suggest it just in case. You should get sustained flow for a good 30 seconds to be sure there are no obstructions.

Good luck, Dave

 

[rick pa]

The reason I don't think it's fuel flow issue is the stumble/miss happens as soon as I throttle up over 1/2 throttle,not gradual like a starvation issue.
With the idle mixture screw turned 1/4 turn out,it doesn't stumble/miss,but has lean hesitation when throttled up.I think main jet is fixed type.
I know rich mixture/choke can hide ignition problems,but everything was replaced on ignition. But I am new to these magnetos.Can the mag coil give weak spark.

 

[Dave41A]

If it is magneto I would be very surprised if that was what was causing the problem. I put a mag on my H for the reliability and it is 100% rock-solid. I cannot think of any reason why a magneto would start to miss upon throttling up unless there was a loose cap wire or rotor that the sudden vibration from increased RPM's shook loose. But if everything is tight, then I doubt spark is your problem.

I will look at my Zenith carb when I get some time tomorrow and get back to you. It has been a while since I have even looked at one and I need to refresh my memory.

Good luck,
Dave

 

[rick pa]

So to recap.It idles good always,it runs good up to 1/2 throttle,above 1/2 throttle with no load it has a miss/stumble, it is more of a carb stumble than ignition miss,add some choke and the miss/stumble goes away.
It runs good climbing a hill above 1/2 throttle but when you crest the hill and the load comes off the engine the miss/stumble comes right back.
With the idle mixture screw at 1/4 turn out it doesn't miss/stumble above 1/2 throttle without load but it doesn't seem up to power.
Sorry to be a nuisance, I have a Ferguson to20,and some old 2 stroke motorcycles so I have to work on carb issues,but this has me stumped.

 

[Dave41A]

No problem. I am not going to be able to look at the Zenith carb today. It is at a different location and my plans have changed due to other obligations.

I think the key here is the idle adjustment. If I remember correctly, the tractor actually runs on the idle jet even when at "high idle," and the "main jet" starts to kick in when under load. I think that is what is happening here. At high RPM's under load, the throttle opens up and the main jet kicks in full force. This is why it handles going up hills at high throttle just fine. So I think the main jet is good.

However, when at high idle, but under no load, the idle jet may not be keeping up. You can look at the position of the throttle linkage when it is at high idle but not moving...I think the throttle butterfly will still be almost completely closed, meaning the tractor is running off the idle jet.

Another explanation is that there is slop in the governor linkage and the carb is getting caught between idle and main jets. I could see this happening where slop in the linkage means it doesn't get the gas it needs and tries to die, but then the governor over-reacts and throws it to full throttle. You would get "hunting" or "surging" of the governor in this situation. This is hard to explain without an example. Sorry that wasn't as much help as it could have been. Someone else will probably chime in and set me straight. Dave

 

[rick pa]

I wont get a chance to look at it till the weekend. It makes sense about running on idle jet till governor opens throttle for load.
My Ferguson surges under load(governor or mixture not set just right) but I would say this is more of a miss than surge, but I will check throttle shaft for movement.Thanks

 

[rick pa]

You were right about full throttle running on idle jet, when not under load but full throttle, the throttle plate is only slightly open.
I guess my next move is to open carb again and check idle circuit, but really don't think I will find much, I have been through it many times now.
It isn't surging, only very slight movement on throttle arm.thanks again