BN Surging/Hunting like mad!

Don't fix what isn't broken right?

So the throttle shaft in the Marvel on my BN is really worn, it still runs OK, not as good as it should have but better then you would expect with that much play. On the shelf I had a Zenith that was freshly rebuilt for a tractor that isn't ready for yet so I figured what the heck, let throw it on.

I bolted it on, syced it up (WOT trottle, WOT on carb, pin slides in freely) and fired her up.

Well, it idles nicely but anything over 1/2 throttle causes some massive surging. Not just a little hunting, like full range of RPM surging, non-stop.

At first I thought it was the carb but if I held the governor shaft steady it ran well.

To sync the carb I had to adjust the yoke on the rod a lot, it surprised me as I was doing it but I figured two different carbs...

I lengthened the rod back out to about where it was on the Marvel and it now doesn't surge but it's really the exact opposite of where it should be (the pin sides in when the carb and the throttle are at low idle). I didn't test governor response but I'm guessing it's not going to be great.

I didn't try the bump spring but isn't that more of a minor correction adjustment?

Everything is moving freely/not binding up.

K

Is it too lean?

(quoted from post at 17:45:21 09/09/13) Is it too lean?

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Adjusting the jet didn't have any real effect. The Zenith only has the one adjustable idle jet, the main jet is fixed.

I guess one test would be to put the old carb back on and set it up the right way and see what happens.

K

Are you sure the carb is the right size for the tractor?

I would compare the throttle plate sizes.

You are supposed to set the yoke with engine off then open throttle and adjust the yoke. Your bumper spring could be rusted broke or out of adjustment.

It's off an A, same engine as the BN. Given I did buy it off eBay but the part numbers match up, it has the right vents and jets...

I'm going to reinstall the Marvel, sync it properly and see what happens. I also have another Zenith that I know is correct, it's off a running A that I know the history of.

K

(quoted from post at 18:39:34 09/09/13) You are supposed to set the yoke with engine off then open throttle and adjust the yoke. Your bumper spring could be rusted broke or out of adjustment.

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That's exactly what I did. I have a service manual and I've done it before.

Would a bad bumper spring cause that bad surging?

K

Most of those bumper springs are broken or bent or just plain worn out. That would be the place I would look first. Most any similar spring will work, at least for a test to see if it helps before you can get the proper one. Just keep screwing it in until surge stops. If it will idle down after that adjustment you are good to go. A good lively governor will surge slightly once or twice when going from slow idle to wide open throttle.

(quoted from post at 19:15:28 09/09/13) Most of those bumper springs are broken or bent or just plain worn out. That would be the place I would look first. Most any similar spring will work, at least for a test to see if it helps before you can get the proper one. Just keep screwing it in until surge stops. If it will idle down after that adjustment you are good to go. A good lively governor will surge slightly once or twice when going from slow idle to wide open throttle.

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Sounds like I will be looking at that spring tomorrow.

Lengthening the rod like I did to make it work essentially loading up the governor spring and makes it not do it's thing correct?

K

No not having the rod adjusted does affect throttle response. Bumper spring is for surging but throttle rod needs to be adjusted correct. I would take the surge screw out to check that spring and make sure its all there and not rusted fast.

I know this might sound ridiculously simplistic - but - are you sure your tank's full?

On mine I know I'm running low when it start surging like that. I can look in and see gas, but it's just not enough.

It'll run like that for a long while before I finally break down and re fill it.

Agree with others about the spring - but definitely check your fuel level before digging in.

(quoted from post at 08:58:11 09/10/13) I know this might sound ridiculously simplistic - but - are you sure your tank's full?

On mine I know I'm running low when it start surging like that. I can look in and see gas, but it's just not enough.

It'll run like that for a long while before I finally break down and re fill it.

Agree with others about the spring - but definitely check your fuel level before digging in.

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Funny, that's the first thing I checked!

My grandfather wouldn't put more then 2 gallons of gas in the old farmalls at a time (unless he was in the field). I know all about running out of gas.

Good thought though.

K

So...

I pulled the retainer/adjuster/threaded thing out and the spring is there and sticking out maybe 1/4" to 3/8". Does that sound about right?

The interesting part is the jam nut and the acorn nut were missing. Never played with them on this tractor so I didn't notice them. I've got the thing sitting in some kerosene to degrease it before I put it back in.

I would say it was only threaded in 1/4 of the way. Not sure what's normal...

I'll stop and get at the very least a jam nut tomorrow.

K