NAA High RPM Stumble

VTPSD · Mar 31, 2017

I am helping my dad fix up his NAA. It was 'restored' by some jackarse who basically messed up everything he touched. After fixing about a dozen leaks, and various other problems, I am trying to get it running right.

The engine is rebuilt, and seems very solid. It has a zenith carb that appears to be new. I have tried every adjustment known to man, and I can't get this thing to run right under load above half throttle or so. It hesitates like it might be running a little lean, but giving it choke doesnt really clear it up. The ignition timing is at 30 degrees at 2000 rpm. I have the carb set so it idles and runs at partial throttle perfectly. For the life of me, I cant get the top end powerband right.

The bottom adjustment needle appears to be the main jet. This is supposed to be set 2.5 turns out. No amount of turning it further out makes any difference. I can turn it in, and eventually the engine starts to stumble.

I am just basically looking for a shot in the dark. It is strange that it runs so well at all other RPM's, but I just cant get it smoothed out on top.

flynlow8740 · Apr 1, 2017

What you describe sounds like ignition to me. I'm not familiar enough with those to know much about the distributor, but points can cause your symptoms. So can a coil, but not normally they usually just quit entirely. Also, be certain the plugs are gapped to spec, as those old ignition systems don't make enough voltage to jump a big gap.

sweettractors · Apr 1, 2017

VTPSD said:
I am helping my dad fix up his NAA. It was 'restored' by some jackarse who basically messed up everything he touched. After fixing about a dozen leaks, and various other problems, I am trying to get it running right.

The engine is rebuilt, and seems very solid. It has a zenith carb that appears to be new. I have tried every adjustment known to man, and I can't get this thing to run right under load above half throttle or so. It hesitates like it might be running a little lean, but giving it choke doesnt really clear it up. The ignition timing is at 30 degrees at 2000 rpm. I have the carb set so it idles and runs at partial throttle perfectly. For the life of me, I cant get the top end powerband right.

The bottom adjustment needle appears to be the main jet. This is supposed to be set 2.5 turns out. No amount of turning it further out makes any difference. I can turn it in, and eventually the engine starts to stumble.

I am just basically looking for a shot in the dark. It is strange that it runs so well at all other RPM's, but I just cant get it smoothed out on top.

It sounds like the governors are wore. Ken Sweet

3930dave · Apr 1, 2017

Fuel filter is brand new ?

Should be cheap to change, and even if it is not that old, the existing one may have plugged up fast depending how long the tractor has been sitting and what state the tank was left in....

Edit - I don't know those tanks - if they have a screen/petcock assembly you can check, see if that is obstructed.

Rgds, D.

KYErik · Apr 1, 2017

Is the fuel fresh? Do you know if the fuel level in the carb is OK (that the float is set properly)? Like someone else, check the ignition first then look at the carb closer.

Ford850 · Apr 1, 2017

It should have a screen in the tank, above the fuel shutoff. Be sure that is clean so it can flow fuel at a high level when needed.

VTPSD · Apr 2, 2017

The carburetor is almost brand new, fuel is fresh, non ethanol, great fuel flow at the carb, new fuel filter. I am also suspecting ignition since the adjustments on the carb do little or nothing to cure my misfire.

I also noticed when setting timing, the light seems to blink out when the misfire occurs, which would lead me to believe it stops sparking for a split second.

The points are new, and gapped to .025", but I will take a closer look at them, and clean all the connections and surfaces up.

Thanks for the ideas, I will report back on what I find. I am so close to having this tractor back into service! All the leaks are now fixed, hydraulics working, axle bearings and seals replaced, brakes working great. One final hurdle!

3930dave · Apr 2, 2017

VTPSD said:
The carburetor is almost brand new, fuel is fresh, non ethanol, great fuel flow at the carb, new fuel filter. I am also suspecting ignition since the adjustments on the carb do little or nothing to cure my misfire.

I also noticed when setting timing, the light seems to blink out when the misfire occurs, which would lead me to believe it stops sparking for a split second.

The points are new, and gapped to .025", but I will take a closer look at them, and clean all the connections and surfaces up.

Thanks for the ideas, I will report back on what I find. I am so close to having this tractor back into service! All the leaks are now fixed, hydraulics working, axle bearings and seals replaced, brakes working great. One final hurdle!

Check any internal wiring in the distributor very closely. Years ago, I had a car that was driving me nuts - there was a small internal wire inside the distributor that looked fine, but was actually cracked in the middle, under healthy looking insulation.

Rgds, D.

VTPSD · Apr 3, 2017

I think I have it solved, and I hate to admit that it appeared to be simply fouled spark plugs. My dad had been running it with some pretty bad carb settings before he brought it to me, then it sat in my shop and only ran for short spurts when I was diagnosing hydraulic issues.

I had cleaned up the plugs once before, but maybe I didnt do a very good job. This time I sandblasted them, and gapped them to .025". They were set a little too open, so that might have also been part of the issue. She started right up and ran great, I was able to get the carb tuned right in. I got a nice smooth idle and clean running all the way through the revs.

We still have 12" of snow, so I took it out for a few miles on the road and it ran great.

I will put new plugs in it before returning it to my father. Is there a preferred brand and heat range for these tractors? It is still 6v, if that makes any difference.

3930dave · Apr 3, 2017

Glad to hear that worked out fine VT.

Many of us that hang out on here like old iron, but it's easy to forget how (almost) maintenance free modern plugs and ignition systems are.

I don't know the plug specifics on those engines VT, but I'd definitely stay with a copper-core conventional (olde school) plug. Most of the modern precious metal plugs are harder to fire than copper-core; definitely not what you want on a 6v system, or even many old 12v systems.

Who is happier, you or Dad ? :thumbsup:

Rgds, D.