N Board Forum - Expanded Thread Page

Subject: Runs rough at higher RPMs

[Back] [Return to Top of Forum]

Tom    Posted 03-18-2017 at 20:08:53 [URL] [DELETE]        [Reply] [No Email]  
  • Runs rough at higher RPMs
  • Just getting my old '49 8N running again. It has the original six volt system and I gave it some freshening up. I've replaced the tank, fuel line, battery, plugs, points, condensor, rotor, wiring harness, ignition switch, and had the carb rebuilt by a professional shop. Fires up and runs fine at idle, but runs very rough at anything more than an idle. Compression is 90-100. I followed the instructions for adjusting main jet in carb and can't get it to smooth out. How do I determine if the rough running is spark related or carb related? Wife told me that if I don't get it fixed this weekend it should go down the road!!! Let's not have that happen!

    Bruce (VA)    Posted 03-19-2017 at 08:08:37 [URL] [DELETE]        [Reply] [Email]  
  • Re: Runs rough at higher RPMs
  • Did you set the timing?

    Change/clean the 3 fuel screens?

    There is more to adjusting the carb than just the main jet.

    Make sure the tractor is at operating temp; that usually takes 10 15 minutes at idle depending on ambient temp.

    Both Ford and Marvel/Schebler (assuming you have a M/S carb ) say to set both the side-pointing idlemix and the down-pointing mainjet to 1-turn as a starting point. I set the down-pointing mainjet to 1-1/2 turns and don't fool w/ it until the final step.

    Then adjust the side-pointing idlemix for fastest idle; not the smoothest idle. Next, adjust the behind the carb idle-speed set-screw for very slow 400-rpms idle. Do that idlemix adjust for maximum idle at least 3-times. Make sure that you turn the screws slowly, like 1/8 of a turn at a time & wait a second or two for the engine to catch up. Take your time!

    Do it like this:

    1. Adjust idle mix jet until RPM increases

    2. Adjust idle-mix set screw until the engine nearly stops (as slow as you can get it unless you have a tach that tells you 400 rpm)

    Repeat steps 1 - 3 three times.

    Remember the side-pointing idlemix is out for lean, in for rich.

    If you do not have any problems inside the carb, it is easy to get the idle down to 350 - 400 rpms.

    Your last step is to go back to the main jet. Remember, in for lean, out for rich. If you end up turning it OUT more than turn for max power (remember, you already had it 1 turns out) then stop right there because you have a dirty carb or a fuel problem.

    Tom    Posted 03-19-2017 at 11:45:01 [URL] [DELETE]        [Reply] [No Email]  
  • Re: Runs rough at higher RPMs-update
  • Bruce,
    Thanks for your detailed instructions. I went out this morning to start the tractor and because it was so cold (20) it wouldn't crank fast enough to start. The battery is about 6 months old. I then got my farmall h out which has an 8v battery and jumped it off of that. Fired right up and ran really smooth. As soon as I removed the jumper cables it started running rough, so now I know it's not the carb adjustment. The ammeter reads that it's charging, but obviously has weak spark. Any ideas? Thanks!

    Bruce (VA)    Posted 03-19-2017 at 12:17:18 [URL] [DELETE]        [Reply] [Email]  
  • Re: Runs rough at higher RPMs-update
  • Have you set the timing?

    The most common cause of a weak spark is a weak battery. Charge it (see tip # 60) and check it .(tip # 49)

    The fact that your Farmall started and your N didn't tells me that you most likely have a weak battery. It does not matter how old the battery is; ambient temp & the state of charge on the battery is what counts. At 20*, you lose about 25% of your battery, even more if it's not fully charged. I keep all of my stuff on Battery Tenders.

    Other causes of a weak spark, in no particular order and probably not all inclusive, bad grounds/connections, worn plug wires or carbon core (automotive) plug wires, wrong plug gap, wrong plugs or burned plugs, worn out rotor/cap/points, incorrect point gap, worn insulator on the side of the distributor, incorrect or burned out resistor & a worn out ignition switch putting resistance in the circuit. Rarely, very rarely when the engine is cold, a bad coil

    A weak battery, along with one or more of the problems I just listed, will result in a weak spark.

    Assuming that the bushings & advance weights are ok (*see below), & that you have correct voltage to the coil (battery voltage with the points open and about half that with the points closed), the most common electrical failure (no spark, weak spark) points on the front mount are:

    1. The insulator under the brass concave head screw & where the copper
    strip attaches. (it's fiber & will wear out; poke & prod w/ your meter leads to make sure it still works) If you need to replace the insulator, use a .250 x 3/8 nylon square nylon anchor nut available at most big box home stores.

    2. The pigtail at the bottom of the coil not making contact w/ the
    concave head brass screw inside the distributor. (With the coil on, the pigtail must firmly contact the brass screw. No contact = no spark)

    3. The copper strip is broken or grounded to the plate. (look very carefully for cracks & breaks).

    4. The condenser wire grounding to the plate or side of the distributor.

    5. The tab on the bottom of the coil not making contact w/ the brass button on the cap. (With the cap on, the tab must firmly contact the brass button. No contact = no spark.)

    6. Incorrect positioning of the spring clip on the plate causing the pigtail to ground. (the open part of the clip goes between 7 & 9 o'clock on the plate. That puts the straight part of the clip opposite of the timing screw at 3 o'clock)

    7. Incorrect seating of the coil on the distributor due to a loose bail
    or no gasket.(the coil must not move at all; if it does, replace the gasket or bail. Or stick some cardboard under the bail).

    8. Water/moisture inside the cap due to gasket failure or the absence of a gasket. (the cap AND coil have gaskets)

    9. Dirty/corroded/burned/incorrectly gapped or misaligned points. I use only Wells, Blue Streak or Echlin brand points (* *see below). If you are using quality points and cannot get the gap to open to .015, chances are you need to replace the bushings. If the shaft has any sideways movement AT ALL, the bushings must be replaced. (*** see below)

    10. Burned rotor, cracked/carbon tracked cap.

    After find the problem & re-check the point gap, do a continuity check before you put the distributor back on the tractor. Before you start, make sure your meter/light works.

    With the distributor still off the tractor, follow these steps:

    1. Coil off, cap off, points open. One probe on the brass screw & the
    other on both sides of the open points. On the side closest to the cam, you should have continuity. Not on the other side! If you do, you will also have continuity everywhere because the points are grounded.

    2. Coil off, cap off, points open. One probe on the brass screw & the
    other anywhere on the body of the distributor. You should have no continuity! Now, rotate the tang on the distributor....as the points open & close, you have continuity (closed) and lose it when they open.

    3. Put the coil on the distributor, cap off, points open. One probe on
    the lead on the top of the coil, the other on the cam side of the open points. You should have continuity!

    4. Coil on, cap off, points open. One probe on the lead on the top of
    the coil, the other anywhere on the body of the distributor. You should have no continuity!

    At this point, I just put the distributor, coil & cap all back on the tractor as a unit. The reason I do this is because it is real easy to get the cap or coil misaligned trying to put it back together, one piece at a time. The result is something gets broken or you get a 'no spark' problem.

    It's possible to put it back on wrong & break it. Look at the slot on the end of the cam shaft. Whatever angle it happens to be, turn the distributor tang to match it. Make sure you can tell the wide side from the narrow side on both the cam & distributor! (close counts). Place the distributor on the front of the engine, gently push it in place & slowly turn the distributor body until you feel the tang slip into the slot. Rotate the distributor body until the bolt holes line up. Hand tighten the two bolts until the distributor body is flush with the timing gear cover.

    * Unscrew the plate hold down screw & remove the C clip to get the plate out. Remove the shaft & weights. The weights should freely move.

    * *NAPA part numbers:
    . Points: FD-6769X
    . Condenser: FD-71
    . Rotor: FD-104
    . Cap: FD-126

    *** There are three ways to replace the bushings in a front distributor:

    1. Buy new bushings (part numbers 9N12120 front & 18-12132 rear). Press out the old ones, press in the new ones and ream to fit. CAUTION: do not try this unless you have a press & know how to use it. If you break the base, a new one costs $130. If you bend the tower which holds the front bushing, a new plate will cost you $30. Resist the temptation to buy a new plate; most are pot metal and the threads will wallow out about the third time you change the points.

    2. Take the new bushings and distributor to your local machine shop.

    3. Send the distributor out for bushing replacement if you do not have a local machine shop

    [Back] [Return to Top of Forum]


    Top