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Subject: idle mixture trouble

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JohnB    Posted 08-30-2017 at 20:24:27 [URL] [DELETE]        [Reply] [Email]  
  • idle mixture trouble
  • I've had my 8N for 2 years and it has ran great . . .except for a very slight, barely noticeable miss at idle. I decided to tackle this problem earlier this week and performed the following: replaced the points, condenser, plug, plug wires (these were due for replacement anyway), replaced the distributor bushing, and R & R'd the carb (completely disassembled, soaked, blasted, soaked again, and blown out).

    The tractor runs just as good as it did before all of this work . . . and still has the slight miss. I also noticed that it idles best with the idle mixture screw all the way in. As much as a 1/4 turn out will result in a rough idle. If I remember correctly, turning the screw in will RICHEN the mixture. If that is correct, then I am lead to believe there could be a vacuum leak that the extra-rich idle mixture is compensating for. If that's the case, then I have to believe the manifold could be the issue.

    Am I on the right track here?

    Tim Daley(MI)    Posted 08-31-2017 at 06:29:56 [URL] [DELETE]        [Reply] [No Email]  
  • Re: idle mixture trouble
  • Yes, what Tony said below, and the brass Idle Air jet, p/n 9N-9596, orifice is very small, gets plugged and/or gummed up. The Idle Air Mix Screw, on the side of the carb, is also brass, and the pointed end that slips in to the that hole often gets worn and won't seal properly. I looked at a few of them up close, old and new, and an old one to the naked eye seems to look fine but when observed under a microscope, you can see the pointed end burred, out-of-round, and/or no longer pointed. It is always best to replace with new parts. You have the unit stripped down to bare bones, so why not do it right the first time and replace all the parts then?

    Tim Daley(MI)

    Tony    Posted 08-31-2017 at 01:51:53 [URL] [DELETE]        [Reply] [Email]  
  • Re: idle mixture trouble
  • You are on the right track.

    You used copper or steel wires not resister wires didn't you?

    You might want to check the carb and manifold bases with straight edge and true them with a flat plate and Emory. Not as big a problem on tractors as both are cast steel, not alloy or pot metal but they can get deformed over the years from gorilla tightening. Same for intake manifold leaks. THe starting fluid BraKleen, AquaNet test around the manifold will confirm any vacuum leaks in those areas

    If you didn't kit the carb at least replace the idle adjusting screw. Not a"known to be good" screw, a new one. It may look good but not seal. It's brass and the threads wear allowing an unbalanced condition. That is the reason a new screw is included in a kit. Same for a new throttle shaft.

    Check compression. You may have a valve beginning to ride causing a misfire at idle. Unlikely but possible. Checking clearances requires a fair amount of effort however.

    Last point and probably the reason I'll get flamed:

    It's a farm tractor with an updraft carb. Not the most efficient induction system. If it makes good power, transitions from idle to power with no issue, doesn't stall and starts quickly why be concerned?

    I have a 2n and a Jubilee along with a JD M. Maybe I'm just used to the pop!, bark!, pop!,pop of the JD to worry about a fuzzy idle from my FORDS.


    JohnB    Posted 08-31-2017 at 07:21:42 [URL] [DELETE]        [Reply] [Email]  
  • Re: idle mixture trouble
  • Great info here. I purchased all of my parts from Just 8N's. Wires are copper and I replaced all jets and adjusting screws. The old screws were mushroomed at the end from over-tightening.

    The manifold looks like it has sat out in the rain for 750 years. It will definitely be a challenge to get it off without snapping the studs.

    Compression checks out at 90-95 on all cylinders.

    And yes, in the end, it does run great, doesn't hesitate, and makes good power. We'll see what challenges the manifold holds. If it's going to be more trouble than it's worth, then I may just the slight miss.

    Tim Daley(MI)    Posted 08-31-2017 at 12:43:05 [URL] [DELETE]        [Reply] [No Email]  
  • Re: idle mixture trouble
  • Well you went to the right place for parts. On the exhaust manifold issue, it is common for these to get corroded out especially at the #4 port, nearest the downward exhaust port/pipe. That area gets the hottest and the mounting surface wears a channel that eventually will burn the gasket away and leak fumes. The manifold is suppose to use 7/16-20 BRASS Hex Nuts with steel lockwashers torqued at 27 ft/lbs. Some just use steel nuts and that makes removal difficult. To remove brass, get engine hot and let cool down a bit then try to loosen nuts normally. Use a good 6-point deep well socket to help avoid rounding off. If hard to budge, try spraying some PB BLASTER on them and let set for a bit. there are two paper gaskets with a new manifold, and some places use to sell the brass nuts with it as well. You do not need to apply any goop on the gaskets when installing. The reason for brass over steel is that the manifold constantly is getting hot and cooled down. With steel, the material becomes work-hardened and then removing from the studs becomes difficult. You might want to drain some coolant from the radiator first as a lot of times a stud or two come out and you'll get a coolant bath.

    Tim Daley(MI)

    Tony    Posted 08-31-2017 at 15:41:17 [URL] [DELETE]        [Reply] [No Email]  
  • Re: idle mixture trouble
  • Good points, especially the 6pt socket. Rusted nuts lose a few thousands surface on the flats. Doesn't seem like much but sometimes enough to allow a 12pt socket to slip. I've beaten a metric socket on a or bolt head that a nominal Imperial size slips on. Ya do what ya gotta do.

    By now most manifolds have steel or a mix of brass and steel. A torch with a 0 or 00 tip can be used to heat steel nuts dull red then quench with water. Most times shock cooling them will break them free. DOesn't work well with non ferrous though. Concentrating the heat on the stud helps but........

    Resist trying to turn off the nuts while hot. The squeak you hear as it makes a quarter turn are the threads galling. At that point the chapter under "extreme measures" notes the use of hand or air chisel.


    Tim Daley(MI)    Posted 08-31-2017 at 18:13:57 [URL] [DELETE]        [Reply] [No Email]  
  • Re: idle mixture trouble
  • Yes, you really need to pay attention to the manifold nuts when loosening. Watch the head of the stud as you turn the nut. If it is turning with the nut, chances are it will come off okay, but you must be careful not to break the stud off. That then becomes major job to fix. I rec'md a deep well 6-point socket because the few threads protruding from the studs often will not allow standard socket to seat all the way down. I use 6-point wrenches whenever I can on these old iron machines. less chance of rounding off and many sizes no longer made so I preserve the originals as much as possible. On brass, the 6-point is even better because the metal is softer than steel and will deform plastically very easy.

    Tim Daley(MI)

    R Geiger    Posted 08-30-2017 at 21:37:36 [URL] [DELETE]        [Reply] [No Email]  
  • Re: idle mixture trouble
  • Or a wrong throttle shaft.

    clayton ingmire    Posted 08-30-2017 at 23:07:16 [URL] [DELETE]        [Reply] [Email]  
  • Re: idle mixture trouble
  • lean it back out till it runs rough get a can of carb clean and carefully spray around carb if you hit the right spot it will smooth out also could be bad intake rotted through from exhaust side to intake side Chinese manifolds had this casting problem

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