1974 CJ Ignition question

Does the 74 jeep Renegade require a ballast resistor? The P.O installed a 401 and I'm not sure what vintage the engine is. My guess is somewhere between 74-77. The stock wire loom appears to be unmolested. My problem is trying to fine tune this thing and get a smoothe clean idle and low speed run out of it, so I'm going through a process of elimination. The engine has plenty of power, starts within one bump of the key and accelerates nicely but will sometimes get backfire through the duel exhaust upon acceleration and almost always between shifts or if I let off the gas after highway speeds. It sounds like the old split manifold sixes with cherry bomb exhaust from the Low Rider days.
I've replaced the points, plugs and adjusted the timing to factory specks, 5 deg b tdc @ 700 rpm, accordig to all my books. At first I noticed that the timing mark was jumping all over the place so I checked the centrifigul advance and shot some wd40 on it and that seemed to fix that problem. After the timing was set I moved the vacuum advance hose to the right side of the carb, Carter afb 4bbl. I dont know if this is the correct placement or not? I adjusted the idle mikture screws to the smoothest lean mixture I could and readjusted the idle back to 700 rpm. This is where I am now. After a high speed run the idle jumps to about 800 t0 900 rpm, I still get some subtle backfire as explained earlier, and rough unballanced feeling engine in the lower gears which makes trail driving very unpleasent and the exhaust still smells rich.
Some local advice I've gotten suggest an ignition problem, possible centrifigul adv springs or to hot spark requireing a ballast resistor. None of the parts books list a number for one in this vintage of Jeep, so I dont know if its needed or not? All of the ignition componants appear to be in good working order so I've hit a wall here and hope its not an enternal problem.

 

Ballast resistor depends on the coil. Typically the coil will be marked "use ballast resistor" or such. Take the coil out of the bracket and see if you can read anything on it. The Standard catalog calls for a UC-12, which uses an external resistor. The Jeep may have a resistance wire between the switch and the coil - if so, it will be indicated on the wiring diagram of the '74 FSM. If you run without a ballast resistor, I don't think that will affect performance at all. You'll burn up the points after a while, but the car should run fine.

The points ignition system is bacsically the same for all the AMC V8s - 304, 360, 401 - so it doesn't matter if you have the original ignition system or the system that came with the 401. I would not spend any significant money hopping up a points ignition, when there are easy and cheap electronic ignitions that will drop in. Electronic ignition will have much lower maintenance, higher performance, and arguably better reliability. You can put one together from junkyard parts, plus some new ignition parts.

You're pretty much on your own with an aftermarket carburetor. The AFB is a good carb, and is still in production as the Edelbrock carb, but you really need to do some research and maybe find somebody that understands that carb well. What do your spark plugs look like?

 

The old plugs looked good, no oil fouling or major carbon. By looking at black and white pictures in a book it looks like a rich fuel mixture, kind of hard to tell without a professional opinion, but all 8 are fireing normally as far as I can tell. My primary wire tester indicates that the plug wires are working good. An old Corvette efficienato I know suggest going with lighter springs on the centrifigul advance. After looking up the carb numbers, I have a 630 something cfm, which should not be to much for the 401. The engine doesn't run poorly at normal speeds, but in the low and mid ranges something doesn't seem right. If I lug it down in 4th gear then step on the throttle I can hear the engine "blubber" from the exhaust, kind of a puh puh puh puh puh sound, and with the reoccuring tailpipe backfire now and again that I cant seem to get rid of is frustrating. The PO clames he had the rumpity rump cam swaped out with a smaller one at a local shop, so that worries me because did the shop put a junkyard special in it or a new one and I really dont want to go there if I dont have to. With my luck of preowned jeeps in the past, the worst case scenario is usually the case. $$$$. Boy do I have some horror stories... The exhust system on this thing looks like it was installed by Bubba the muffler guy or K Mart. Rusty short mufflers and pipes that dump out in front of the rear axle and show well below the rocker pannel. He spared no expense I'm sure. This would be my next guess as far as the backfire problem???

 

There are color pictures of plugs online ... I could search, but I'd prefer to leave that to you. There's a very detailed NGK chart that I'm thinking of, somewhere.

Fuel in the hot exhaust will make a backfire or popping. I'd guess your AFB is setup too rich. 630 CFM isn't wildly large for a 401, but it still needs to be set up right. Look on Amazon, and get a book on the AFB - you should be able to find a paperback for $15-15 or so.

The guys at the AMC or Wagoneer forums will have more experience with the AFB/Edelbrock on a 401 - you might try there. www.ifsja.org or maybe www.bulltear.com

 

Looks like I need a resistor according to the coil. Which side does it connect to + or - The wires comming off the coil do not indicate being resistor wires.

 

Doesn't matter. Usually the resistor is between the ignition switch and the coil. You probably already have one, either as a resistance wire or hidden somewhere.

<edit> Looking at the '72 harness diagram (which should be similar) it shows a ballast resistor http://trailforge.com/TSM/Section_22.pdf

 

Both sides of the coil + and - have two wires comming off of it, one of the - side wires connects to the destributor and all the others go into the loom at some point. My points are not burnt so I guess I'm ok.