I drove it in this morning. There may have been a subtile difference, but not much. One thing I forgot to mention last night was that MAN. is labeled above the nipple on the vac. advance. Is this to signify that the vacuum line needs to be connected to manifold vacuum?
I did connect it to the manifold port and the timing jump up significantly. I have not driven it this way though.
The other thing I failed to mention was I don't have the CTO valve connected. The CTO I believe changes the distributor's vacuum line from manifold to carb depending on engine temperature. Is a CTO valve a crutial component or does it just enable the engine to run better at a cold start up?
Some engines were tuned for a manifold vacuum that goes full advance at idle, some are tuned for a ported vacuum.
Some have both and the CTO in there to change it as you are thinking. They were getting 'creative' with the emissions junk there for a while.
What are you doing about the EGR valve? That really needs the CTO and the thermostat inside the air filter to work properly, but at least the CTO.
It will run like a pig cold if the EGR is kicking in and it will not have the same top power with the EGR disconnected. They made a different cam for the engine without the EGR.
When I killed the computer in mine I went to a ported vacuum. I get the best performance like that and the emissions sniffer likes it that way.
I can get 15-17 ppm HC's at idle tuned that way, If I tune it for manifold vacuum at idle my HC's jump up to 1585 ppm or so and it fails miserably.
There is no difference at 2500 rpm on the dyno though, the timing is at full advance there no matter which vacuum I use. The weights pull it to full advance around 2300 rpm I believe.
The difference is in the acceleration power. I get a smooth pull all the way up to 4500 rpm with ported, it has big dead spots in the pull with manifold vacuum.
I am running the Carter BBD 2 bbl on my 78 engine though and just have a straight pipe with a Dynomax high flow muffler. I don't need a Cat for emissions on my 'utility' vehicle.
So I just went out and sucked on my vacuum line to the advance and I can easily suck it to full advance. Didn't need to put the light on it, when I have it sucked, my fingers cannot move the advance lever any more. I guess you could remove your distributor cap and watch the rotor while sucking. Then when it is sucked hard, see if you can twist the rotor more.
I don't have an EGR. It's port on the carb is capped. What I have on the carb is a capped EGR (port that points straight at the valve cover), vacuum line on the diagonal port that (points toward water pump/front of engine) goes to distributor nipple, and a capped spark port that points toward drivers side headlight. I don't have an emissions diagram for my year 1975, but I believe that the '74 did not have an EGR.
Sounds crazy, but all this power lack came after I replaced the header w/stock manifold and re-built the carb. 65-70 mph is definitely top speed now. Where as before the changes I had much more.
I do the suction thing on the distributor and see if the rotor can be moved more.
Mike, I did the suction thing to see if the rotor moved. It does not. I can physically move it, but it stays where I put it. With or without suction. Does this sound like the distributor weights / springs are about worn out.
Ok, so the vacuum advance just moves the trigger plate then. I didn't open my cap to look, I just saw the plate move from the outside on mine.
You have issues with the weights and rotor. The rotor should be spring loaded and pop back into place instantly. I would try a couple drops of oil down the hole under the rotor and see if that loosens it up. With that seized or sticky, the base timing will never set stable.
Taking the whole works apart can be a pain because that pickup wheel is pressed onto the shaft, you need a gear puller to get it off.
I was just looking at my book and it would appear your vacuum advance is adjustable too, but no need to mess with that.
Here is one for you too. I see this all the time. The manifold and the carb base need to be re-torqued after a couple hundred miles. If/when they come loose, it will really drop in power.
Bill is thinking along the same lines as I am, the shaft is likely in dire need of oil. I would take out the felt pad for the first oil job, then once it is loosened up, oil the pad and put it back in.
Correction! You know, I thought about this rotor moving thing. When suction is applied the advance I have, it pulls the pickup sensor back and forth within the trigger wheel of the distributor.
Hi Bill, If I remember right I have an electronic type not a points type distributor. I can remember where I read it, but it's in one of my manual's vehicle specifications charts for my year and engine.
The pivot pins for the weights could even be messed up and seizing, let alone the tube outside the shaft the rotor rides on.
I am running the stock distributor and carb with an Accel SuperCoil and my engine rocks. This is the second engine for me and the old one rocked just as well, but leaked like a sieve.
I can power up over 50 mph in second and 4th will bury the speedo....
To Bill and Mike, Bill, mine was manufactured in Nov. 1975. I remember I looked for a duell setting for the points, but my manual said it was electronic with no points, but an electronic pickup sensor.
Mike, I oiled that generously late night. I may opt to tear the distributor down again tonight. Or I may just start the search for a replacement. How do you feel about those HEI ones with the high power coil included? Or should I look for a re-manufactured unit?
Great info Guys!! Unfortunately, Bill I use this almost as an everyday driver. I don't have many pick and pulls here in the Vail / Eagle county area. The money might be steep, but how do you feel about the aftermarket HEI stuff through 4wd.com?
I did one of these for my father's 85 CJ. Its a beautifully machined unit. It just needs switched 12 volts to run. It really cleans things up. I put in some autolite platinum plugs that we set to 55/1000s gap and some 8mm wires.
When you call Davis, they will ask you for engine redline, vehicle weight, towing, plowing, transmission, whether the engine is built up at all. They adjust the advance curve to your answer to give the smoothest performance.
I did this upgrade at the same time as a staged Weber 2bbl carb. Combined the response changes where dramatic and the engine compartment went from complicated to knuckle-dragger simple, (though perhaps not smog legal)
Its amazing how easily the thing starts now. Even when left for a week or two.
I'm told you can get the same effect by finding a HEI distributor from a late 80s Chevy six cylinder and having the drive gear changed at a machine shop. A lot less money for probably the same result.
The only negative I saw was that the fatter 8mm wires and much bigger HEI cap come very close to the side of the engine and setting the timing is awkward. Well, that and the price....
If you haven't done a distributor before this might help.
Yours is probably adjustable (the ZJ is bolted in one spot) In that case you have to mark a reference between the distributor and the engine block in addition to the refs marks for the rotor.
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