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Discussion Starter · #1 ·
Just exactly what is a "flat spot"? Is this an engine breathing problem?
How can it be corrected?

CJ8George
o[[[[o
85 CJ8,258,Weber32/36,4" BDS lift,4.10,35"tires,f/r lockers
 

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/wwwthreads_images/icons/crazy.gif You have to understand that even though we have used carbs since before the Model T days, that carbs are imperfect in that there is no linear relationship between the air that passes through, and the amount of fuel drawn in. It's tough, but it's true, and always has been. That's why fuel injection is so darn much better, it has the perfect potential relationship, and is being constantly adjusted to stay perfect. In carbs, all sorts of little tricks and devices are built in to compensate for the diference in mixture caused by the non-linear air/fuel relationship. Because of that, there are often transitional thresholds of airflow which trigger the adjustment in mixture. When the change occurs, it in itself is imperfect, and often causes a "flat spot" in the power output versus RPM. If the engine is doing that with fuel injection, then one or more of the sensors is not reading and reporting just right./wwwthreads_images/icons/frown.gif There...I got it all in print...Whew./wwwthreads_images/icons/tongue.gif

CJDave
 
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Discussion Starter · #3 ·
Great explanation CJDave, I would like to offer a layman's interpretation of the answer as well.

Acceleration should be 'fairly' linear - ie RPM's continue to increase proportionately over time up to the engines ability to increase RPM's. But for the sake of argument let's say up to redline. Typically when you open the throttle wide open the acceleration (RPM increase) should be constant. (ie say 500 rpm per second to redline) A flat spot would be indicated when that rate of increase diminshes significantly during the acceleration process. Example my Jeep would accelerate in third gear nicely to about 2300 rpm in a matter of a second or two or three, then to go another 500 rpm would take several more seconds, basically it would sneak up on 3000 . I installed a GM-HEI and nothing else and this problem went away completely. It will accelerate to redline(or as close as I dare) with out any flat spots. So it is not necessarily caused by a carburator, but ignition is factor as well. Probably not a well matched set-up to begin with since the 258's are notorious for flat spots.

Hope this helps too,


CB
 
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Discussion Starter · #4 ·
If I may offer one more explination....
Flat Spot is some times used to refer to a 'Flat Spot' or a 'Flat Lobe' on a cam, i.e, the lobe has worn down. This is more of a back yard mechanic term so the previous explinations are more accurate. If you remember your high school math (yes, there was a reason for it), draw a sloped graph where the line going up is your acceleration, then any drop off of the acceleration would be the flat spot.

Now that we have that out of the way...did the HEI really help that much?

Regards.......John 84CJ7

 
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Discussion Starter · #5 ·
Yes the HEI was impressive. Like all other owners of Carter BBD carburated Jeeps I was suspecting that the trouble was fuel. The Carb I have was a rebuilt from PAPA auto parts and was doing okay. It searched for an RPM at idle or would idle way down to about 300-400 when braking hard. The dead-on flat spot was a nusiance. I was looking for carbs probably the weber, but the prices were too high. I priced the HEI from parts mike, and took a poll about Jacobs products and decided to go to the junk yard instead. Picked up a complete distributor for $65 (already pulled) cleaned up the inside a little and oiled the advance weights. I followed Larry M's directions and put it in and fired it up. I couldn't believe the improvement. All symptoms gone, and I am kind of fond of the Carter BBD now. Gas mileage is near 18mpg mostly highway. I have maybe $140 into the entire upgrade - new cap, rotor, wires, AMC V8 drive gear, couple pieces of 12 gauge wire and a couple 1/2in female connectors. Tach worked perfectly, I left the original coil and ignition box in place so I can carry the original distributor and wires as a spare if needed. Maybe my ignition systom was weak, but I had all the symptoms everyone describes of the pre injection Jeeps and it cured them all.



CB
 
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