258 hei vacume advance issues - JeepForum.com

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post #1 of 3 Old 06-30-2013, 05:13 PM Thread Starter
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258 hei vacume advance issues

I have a 258 with offy intake holly 390 and mild cam. Runs great with vacume unplugged. Plug it in and it skips and pops. Base timing is 8 btdc and i have moved it both ways with no change. Please help im stumped. This is all brand new fresh rebuild on engine,new carb and new hei.

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post #2 of 3 Old 06-30-2013, 07:45 PM
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1977 CJ5 
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Location: Arlington TX
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You could have stabbed the distributor wrong, don't just trust the balancer to be accurate as many have slipped throughout the years. Turn the engine over by hand with the spark plug out of the #1 cylinder until you are close to tdc on the compression stroke by having a second person plug the sparkplug hole with their finger. They will feel the compression as the piston moves up on the compression stroke. Once close to tdc put a wooden dowel rod or chopstick in the sparkplug hole and turn over slowly until it goes up as far as its going to before falling further back in and check your harmonical balancer and see if it matches with tdc on the timing cover. If not mark it as it is true tdc and the outer ring has slipped. Also pull off the distributor cap. The rotor should now be pointing directly at where the #1 spark plug wire would be. If it is not then you have 2 options. 1 move the spark plug wires around the cap to match where it is pointed to or 2 pull and restab the distributor so the rotor matches the cap.

Also make sure you are setting your timing with the vacuum advance disconnected. Most HEIs for the 258 are curved to use manifold vacuum not ported vacuum. And lastly you may need to tune the distributor using different springs which will change the rate that the mechanical advance comes in and most of the HEIs come with an adjustable vacuum advance where you put an Allen head wrench in the vacuum hole and turn to increase or decrease the amount of vacuum required for the vacuum advance to come in. You can even go a step further and get a vacuum advance limiter to decrease the total amount of vacuum advance.
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post #3 of 3 Old 06-30-2013, 10:20 PM
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If you have a mighty vac.( a small hand held vacuum pump) hook it to the HEI and apply vacuum. If it sputters then the advance if internally shorting. I have seen this problem on older cars years ago. Does the tach work and is it smooth or does it flucuate when the advance is hooked up?


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