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Intermittant "miss" at idle
#814203
09/25/10 08:00 PM
09/25/10 08:00 PM
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Joined: Sep 2010
Posts: 14
Iowa
Aloyisius
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OP
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Joined: Sep 2010
Posts: 14
Iowa
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Tried "search function"...didn't find what I was looking for.
Got a stock '68 Chrysler 440 (standard motor), single point dist and factory Holly 4bbl carb.
Motor was beginning to run a little rough, found the points were a bit too close, adjusted them to within specs.....replaced the sparkplugs and plug wires. Left the condenser and rotor alone as they "looked" ok.
The motor starts right up and runs fine at street and highway speeds with no issues. However at idle, every few revolutions, it "skips a beat", like a spark plug sometimes just doesn't fire....only at idle, though.
I even went so far as to put the car in the garage at night with all the lights out and looked for indications of any arcing going on, but none was seen nor heard.
Anyone have an idea what could be causing this?...and is it something I should be concerned about, or just leave it be, since it does not occur consistently and other than that, seems to run just fine.
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Re: Intermittant "miss" at idle
[Re: polkat]
#814206
09/25/10 09:31 PM
09/25/10 09:31 PM
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Joined: Sep 2010
Posts: 14
Iowa
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Quote:
Just a guess: How tight is the shaft inside the distributor? If when grabbing the rotor can you wiggle it at all in any direction, even the slightest amount (other then in the advance direction)? What is it timed at?
As a matter of fact, I can wiggle it a fair amount. I just figured it was a "normal" amount of play in the advance plate...but it did (in my unprofessional opinion) seem to be awfully easy to move.
Can't really say, if I could move it in the opposite direction of the advance, though.
I haven't yet figured out what the timing is....can't seem to find any numbers on the balancer. The motor doesn't "ping", overheat, nor "run-on" after shutting it off, so it must be pretty close to factory specs.
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Re: Intermittant "miss" at idle
[Re: davenc]
#814207
09/25/10 09:39 PM
09/25/10 09:39 PM
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Joined: Sep 2010
Posts: 14
Iowa
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Quote:
What plug did you put in? Same as the previous ones? Did you make any adjustments to the carb? Could it be a little too lean?
It had some really dirty and rusty Champions in it. I replaced them (because I read somewhere that the quality of Champ plugs has gone downhill over the years), with AutoLite plugs of the same heat range.
After replacing the plugs, wires and setting the points, the idle went from 600 rpm in neutral to 850 rpm. I might mention that the idle was very uneven and choppy before changing the plugs and the rest of what I did.
So I set it back down to 600 and had to re-adjust the A/F screws accordingly to get the smoothest idle without causing the idle speed to drop.
I don't know how to check for "leanness".
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Re: Intermittant "miss" at idle
[Re: davenc]
#814209
09/25/10 11:05 PM
09/25/10 11:05 PM
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Joined: Sep 2010
Posts: 14
Iowa
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Quote:
You need specific equipment to know the actual Air Fuel Ratio, but you can experiment with the mixture screws to see if it affects the miss. Try turning each screw in 1/8 turn (which makes it leaner on most carbs) and see if it gets worse or better. Then put back into the original position and try turning each screw out 1/8 turn. If turning them out a bit helps, then the mixture was too lean.
Do you have a vacuum gauge? Did you use it to adjust the mixture, or did you do it just by ear judging the affect on the motor?
No fancy gauges here...do have a vacuum gauge though. I set the mixture "by ear"....with practice, it can be done that way and reasonably accurately too.
What I do, is turn the screw in until the "r's" drop abt 50, note the position of the screw head....then turn it the other way until the "r's" drop by 50 again, note the screw head position....then split the difference.
"Old skool" stuff 
Anyway, no, it didn't affect the miss either way.
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