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Ford 9N, 2N & 8N Discussion Forum
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Irregular Spark

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Pete in OH

07-26-2006 07:43:14




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Hi All, I'm looking for some advice in troubleshooting my 8N (front mount dist) that idles well but runs poorly and backfires (flames out exhaust pipe) above half throttle. Here's the setup: all new wiring, new 12V 2.5ohm squarecan coil, new cap-rotor-points-condensor-plugs(AL437s) & wires. I've checked and rechecked the gap on the points (.015) and plugs (.025) and replaced the old point screws with new ones (3/16" for the points, 1/4" for condensor). The copper strap broke off when I changed the points so I used the copper strap from the old points to make a new strap. I've installed Dell's trick resistor between ignition switch and coil and removed the original ballast resistor. I installed a new ignition switch too. Last night I took apart the carb, cleaned it thouroughly and reinstalled. Before dark I pulled the #3 spark plug and noticed it was carbon fouled. I checked the spark with the engine running and noticed an irregular miss in the spark. When it did spark it was a nice blue but every few seconds it would miss a spark or two and occasionally I'd get a yellowish spark too. Any advice would be greatly appreciated (by the way I can't check the site much during the day so I'll go ahead and say thanks now for your help).

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Phil (NJ,AZ,Sask)

07-26-2006 12:24:00




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 Re: Irregular Spark in reply to Pete in OH, 07-26-2006 07:43:14  
Pete, The reason you set (check) the points (.015)on all four lobes is for bushing wear and/or wow/off center of the points plate. The front mount lobes are unique in that transitions are "Sharp" to a machined "Flat", this predetermines the dwell. The Sharp transitions can result in points bounce if spring tension is increased.

The normal practice, on traditional distributor lobes, of setting point gaps wider affecting dwell does not work on the front mount.

BTW: The 2.5 ohm coil with Dell's .5 Ohm resistor will result in > (more than) 3 times the COIL heat than the correct OEM Ballast. You will be replacing your coil sometime in the future.

JMHO

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Jerry/MT

07-26-2006 10:27:06




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 Re: Irregular Spark in reply to Pete in OH, 07-26-2006 07:43:14  
Very likely your distributor shaft/bushing is worn and needs replacement. To check, get the points closed and push radially(from the side)on the shaft and look for motion at the points. There should be none. Do it pushing from several different directions, noting any motion at the points. My Ferguson distributor was worn so badly that the points opened to 0.030"!



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Bob

07-26-2006 10:04:31




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 Re: Irregular Spark in reply to Pete in OH, 07-26-2006 07:43:14  
Have you checked for wear in the distributor shaft bushings?

If the fit of the shaft in the bushings is sloppy, the shaft may move around enough, affecting the point gap, and causing misfire.



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Dan (NYC)

07-26-2006 09:30:10




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 Re: Irregular Spark in reply to Pete in OH, 07-26-2006 07:43:14  
I have a similar occasional miss. Last weekend I checked the Distributor( front mount) and saw evidence that the rotor was actually touching 1 contact on the cap not clearing it with a gap. I am checking this on mine. Does your cap have evidence of the rotor contacting any of the 4 pads in it?
Dan



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wfw

07-26-2006 08:59:21




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 Re: Irregular Spark in reply to Pete in OH, 07-26-2006 07:43:14  
most educated guess? plug wire on number three or dist cap. This is indicated by only trouble with number three cylinder. lots of luck.



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