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  #1  
Old 01-14-2017, 07:04 PM
Yadkin Yadkin is offline
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Default Pertronix Distributor

Still tweaking the engine for best performance. As you know, I have a FAST EZ EFI system. I swapped my OE distributor with a Pertronix and that made a noticeable improvement. This has a GM style adavance so is a lot easier to tune. I've been working with FAST on their help forum for the correct settings. However still running rough under load, and mpg is less than 13.

I opened up the distributor cap and found that the carbon button was worn to nothing. So I replaced the cap and rotor. No difference at all.

I analyzed using an oscilloscope and identical clamp type probes. One on the coil wire and the second on a plug wire. Here's a picture of representative results. It looks like I'm losing a lot of energy through the distributor cap. I did this with all eight plug wires and all results were similar.

Yellow is the coil wire and blue is a spark plug wire.
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File Type: jpg scope_image.jpg (190.4 KB, 86 views)
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  #2  
Old 01-14-2017, 07:07 PM
Yadkin Yadkin is offline
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This afternoon I cut a large window in the old cap and used that to measure clearance between the cap and rotor electrodes. I get about 0.039 to 0.048. This is larger than my plug gap.
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File Type: jpg Copy of Cap window.jpg (34.5 KB, 97 views)
File Type: jpg Copy of Cap installed.jpg (39.7 KB, 98 views)
File Type: jpg Copy of .048 clear.jpg (29.5 KB, 98 views)
File Type: jpg Copy of Next post 0.039.jpg (30.3 KB, 98 views)
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  #3  
Old 01-14-2017, 09:20 PM
Joe Johnston Joe Johnston is offline
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Even though that gap seems large in the cap you cut up, you need to compare the diameter distance between the contacts across the cap from the cut up cap to the new cap you are running. Its not quite apples to apples as you have two different caps.

Also you should try to get a good guestimate of the length of the 2 rotors by measuring from the center mark made by the carbon center post to the end.

I would assume the differences to be slight, but y'all know how that goes and you are trying to track down something important. Easy to check and perhaps another dead end, but it would be a known dead end! Perhaps a different combination of brands would make the gap tighter??
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Old 01-15-2017, 11:14 AM
Yadkin Yadkin is offline
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The old and new cap and rotor are all pertonix. Should be identical. Also I don't have deep calipers to measure the new cap. I figured this would give the most accurate results.
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Old 01-15-2017, 11:58 AM
Joe Johnston Joe Johnston is offline
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I agree, dimensions should be identical across the same brand. Still would be interesting to even fit a wood strip across and compare how it fits to an old Ford cap.

That gap seems really big to me too. I've never liked aluminum contacts either in a cap, but that is just me. Keep checking - you'll find the problem.
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Old 01-15-2017, 02:35 PM
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I'm going to take the dremel to the old cap and make the contact closer, see how that works.
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  #7  
Old 01-15-2017, 03:48 PM
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Quote:
Originally Posted by Yadkin View Post
This afternoon I cut a large window in the old cap and used that to measure clearance between the cap and rotor electrodes. I get about 0.039 to 0.048. This is larger than my plug gap.
But the spark doesn't have to fight compression, possible fuel wash, heat etc inside the distributor cap so I don't think that gap is unreasonable. (I have run motors with 40 thou plug gap)
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Old 01-15-2017, 04:31 PM
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Good point, but how do you explain the oscilloscope graphs?
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Old 01-15-2017, 04:59 PM
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I'm on par with Tom's explanations.
As far as the graphs, did you try swapping oscilloscope leads? Do they give exact same results?

If you're worried about .03" gap, solder a wire extension onto the brass rotor (or peen it closer). My bet is, it won't make a bit of difference.

Did you ever have a plug that wouldn't fire? Sometimes pulling the boot off the plug 1/2" will wake it up again. To rationalize, I think the collapsing field in the coil builds to a higher voltage before it can jump across the gap. That's why more modern vehicles incorporate a resistor-plug. - Dave
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Old 01-15-2017, 10:38 PM
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Yes swapped the leads from the coil, same results. Also tried every plug.

I decided to experiment a bit and apply the dremel to the rotor contact. The first picture is my first grind and the second is my final, after several trial and error fits. I brought the clearance from 0.040 or larger down to 0.018.

Too late to fire her up, I'll try to find time tomorrow and get a new scope reading.
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File Type: jpg rotor contacts.jpg (26.8 KB, 129 views)
File Type: jpg IMG_20170115_145712602.jpg (32.6 KB, 130 views)
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