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Detonation after distributor cap install.

kaj750

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I'm going to try to explain this best I can since I couldn't take pics/vid while also timing the car. :lol:

So. Car ran fine before I started all of this and *all* I did was swap to a new distributor cap. I replaced a Mallory cap with a Mallory cap. Went for a drive and car is detonating at any RPM over 2500 or so, I believe it was.
I bring the car back in and check timing. There is a stripe painted that I *think* is my target timing mark, as that's where they car has been timed the entire time I've owned it; no detonation. When the pulley was timed to this paint mark, my timing gun says "0". Yeah, I know.. but I have no info on this engine build so just timed it until it seemed to run best. LOL
If I adjust the dist so that the timing mark is at the stock location, my gun says "5" degrees of advance. It also detonates at that timing, of course.

So.. timing may be off and all, but why would the car start detonating after swapping distributor caps? Any help would be appreciated. Thanks, in advance, guys!
 
If the hold-down bolt wasn't tight, you could've turned the distributor inadvertently, thereby altering the timing.
 
Or the location notch is somehow different between the two caps.
 
What if you accidently put the wires in the NEXT terminal? You know, each wire now plugged in one socket forward of their correct position?
 
If it ran fine, what was the reason for swapping parts?
 
If the hold-down bolt wasn't tight, you could've turned the distributor inadvertently, thereby altering the timing.

100% tightened it down, but I'll check again, since it's free LOL. timing checked before and after tightening bolt.

Or the location notch is somehow different between the two caps.

That's what I was wondering. They look identical, but I'll take a closer look tomorrow.

What if you accidently put the wires in the NEXT terminal? You know, each wire now plugged in one socket forward of their correct position?

To avoid any confustion, I swapped wire for wire... but anything is possible, so I will to back to be sure. Dumb question, but I guess the only way to tell where plug 1 goes is to rotate the crank to TDC and check location of the rotor?

As y'all can see, I'm mostly new to all these older cars. I got spoiled with all my OBD-I/II stuff LOL. The only cars I've dealt with, that had distributors, were Hondas, back in the 90/early '00s, and it REALLY obvious if wires were in the wrong spot.
 
Adjust your timing to 0 degrees with your timing light before checking your wire locations. Then rotate engine to top dead center on #1 cylinder and then check your rotor location. My bet is you are one plug off in one direction or the other if the two caps are identical.
 
Put the old distributor cap back on and start from there. Recheck everything you took for granted. Firing order to plug wires. Did you also change the rotor? If so put the old one back on and reestablish the base condition and you will find your problem.
 
[1] Assuming the dist was not moved.......
[2] Idle quality has not changed which means leads are in correct position.


Only thing I can think of: the gap between the rotor tip & segments in the new cap are different, changing the spark profile. I think this is a long shot.....
 
Any chance you change the rotor? Sometimes they are mis boxed and the later one is taller and really hard to put on the shaft.
 
Adjust your timing to 0 degrees with your timing light before checking your wire locations. Then rotate engine to top dead center on #1 cylinder and then check your rotor location. My bet is you are one plug off in one direction or the other if the two caps are identical.

Will do. Let's hope my wires just need to go a space counter clockwise.
Weird that the timing light would still show the same timing after , though, wouldn't it?
At this point, I'll check everything, regardless.
I'm over it lol
 
Another possibility. The resistance of the carbon button in the new cap is different & affected the spark. You can measure with an ohmmeter & compare.
 
Another possibility. The resistance of the carbon button in the new cap is different & affected the spark. You can measure with an ohmmeter & compare.

You know what? I just went and looked. One seems to have a normal, flat spot in the middle. Slightly corroded. It's part of the reason I replaced it. The new one has a spring-loaded button in the middle. Definitely not the exact same distributor cap.
That's what I get for using summit racings advice
 
You know what? I just went and looked. One seems to have a normal, flat spot in the middle. Slightly corroded. It's part of the reason I replaced it. The new one has a spring-loaded button in the middle. Definitely not the exact same distributor cap.
That's what I get for using summit racings advice
Then the new cap works better so I wouldn't be disappointed with summit.
 
I've had goofy ignition issues that ended up being the new cap even though there was nothing wrong with it that you could see or test. Timing was all over the place, swap in a used cap and it ran good, swap it back, timing all over the place again. If everything else checks out, maybe someone has a new Mallory cap that was made ten years ago instead of the more recent one that is obviously different. In my case it was an '89 Range Rover and it is known that the new aftermarket caps were a crap shoot.
 
Forgive me Kaj, but I have to ask if your dealing with detonation (ping and knocking ) or a miss fire ?
 
Forgive me Kaj, but I have to ask if your dealing with detonation (ping and knocking ) or a miss fire ?

Sounds like detonation. Not sure how a misfire would sound, so maybe. ...?


Swapping the old cap back on this weekend. We'll see how that goes. Wish me luck, guys!
 
Update:
1. Here are pictures of the old and new distributor cap. I don't think the cap is the problem.
2. I just remembered I swapped out to a different coil. I thought I researched to make sure it was the same ohm rating, but maybe not.

1000007074.jpg


1000006466.jpg
 
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