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Offline Pinto5.0

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Can changing a distributor cap & rotor cause pre-ignition?
« on: August 06, 2013, 05:21:09 PM »
I installed a late model distributor cap & rotor along with a set of MSD 8.5 wires on my wagon. The car ran fine on the old stock parts but now I have spark knock under load & run-on when I shut it off. All I can think is the increased rotor sweep is causing the problem.
 
This is the standard rotor I took out.
 

 
This is the late model rotor with about 3 times the sweep of the stock version.
 

 
All I can think is that increased sweep is the cause of all this. I have an old rotor & I'm going to trim & file the contact to the size of a stock one & see if it helps.
 
Anyone run into this before??
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Offline 74 PintoWagon

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Re: Can changing a distributor cap & rotor cause pre-ignition?
« Reply #1 on: August 06, 2013, 08:08:21 PM »
That shouldn't have anything to do with it really it just fires and that's it, but you may be getting way better spark with the new parts and causing more/better combustion and making it leaner, I had that happen before when I upgraded my ignition, fattened it up a bit and it was all good.
Art
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Offline 72pair

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Re: Can changing a distributor cap & rotor cause pre-ignition?
« Reply #2 on: August 06, 2013, 08:38:17 PM »
Did you reset the ignition timing? The wider rotor advances the lead. ie: the rotor gets to the terminal sooner than it did.
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Offline Pinto5.0

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Re: Can changing a distributor cap & rotor cause pre-ignition?
« Reply #3 on: August 06, 2013, 09:55:22 PM »
I had the timing at 20 with the advance plugged but the pinging was so bad I dropped it to 16 just to quiet it down some. I'm having to shut it off in gear now because the run-on is insane.
 
It was running fine with the new carb at 20 degrees with the 20 year old cap/rotor/wires but it went crazy when I switched to the brass terminal cap, brass rotor & MSD wires. I'm still running the 37 year old dizzy & coil.
 
Which jets richen the carb? Is it the set under the choke butterfly or are they internal?
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Offline 74 PintoWagon

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Re: Can changing a distributor cap & rotor cause pre-ignition?
« Reply #4 on: August 06, 2013, 10:09:22 PM »
The main jets are in the float bowl area.
Art
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Offline Pinto5.0

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Re: Can changing a distributor cap & rotor cause pre-ignition?
« Reply #5 on: August 06, 2013, 10:39:49 PM »
The main jets are in the float bowl area.

That should be the ones I can see from the top. It looks like they can be changed easily. I'll have to try it in a couple weeks & see if it helps along with trimming a rotor down.. I know racers file them to points to dial in the distributor so trimming one should be simple. 
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Offline 74 PintoWagon

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Re: Can changing a distributor cap & rotor cause pre-ignition?
« Reply #6 on: August 06, 2013, 11:46:11 PM »
Yep they be the ones, you can try trimming the rotor too won't hurt anything, just do one change at a time.
Art
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Offline 78_starsky

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Re: Can changing a distributor cap & rotor cause pre-ignition?
« Reply #7 on: August 06, 2013, 11:50:20 PM »
Wouldn't it be easy to put back in the old rotor and see if that fixes it?  this on what size motor?

Offline Pinto5.0

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Re: Can changing a distributor cap & rotor cause pre-ignition?
« Reply #8 on: August 07, 2013, 12:26:03 AM »
Wouldn't it be easy to put back in the old rotor and see if that fixes it?  this on what size motor?

It's a 2.3 in my wagon. I probably could put the old parts on long enough to see if it goes away. I have a used late model rotor to grind the tip of so I wanna try that since it's a 2 minute job.
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Offline 78_starsky

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Re: Can changing a distributor cap & rotor cause pre-ignition?
« Reply #9 on: August 07, 2013, 12:58:48 AM »
have you switched octane levels on your fuel in the last couple days / or / since this started?   one would think that for the rotor to do this on a 4 cyl. motor the rotor would need to be huge on the contact travel length.  head shaker for sure.

first i have heard this, but in our whacky world anything is possible today...  cheers

next thought,  have you checked to see if you happened to accidently knocked the distributor to the advance/retard by a loose turning?  is the hold down bolt tight?

Offline Pinto5.0

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Re: Can changing a distributor cap & rotor cause pre-ignition?
« Reply #10 on: August 07, 2013, 01:14:01 AM »
It did it on the same tank of gas. I drove home, changed parts, double checked the timing & called it a night. The next day I had so much pinging under the hood that I backed off the timing because I was afraid of causing engine damage. It really has me perplexed but it must be the increased ignition efficiency. Those are some of the best wires on the market but if that caused this I'm gonna be shocked. All that leaves is the rotor sweep which has to be adding spark duration to the fuel mix.
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Offline oldkayaker

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Re: Can changing a distributor cap & rotor cause pre-ignition?
« Reply #11 on: August 07, 2013, 05:33:10 AM »
A few things to check/consider.  Good luck.
1) Since you replaced the wires, cap, and rotor, just wondering if the plugs were replaced also.  If so, check the heat range on the new plugs.  Too hot a plug could cause both these problems ("pinging" & run on).  If not replaced, check plugs for deposit build up and general condition.
2) Not familiar with the 76, but looks like you are mixing parts from different years.  May want to check that the old and new rotors & caps are indexed the same (both point to the same terminal at the same time).  If out of whack, this should only affect "pinging".
3) Again not familiar with the 76, but some carbs had a solenoid that would close the throttle when the ignition was turned off.  This was to reduced run on.  With all your activity in the area, maybe it was disturbed (if so equipped).
4) My 71 2000cc had a run on problem caused by bad valve seals.  The extra oil would cause excessive deposits which acted like little glow plugs.  Messed up one clutch disk trying to stop run on by putting it gear to stop it.
Jerry J - Jupiter, Florida

Offline Pinto5.0

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Re: Can changing a distributor cap & rotor cause pre-ignition?
« Reply #12 on: August 07, 2013, 11:39:53 AM »
You may be on to something with the plugs. I forgot that I swapped in a set of Denso platinum plugs in the stock 6 heat range. The AC plugs I took out had a nice tan color but they were white at the tips & a little crusty. Those were probably on the verge of detonation but the tune up put it over the edge. Maybe I'll try a colder set & see what it does. I still want to richen the carb a jet size because I think it's a tad lean on top of the rest.
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Offline amc49

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Re: Can changing a distributor cap & rotor cause pre-ignition?
« Reply #13 on: August 23, 2013, 04:52:18 AM »
The main jets are INSIDE the carb under the top cover. Any jets you see around choke plates are AIR jets and messing with them will put you in big trouble quick.

You can narrow the tip of that other rotor and it will run fine, it's wide enough that with advance swing it's probably occasionally sparking to the wrong cylinder. I'd be looking at the tip versus notch inside bottom hole of rotor for proper index to each other too, they must be the same, or match.

I used a late tall ignition cap on a 2.3 that had a 2.0 intake, the closeness of parts meant I had to take two plug wires out of the SIDE of the cap, not the top. It worked perfectly.