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Discussion Starter #1
I've got a difficult to diagnose problem that I hope you guys can help with.

I've got a '69 vert with a 383 (not stock) that I use as a daily driver. The problem is that the #8 plug fouls and fouls very quickly. I had the engine re-built 3 weeks ago and dyno'ed before putting it back in the car. It dyno'ed great and ran perfectly. Once back in the car, it was missing right away and fouled #8 which was why I rebuilt the engine. Before rebuild, there was blowby past the rings AND the valve stem seals leaked. Now all cylinders hold pressure (with a leakdown test all near 10% loss) and all cylinders check out ok with compression (between 190 psi and 170 psi).

I switched the #8 and #6 plug wires expecting to see #6 fouled, but within 30 miles of driving, #8 was fouled again. Could this be a distributor problem? How would I diagnose that?

Thanks for any insight.
 

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Hi, can you tell us what you are running for a Distributer setup MSD or HEI and is it old? Did you replace the wires, plugs.

You mentioned the motor was dynoed, was that with another distributer setup?

Thanks
Jeff G.
69 Vert RS/SS 396
 

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Discussion Starter #4
Oh yeah. The wires are 2 years old too. The plugs are brand new and #8 is replaced with new each time I look at it. I'll save 'em and clean 'em for later use.
 

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Discussion Starter #7
The fouled plug is black and completely carbon coated. All of the other plugs are either still white or a little bit tan.

The engine oil is clean not milky and I haven't smelled any fuel in the oil either.

Jeff, I don't follow what you mean by search here...
 

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One way to diagnose a misfire is to put it on a scope (are there any still out there?). Back in the day, you would hook up the scope and look for 8 beats (just like a heartbeat so to speak). If you didn't see 8, you had a misfire.

Bad tower in the distributor cap?

Dull black (carbon) is fuel fouling, shiny black is oil.

alan
 

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Post a photo of the plug.....are you using any oil? You have eliminated the wire and plug. It's got to be cap or mechanical.
 

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I'd change the cap, cheap enough. I don't know why it wouldn't foul when engine was out of the car but sounds like you're not getting spark to that plug intermittently. Like a cracked cap.
 

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bb,
Is it smoking at all ?? I did a 331 stroker a few years ago and got a single second ring on the piston upside down. It did exactly the same thing and also smoked a little. The second ring will not affect compression readings but if it upside down, it will scrape all the oil on the cylinder up into the combustion chamber and foul the plug etc :( You might have to take it apart to find the problem.
 

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I agree with BillK, I have seen same ring upside down also, not on mine, but others who were rebuilding.
Might look down the intake runner with a borescope to see if intake valve head is covered in oil resulting from a leaking stem seal.
Use a timing light to 'see' if spark is going to cylinder.
Use a temp gun on the exhaust manifold runner to 'see' if cylinder is firing.
 

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Discussion Starter #14
Would the upside down ring be a mistake that could be made by a reputable race engine builder? The guy has a good rep but I don't pretend to know the diff myself.

it does smoke a little but only sometimes. Also sometimes a larger puff at startup. The same guy that rebuilt the engine also replaced the Trick Flow stem seals with Teflon seals. And it consumes a lot of oil without a drop on the ground.

It's a 4-speed. And the cap looks fine outside and in (visually anyway).

I'll try to find a shop with a scope first. If it's not misfiring, is the upside down ring the only other diagnosis left?

All this info is great. I feel a lot better about doing my own engine work.
 

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Sounds like she needs to come apart. Check the back of the intake valve first, if it's clean then it's time to look at the rings. Anybody can make a mistake.
 

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I switched the #8 and #6 plug wires expecting to see #6 fouled, but within 30 miles of driving, #8 was fouled again. Could this be a distributor problem? How would I diagnose that?

Thanks for any insight.
If you spend some time searching, I know I've seen some explanation as to why the #8 plug could have a tendency to foul. I can't remember for sure though.... If I run across it, I'll post a link.

In the mean time, it's easy to check out the distributor. Rotate the motor around to TDC on #1 (do it with the cap off to verify). Pull the distributor out, rotate the rotor forward or back one post/cylinder (you'll have to move the oil pump shaft as well), and reinstall. Have someone help if you're not sure about getting it back in correctly. Reinstall the plug wires accordingly (moved one spot over). That will put the previous #8 post at either #1 or #4 depending on which way you turned the rotor. What you are essentially doing with this is moving your #1 spot to a different place and thus moving the entire firing order one spot over on the distributor.
 

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Wouldn't it be easier just to replace the cap?

alan

Yep. But he did ask about diagnosing the distributor, not buying parts to try and find the problem.

Plus, some people are hard headed like me and will climb a tree to keep from having to go to the parts store yet again.

I was a commissioning engineer in a former life and some of the old ways die hard. If I had a problem with something on site that made it through FAT testing, I had to prove that it was bad by at least making the problem move somewhere else....especially since a lot of the controllers and such had an eight week lead time and cost $40k. I still do it with a ten dollar part. Weird, I know....
 

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Would the upside down ring be a mistake that could be made by a reputable race engine builder? The guy has a good rep but I don't pretend to know the diff myself.

it does smoke a little but only sometimes. Also sometimes a larger puff at startup. The same guy that rebuilt the engine also replaced the Trick Flow stem seals with Teflon seals. And it consumes a lot of oil without a drop on the ground.

It's a 4-speed. And the cap looks fine outside and in (visually anyway).

I'll try to find a shop with a scope first. If it's not misfiring, is the upside down ring the only other diagnosis left?

All this info is great. I feel a lot better about doing my own engine work.
You guys did read this right?
 

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Yep. But he did ask about diagnosing the distributor, not buying parts to try and find the problem.

Plus, some people are hard headed like me and will climb a tree to keep from having to go to the parts store yet again.

I was a commissioning engineer in a former life and some of the old ways die hard. If I had a problem with something on site that made it through FAT testing, I had to prove that it was bad by at least making the problem move somewhere else....especially since a lot of the controllers and such had an eight week lead time and cost $40k. I still do it with a ten dollar part. Weird, I know....
Actually, as I read your response, I had a thought (no, it didn't hurt :) ). What if one of the lobes on the distributor is worn? That would cause a misfire as well.

alan
 
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