Misfiring while cruising

[jalexquijano]

30 minutes ago, siteunseen said:

BINGO!!!  We have a winner.

I only replaced the camshaft with a schneider 274f and new rocker arms. The timing is set to 14btdc.

Edited September 27, 2018 by jalexquijano

[jalexquijano]

28 minutes ago, Zed Head said:

That's what Mark was saying to do.  Put the plug in the end of the wire then measure from contact inside the cap to the center electrode of the plug.  He said body but I think he meant center electrode.  Do it for all six so that you have a good comparison.

 

[Zed Head]

Yes, the center electrode should have a good path back to the contact in the distributor cap.  You might see high resistance since you probably have resistor wires, but all six plugs and wires should show the same number.  Wiggle the wires around once you get your meter set up, for each wire but especially 3 and 4, to be sure that you don't have an intermittent break in the circuit.

[jalexquijano]

4 hours ago, Mark Maras said:

 I'd grab a volt-ohm meter and measure the resistance from the contacts in the dist. cap to the body of the spark plug. Basically, remove the cap leaving the wires plugged in. Next remove the spark plugs and plug them back onto the wires. One of the meter leads will go onto the contact inside the cap. The other meter lead will go onto the body of the spark plug. Measure the resistance in each of the cap, wire, and plug. When you find one with a lot of resistance remove the cap and measure the resistance in the wire and the plug. Then reinstall the wire into the cap and pull the plug from the wire. Then measure the resistance in the wire and cap. Ask if you have any questions. It's the best way to learn.

Mark,

Ill follow your advice and do the resistance test with my Digital Multimeter. I also have a set of old NGK Blue spark plugs which i can test. I called my mechanic today and he argues that if the seals should be good or else the engine would present problems at high rpms. He doesnt see any oil on those plugs, more gasoline. Weird!

[jalexquijano]

4 hours ago, Zed Head said:

That is a great picture.  It shows a lot.

We might all ponder the various causes of fouling for a short while.  As I recall from much past experience with poor running engines, a fuel fouled plug often looks like 3 and 4 here, but then when it  fouls, it gets wet with fuel.  And it fouls quickly and the carbon coat is not so smooth.   These plugs look fairly dry, and the other plugs look almost lean, like they're not even close to fuel fouling.  Weak spark fouled plugs typically look wet also, like they were wet from the beginning of missed sparks.  These plugs look like they were firing well, with a good even coat of sooty carbon on everything exposed, until the spark started shorting to ground through the carbon.  In short, they look like they had great spark until they didn't.

#2 (or maybe #5) looks like it is fouling also.  But the other three look very clean.  Considering how the carbs and manifolds are set up it doesn't seem like it could be a fuel problem, unless fuel is running down the intake runners and in to those cylinders.  Is that possible?  Is there a path for raw fuel in to the center runners?  If he parks the car on a slant so the fuel runs away from the cylinders would that be a clue?

If the spark is good and the fouling is fuel or oil related, and fuel is discounted, that leaves oil.  Which could be rings or valve seals. 

It could also be valve lash, possibly, causing poor combustion.

Just some thoughts on other possible causes.  On situations like this, I tend to go around in a big circle on all of the possible causes until one wins.  There's definitely a cylinder specific problem though, which narrows the possibilities down.

Look at this fórum, this user has a similar problem. Mine is not oil related as the spark plugs look sooty matte black and not oily:

https://www.zcar.com/forum/10-70-83-tech-discussion-forum/268092-spark-plug-fouling.html

 

[Zed Head]

Depends on how much oil is entering the cylinder and whether or not it's burning with combustion.

You need to work on the simple checks so that you can zero in on what it really is.  Focus on those spark plug wires. 

Here's another possibility - the newish spark plug wires aren't fully seated in the cap.  That would give a sporadic misfire if there's no contact between wire and cap contact.  That will show up in the multimeter test, which I'm sure you're doing while also searching the internet.

[siteunseen]

The internet searching is what's hurting him in my opinion.  Stay focused on each issue.  We've read weather stripping to u-joints and differential mount issues, cam and idling for 20 minutes back to spark plugs soot. Your all over the place. Sorry Jalex but common sense is called too many irons in the fire.

You should ship your car to the States, Datsun Spirit would be my choice if I couldn't help myself.

[Zed Head]

He's so close, right now, to figuring out the poor idle in the heat problem.  Those two center plugs and the dirty one next to it are huge clues.  Much more informative than many of the previous posts.  The clean plugs, 1, 2 and 6, tend to remove any choke setting, or carb-tuning, or fuel percolation, or Far311 camshaft issues.  Unless, the fuel overflow runs straight down the 3 and 4 intake runners.

With that focus a bunch of new ideas come to mind.  But we need to know if the plugs are getting their voltage or not.  Gotta test those plug wires.

[siteunseen]

Dielectric grease is good also on all the plug wire connections, IMHO.

[Zed Head]

Seriously?  This advice is not good enough?  I don't see how it could be better.  Not clear that Jalex really wants to fix the problems.  I have to admit that sometimes I miss my car's old problems.  It was something to work on.

Good luck. 

https://www.zcar.com/forum/10-70-83-tech-discussion-forum/418272-spark-plugs-3-4-getting-sooty-black-my-1972-240z.html

[jalexquijano]

6.89 k spark plug 5

7.17k spark plug 4

5.83k spark plug 3

Edited September 28, 2018 by jalexquijano

[Zed Head]

One...

https://www.taylorvertex.com/faqs/#ohmsonwires

http://www.ngk-sparkplugs.jp/english/techinfo/resistor-wire/01/index.html

Edited September 28, 2018 by Zed Head