Misfiring while cruising

[Patcon]

That's progress! At least we know it's cylinder specific

[jalexquijano]

Now what? Tear all the block down? I know for certain that if drive the car the fouling goes away. 

[jalexquijano]

16 hours ago, Mark Maras said:

 I didn't give up on #5. That plug looked pretty darned good, imho, having idled for a long time. I was just glad to see that all the plugs weren't black. It doesn't matter to me if the other five were black. The fact that at least one plug looked good narrows the problem down. Also, the original problem was always isolated to #4 until the dist was removed and reinstalled. Then #5 started doing the same thing as #4 did previously. Seems to me that moving the wires and or disconnecting them and reinstalling them is the cause for the #5 gremlin and probably #4 too.

 I considered valve seals too but the two dark plugs looked sooty not oily. I'm hoping that re-seating the wires will eliminate the misfire. With Alex removing and reinstalling the wires the misfire may move to another cylinder until all the wire connections are right. 

Thats correct. 3 and 4 are sooty not oily.

[jalexquijano]

16 hours ago, Mark Maras said:

 Sometimes you just have to go with the limited info and have faith that it will work out.

Mark,

What would you recommend now that i did the test?

[Zed Head]

14 hours ago, jalexquijano said:

Just tested with ngk bp6es brand new plugs at 1200 rpm idle for 20 minutes in parking lot. 3 and 4 are fouled. Already tightened connectors.

 

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.

[Mark Maras]

 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.

[Zed Head]

Another quick test of the wires would be to swap 2, 3, or 4 for any of the clean cylinders.  It might be a stretch because the wires are custom fit  but should be doable.  Or just buy a single new plug wire and replace 3 or 4.  If it's the wire, the problem will follow the wire.

You're in to the process of elimination now.

[Zed Head]

14 minutes 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. 

That's a good test.  But, does Jalex have a meter?

[Zed Head]

On 9/24/2018 at 8:17 AM, jalexquijano said:

 We already replaced cap and rotor, check cylinder compression. 

Cap and rotor.

On 9/23/2018 at 8:54 PM, jalexquijano said:

Ill try that but these cables are not that old. I thought this misfire problem was over. 

Circling back...

Oil rings don't show up in a cylinder pressure test.  This a California Datsun engine after all.  Valve seals can leak a lot of oil also.  

Focus on the plug wires and spark for now though.  If you can show that it's definitely not spark related then you can move on to other possibilities.  Eliminate one possible cause at a time.

[jalexquijano]

I have a multimeter and can measure the resistance with it. I will need to place the spark plug inside the boot or otherwise the lead wont reach the terminal.

[siteunseen]

4 minutes ago, Zed Head said:

This a California Datsun engine after all. 

BINGO!!!  We have a winner.

[Zed Head]

4 minutes ago, jalexquijano said:

I have a multimeter and can measure the resistance with it. I will need to place the spark plug inside the boot or otherwise the lead wont reach the terminal.

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.