Misfiring while cruising

[Jeff Berk]

Could adjacent plugs mean a gasket breach between them?

[Zed Head]

4 hours ago, Dave WM said:

how about air leaks at the intake manifold? clean plugs have leaks. this is assuming there are both rich and lean readings coming from plugs fed by the same carburetor.

That's a definite possibility.  If the fouling is from excess fuel, then 3 and 4 could be getting a rich fuel mixture from the carbs and the outside cylinders have air leaks that lean it out.  The difference is very distinct though.  3 and 4 are terrible and the others look almost lean.

 

[Zed Head]

13 minutes ago, Jeff Berk said:

Could adjacent plugs mean a gasket breach between them?

Seems like that would show up on the cylinder pressure test.  He says that they're all 180 psi.  which is actually very high for an engine with a performance cam.

It's an odd problem, but at least it's a new one.

[Zed Head]

1 hour ago, jalexquijano said:

there is no air leak at the intake manifold. i have sprayed carb cleaner many times and have not noticed any idle drop. All my mechanics have concur there is no air leak. Come on guys. Both of my carbs were recently remanufactured in Dec 2014 by Ztherapy. So that must also be taken out of the equation. Why plugs 3 and 4??

If it's not fouling from excess fuel and all of the spark plugs are getting good spark that leaves something specific to each cylinder.  Oil fouling from leaking valve seals or bad oil rings seems reasonable.

Oil fouling from bad valve seals would probably show some signs in the intake runners in the cylinder heads and the backs of the intake valves.  3 and 4 will be oily, the others not so much.  A bore cam ran down through the carb throats might show something.  Or remove the carbs and intake manifolds to get a really good look.

There might also be signs from above, under the valve cover.  Obviously misplaced valve seals can probably be seen through the valve springs.

[jalexquijano]

Should i drive the car for 30 mins and then let it idle for 15 to see the results?

Edited September 28, 2018 by jalexquijano

[Zed Head]

Driving and looking at plugs has already been done.  What will doing more of it tell?  You need to find out where the "fuel" is coming from that's fouling those plugs.  Why are cylinders 3 and 4 dirtier than the others?

This assumes, of course, that spark is good and strong in 3 and 4.  What are you using to hold your plug wires as they cross over the engine?  And what brand are they?  Maybe 3 and 4 are shorting to the engine somewhere and even though the resistance is good, the spark is still bad.

 

[jalexquijano]

Pertronix ignitor module 1761 and petronix flamethrower 1.5 ohms coil installed with original hitachi distributor and ngk blue spark plug wires. 

[Zed Head]

On 9/26/2018 at 5:48 PM, 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.

I don't know who your mechanics are but a good one should be able to do something with this picture alone.  Show them this picture and tell them that it happened immediately after installing new spark plugs.  Don't say anything else.

 

[Mark Maras]

 @jalexquijano

We can't eliminate poor wires or connections until you complete the tests on all six. Three separate tests that includes testing with everything (cap and plugs) connected Then another test on all six with the cap disconnected, plugs connected, then a third test on all six with the plugs disconnected and the cap connected. It would be a good idea to do a fourth test on the wires only. No cap or plugs connected. I still think you'll find the problem somewhere in the cap (new), rotor (new), wires, or the wire connections. I think the carbs are close enough based on some plugs looking good. You've checked for intake leaks and found none. Compression ratio is OK. There's really only a couple of things left. Valve seals are a possibility (seals could have been damaged due to improper installation) but lets eliminate the easy one first. Try not to get too frustrated. WE will figure this out.

[Mark Maras]

21 minutes ago, Zed Head said:

I don't know who your mechanics are but a good one should be able to do something with this picture alone.  Show them this picture and tell them that it happened immediately after installing new spark plugs.  Don't say anything else.

 

 If I were that mechanic, I'd grab my volt-ohm meter first.

[Zed Head]

He has an old set of NGK plug wires.  Swap those on and if the problem is exactly the same odds are it's not the wires.  Put an old cap or a new cap on, see what happens.  Odds are the problem won't stay with 3 and 4 through two different distributor caps.

A good mechanic, even with no meter but with the thought that it might be plug wires, would just swap a "good" wire for a "bad" wire.  Clean the plug, let it idle, recheck.  The good news is that after all of these years (yes, years) "the problem" is narrowed down to 2-3 cylinders (still not clear if the half-dirty plug is #2 or #5), and the problem shows up in 5 minutes of idling.  This is Saturday afternoon futzing around kind of work.  Make guess, make a change, run a 5 minute test.  Learn, rerun process.

Our guesses almost don't matter any more.  A good mechanic should be able to look at all six plugs and have a good diagnosis within an hour.  Three really dirty plugs and three really clean plugs, and test results in 5 minutes.  Can't get much easier I think.  For all of the time involved, he'd probably just put all new parts on to discount any wire, cap or plug issues.  A basic tuneup.

The basic problem here, on the internet, is still the same.  Incomplete information, incomplete followup (resistance readings on the bad three but not the good three), actions suggested but not taken.

[Zed Head]

Still significant that the other three cylinders run clean.  That's something that removes many other potential causes.  Adds focus.