Random Misfire while cruising

[siteunseen]

Do you think the piton is making contact @Mark Maras with the plug? The hotter plugs are longer right?

 

[Zed Head]

I've read that detonation can cause the plug gap to close.  Detonation does a lot of damage, it can look foreign object damage on the piston top.  All it takes is a hot day, some low octane fuel, maybe having the timing off a bit.

I like Mark's suggestion.  Try a new plug and see how it does.  Everything else looks pretty good, the pressure numbers and the other plugs.  The pressure numbers are on the high end though, which might mean a high CR, which might be a reason for detonation.  If you do hear knocking you can just dial the timing back a few degrees and should be safe.

Edited October 20, 2020 by Zed Head

[Johnny wick]

Just tried a new plug. Car starts to misfire once it gets stucked in traffic.  Im about to get a new valve cover gasket, open the valve cover and adjust the valves to see if there is something wrong with that 4th cylinder.  

[Zed Head]

Recheck the plug first.

[Johnny wick]

2 hours ago, Zed Head said:

Recheck the plug first.

Plug 4 after 20 mins of driving and stopping on traffic lights back home

[Johnny wick]

Plug number 3  - front carb

[Mark Maras]

7 hours ago, siteunseen said:

Do you think the piton is making contact @Mark Maras with the plug? The hotter plugs are longer right?

 

 That has been my understanding. I hope I haven't been wrong for 50 years. I once owned (before Zs) a dragster (BB/GD) with a blown (671) 327 small block chevy. The center electrode was recessed down in the porcelain on the plugs that came with that engine. 

[Zed Head]

8 hours ago, siteunseen said:

Do you think the piton is making contact @Mark Maras with the plug? The hotter plugs are longer right?

 

I think it's only the insulator length that is changed.  Your drawings show a longer threaded portion but I don't think that's right.

Here's NGK's stuff.  Look at the porcelain.  Threads are the same though.

https://www.ngk.com.au/technical_info/heat-range/

[Johnny wick]

So whats next??? Should i open the valve cover and check valves?

[Mark Maras]

Yes, and after they've been reset another compression test would be helpful to rule out low compression in #4.

[Johnny wick]

14 minutes ago, Mark Maras said:

Yes, and after they've been reset another compression test would be helpful to rule out low compression in #4.

Compression readings attached.

[Mark Maras]

 @jalexquijanoWere those readings with the throttle wide open? Also before or after checking the valve lash?