Lost comp. on cyl. #4

[Robert_6]

I pushed the car too much yesterday coming back from a car show. There's a good stretch of highway 7 miles from home and I usually clean the plugs and carbon deposits in general by pushing the motor a bit. Nothing serious, just accelerating from 70 to 100 with the O/D on and revs not going above 4,200. Done it plenty of times in the 11 years I'vehad the car. Now 72K miles on the odo..
As I got to the stop light, I noticed it idled a bit rough and when I took-off I felt as if there were only 5 cylinders pulling the car. I limped it home and waited until it cooled-off a bit.
Checked if there was spark at every cyl by pulling the wires one a time while at idle and #4 didn't make a difference. I checked for compression and when I pulled #4 plug it looked like the pictures I included. All the others were ash in color and looking good. When I checked for compression, the others were at 120 lbs, but #4 was at zero,zilch, nada!
Took the valve cover off and checked the valve clearance on 7 & 8, good at 0.010".
Last night I ordered an endoscope camera for the iPhone which should arrive tomorrow PM. I'm hoping it's just a stuck valve. Maybe looking inside will confirm my theory. I will also adapt the compressor hose to push some air in through the plug opening and check for air flow while the valves are closed.

Any other ideas on how to inspect the damage before I decide to pull the head? The debris found on the plug is not metal. I scraped it off after taking the pics and tried to pick it up with a magnet and nothing stuck to it. The plug also smells of gas heavily. The car never smoked on the way home, I never heard any noise coming from the engine while at speed, it holds idle, although the valve train sounds a bit louder than normal.

Any thoughts, suggestions, reprimands for going too fast on a 41 yr old car with no airbags are all welcome. B)
Many thanks,
Robert

[SapphireBlue72]

Try putting some SeaFoam or Marvel Mystery Oil in a squirt-bottle and spray it through the rear carb (for #4) at a fast idle.
If valve is indeed stuck, it may free it up.
Remove spark plug & check compression afterwards.

[SapphireBlue72]

Another thought:
try switching ignition wires,- maybe swap 4 with 5 and see if you get a spark.
Try a new spark plug in #4 cylinder, plugs can fail.

[Tush]

But...zero compression Walt ...

I was thinking the same thing...even check the inside of the cap but that's a moot point now ...

Cheers
Tush

[poolboy]

I guess we'll know soon enough, but it might be either a chink or a chunk missing from the piston.

[tr6harris]

Sounds like you have a logical plan, especially with the air/leakdown type check.

Burned/broken exhaust valve? Would one hear a huff/puff or hiss as cranked during the compression check?

[Rick]

Hold a piece of heavy paper to the tailpipe at idle. If you have a burnt exhaust valve it will slap against the pipe every time that cylinder should fire.

[Greg_M]

A leaking cylinder at #4 set to TDC on the compression stroke, you should hear the compressed air you put in the cylinder hissing out someplace; tail pipe, carb or from around the piston via the oil fill hole.

Greg

[Robert_6]

Thanks for all your advice.
I sprayed Sea Foam into rear carb following the instructions on the can, no change. I haven't had the chance to attach the compressor to the cylinder and listen where the air comes out from. Also waiting on UPS for the endoscope tonight.

[dicta]

Thanks for all your advice.
I sprayed Sea Foam into rear carb following the instructions on the can, no change. I haven't had the chance to attach the compressor to the cylinder and listen where the air comes out from. Also waiting on UPS for the endoscope tonight.

Robert_6 --- Hope you ordered the WIFI Jetion endoscope. The unit as sold from Harbor Freight has a head that won't fit thru the spark plug hole. (The Jetion will)

Dick

[Robert_6]

Thanks Dick, I ordered this one. It's small enough to fit through the plug hole.
https://smile.amazon.com/gp/product/B06XTG5GM9/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1

[Robert_6]

I pulled the head today and here is a pic of the culprit.



I will check the cyl wall and piston head tomorrow for any scratches. They looked fine at a glance today, but couldn't take a good look.
Here is a picture of the carbon deposits on the piston heads in just 22,000 miles. I always use premium gas with "Sta-Bil" as an additive. Go figure...

[Robert_6]

I pulled the head today and here is a pic of the culprit.






I will check the cyl wall and piston head tomorrow for any scratches. They looked fine at a glance today, but couldn't take a good look.
Here is a picture of the carbon deposits on the piston heads in just 22,000 miles. I always use premium gas with "Sta-Bil" as an additive. Go figure...

[Robert_6]

Pics are too large. Working on it...

[Robert_6]

Can't get the pics to post for now.
The exhaust valve is chipped. The head will be taken to the machine shop tomorrow. Any suggestions as to which valves I should get? The shop replaced these only 22K miles ago in 2009 when I had to remove the head after a boil over coming back form TRials in NJ. It seems like they used the cheapest ones around if you ask me. I want to make sure I bring him the valves this time. I don't wish to spend a fortune on some high-performance-overkill-racing-type, but there may be some that are better than others. I don't have any knowledge on this.
Thank you for your help.
Cheers,
Robert