The following is the spark plug from a bad cylinder #7 with a psi of 90.







I know a lot of you have the ability to diagnose a problem just by looking at the plugs so im hoping i could get some info.

i first discovered the weak cylinder a couple of months ago after a compression test, i have been running the motor since on a daily basis and decided to do another compression test recently. Seems like the psi has stayed the same over the past few months which of course is good news for me, car is still driving fine with a slight miss but nothing noticeable.

Im running ac delco r45s, which seemed to help quite a bit compared to the r43s. Im leaning more towards a bad valve than bad rings but im not sure.

I do have a valve cover leak which explains the thread being oily on the plugs. Seems to me like the plug is revealing a lean condition? although im not experienced so id like some support/general input.

thank you

 

I'm no expert, but I'd say it doesn't look too bad. Higher resolution pics might help. What do the other plugs look like? The fuel you use will have an effect on the color of the plug as well. Up here, with the gas we have, it usually leaves a tan color on the plug.

 

the rest of the plugs look about the same, didnt take a picture of them.

I dont understand though, with a cylinder giving a reading of 90 psi, shouldnt i see ovbious signs of damage on the spark plug? or anything that might hint to the problem?

car doesnt smoke, doesnt do anything out of the ordinary, it does have some blow by and a slight miss, but it is very faint.

Edit: ive been pumping regular.

 

what about the rest of the system? are you sure your miss under load is the engine? 90 isnt all bad compared to the other cylinders could be a low gauge but you for sure have spark and fuel?

 

car is a carbed 305 that has had work done at some point.

Im running accel hei distributor with the integrated coil. Also running accell super stock spiral core wires.(red)

yes theres spark, got shocked before pretty bad (lol)

the miss is mostly when taking off/accelerating but that could be related to the accelerator pump on the carb needing adjustment.

my idle is also pretty high, its at 1500 on park and 1200 on drive.Ive tried lowering the idle through the carb screws but the car refuses to idle any lower..

However that does not explain why that cylinder is gving a reading of 90 and the rest are in the 140s?

 

did you bore out the holes on the carb to adjust even more?

 

John-John, check compression on that cylinder again, but first squirt some oil in the cylinder,if the compression builds or is higher the rings are worn if not you have a burnt valve"" Terry

 

one other thing you might try is re-adjusting the rockerarms on that cylinder.