So had a microscopic piece of rubber hose stuck in the fuel inlet valve of the carb, which caused the carb to flood and would cause the car to stall when coming to a stop. Fixed this issue and now have discovered that the #8 cylinder is not firing causing a rough idle, still drives great, except when stopping the rough idle makes it feel like it still wants to die. Im curious is if the two issues are related? Could too much fuel go into #8 causing it to wash out the cylinder walls? I swapped out the spark plug (it looked normal), I have spark thru the spark plug wire all through the spark plug, so I have ruled those two issues out as a possible problem. So now Im going to do a compression test on that #8 cylinder and see what that tells me. Any other thoughts, ideas or advice Id greatly appreciate!! My F-I-L said to drive it and see if it will work itself out but Im afraid of doing that, I dont want to make a bad problem worse. So any advice is GREATLY appreaciated!! I want to add that it drove and ran perfectly fine before that issue with the carb.

 

Check the compression as it could be the cause of your issues. I had a small block that used to break the piston between oil and compression ring which killed compression making my engine a 7 cylinder.

If the compression is low you might want to do a Leak Down Test on the engine to identify the cause of the loss of compression. Leak Down Tests are better at identifying issues that cause the loss of compression. They require a good source of air pressure to perform the test, the tool is not that expensive but very useful in diagnosing problems. While you are pressurizing the cylinder you might hear air coming out of oil fill hole which would indicate that you have excessive blow by caused by worn piston rings. Air leaking out the exhaust would indicate a leaky or non-sealing exhaust valve. Air in the intake would show a leaky intake valve or bad valve seals. There is plenty of information on Leak Downs on line but I learned them when I owned a Piston powered Aircraft and they performed a Leak Down every year at the annual maintenance. It would allow you to monitor the wear and indicate problems if they are there. A good tight engine will leak very little air where as a older worn engine would have greater leak down losses. My airplane engine used to stay around 80-85/100 so with 100psi of air going into the cylinder the engine held 85% of the air indicating 15% leakage. An older engine would be 70/100 showing more wear. These numbers are not fixed in stone. When doing the test you need to get the cylinder being tested at TDC with both valves closed and then apply air pressure. On my airplane I had to hold the propeller to keep it from spinning while the cylinder was pressurized as it would try to drop to the BDC with the air pressure. On my 427 I do it with the engine in gear (4-Speed) and the parking brake ON. Wheel chocks are a good extra layer of insurance. Since my engine is a Higher than normal compression the spark plugs are left in the other cylinders to deter it from rotating under the air pressure.

Sometimes a bad spark plug wire can be enough to loose a cylinder and this is more common than something killing the compression. The wire might make a spark with no compression and not spark under compression. Try a different spark plug wire if the compression is not the cause.

Once I put a bit too much die-electric grease inside the boot of the spark plug wires and this caused my spark plug to not fire. Instead it would ground out someplace before the plug end and make a great light show in the dark. If you have been using Die-electric grease that could be a reason. Most people don't understand that Die-electric grease IS NOT Conductive and can stop the spark if between the spark plug wire and end of the plug.

Try another plug in the #8 cylinder and a different Spark Plug wire before giving up hope.

 

My 68 427 had similar symptoms . Rough idle. After replacing plugs, wires and cap, stil rough idle. Compression check found #8 cylinder very low but variable. By the way, you have to access #8 plug from under the car! Took it to a shop that diagnosed a broken valve spring. It turns out 427 valve springs that are 50+ years old don’t last. The shop put in new springs all around and check the cam for wear. It was good. Runs great now

 

Temps of the cylinders. Replaced spark plug and spark plug wire, took valvve cover off and turned motor by hand, watched both valves open and close everything seems normal there, triple checked firing order, everything is correct but still no fire in #8. Im grasping at straws now. Not sure where to go from here. I dont want to drive it for fear I might make something worse.

 

Forgot to mention I did compression test on that cylinder and it was 154….so compression is good

 

you dont do a compression test on one cylinder. You do all of them and if one of them has a reading that is 10% less than the highest reading that cylinder is considered problematic.

 

I understand that but what I read online is that if i had little to no compression on that cylinder that would tell me the piston rings were the culprit. It tested at 154psi which i found online that normal range is 150-170. Either way I still have no fire in that cylinder. Im wondering if I should pull all the plugs and do a leak down test? Im running out of ideas. Ill try anything if anyone has any good ideas or had similiar issues.

 

Cap? Rotor?
try some ngk plugs..

 

How did you determine 8 is the non-firing cylinder?

 

I have NGK YR5’s in there, just bought a new one and no difference. Pulled off #8 wire from distributor and absolutely no change at all in motor. Go thru and do the exact same to every one and the motor bogs down. Theres a very obvious miss, u can hear it when its running. I have not changed cap and rotor yet. It ran perfectly for weeks but i guess it could be worn on there. I will try that next.

 

Year?

Modifications to Ignition System?

 

As a test, is the #8 plug itself firing when out of the head during cranking? If not, swap #8 wire with #6 or 7 and see if the problem moves with the wire. If it does not, inspects the cap's terminal for #8.

 

Its a 74 with 350, no mods to ignition system, i pulled the plug out and hooked it back up to spark plug wire and turned the motor over and it has spark all way thru wire, thru spark plug. I read that the spark could possibly be weak? , so I changed them out and put new in and no change.

 

So that seems to indicate the wire is at least OK part of the time. Might still be a good idea to swap with another to make sure it's not an intermittent wire.
You could also hook a timing light up to the #8 wire and run the engine. Judge the strobe consistency at idle and at various higher rpms. Even though the wire appears to fire the plug while cranking, it may drop occasionally drop out. Compare the strobe pattern to #6.

If that doesn't show any anomalies, perhaps a leak down test is in order to make sure the rings and valves are truly sealing. Out of curiosity I might even stick a borescope in there to inspect the top of the piston.

 

Sometimes on some engine the distributor caps can develop Carbon Tracking inside the cap which can cause a misfire of that cylinder. I have seen it more often on smaller distributor caps when using an aftermarket more powerful coil. Try a different cap or take a look inside with a Black Light. Replacing the #8 spark Plug wire did nothing? I frequently find a bad spark plug wire causing issues like you are describing. More often the wire is bad and then the distributor cap would be the next most likely to swap or replace.

On Compression tests it is always a good idea to start with a fully charged battery and perform the test with a battery charger attached keeping the battery strong during the test for consistent results. You also have to wire or tie the throttle plates wide open during the test. I also remove all 8 spark plugs before performing the test so the engine spins as freely as it can. If there is an electric fuel pump I also disconnect power to it before doing a compression test. There are no fixed compression numbers for most engines. What you are looking for is Consistency between the cylinders with less than 10% variance between all 8 cylinders. I never test just one cylinder with a compression test, I do all the cylinders and record the data for later testing. This is the same for the Leak down testing as the results allow you to see the engine while it is wearing. You can start to see patterns forming by doing this.

Best regards,
Chris

 

Pull a DIFFERENT spark plug (one that you find is working) and observe intensity of that plugs spark. How does intensity of that spark compare to #8?

 

I will try this tomorrow, I bought a new cap and rotor.

 

You only need three things for a cylinder to fire, 1) Compression, 2) Fuel, 3) Spark. You already verified you have compression and it's safe to assume you have fuel because the engine is running so that leaves spark. You can rule out the ignition coil and rotor because again the engine is running, that leaves the plug, plug wire, and distributor cap.

I would swap the #6 & #8 plug and plug wire and see if the problem moved to the #6 cylinder and if not than change the distributor cap. If that didn't solve the problem I would look at the intake and exhaust valves next but you already verified they are opening and closing.

I'm leaning towards this being an ignition problem vice engine mechanical issue.