Dead Cylinder
 

I have had this car for going on three years now and it took me most of that time to get my Webers running right! After a long winter here in Middle Tennessee I took the car to work a few weeks ago and promptly fouled a plug. I had to drive the thing about forty miles back home with it popping and backfiring.

A couple of days ago I put some fresh plugs in and cranked it up, but still had the miss. Using my flow meter I quickly determined it to be the rear left (driver’s) cylinder that was not firing. It was reading around 5 while the others cylinders were right at 10. I played with the carb a little, but soon figured it was not the problem.

I changer the plug again and noticed a slight amount of oil on the electrode…Not wet, but slightly discolored. I changed the plug and swapped the plug wires with the plug next to it. No change.

Right after I changed the plug the first time, I dove the car about 10 miles and it seemed that the offending cylinder would fire every once in a while, as the RPMs would climb some and the engine would smooth out.

This engine only has about 5000 miles on it and was professionally built by Performance Engineering, so it is hard for me to believe it is a serious engine problem (At least I am keeping my fingers crossed!) that is causing this problem.

I am running an MSD ignition and distributor/cap with Taylor wires and Autolite Platinum Plugs (AP3924) . I am somewhat concerned about the oil film on the plug.

Any thoughts??

Thanks,

Gun Doc

The problem is in the idle circuit.

if the webers were set wrong, and that weber stack was washing that cylinder with gas, the rings could be trashed.


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Perhaps some sort of insect plugged up a circuit?

I'd be doing a compression test first to rule out the engine.

Then I'd swap two carbs around to prove the fault to one barrel of the suspect carb.

Airflow of 5 compared to 10 of the rest does not sound good, could be a twisted throttle shaft or idle air issue.

sounds like no spark problem
 

Gun Doc If the spark plug is wet, I would n't start with the carbs. Start with the cap and rotor on the MSD. REPLACE both. MSD has had cap failures and burnt holes in the rotors. Make sure you have good light in that cylinder.
Next You may have to do a little search on this forum and the FE forum but running Platinum plugs is no good for MSD systems. Running just 3924 with a wider gap of .044" Is what I believe is to be run with stock plugs.
As Gaz said the easy thing is to swap carbs and retest. If you have poping and back fires, this is more ignition than carb. Get a spark tester and make sure you have a good large blue or yellow spark. If not the coil could be going. Lets try the basics first, Check for good spark in all cylinders and see if you can shoot a picture of inside the cap and rotor. Rick L.

I guess the big question is, what is fouling the plug, gas or oil. He mentioned oil.


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Thanks for all the replies…I have not had the time to do any more testing, but I think I’ll start by blowing out the idle circuit on the offending barrel.

I did say there was some residual oil on the plug. It is NOT wet, but has a slight film on the electrode. I smelled both plugs that were recently in the cylinder and neither has an odor of gas. I really don’t think that plug was firing at all. The original plug I took out HAD been firing and looked to be fuel fouled (black and sooty).

The last drive I took in the late fall I remember the car running pretty well. The problems started with this first spring drive.

If the carb doesn’t fix it, I guess my next step is to run a compression test on that and a couple other cylinders.

If I have destroyed the rings in that cylinder, what are my options? Is it as simple as honing and replacing the rings with oversized, or is it a complete overhaul of the engine? I have just over 4300 total miles on the engine.

Thanks,

Gun Doc

You'll find the problem without going to that extreme.

Easy big GUY
 

Gun Doc Doc you are helping to fill in some things. First off, last fall, did you empty the fuel system and run the carbs out of gas?? Did you replace the gas filter and open it up, this is to see if there is any debry in there? Here's the mean question, If the fuel system was not emptied, did you fill and run some gas stablizer through the system? How did the MSD system check out?? Before you start disassembling the motor, leak down test and compression test will tell you if that cylinder is fuel washed.
Is there any change that you red lined the motor? bent some pushrods? Using a vacuum gauge might help. You all ready know the bad cylinder, Check out theses parts too. The motor will run with a bent pushrod. The other question, does the motor run better ot worse when hot or cold? This is also important. What hoses are you running on the Webers? There are starting to be problems with running rubber hoses. The rubber is breaking down because of the 10% ethonal ( spelling is off ). I have had to replace all the rubber hoses in my chain saws, and now the fuel system in my car for the fi system. The rubber turns into little balls of goo. The one carb could also have tarnish in the bowl and causing this problem. Keeps us posted. Rick L.

Did a little more troubleshooting after work today... I started by blowing out the idle circuit (mains, too) with my blow gun. I then changed the idle jet in that barrel from a .60 to a .65 and started the engine.

I only let it run for a couple of minutes then shut it down. It wasn’t missing too badly at idle, but after I rev’ed it up a couple of times, it did start to miss. I shut it down and pulled the plug.

I could now smell fuel on the plug and when I wiped it off with a shop towel, the electrode wiped clean – white. The gas cleaned off the slight oil film.

I am beginning to think this may be an ignition problem. The plug is obviously not firing at all. I don’t think there was enough oil on the thing to foul it. Before I changed out the idle jets a couple of years ago the plugs would come out black and covered in soot, but would still fire. I guess the cap and new, non platinum plugs are my next step…

MTF

Gun Doc

I am confident that Rick is close to the solution; fouled idle curcuit. And to check if the plug is firing or not is quite easy; put it close to ground and crank. Although, a combination of faults is also possible...

hope you get it running.

I took a little break from the car to get my head right…I have two bikes that I have been neglecting since I got the Cobra. One of the bikes has been sitting since I got transferred to Tennessee a couple of years ago; the other has sat since I got divorced four years ago…So I spent some time on them and got both running. I just now started back on the Cobra.

I did a cold cranking (throttle closed) compression check of the dead cylinder and got just over 150 psi. I have not checked the other cylinders yet, frankly because it is a real pain to get the tester started in the spark plug holes!!

I am not sure how much compression I should have, because of cam, etc, but I am pretty sure 150psi cold with starter motor only should be plenty for that cylinder to fire if it has fuel and spark.

I am now debating whether to pull the carb, or just go out and buy a new cap and rotor. I did look at the cap and rotor and they both look okay…Nothing obvious, like a visible crack, or burn.

Gun Doc

Suggestion: Try this before you drop the cash for new ignition parts. On a cold engine, drain the fuel from the suspect carb. remove the idle and main jet holders. Spray some carb cleaner in the holes where the jets screw into. Then disassemble the jets and holders and spray THEM with carb cleaner, pay close attention to the small hole in the end of the idle jet. THEN when you reassemble the idle jet in its holder do not fully bottom the parts out when you insert one in the other. Then mark the taperd face of the idle jet with a felt marker or machinists dye, do the same with the tapered face of the main jet. When you screw them into the carb body the jets will first bottom out in the wells, marking the faces of the jets, pull them out and inspect the markings. They must seal in the bottom of the wells or fuel will not flow correctly. Sometimes over the winter some of the fuel can go bad and plug very small passageways. Check it out and report back.

Did u check the plug wire? Put an ohm meter on it, it could be open.


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Cobra Ed;
I did not ring the plug wire, but I did swap it with the one next to it with no change...Problem did not move.

Rick,
Funny you should suggest that...I was just reading an article on Aircooled.net that details rebuilding these carbs and they show the felt tip trick on the jets to check for seating. I'll do that tomorrow. I have taken the main and idle jets (with holders) out and blew through the idle and could force air through it with lung pressure only.

Gun Doc

Today after work I took out both the main and idle jets, as well as the mixture screw and cleaned everything with carb cleaner. Once clean, I marked both jets with a Sharpie, seated them and then removed them for inspection. Both jets were fully seating as evidenced by the absence of any ink.

I then put everything back together and started it up. It felt like that cylinder was pulling a little more. Not as much as the one next to it, but better than it had been. I never got a chance to put my sync meter on, because I was running it in the garage and after about 3-4 minutes every smoke alarm (no smoke, just fumes) in my house goes off! By the time the fumes cleared out of the garage it was too late to do any more with the thing...

I think I'll fiddle with that carb a little more before spending the cash on a new rotor and cap. I may take it off to see it it is seating evenly...Maybe a leaking gasket under that barrel??

Gun Doc

Cleaning the jets is one thing.

Cleaning the screen filters (at least I have those in the IDF's fuel intakes) and also cleaning the internal piping with compressed air (can also be done with thin, long stalks of copper wire-or pipe-cleaners?) is mandatory.

Here is the latest update: I put my timing light on the offending cylinder’s plug wire and sure enough, I had a strobe. Just to be sure, I pulled the plug and grounded it out while the engine was running. I got a bright white spark. I then put my compression tester on a couple of other cylinders to compare what I had gotten previously from the dead cylinder and they were almost exactly the same (approx 150 psi).

This is really beginning to puzzle me…I have good compression, appear to have good spark and when I remove the plug I can smell some fuel on it. Also, that cylinder will back fire (in the pipe) when the engine is running; telling me there is un-burnt fuel in the header. Maybe the spark is too weak once it is under pressure??

I sure would like to get this car running right. I have to go back to Afghanistan in mid August and would really like to drive it some before I leave.

Any suggestions???

Thanks,

Gun Doc

This might be 'too' simple, but have you checked the plug leads for the correct firing order sequence as they go around the cap, might have a couple crossed..