79 308GTS The front bank #1 cylinder keeps getting carboned up until it eventually stops firing. I have just replaced the spark plug wires, new plugs and rotors and cleaned the distributor contacts. All of the other plugs are firing clean after inspecting each of them. Any help or suggestions would be appreciated. Thank you
What spark plug brand/number are you using, and have you confirmed that the mixture screw adjustment for that cylinder is reasonable (maybe just way too rich)? If the choke spool is stuck open, it should be noticeable as being unable to be adjusted to go from lean-missing to proper-running. PS Your description of which cylinder is having the problem is a little confusing -- the physically forwardmost bank are cylinders #5-#8. Do you mean cyl #1, cyl #5, or cyl #8?
I replaced the old NGK BP6ES to Champion N9YC. The reason was my small town store didn't have the NGK. Sorry Steve, front bank, closest to the distributor. Cylinder #5 I set all the adjustment screws out 3 turns per the manual. However, now that you mention it, I don't believe I could hear any difference as I was turning the screw
I hate to agree with this because it does not bode well..... but, yes. If you are lucky, only the valve guides / seals are leaking on that cylinder. If the fouling is from oil ? If it were a "fuel rich" condition, you should be having problems affecting more than one cylinder and/or poor running conditions. If you are very lucky, just the rings (on that cylinder) are stuck = Italian tune up
The plug is sooty and black. cleans off easily with a brush. The plug is not wet Driving symptoms: - I can feel the engine missing then firing and missing again at high revs/speeds - At idle, the engine rpms go up and down slightly I like the Italian tune up fix
engine has been rebuilt 10000 miles ago. new rings, valves, guides and seals were replaced as part of the rebuild.
Why? On the carb cars, the A/F ratio at idle of each cylinder is independently adjustable, and even if the A/F ratio isn't correct at idle, carbs tend to be self-correcting at higher RPM/airflow (so a single cylinder being a little wonky at idle can be not so noticeable overall). That's a reasonable place to start, but it's important that at warm idle you are able to intentionally cause a lean misfire at each cylinder by closing the mixture screw and then bring that cylinder back into idle operation as you slowly open the mixture (and then usually add a bit more richness by opening a small fixed rotation amount beyond that). Not being able to do that is a clear sign that something is wrong, but checking for that behavior at the bad cylinder and a good cylinder for comparison is where I'd start -- JMO. Good hunting!
I suppose I am just going with the odds / assumption that it would be more likely to be a general air or fuel delivery issue affecting more than one cylinder and / or present at varying (not just idle) conditions --- but, you are right of course, it could be specific to a cylinder. And even a totally dead (not firing at all) single cylinder is hard to notice at throttle if the other 7 are working just fine. With EFI + electronic ignition (each cylinder fully independent), it is more common to have a single bad firing cylinder..... on carbed engines, it is less common. And, if only a single cylinder (carbed engine) is acting up, it more often than not relates to a mechanical sealing / leaking problem on that specific cylinder. That said, probability does not equal certainty . .
You might have a plug wire issue. Check the plug wire resistance from the inside cap terminal to the end. Sometimes the pointy set screw doesn't quite make contact with the conductor.
This is telling you the mixture is to rich. A setting of 3 turns is not the correct mixture setting it is simply a setting that has a high probability a cold engine will start and run reasonable well. Most manuals tell you to set the idle mixture screws to a specific co reading once the engine has warmed up. Generally I like to adjust each idle mixture screw to maximize the idle speed. Note if the mixture is a fair bit off you may have to set the idle speed several times before you are done. To high of an idle speed can cause issues getting the correct mixture because the progressive circuit starts to kick in. Cheers Jim
Thank you for all the help guys, I really hope its a carb issue. I will prove that there is spark and continue to adjust the carbs per the workshop manual and help here. Failing that, a leak down test is in order. I have to go out of town for a few days and will have Thursday/Friday to work out these issues. Will report back on my findings.
The reason I went with mixture is it was implied to idle mixture was never dialed in correctly and the sooty spark plug imply combustion is occurring. Rereading the driving description I missed that the miss came back at higher RPM. That would indicate your assessment is correct, it is most likely an ignition problem. Cheers Jim
How does the inside of the distributor cap (post for that cylinder) look ? Yes, this is a common problem --- can be tedious to get this right sometimes.
Could also be a plug wire arcing to ground. Very common with the extenders used on 308s. Check carefully for a burn through. Image Unavailable, Please Login
I did inspect the cap and cleaned all the contact points. Also installed a new rotor. I will check again though. I'm out of town til Wednesday, will look on Thursday and post a pic
I purchased one of those in line spark testers that connects to the spark plug wire . I got a good steady spark on the wire and also through the resister. I compared it to the other wires and the spark was the same intensity all around. I then when to the carburation. Geez, I forgot how much fun it was to remove the air box and the nuts around all the trumpets. Any way, with my flow sync meter I discovered that the carbs settings ranged from 3 all the way to 7 or 8 on the syncrometer. Also the turnbuckle that marries the front and rear bank was out of whack. It was causing idling and sync issues. Disconnecting it, I adjusted the carbs and attempted to sync them up to an equal setting. One thing that struck me odd is that 6 out of eight, I had the air mixture closed, while the other two I opened about 2 turns to bring all 8 into sync. They are all setting at #4 on the sync scale except 1 carb at cylinder 8 is at the #5 setting and I can't bring it down any further. The good news is that cylinder #5 that was causing me grief from the start is running strong now. The carbs are still not 100% as my test drive proved some popping through the top of the carbs on occasion. But it is not fouling that plug any longer I will take it to a proper mechanic to set the timing and have him look at my carb settings. Might as well change the cam belts too . lol Thanks to all for your help
that's good news - you mentioned earlier that the engine had some fairly extensive work not long ago. as this implies significant mileage, were the carbs rebuilt at that time? given the symptoms and cure you've described, this might be an area to explore further. - r
