308 GT4 on seven (six) cylinders..

I had the same problem with cylinders 1 and 8.

This is a long shot but it turned out to be something quite simple in my case

I replaced all of the black collar rubber seals that go around the lead extensions and that sit over the lip for the plug holes on the heads.

the new black rubber seales were far more elastic than the old ones and over time even though there was no appanrent difference when you looked into the engine bay the new black rubber had actually pushed the extensions fractionally up off the spark plug so there was not a proper spark going to the plug.

For me it just required moving the black rubber seal back long the extension so it was not exerting as much pressure on the extension/ lip.

problem solved

Obvious point but see when you remove the lead form the difficult cylinder will it spark againt the head directly. If it does then your problem has got to be the plug.

If it does not spark the the problem is teh extension, the lead or the distributor. Its an easy process of elimintaion to remove them one by one and check if they spark!

 

Same here.I have replaced leads,plugs,extenders and have gone for a Crane electronic ignition set up.Now I have Iridium plugs and haven't fouled a plug since.I pulled the plugs the other day and found some were sooty.I'll probably have to adjust the A/F mixture on those cylinders.Also what jet sizes are you running?I'm currently running 50 idles,130 mains and 220 emulsion.I have tried various other jet set ups and this seems to work best for me.I'm not sure about the Dinoplex.You probably might want to go with something more modern.

 

What heat range plug are you running now? I'd second maurice's suggestion to try one of the small center electrode (less than 1 mm diameter) technologies (either Platinum or Iridium in the Denso or NGK brands), but, unless you're really pushing the engine for extended time at high output, I think the concensus here is to also go up at least 2 heat ranges from the early stock 308 heat range spec. (My data point is going from the standard 2mm plug technology to the NGK Platinum BP6EVX extended the fouling interval from just several hundred miles to 5~10K miles).

Your description of "carbs cleaned" would imply a very significant amount of carb work/verification and that the Mechanic reported no trouble (or difference) in tweaking up 7 & 8 versus the other cylinders (for example, all mixture screws out roughly the same amounts) -- is this the case?

One thing to check is that the choke function on the 7&8 carb is working properly:

1. that the exterior linkage goes to the full rest position when closed, and

2. that the choke spools inside the carb are well-seated (although if these are "stuck" open, I'd think that your Mechanic would have reported some difference/difficulty in tweaking up those two cylinders mixture-wise).

 

Is this a problem at idle or throughout the rev range?

If at idle, check whether the 7-8 carb is flowing enought air to support combustion. If it is too low, the plugs will simply not fire on the these banks. Try increasing the throttle for the affected cylinders and re-adjusting the mixture.

Bill