XJ6 Series 1 Running rich on front three cylinders
 

Recently purchased a lovely 1970 XJ6 Series 1, Need some help on a running issue.
Problem: The front three cylinders are running rich; spark plugs foul up after 50 miles. Back 3 plugs are ok.
Car has WEBER 38 DGAS. Been on the car for a few years. Carbs have been serviced, new gaskets, jets. Plugs leads replaced. Distributor cap replaced. Compression on all cylinders is good between 115 and 121 PSI. When plugs cleaned car runs really well.
I have swapped the carbs around to eliminate the carb and the problem stayed with the front 3 cylinders.
Any ideas?
Thanks Kevin

No idea on the Weber situation.

But the inlet manifold does feed all 6 cylinders, OK, the front carb the front 3 cyls, and the rear carb the rear 3 cyls is the favoured flow.

I will assume a manual choke??????.

If its still using the 3rd carbie for cold start, OR the Lucas AED for enrichnment, and either of them can/will cause rich running of the front 3 cylinders.

The carby swap eliminates th cardies, maybe.

Somethng is overfueling the front section, and the carby at the front is still a suspect in my thinking.

If it runs sweet after a plug clean, for the distance you say, then distributor cap/rotor/leads etc are not really a part of the issue to me.

Grant,

Thanks the car does not have a AED (or at least I dont think it does). With the Weber swap it has 2 seperate manifolds (Pearce Manifolds). That was going to be next step to to take the off to check the gaskets etc.

Thoughts and thanks?

Almost solved
 

The issue(s) was cause by a vacum leak on inlet and exhaust manifolds at the firewall eng. Once the gaskets were replaced plugs no longer getting fouled up. Still have an annoying leak somewhere, but I can drive the car again!!!!!

Now the next on my list is to fix the lovely petrol smell in the car.