So I just bought a 1976 Jag XJ6L with duel carbs that have been converted to webbers. It's smelling really rich and only getting 5.4mpg... It really sucks. Any ideas what this could be?

 

A couple of things maybe:

I will assume you are in the USA, and this is a US spec car ?????. Your sig etc give NO idication where in teh big world you are.

Your a bloody leadfoot haha.
The carbies are just plain jetted wrong.
The SU carbies are a very fuel efficient device, and Webers are really a "racing type" carby, particularly the side draught style.
If it smells rich it probably is, and therein is your problem.
Over fuelling from carby faults (stuck needle/seats etc) are always on the table, but my bet is wrongly jetted for teh engine.

 

Yes. I am in the US. the Jag is a cali car, I'm in Oregon. I'm sorry I'm not super mechanical. and I really don't know much about carbs but i've been reading. What does it mean if they're "jetted" wrong?

 

OK,

All carbies have a jet or 2 depending on the carby and application. these come in various sizes, the bigger the jet the more fuel it can/will pass at any given condition.

Each manufacturer has a base specific set up for engine size, max rpm, number of carbies being used etc, and this will be fine for actually getting the thing running, then fine tuning by guess work, or using an exhaust gas anyliser gets it correct.

The US S2 had a "dual layer" inlet manifold, with 2 sets of butterflies etc, and I have NEVER seen one in the flesh, only pictures, but it looks messy, and if the synchronisation of the 2 sets of butterflies was out of whack, it would run quite badly, my opinion only. Now this may have been removed to do the Weber thing, probably has been removed, so you are back to a standard manifold, which would be a bonus.

I think that you will need to determine why this is overfueling, and experience with carbies will be required, no put down on your ability meant, just that fiddling with Weber carbies (which type??) can be dawnting, even for the experienced.

Many, many years ago I had a rally version of a 1964 Renault R8 Gordini, 1300cc engine, twin side draught Weber DCOE carbies, and 130HP, and it took a lot of time and even more patience to set these up correctly, and keep them that way.

So not knowing what style/type of Weber you have on this car makes it almost impossible to give more than just "broad advice". Maybe you can find someone in your area that is familiar with carbies (old school), and even better if he has Weber knowledge.

 

I also do not know much about Weber carbs but they were once very popular for "performance" cars.

Your problem could be as simple as a punctured float in one of the fuel chambers, or the float needle not seating properly thus causing overfuelling. As far as I know, all auto carburettors including Webers have fuel levels set by floats.

 

As above, setting up multiple carbs can be 'fun'. Supposing that your choke is not stuck out.

To check that you are running rich and not leaking fuel. Look at the colour of your exhaust black and sooty is an indicator.

Now pull a spark plug or two, if they are blackened or wet, that is another indication.

Now check your air filters (if present) they need to be clean and free flowing. If these carbs can not get enough air, then no amount of adjusting/tuning is going to help.

I have had various cars with multiple carbs and it was always a regular issue with them going out of tune.

One of the methods of getting a basic tune for this type of carb can be found here
http://www.carburetion.com/Weber/adjust.htm I am not going to go through all the steps, you can read this for yourself.

Apart from trying for a basic tune, you also need to ensure that the various mixture jets are not worn or incorrect. You can check for correctness by looking at the standard jet no for your model of carb. (someone may have fitted larger 'higher performance' jets).

I am afraid that if you are not greatly familiar with tuning older type sports cars etc then this is a job best given to a specialist. The money you spend on the tune will quickly be offset by the money you save on fuel.

They will also be able to check for wear, and will probably suggest changing the jets before going for a good setup, including balancing the carbs, which I doubt you will be able to do yourself.

I would think that you should have no trouble finding an old muscle car specialist in your country.

All the best,

 

Generally speaking, altering the carburetion spec. of almost any of the old cars would result in much heavier fuel consumption. It may not be well known, but SU not only designed the carburettors, but the inlet manifold too. One of their features was the "hot-spot" where the hot exhaust gasses hit a metal disc, where on the other side of it, the cold inlet fuel/air mixture impinged. This warmed up the inlet considerably, making the necessity for choke enrichment less, and for a shorter period. You could push the choke in a lot quicker, saving some fuel. The choking system on SU carbs. dropped the main jet underneath, making the mixture richer without altering the airflow as allowed by the air-slide and needle assy. It was faster than a water-heated manifold, esp. in winter. The SU carb + manifold were so good, it was used by Rolls Royce. If I were you, I'd restore the carburetion system back to exactly as it was before as made by Jag., and get a tuner to set up the carbs. properly. He'll most likely use a stethoscope to balance them. He'll make sure the dashpots have oil in them and he'll want to see that all ignition timing, spark gaps, plug types, cb points and capacitor are renewed. Steady driving should then make about 15mpg or so. The 1970s Jags. were not exactly fuel-efficient, so don't expect too much!
Leedsman.
nb. Petrol in the UK has now hit £5:50 per gallon IMP.