V12 Weber Conversion

After several years of straightening out the car (1972 coupe) I have finally been able to run the 6 Webers without killing myself with exhaust gasoline. The car was purchased with a spun main directly blamed on the flooding carbs. Much has been done including using the original block but top ended with the Mays heads from an 89 Xjs.
Original configuration had the Webers with the little mushroom filters. Pretty but absurd! We all know you can barbecue under the hood and so imagine that air being sucked in!
After several variations I built an intake chamber that is sealed on top of the carburetors! This was obligatory as I experienced a couple of fires from flooding carbs and the snake pit of ignition wires in valley.
The sealing was also to have a polycarbonate cover that contoured the bonnet bulge. The chamber is fed fresh air from the top of the radiator space.
These mods followed the removal of the distributor and installed Electromotive electronic ignition similar in principle to Ray Livingston’s 6 cylinder kit.
The crankcase is vented to the above chamber using the distributor opening in the valley. The carbs require shorter idle jets as well as cutting the screwdriver slot adjustments for a hex head, the valve covers interfere.
Float adjustments on these carbs required removal of the tops and I got tired of removal and redesigned the lids to permit float control externally. This makes for a dead nuts setting because you can drop a tiny float that sticks out of the openings where the main jets protrude. This beats the bending float adjustments that you can’t see the results when it’s reassembled.
Most critical to all this is the installation of oxy sensors on each exhaust bank and coupled to air fuel ratio gages. No way in God’s creation your eyes and ears can tweak 12 main and idle jets! Not to mention the myriad of choices for each jet. The details of this probably require a textbook but if the desire and perseverance is there, it can be done.
A tangential teaser is that during all this, my rear main started leaking. After 2 attempts of doing it in place I uncovered a method of doing it, contrary to the man, Stew Jones, who knows these machines cold.
I’ll try to put up some shots of the conversion


The usual reason for a rear main leak is that the PCV system isn’t functioning as intended. Since you have $%#$%ed up the intakes big time, how does your PCV system work?

Downdraught Webers on a V12 look magnificent! I have my eye on an old set we have in the workshop which once ran a race boat. I have this plan to put them on an XJS.

How about some pictures!

Couldn’t figure how to post the shots

Use a 6 cyl bonnet (deeper bulge) on subtly raised hinges and wings (washers are your friend) and the engine dropped as far as you can manage. Every millimeter counts, and when you tweak dimensions in several places (slotted engine mounts etc.) you can find you have generated a centimeter or two of added clearance, which is like gold dust for V12 Webers.

There is a system of six DCOEs on long horizontal runners out to the sides but I’ve only seen it on E-types and I agree with your eye-candy assessment.

As I stated, the opening in the valley where the dizzy used to be is the vent point into the chamber which gets suction from the 6 carbs. I’d photo it but apparently I’m to challenged to post them!

The seal was trash as it had shrunk away from the butt contact. I designed a seal out of Viton which takes temp limits to 400F. The added benefit is the better slickness similar to Teflon. I had installed one fresh when I did the rebuild and that followed up with at least one other attempt with a sneaky Pete to no avail.

IOW, you’ve connected the crankcase to the air filter housing? That’s not suction, at least not at idle.

I guess I don’t really understand this. You replaced the rope seal with something made of Viton?

It would be a lot clearer if I had some grey matter to figure out posting a pic. Email me azlabsnc@gmail.com and I’ll send pics
Sorry about the obfuscation, but yes the rope seal was replaced by viton and after several drives no sign of oil, knock on wood!!!

Cute trick! Where did you get a Viton seal that would fit? Did you have to have the crank out to install it?

In the past we have discussed using the Ford 460 seal, which is a rope seal similar to the OEM except that it is graphite-impregnated and a bit longer so it needs to be trimmed to fit.

I just recently posted my first pic here. It’s really easy. Just open up your File Manager and click-and-drag a photo file from there to the message you’re posting.

About mid way alone the top of the reply window there is an upward pointing arrow, click on that and you can download a picture from your collection.

A few years back we heard a sob story from someone with Webers on their V12. It had grown into a huge debacle, possibly partly because the engine also had high-performance cams in it. The cams, having long duration, caused a certain amount of backflow up through the Webers at idle. When air came down through the Webers, it would draw some fuel in with it. When the piston came back up with the intake valve still open and pushed air back up the intake through the Webers, it’d suck in some more fuel. And when the next intake came around, it’d suck this cloud back in, adding yet more fuel. The end result was that the car ran very rich at idle, and it was a fire hazard – there was a cloud of fuel hovering above the intakes when it was running.

Holy Rubber Bands Batman!!! :open_mouth:

Details please, for all of us V12 owners. This has to be the biggest bugaboo for any V12 using the rope seal. Even with a properly working PCV system and the rope seal installed correctly with the factory tool, it eventually leaks. In my case that was after only about 3000 miles.

The sneaky Pete method using something made of Viton that doesn’t leak sounds like a godsend.

To post a picture just hit the icon at the top of the reply box that has an arrow pointing up and then browse to select your picture file.

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The pics have:
Polycarbonate cover on and off
Forward view of air filter canister courtesy of 1990 Toyota Cressida but not showing the gooseneck that sits above the radiator
The limited space to the idle jet because of the cam cover
The modified idle jet
A shot of the Chinese knockoffs with the nice imperfections of 4 breakthroughs on the main jets circuit. Not enough casting metal. Otherwise would have been a great deal, 6 for under $500!
Chrysler AC instead of the monster that would interfere with my chamber.

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The design addresses the crappy rope seal that I hear is not unique to the jags. I’ve spent many hours prototyping and the final iteration (simply?) requires dropping the oil pan and loosening the crank caps to get the crank to drop about 40 thousandths; about as much as it can with everything else bolted on the crank. This provides a little more room for drawing it up and a squish when bolted back. I’m reluctant to say more only because I’m looking into the patentable possibility.
I wish I could detail further but when Stew Jones says in place repairs are impossible, I realize that this saves a ton of $ and time. I have thought of collaboration with a local who could perform the repair under confidentially. The reality is that because of the viton property it may well outlast the engine leak free.


Local? Where are you located?

I hope you can get through your patent process soon, I and probably others have been waiting a long time for something like this.

Best of luck to you!

sounds very simple , amazing that NO one else has done it?
i have replaced many rope main seals on many,many types of engines!
far as i know nobody has ever complained of leaks!

the Jag V12 rope seal seems to be a major problem engine, also most seem to leak after a run setting like overnight(lot of guys put a drip pan under them).
i use a vacuum in the case ,connected to the PCV piping and when running it doesnt leak , but right after shut off it seems to drain the oil from the seal area?

i,m guessin that a piece of Viton just may be the trick, after shut down it may tighten up the space between the crank and slot it fits into? or maybe a slot cut into the main cap to drain back into the pan.
just sounds too simple, DAMIFINO?

I think you will find that trying to patent a seal–which is not a novel idea–to be…challenging.

Wiggy, Ron
I’m certain there’s thousands of patents but the question remains why hasn’t it been a solution to rope seals? I can confirm it’s a whole lot easier working an elastomer than a fibrous structure. It’s just surprising that rope seals aren’t obsolete? Or, are they still installed in new vehicles?