Fuel System - 3000M
System Overview
The 3000M models came with a Weber 38DGAS carburettor, possibly some with a DGMS (manual choke) version. As the engine had the standard V6 Essex inlet manifold, the carburettor was a twin choke version and was non progressive - i.e. both butterflies open at the same time. Each choke roughly fed the cylinder head on its own side - therefore 3 cyliders. The manifold mounting plate for the carb. was oval and not ported to each side, to allow an even vacuum to the whole carb. & therefore reducing any pulsing to a minimum. This also helped with vacuum signal, and general tuning, as any minor adjustment errors would be forgiving due to the smoother airflow from the 6 cylinders.
Insulating Spacer 38DGAS
There is a spacer plate originally made from bakelite - incorporating a W shaped plate underneath to help diffuse the fuel once it had left the carburettor & passed down into the manifold. The main idea of this was to ensure the mixture was slightly biased away from the centre cylinders, as these were the closest to the carburettor. The spacer also had the breather port built into it, to allow a certain amount of engine casing/rocker cover oil fumes to be drawn back into the engine, from today's standard it was a very crude effort at an early EGR system, but better than nothing. The idea was that at maximum vacuum, the engine would draw in any positive crankcase pressure/fumes and prevent them being released directly to the atmosphere. Another reason for the spacer was to insulate the carb from overheating in hot temperatures. On a hot summer day, the temperature in the centre of a V engine can be immense, and any conductive heat from the inlet manifold to the carb could cause the fuel to boil. In fact, this can happen even with the spacer fitted, especially after turning off the engine. A cooling system fault involving the thermostat sticking closed can also cause the fuel to boil, which I've actually heard on one car after the the engine was turned off. A downside to insulating spacers can be carburettor icing, but on the Essex fitted under a TVR bonnet, things usually warm up pretty nicely. Later cars had a winter/summer PCV parted vacuum switch which worked on a wax controlled valve opening - to allow vacuum to open the valve from the air filter, to take hot air from the left hand exhaust manifold until the engine was warmed up.
The spacers are hard to find these days in Bakelite (a form of asbestos based heatproof material, similar to plastic). I've seen more recent versions being made from Nylon & look 3D printed even from the Weber Webcon store. In fact I've made a design myself which is handy as the crank breather port can be omitted if using an oil catch tank/filter system. Nylon is pretty much fuel proof and good to engine temperatures, although I have found personally it can expand/contract quite drastically, unless homogenised after printing using an oven process.
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Inlet Manifold
The standard inlet manifold is a production part, but there are two main types available, and you have to make sure you get the right one for your cylinder heads. The difference is the shape of the ports where they meet with the cylinder heads. Early ones are oval, later ones are arched - or have 'D' shaped ports as commonly known. Most manifolds should be easy to fit without coolant leaks, just make sure you screw down the manifold in the correct order, and carefully keep the bolts turning the same amount to make sure it presses the gasket down evenly.
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Fuel Pump
A mechanical fuel pump was fitted as standard, which fitted to the left hand side of the engine block, above the oil filter. This is a reliable item and should last for years. Many people change over to using an electric fuel pump, to reduce the mechanical load of the engine, and also to make aftermarket modifications when using fuel filters, extra carburettors etc. Many electric fuel pumps will have a higher than standard fuel pressure, and so a fuel pressure regulator would be needed to reduce the pressure down, preventing the carb. float needle valve to be forced open. Something like 3-4.5psi max is all you need, the standard pressure being more like 2.5 - 3psi depending on age of pump & type. Different carbs have different advised max. pressure settings. Be aware if purchasing a new electric pump that firstly it is for low pressure use i.e for carbs - many pumps are approx. max. 7psi for carb use, as injection types can be much higher and way too high. Secondly you'll need a regulator - and a good filter. I use a Malpassi Filter King, which incorporates a cleanable/replaceable filter element, plus a regulator. You'll need a fuel pressure gauge to set this up - some come complete with such a gauge. Fitting of the fuel pump is best right at the back, near the fuel tank, as in faster acceleration it's possible the fuel delivery can suffer if the pump is being sucked from the front (i.e. upstream of the tank). I've read about this and have no proof - ok it may be only an issue with a top fuel drag car but I can see the theory! The idea of a pump at say 7psi is that it can feed higher pressure fuel to the front engine area, where it can then be regulated down to a pressure suitable to be just under the carb's max. fuel pressure, therefore allowing max. fuel delivery to the carb at all times.
Fuel Return
Not all M Series cars had a fuel return (as far as I know), but mine has all the pipework, but the carb is not set up with a return outlet. The fuel return is to prevent over-pressure of the float needle valve, much as the fuel pressure regulator dows on modified cars. Any fuel that's not immediately required to enter the float chamber is fed back in a return pipe back into the fuel tank.
Fuel Tank & Sender
The original fuel tank was a TVR made tank, positioned within the back of the bodyshell, under a carpeted cover. Despite this safety issue, it has the benefit of keeping the outside of the fuel tank in excellent condition, The downside is that the inside can get badly corroded due to heating up/cooling down - being under the huge rear screen. Condensation eventually means the tank will corrode from the inside out. Fuel is picked up from a tube brazed into the tank, and originally is fed to the front of the car with a plastic hose. Changing the hose to a 5/16" (8mm) copper nickle (Kunifer) pipe is a great mod, allows max fuel delivery, no corrosion, and a robust kink free pipe. The downside is you'll need the bodyshell removed to do this properly. The fuel level sender is much like many old Austin/Leyland fuel tank senders, and is held in place with a bayonet type locking ring, and a cork/rubber gasket. Note the gasket needs to be a good one, otherwise you'll soon smell of petrol after only a few minutes inside the car.
Safety has to be mentioned here. The fuel tank would never pass any health & safety laws today, being inside the vehicle. A rear end shunt is not worth even thinking about...and so it would be wise to consider replacing the tank with an explosion safe version at the very least. I've also seen mods to the chassis which are like an extended chassis loop, that wrap around the fuel tank. The original pieces of 2" x 3" wood that support the back of the car are amazingly all that exists as any sort of strength to the back of the shell. The chassis diff. box area sits in front of the tank, and so any moderate impact will likely result in the tank being crushed. if you look at Peter Wheeler's 5000M you'll see that it had a bulkhead screen behind the driver's seat, which is another option to keep any fuel away from the occupants in the case of an accident.
Fuel Filter
I mentioned earlier the fuel filter in the 38DGAS Carb. That is all that was fitted originally, but due to the tank corrosion issue, plus the fuel filler being horizontal, it's a good idea to add a good quality fuel filter just after the tank outlet - to make sure any debris is caught before it gets any further into the fuel system. The DGAS filter is like a small thimble and it doesn't take a lot to clog. The issue here is that the tank is in the car, and so it would make sense to have the filter somewhere easy to get to, but preferably not within the vehicle. Fitting an extra filter near the tank - pre pump - will ensure nothing gets into anything - pump, regulator, carbs, although there should be a mesh filter at the carb inlet using original DGAS carbs as a final backup, as rubber hoses can still degrade over time.
Fuel
Modern fuel appears to have a much shorter life before degrading than fuel in the 1970's, and can also damage some older components that appear to be unable to cope with some of the chemicals. If you wonder how to tell if fuel has gone off, it's usually pretty easy to smell. It loses its potency in terms of vapour and instead turns more yellow, smells like an old lawmower...in fact that's where you'll first probably come across it....
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For an example of modern fuel damage, I had a Facet shuttle valve type fuel pump which stuck every time the car was left for more than a few weeks. Of course it could have been a faulty pump, but i've found the same to happen on another pump. A new modern version of the same pump has however been fine, pointing towards a design change or new seal materials being used. Viton is normally the special rubber used for fuel pumps, however after trying various viton seal hand spray pumps for brake cleaner, I can vouch for some lasting years, where others last minutes! If you need a brake cleaner hand spray buy one from Wurth, you'll not be disappointed. Not an advert btw, just a tip from experience.
Triple Weber DCNF Conversion
I will import all my data on this mod. from my old website, but generally the job is fairly easy to do. The main consideration factors are:
a) Cost - prices are pretty insane for the manifolds now, and the carburettors too. I had my DCNFs "manufactured" by Webcon in the early noughties 's and they cost £350 each then.
b) Space - for air filters is limited by the small bonnet to carb clearance. A ducted plenum may be the answer.
c) Individual carb choke for each cylinder is optimum for maximum performance (as far as carburettors go), however they need a lot of time in the tuning stage as the pulses from each cylinder at idle and just above mean that hydrocarbons (unburnt fuel) can be a major issue at MOT time.
d) Balancing the carbs - this is essential, and the linkage between them needs to be 100% accurate and remain that way. The throttle spindles are pretty soft and any force (i.e. if the throttle pedal stop isn't set up correctly) can twist the carb spindle(s) and put them out of balance. New spindles aren't cheap or easy to source. I had one of my DCNF spindles bent by an MOT tester years ago using a screwdriver to 'look into the carb' due to a high emission issue. Don't let anyone near them!!!
e) Distributor & vacuum advance- vacuum is not simply possible by tapping in to one manifold branch with DCNFs. Instead you'll need to tap in to all 6, join them all, and take the vacuum reading from the combined cylinders. Without this the vacuum signal will be a strong on/off pulse and make the timing scatter at lower throttle openings. The other option is to fit a mapped ignition setup - I use an Aldon Amethyst - and this can be set up using a throttle position sensor (vacuum option is available). I will be doing a separate section on this in spring 2021, as have yet to fit the throttle pos. sensor. Using a distributor alone with no vacuum advance is possible, however unless using for track use only then low throttle driving i.e. cruising, mild acceleration etc will never be all that good, as the timing advance needs to be increased in this situation, certainly more than a centrifugal only dizzy is going to cater for. Your fuel consumption will suffer, and lower speed driving will be sluggish.
Holley Conversion
Something I've not yet tried - but it is possibly a better option than triple DCNF's simply as there's only one carburettor to tune.
Weber Retroject 38/38 Throttle Body
A new design - replaces the 38DGAS directly with a modern fuel injected throttle body. In addition you'll need an EFI Swirl Pot, plus an ECU for engine management.
Throttle Bodies (using a triple DCNF manifold or tailor made to suit)
Perhaps the ultimate mod - along with this you're going to need a mapped igntion system with crank sensor etc. Ric Wood - https://ricwood.com - is probably the best person to contact if you're considering this.
Rolling Road/Road Tuning
This is essential after fitting a new carburettor or ignition system. It is possible to tune a car on the road, using the correct equipment. I use an Air/Fuel meter with a data logging set up for RPM. I'm just working on a throttle position sensor & knock sensor signal data logging setup for maximum performance potential. The only missing data will be for horsepower & torque, but these can be done at leisure using a rolling road as a final stage. See the 'Engine Tuning' section for more information.