Whenever I do a rebuild on this engine, I will probably keep some type of balance tube. It was most likely for emissions for the most part but from reading Tim Engel's detailed posting several years ago on the Yahoo Europa mail list, there appears to be some tuning advantage to having the balance tube in place. He doesn't address the effect of changing the relative position of the tube has on running condition. Also the European Stromberg Elans and Europas have a non emissions manifold with a high balance tube. This is the snippet of that section dated 2/2/2019.
Post by: Esprit2 » 02 Feb 2019 12:58 pm
lotusbzz, The integral Stromberg manifold connects the 1-2 intake ports to one carb, and the 3-4 ports to the other carb. Then the 1-3-4-2 firing order results in each carb receiving two consecutive intake pulses ('sucks') in a row, followed by a two-pulse rest period. Fuel is heavier than air, it has more inertia, and it takes it longer to respond to an intake pulse and get going. As a result, the first port in a pair will receive a relatively lean mixture. By the time the second pulse hits, the fuel will be moving better, so the second port gets a richer mixture than the first port. Then the rest period hits, it all comes to a stop, then starts over again when the next two-pulse hits that carb again. The firing order is 1-3-4-2, and it starts with '1' simply because that's the way our simple minds work. With the engine running continuously, you can jump into the sequence anywhere, and 1-3-4-2-1-3-4-2-1-3-4-2-1-3-4-2 can just as well be... 2-1, 3-4, 2-1, 3-4, 2-1, 3-4, 2-1, 3-4 or..., if L = Lean and R = Rich... L-R, L-R, L-R, L-R... on and on 2-1, 3-4, 2-1, 3-4... When you check the sparkplugs, have you ever noticed that two cylinders consistently appear to be running richer (black & sooty), while the other two appear leaner (cleaner, tan colored)? All the above is why. The balance tube connects the small plenum area that occurs where the two manifold runners join just before mating with the carb. And the balance tube adapter manifold enhances that plenum area a bit. The balance tube allows for a little cross-talking between the 1-2 and 3-4 pairs. Not a 'full flow', but enough vacuum reaches the resting carb from the active carb to keep 'some' flow going in the resting carb. That keeps the heavy fuel flowing a little bit. Not fully up to speed, but also not starting from stopped each time the next pair of intake pulses arrives. The balance tube has to be large enough in diameter to flow enough between the carbs to help, but small enough in volume to be able to reverse directions quickly... almost instantaly. There's complicated black art involved in balancing carb size, engine size (ie, suck pulse strength), plenum volume, balance tube diameter/ length/ volume, tube location (height), and a bunch of stuff. It looks simple, but it's very complex. For the Twin Cam, Lotus' High Balance Tube mounting adaptor seems to be the best. Go ahead, eliminate the mounting adaptor manifold with it's high balance tube. Then notice how crappy the engine runs without it. Try re-tuning the carbs to improve the situation, and you'll find there's always a conflict between the two ports in a pair. Get #2 rich enough to run half-way decent, and #1 is way too rich... fowled-plug rich. Lean out #1 to where it wants to be, and #2 is so lean it barely runs/ burns valves. Same with the 3-4 pair. The Balance Tube Mounting Adaptor doesn't totally eliminate the problem... that's an unrealistic expectation. But it DOES improve the situation to the point that the engine runs fairly well, if not a 'little' rough at idle. There's still a difference in plug colors in each pair... but to a far less degree. * Acceptably manageable with the balance tube adaptor. * Problematic without it.
