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MG MGB Technical - Vacuum tube

I am getting an MGB 18V rebuilt for my MGA. I have/had a rubber vacuum advance tube and now rebuilt 1500 carbs (H4s?). My engine builder said the vacuum tube connector (what ever it's called, "vacuum line union") on the SU was buggered so he installed one "in the middle of the intake manifold." I haven't seen it yet (it's 75 miles away). He says it should work better.... I don't remember why. I was thinking that the tube located there would pick-up the fuel charge that would be sucked into the distributer advance mechanism. Anything to be concerned about?
Steve Meline

Your engine is under vaccum so it is sucking in air not pushing air out. Your line there should be fine. I presume the line will be no bigger than the copper tube that was stock. My only concern is if the line is too big, it might create it to advance the timing too soon. What problems did you encounter with your rebuilt engine that Sid did to cause it to have to be rebuilt again.
JEFF BECKER

Steve... If you are to use the 1500 distributor and carbs, then the vacum line needs to go from the port on the carb (which your guy said was 'buggered') to the distributor for the advance to work properly, it won't work properly from the manifold.

If you are using the 1500 carbs.. then the needle and springs will not be right for the 1800 B engine and the car will not run right, unless someone knows the right needles and springs to fit the H4 1500 carbs on an 1800 engine (I don't know if the later HS4...63 to 67..needles will work in the H4's). You also need to know the jet diameter in the 1500 carbs.

Your distributor advance curve is also matched to the Compression Ratio and the Camshaft profile of the 1500 engine, which is different than the MGB engine.

Since the A is no longer 'stock'.. I would use the carbs from an MGB (pre 1968) and the matching distributor. It is still in the spirit of a B series engine!

Marty Schmitz-Hertzberg

Can't speak for an A but on an MGB the only difference between carb vacuum and manifold vacuum is at idle - carb gives zero vacuum and additional advance whereas manifold gives near max. As soon as you start to go off idle carb rapidly rises to meet manifold, then over the rest of throttle movement they both gradually reduce. It was moved from carb to manifold as the latter results in less pollution at idle. The down side is that it can make starting a bit more difficult and possibly 'stress' the vacuum capsule diaphragm more causing earlier rupture.

As long as the carb port is on top of the throat it should pickup any more fuel than when on top of the manifold. However HIFs with bottom vacuum ports *do* send fuel onto the vacuum capsule, which rots the diaphragm.

You will surely need the MGB distributor with the MGB engine, and matched carbs as well, I would have thought.
Paul Hunt 2

Jeff, I had only about 13,000 miles on the engine, There was some kind of lubrication failure with scored mains and premature cam wear. You may remember, on anther front, I was dealing with incessant pre-ignition knock. With the head removed we found wet pistons tops and glazed cylinder walls suggesting that the rings never sealed allowing oil into the cylinders. This may have caused knock from dieseling. Lots of speculation without much evidence to explain this combination very well. I am also now using different oil, Valvoline VR1, per the "New Oils..." Thread on the T Series TD TF board, important stuff.

Marty & Paul, Dave at APT sold me some #5 needles described as "Rich MGB needles" which I used on the last engine.

I was using an Aldon 101BR2 distributor. I lost confidence in it with all the pre-ignition knock so I have switched to a Lucas 45D with a pertronix (good price at Northwest British Parts). Anyone need a low milage distributor?
Steve
Steve Meline

This thread was discussed between 13/01/2007 and 15/01/2007

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