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MG MGB Technical - Mostly Weber Questions about the new B

The 77 US spec B roadster that I just got last week has a Weber dual choke downdraft (32/36) on it. In addition, it has an exhaust header and aftermarket cat. I have a few questions about it.

I did look in the archives for these questions, but the info I was able to find was from 5 to 8 years ago, so I thought I would check for current opinions.

1) The Weber has the common rectangular air cleaner (about 4 x 6 x 1-1/2 inches). The filter element is constructed like a K&N filter with a metal screen on each side of the fabric. I see no manufacturer’s markings or numbers on it. Should I treat it like it is a K&N? That is, should I clean it with K&N cleaner, and oil it with K&N oil?

2) The intake manifold has an aluminum tube running straight through its base below the carburetor. Is this a water-heated manifold? If so, was the intent to plumb the old hot-water choke hoses to it, and what are the opinions on the pros and cons of doing so? Although the diameter is a bit small, its location makes it tempting to run the heater hose through it so that it would heat in cold weather and not in hot weather.

3) Can the sector that fastens to the Weber throttle shaft (the one the throttle cable attaches to) be removed, rotated a bit, and reinstalled for an angle more favorable to achieving full throttle?

4) Not Weber related, but above and rearward of the brake booster there is what appears to be an electrically operated vacuum switch. It is connected electrically (at least at the end I see) but there are no hoses attached. I tried to find it in the wiring diagram, but since I don’t know what it is, it is kind of hard to find on the diagram. Could it be related to an anti-run on system that has gone missing in action? Anyone know what it is?

Thanks,
Charley
C R Huff

That is the filter that comes standard with the Weber for our application. I have always treated it like a K & N and had no problems. Sounds like your intake does have a heated manifold. Some people use it, some don't. If yours doesnt leak, go for it. As for the throttle shaft position, I did not move the throttle shaft but I did modify the throttle plate that holds the cable in place. I did not have smooth movement with mine so I drilled a hole in that plate and moved my cable to the right slightly. I have only had to do that on one car. Hop this helps.
Bryan

Thanks Bryan,

It makes sense that it would be the original filter because it all looks pretty new.

In my case, for the throttle cable, I may need to drill an new hole below the original. But, that may require that I weld another piece to it to get it low enough if the sector doesn't adjust.

Charley
C R Huff

Charley,

Filters such as you describe are available from BAP auto parts and, as Bryan notes, can be treated like a K&N. You do, however, want to ensure that you have one with the 90 degree elbow fitting to attach to the front cover of the tappet chest. If not, you will find some minor oil leaks associated with running a "road draft" conformation. I tried the road draft took when I installed the Weber on my 79 LE to get it through emissions testing. Did not work as well as my daughter's 77B with the tube running to the air cleaner. We have this system on three cars and have had no problems with it over a ten year period.

The manifold you probably have is the TWM intake manifold which has a square extension cast into the manifold below the carb with an aluminum tube running through it. Water flow through this tube helps to heat the intake manifold and provides more accurate mixture control. With out it, the cooler, outboard, portion of the intake manifold will tend to condense fuel out of the mixture, collecting in droplets at the base of the manifold and into the arms. When the throttle is closed, resulting in a higher manifold vacuum, these droplets are drawn into the cylinder head resulting in a richer mixture. Several people have had their Pearce (sp?) manifolds, which do not have the water heater as standard, modified to include the water heater so as to improve driving efficiency.

If you decide to use the water heater feature, plumb a hose from the water choke tube, at the rear of the cylinder head, to the rear of the intake manifold. Then, run a hose from the front of the manifold to a three way connector at the lower radiator hose. The lower connection of the T goes to the radiator hose, the middle connection to the manifold, and the upper connection to the heater hose. An alternative is to install the three way connection at the rear, running the hose from the heater and the rear of the cylinder head into it, thence into the intake manifold and use a single hose forwards. Both ways seem to work, here in Arizona, but the former may provide slightly more cabin heat in cold weather areas.

The throttle linkage mechanism may be adjusted. But, there are at least two different versions of the throttle linkage. The more common is an L shaped bracket that is attached to the carb by two of the carb to intake nuts. This is the factory "universal" kit and can be modified as necessary. This is, most probably, what you have as it is the most commonly available system.

The unidentified item on the top cover of the pedal box is the "Valve, vacuum advance", item number 59 on the "Emission Controls" page of the Moss catalog (page 20 in the catalog I am looking at). Its purpose was to allow the vacuum advance feature on the distributor to function only in fourth gear. This was an emissions related item. All of my cars have the vacuum advance plumbed directly between the opening at the base of the carb and the vacuum advance can. They pass emissions testing, every year, when plumbed in such a manner. In my opinion, it is a feature which is not necessary.

Ask if you have further questions.

Les
Les Bengtson

Les,

Thanks for the great info. I will indeed hook up the manifold heater, because these days efficiency is what it is all about. But, it looks like I already have both a blocked off inlet and blocked off outlet left over from the old choke heat. So, it seems that I could hook up the manifold heat without adding any Ts. Am I missing something here? And, yes, I think you are right that it is a TWM. At least it has three letters embossed on it (that I can't remember at the moment).

I do have the inlet at the bottom of the air cleaner housing for connection to the tappet chest. I was wondering if this was sufficient, or if I should get a PCV valve and go straight to the manifold, which has at least one unused, untapped boss on it. Opinions on this?

Yup, I have the L-shaped bracket bolted carb, but it is bolted to the top section of the carb and not to the base. Maybe that is why it won't go to full throttle? It would have to be holding the cable lower to get full throttle.

Thanks for the info on the vacuum switch. I will leave it disconnected, as it is now.

The air cleaner element looks nearly new, as does the carb, so I will clean and oil it with my K&N kit.

I got to drive the car for the first time today, just back and forth in front of the hanger, but it was fun to see it go under it's own power. And hey, no rust. How cool is that?

Charley


C R Huff

Charley. Let us see if I can manage to cover all your topics.

. But, it looks like I already have both a blocked off inlet and blocked off outlet left over from the old choke heat.

Do not know what this means. Several years ago, I wrote a monograph, later published in MGB Driver magazine, about how to make a blanking plate to fit over the hole in the rear of the cylinder head. Sometime later, Moss Motors offered such a blanking plate in their catalog. If one is not to use the water take off this is a good thing. When we purchased my daughter's 77B, the water take off was closed off by a short length of hose with a bolt clamped into the end. Not a good solution and what prompted me to write the original article on using a blanking plate.

But, in your case, and in ours, there is a better solution and that is to use a "top hat" piece in place of the factory 3/8" rod and connect it to the TWM intake manifold.

Please contact me, off line, through my website, www.cutompistols.com/ and I will provide you with one of my "top hat conversions" for the factory piece.

As to the connection between the air cleaner and the front tappet chest cover, my experience is that is sufficient. We have used that system on Theresa's car for ten years, on my 79LE for eight years, and on my 79 Roadster (My Roadster) for two years. No problems have been noted. Problems were noted when I followed the advise to use an air filter without the elbow connection and run a piece of tubing down as a "road draft tube". The MG factory decided that the road draft tube was insufficent back in the 1960's. My own experience demonstrated the wisdom they displayed. Keep the tube hooked up to the air filter and minimal, if any, oil leaks will happen.

The "base" of the carb is the lower, enlarged section. Not below the base of the carb. The Weber Universal Throttle Kit (from memory, of the designation) fits on top of the "base" of the carb and is held down by two of the bolts which hold the base of the carb to the intake manifold. Thus, it fits on top of the base of the carb and does not interfere with the seal at the actual bottom of the carb.

I use the basic DGV carb, which is a manual choke model, on my daughter's car and both of mine. I had to cold form the bracket, outwards, to not interfere with the manual choke cable. Do what you need to do. I have not had time to make up some form of "ultimate bracket" for my own, test, use.

The Weber DGV seems to wear the throttle cable more than the SU carbs do. I purchase a bike hand brake cable, which has a "slug" on each end, and make up two "Carb cables" from each one. One for the trunk and one for the garage. A very good idea for either the SU carb system or the Weber carb system. I have broken throttle cables with both. And, having a spare in the trunk, have replaced both in a matter of minutes.

If you have questions of general interest, post them here. If you have specific questions, which may not be worth posting here, contact me through my website.

Les
Les Bengtson

This thread was discussed between 17/07/2008 and 20/07/2008

MG MGB Technical index

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