Moss Motors, Ltd

I am new to the Forum and am wondering if any of you have installed a Weber two barrel side draft carb to your MGB. I have a 1976 B with the stock Zenith carb and want to convert, but do not have much of an understanding of the vacuum system and what I can disconnect, etc. Any help would be appreciated.

kshields-

The sidedraft Weber typically installed is the 45 DCOE. This carb does Not use a vacuum advance so you will have to disconnect the vacuum advance and remove it on the distributor and then have the distributor "recurved" to regain the correct advance.  Also, the 45 dcoe tends to have a big flat spot off idle and can be boggy at low rpms. If you do alot of around town/ street driving I would go with the old school twin S.U. setup instead. You'll get much better bottom end and tractability. Unless you are really souping up the engine you will most likely not lose anything on the top end either. If you can get your hands on a Dellorto 45 sidedraft carb this would probably be a very hot setup. They have a much better progression circuit and better atomization of the mixture. Also have a much finer idle adjustment. Hard to find though.

I had a weber side draft on my B back in the day and I switched back to my old S.U.s. Was glad I did for the above reasons. It could be I didn't have the carb set up right but I've heard other people have the same comments. The jetting I used was what was recommended for the "B".

Good Luck!

Ask more questions and I'll try to answer.

LCJUTILA
 

I agree that a sidedraft Weber isn't such a good choice for an otherwise stock engine.  I just did a twin SU conversion for a fella and it made a huge difference.  There are considerations however.  You will have to change at least the manifold & downpipe on the exhaust or get a header, Install a choke cable, get the special K&N air filters to clear the brake booster, change the throttle cable and do some plumbing.  Another popular alternative is to install a 1 1/4 SU on the manifold you have.  Probably not quite the performance as the twin setup but much less work and expense.

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72 MGB 80 TR8
Pittsburgh PA

Guys,

I have installed and tuned a number of DCOE 45s on MGBs with great success and no flat spots or bad low end performance. You have to do some digging and find the right tube/choke/jet combinations. Once you get there, the carburetter works just fine and has the added bonus of staying in tune permenantly. I'm not sure why you fellows have had these bad experiences. The linkage, cold start cable, and air cleaners are, in my opinion, non-issues.

Underdog, I'm sure you meant to mention a 1 3/4 inch SU and not a 1 1/4 incher, which wouldn't even fit on the manifold, let alone perform worth a cent. The carb to which you refer is an HIF style, and it don't come cheap these days either. Concerning dual SUs, you will have to install a choke cable and a different throttle cable setup if you change to dual SUs anyway, so that's a wash. You will also need air filters and alternative plumbing for them. A used weber and manifold can be had at a very good price. and a header is a logical step for anyone who is trying to improve the performance of a late B. The stock exhaust manifold and system isn't bad either, if you can find one. In fact, testing has shown the stock manifold and collector style downpipe flows almost as good as some of the header systems out there. However, if you go that route, you'll likely have to replace virtually the entire exhaust system, including gasket rings, brackets,hangers, and clamps. The header can be attatched to the pipe which is already there. 

I think Mr shields has made a wise choice and will enjoy a fine performing MGB.

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Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon

Thanks for the correction bill.  I need to refrain from posting until I've had at least 2 cups of coffee! lol  The one I just did with the twin SUs, I was able to connect into the existing exhaust with the early manifold and downpipe.  I actually had to cut off some of the existing frt exhaust pipe.  I had most of the stuff left over from my car which I was able to turn into cash so that had some infuence on the decission.  I admit to not haveing any personal experiance with the side draft Weber and was baseing my opinion on others horror stories.  Obviously, you have the knowhow to dial one of these in.  I have a good friend that tried to tune a set on a TR250 for 3yrs and finally gave up.  Guess he got tired of "digging" LOL  I do know that the side draft Mikuni works great right out of the box but they are no longer available as a kit so it's a moot point.  Several choices for sure and any of them will run better than the Stromberg with the converter in place.

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72 MGB 80 TR8
Pittsburgh PA

Under,

Yeah, multiple DCOEs can be a pain, though not so much because of synchronization. The problem is usually that people use too big a "choke" . In weber parlance a choke is not the thing you pull on when starting from cold. It refers to the size of the main throat through the bore of the carburetter. The bigger the throat, the slower the air going through it becomes for a given RPM. At some point, the "signal", or velocity of air drawing on the venturis and jets/transition holes drops to the point that fuel precipitates out of the airstream, resulting in lousy fuel distribution and generally poor and sluggish performance. Once the velocity picks up at higher RPM, things get better. Fortunately you can change to a different choke size. That, of course, is why race cars have crappy idles, bad low performance at low speeds, and come on like gangbusters once they are really revving.

We recently rebuilt and reinstalled a triple set of DCOE 40s on a TR-6 at the owner's insistance, during a restoration. They ran AWFUL. It turned out they had 36mm chokes installed. That meant, in effect, six carburetters, each bigger than the intake port and valve each served. Gee, did this have any chance at all of working? We wound up with 26mm chokes. The car is tractable at low speeds, and goes like a shot. I'll leave it to the owner to find out just how fast it will go. He claims to have exceeded 100MPH by a decent margin with the engine still pulling quite strongly.

Weber DCOEs are remarkable carburetters, but must be understood and worked on with patience.

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Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon

Motorbill-

Please post the jetting/choke sizes etc you used on your conversions.

Thanks!

LCJUTILA 

I'm going racing for the weekend with a customer and his vintage TR4. I don't have time to look up the data right now as I have to get home and pack. I'll try to remember on Monday, but I'm not sure it will do much good. We're at five thousand feet here at the BASE of the Rockies, and the jetting would be quite different, as would most everything else that's changeable. Still, I'll try. 

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Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon

I used to run a Weber 45DCOE on my MGB for many years, cos it was easier to put one together than to rebuild the SUs. I was at pretty high elevation too, but this was my setup.

F16 emulsion, 145 main, 200-220 air corrector, 34mm chokes. The pump return valve was blocked to increase the pump shot. Sorry, I can't remember the pump jets, but they were pretty big.

Going to the smaller chokes helped the idle and transient response dramatically, but decreased top end a little. It's unusual to have such a wide spread between main and air corrector sizes, but it seemed to give the best response following many test drives with different combinations.

This was on an early MGB with a Cannon, then Derrington manifolds.

I would not recomend the DCOE on the later engine as the manifold is very short with tight curves and the pump jet will tend to spray on the manifold rather than into the valve chamber.

The best response I had was to run a pair of 45 DCOE carbs on a Special Tuning split manifold. Only one choke of each carb was used, the other had blocked off passages. This gave a straight shot at the head with good pump dispersal. This manifold design was popular with Minis, which required cutting one of the carbs, but was also available for the MGB.

Personally, my suggestion would be to install a pair of SU carbs. They work well with the siamese port head and will fit if you use the K&N tapered air filters.

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Kelvin Dodd
Global Sourcing Engineer
Moss Motors, Ltd.
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Kind of in the same vein. I'm having a lot of trouble with my little 67 Sprite. It has a 1098 (I know it's the wrong motor, the previous owner was a moron, I won't go in to all of that) a DCOE that could probably use some jet size changes. However, I had it running pretty good, when I would get off the gas and turn a corner it would sputter, but I just attributed that to the lack of a gas tank baffle. The bigger issue came shortly after installing new cobalt plug wires and filling up with super unleaded. At first the car had a lot more umpf, then it started really coughing and sputtering when I would accelerate. If I baby the gas pedal, I can get the revs up, but as soon as the clutch comes out the engine just coughs and wants to die. 

 I'm thinking that this is most likely ignition related, and have thought about converting from the dual point mechanical advance Malory to Petronix Flame Thrower, but I would need to add a vacuum port to the intake manifold. We have a michine shop where I work, so drilling and taping a hole isn't a problem, but should I do that, and where on the manifold? 

 

Thought's?

 

Jim 

Jim,

The first thought that comes to mind this morning is that you should certainly diagnose the problem you currently have instead of "throwing money" at it. You need to figure out if you have an ignition problem, or one with the fuel system. Usually, I say that 99% of carburetter problems turn out to actually be ignition, but something you stated early on was that the car sputtered in turns This can often indicate a float/float valve/ float drop problem.

If the ignition system breaks down under load (accelleration), but not otherwise, you could have a high tension ignition circuit failure. Look at your ignition rotor carefully. Check for signs that look like a carbon deposit or "trail". Some of them fail by shorting to ground right down through the rotor's plastic body to the distributor shaft, which is indeed grounded. This is quite hard to see, so substitution is the only sure diagnosis. Fortunately the rotor is about the cheapest thing in the system. The spark will always take the path of least resistance. When you call for increased load, the resistance across the plug gap goes up considerably, as increased cylinder pressures AND increased heat ciontinue. Look also at the distributor cap inside and out. Clean the cap thoroughly and look for signs of "tracking". Tracking looks like a miniature lightning bolt depicted in a light grey against the black of the cap itself. Also make sure that the center carbon button that contacts the rotor is indeed doing so, and is in good shape. Just as a dirty cap can short the spark to ground, so can a dirty or tracked coil nose. Folks tend to forget this one, but it is just as susceptible as is the cap and rotor. Bad coils are rare, though loads of them are replaced. My experience tells me that about one of every twenty coils replaced actually needed it.

I must say though, that the need to coax the engine to rev sounds like a lean condition in the carburetter. Since it was once running well (Was it?) I will assume that it had been jetted correctly. So, there must be something blocking a passage, or a float drop or valve fault. I'd take the carburetter apart and look for obvious dirt or clogged passages. Be CAREFUL! with that baby apart, as there are some small pieces easily overlooked. I'd disassemble it over a clean surface with a "dam" to keep stuff from rolling away.

As stated, eliminate everything in the ignition system first,(Have you replaced the spark plugs? With the correct ones?) then tackle the fuel, unless a quick check reveals that the fuel pump is simply not keeping up. This is a distinct possibiliyy, so check the fuel output volume as well.

Motorbill

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Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon

Hi Bill, 

 

I guess I should have been a little more detailed in my post. The Cap is a new one from Mallory, the rotor too. The wires are new cobalt wires, and the plugs are new Bosch, same as the old ones. The car was running, but could have used some fine tuning. Immediately after the problem started, I looked at the plugs and they we're covered in soot, and since the cap rotor and wires we're of unknown age, I replaced them as well. I had it running tonight and was going to try to set the timing, when I hooked up the light i noticed that it wasn't strobing very steadily, in fact it would go out for seconds at a time. I've done the TDC check and have the rotor pointing at number one with in a few degrees, but the engine doesn't run smooth enough to set it accurately. 

The has been freshly restored, but not the engine. I had it out, but project ran out of money and time so I put the one I had in and ran it. Having said that, the gas tank, fuel pump, pipe and hose are all brand new as well. The fuel pump seems to be putting out quite a lot of fuel, and when connected to the carb will stop pumping, at times so I believe that's a good sign. 

When first took ownership of this car, it didn't run and at that time I found that the passages in the carb were plugged and that was the cause. So the first thing I did this time was to pull all the little plugs off the top of the DOCE and make sure they were clear, they were. But I think that the soot on the plugs suggests that the engine is running too rich? I was thinking that this pointed to a weak or lack of spark.The only things that haven't been changed in the ignition system are the points, and the condenser. I'm a little hesitant to shotgun the points and condenser at this point because it's ~$60 for the Mallory parts, and I already spent $60 on the cap and rotor. A petronix is $240 and that would relieve me of the need to mess with points, and mechanical advance in the future. 

 I did tear down the distributor and verified that the mechanical advance wasn't bound up and that the points were clean. The advance was fine, but the points looked a little carbonized. I cleaned them up with a little 400 grit sand paper and then found that they weren't making a flat contact anymore. When I re-asseblyed the distributor it didn't seem to have an affect good or bad. 

I'm a little lost at this point but you're right. Before I blow some money on a new distributor and coil, I'd like to find the root cause.

Thanks for your advice, I don't take it lightly.

 Jim 

Jim,

It's entirely posssible that your spark plugs, if they are fouled, are gonners. Even when the mixture problem is solved, plugs that have been fuel fouled can break down under load just as anything else in the ignition high voltage area. Bosch spark plugs are, in my experience, one of the worst at this. I hope you didn't splurge on the platinum tipped ones. They are for modern cars which run on a much leaner, less fuel laden charge. In my experience, platinum tipped plugs in a carburetted engine are too sensitive to fuel mixture and load. Try a set of NGKs. Use a BP6ES, but first find your problem. You may have to go further into the carburetters to do so.

The timing light cutting out doesn't always mean anything. It depends on the timing light and on good connections, plus detectable induction from  the wire. I wonder (though doubtfully) if the Cobalt wires somehow inhibit this. Hmmm.

Motorbill

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Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon

Back on the Road! Well, lesson learned, when in doubt step back and think about how this system works again. The ignition condenser is just a capacitor that provides a load for the coil when it's not busy discharging to the spark plugs. So if the coil is getting really hot, and actually constantly discharging itself to ground through the old, shorted out $3 condenser that kept looking past because it just couldn't be that simple, change it anyway, it's 3 bucks and takes 5 minutes. Incidentally, I didn't want to wait for the correct Mallory part to arrive in the mail, so I just went to Napa and asked for a general condenser, which they provided me with, and it worked great. 

So, if your car is running really rough, you can't accelerate, your spark plugs are sooted up, and the problem came on gradually, replace your condenser. 

 Thanks for all the input, and suggestions! 

 

Jim