Carburettor Madness Part III (bright orange exhaust)

• Further fine-tuning of the Weber DGV carburettor
• Adjusted timing at the same time
• Removed air-intake heating

• Exhaust manifold does not glow orange anymore
• No more vapour-lock

There are many sceptics on the performance of the Weber DGV (in fact, only hours after posting this I already got a nasty comment, see the comments section for this entry). It's often called inefficient, not performance-driven, needlessly complicated. Many 'other' MG owners swear by their SU's. I think I've even heard of someone liking their Zenith!

Well, they're all wrong. Completely wrong, in fact. The Weber DGV is a fantastic carburettor. It was not specifically designed for this car, so odds of it working right out of the box are often tiny (versus an SU that would probably work right away). However, online retailers are selling kits specifically tuned for your MG (Moss, Brittek, etc., see links on right) that make tuning much easier. You may even be able to take your MG for a spin the same day you install it!

For others, such as myself, who bought an MG with a Weber already fitted, you may find yourself in a bit of a bind, trying to figure out how to undo what others did. My Weber was set to over-rich, compensating for bad plugs and a clogged exhaust. My horrible rebuild attempt did nothing to improve the situation; throw in the new Peco exhaust and you've got a recipe for disaster I only overcame yesterday, about 4 months later (on and off, mind you).

That's where the internet comes in handy: through fora and blogs such as this one, we try to make owning an MG pleasant for other people so they don't have to deal with the same non-sense we had to. (Unlike Jaguar owners, I might add, who for whatever reason are part of some tightly knit club. How do I know this? Stay tuned...) So when in trouble, my recommendation is to search the internet.

Update 3/11/2010: OK, so I was being a bit harsh on the Jag people. Though I still think they are a very different breed to MG people, most are very willing to help newcomers. Being a Jag/MG person myself I am always ready to help a fellow Brit-car enthusiast.

My main point today is that it is crucial to set the timing right (about 14 BTDC for a Weber DGV) to make everything come together. Bad timing could be one cause of misfiring or backfiring (where the explosion happens too late, and ends up in the exhaust manifold). The mixture on my DGV was set properly, but the exhaust manifold glowed orange. Checking the timing, I noticed it was set to 5 BTDC. Resetting it to 14 fixed the exhaust problem.

Another problem I've been having lately is that after running for a while and stopping, the car would not start up again. All signs pointed to not enough fuel reaching the carb. The pump was working and the filter was full (one of those stupid clear Lucas filters actually came in handy for once), but the fuel kept evaporating when it hit the bowl. Without manifold vacuum that vapour was going nowhere. My solution was to get rid of the hoses used for intake heating (plugged them up with a bolt), which dramatically reduced the temperature in the bowl so fuel would stay liquid. The intake heater is a fuel-economy thing that I do not think really necessary. I may attach it again in the heart of winter.

So:

Bright red exhaust manifold?

• Mixture too rich
• Timing too retarded (advance to 10-14 BTDC)
  

Misfiring?

• Mixture too lean
• Timing too advanced (retard to 10-14 BTDC)

It's that simple. Please see my other posts (click 'carburettor' on the right) on fine-tuning the Weber DGV.

Also I passed my qualifier exam!

Wait, what's this??? :
More very soon...

Carburettor Madness Part II (pictures this time)

• Carburettor (Weber DGV) fine-tuning
• How to adjust the idle speed screw, the long way (when all else fails)
  

• Actually got to drive around the neighbourhood for once, reliably.
• Managed 60 mph, easily (couldn't go over 30 before...)
• Fixed rough idle
  

Last time I discussed some general things concerning Weber DGV's and MGB's. I did some more fine-tuning today and learned that the 1.5 turns out on the idle mixture, as suggested by the webpage I discussed last time (by the way, still the best Weber DGV tuning page I've found) was not enough. I'm at about 2.25 turns out now which seems to be necessary to keep things running well (possibly due to the Peco header). The idle mixture screw is the one on the bottom of the carb, closest to the intake manifold (in an image further below I show where the idle mixture screw leads to). It's not the easiest screw to get to, but with some persistence you can get to it.

Much more frustrating is the idle speed screw, which is conveniently located in a completely inaccessible position while the carb is mounted (this may have been fixed on newer Webers). I found another screw next to the choke that links some stuff to that screw, and so in essence you can directly adjust the idle speed screw from there. I haven't tested the long-term effects of this, and I could be completely wrong, so if you try this be careful. The screw I'm talking about is shown in the picture below. Note that it is not directly attached to the choke, and as far as I can tell only affects how far the choke opens, but has a larger impact on the throttle plate.


My carb was so far out of wack I had to visually inspect the idle speed screw and the location of the throttle. When you turn the carb upside-down and look in the shafts you should be able to see three holes (open the throttle; see image below). These holes are outlets of various fuel circuits, and are activated based on how open the throttle is (interestingly, note that the secondary shaft only has two holes which are only activated when you're really flooring the accelerator; not shown).


I adjusted the idle speed screw directly linked to the throttle (so, not the screw near the choke as described above) so that the right hole, the 'highest' of the three holes, is just covered. The image below shows the right hole just peeking out, and thus how you should not have things. Make sure you cover that hole entirely. Put the whole thing back together, and make fine adjustments from the screw near the choke.


In a previous post I called the Peco exhaust noisy and unpleasant, leaving me with a ringing in my ear afterwards. Having properly adjusted the carb the sound, though still rather rough, was pleasant in the sort of roar one expects from a performance vehicle. Idle was very quiet, full throttle ear-deafening, in between pleasantly throaty. I bought this exhaust for it's performance aspects (which I have already noted - it works), but if you're looking for a comfortable drive I wouldn't recommend this.

New exhaust, new woes

• Installed new complete Peco exhaust system
  

• No more exhaust fumes inside the cabin
• Engine can breath freely
  

I plan on eventually turning this car into a real performance car, so the first step is always to put in a decent exhaust. The previous exhaust had bulged open at the end of the exhaust header and was letting exhaust fumes into the cabin, which led me to believe that it may be clogged somewhere (which is not surprising if you consider how much time this car has spent under a tarp over the past few years). So I decided to pull it and replace the whole thing.The exhaust header, though rusted, was still in great shape. I thought of just sanding it down and repainting it, but had a hard time finding head pipes to connect it to a new exhaust. The rest of the exhaust was in good shape (barely any rust), but seemed clogged. In the end I bought a new Peco header (see image above) with the rest of the exhaust system, a 2-inch straight through system (and a big FU to the EPA). Note that in Maryland classic cars are exempt from state emissions testing, and so removal of the catalytic converter (if there is one, mine did not have one) is legal. This is not the case in California, where pretty much any exhaust or emissions modification is illegal. So check your state regulations to make sure it is as cool as Maryland :-) (if you're in a southern state or Michigan, you're good to go).

Update: It has been brought to my attention that removing the catalytic converter is illegal by federal law. Though Maryland is not going to check (assuming you have historic tags), you would apparently still be breaking the law if you remove the converter. A car must always retain its originally installed emissions regulation equipment, even if it fails (!). So draw your own conclusions...

There are several considerations when purchasing an exhaust system, but it generally boils down to whether you want a comfortable or a sporty ride. My Peco system is incredibly loud, and after a short drive my ears were ringing (this will probably improve when I put in sound insulation). It only has the one straight-through muffler, so not really ideal for comfortable riding. If you're looking for a comfortable ride you'll need a decent muffler or a 2-muffler setup (more common on the MGB). If you're a tree-hugger you'll also want to throw in a cat convertor.

When picking a header one should pick one with the least amount of curves, thus improving the flow of gasses. More angles mean a slower flow, though this is generally a very modest decrease. On the MGB there are only 3 exhaust ports for a 4 cylinder car, which seems wrong. This was a conscious choice by the engineers to save manufacturing costs, but was designed in such a way that with it shouldn't impede exhaust fumes too much, however, efficient expulsion of those fumes is very important. The centre two exhaust valves are connected (the so-called 'siamese' system) and go out through one pipe, so naturally that pipe should be larger than the other two (which each serve one valve respectively). The Peco header has a larger centre port which merges with the other two towards the end of the pipe, thus enabling a better flow of exhaust fumes. There are MGB engine upgrades available that allow for four intakes on one side and four exhausts on the other side of the engine (but be prepared for some machining).
A major issue when picking exhaust systems for the MGB is picking the right setup. Over the years the exhaust and intake system changed many times, so a 1980 MGB will have a completely different setup to a 1965 MGB (and I mean radically different). Later models have catalytic converters (under the hood, not in the back) and different intakes. I would strongly recommend retrofitting your car to the old-fashioned exhaust system as I did, but you will also most likely need to change your carburettor and intake manifold for this. This allows for a better freer flow, and will bring your engine closer to the original 98 hp it once had. Consider the Weber DGV (downdraft) if you do this conversion.

The exhaust arrived and I set out to install it, which is a very simple procedure in principle. As it differs on most cars I'm not going to give detailed instructions. Rather, just take a look at your old exhaust and figure out a way of putting the new one in. Don't know what to do? Just start taking it out, and make sure the new exhaust system fits snugly and doesn't move. If it is designed for the MGB it will fit, you just have to keep trying. Here are some tools you will need:

• An electric saw (to saw through pipes and bolts that won't give way)
• A wrench set
• Ramps !!! (I used jack stands and a floor jack, which I do not recommend)
• WD-40
• Muffler clamps (to hold together the pieces of the exhaust system; typically not included)
• A friend to hold stuff
  
• Patience. Lots and lots of patience. This took me 2 days and was incredibly frustrating.
  

Note that on some exhaust systems, as was the case on mine, the intake header has to be in place before you put in the exhaust header. Use a mallet (or a hammer if you get fed up like I did) to get everything in place. The Peco seemed to touch the rear suspension, but that was because I had the pipes in the wrong order, so be mindful of that.
Now, I was planning on posting a video showing the before and after sounds. Unfortunately my carburettor is acting up, and the damn car won't rev lower than 3000rpm. As I had the intake out anyway I thought I'd rebuild the carburettor. Apparently I did something wrong. As soon as I figure out what I did wrong I'll post it and my solution. In the meanwhile, if anyone has any suggestions....