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.

Carburettor Madness

• Rebuilt the carburettor (Weber DGV)

• Fixed fast idle
• Got normal gas mileage (~23 mpg instead of 7)
• Misfiring and backfiring resolved
  

Diligent readers will have noticed that it's been a while since I've last posted. There are various reasons for this -work, travel, family, etc, I'm writing this in Poland- but the most pressing one would have to be that when I replaced the B's exhaust, as described in the previous post, I killed the car.
It just so happens that the various systems of this B were working together in perfect harmony. I was always aware that the engine was running rich (7mpg is not very good, not even for an old MG), and that the exhaust had a massive gap under the hood and was sending exhaust fumes into the cabin. But I didn't know that the exhaust gap was indicative of a clog further down the line, and the exhaust fumes not being vented were choking the engine, which was compensated for by having a richer fuel mixture. When I replaced the exhaust system the engine was free to breath again, and the excess fuel was burning off as it should. However, this resulted in an engine refusing to go lower than 3000 rpm (and would stay high if you tapped the accelerator), an exhaust manifold turning bright orange, and flames shooting out the carburettor and the muffler (worse than before). My car was shitting itself while throwing up, but still devouring gas. It was bad.

There were several problems with the carburettor after I rebuilt it, mostly linked to the previous owner not having the right settings and overcompensating for the asphyxiated engine. Rebuilding a Weber is quite straight forward (I bought a manual but have yet to read it); essentially just grab a screw driver and pull stuff out. In a next post I will post pictures with descriptions (once again, I'm in Poland right now). Here's what I've learned, and some tips on what to do when rebuilding your carburettor (specifically, the Weber DGV):

• Unless you really need to change everything, don't bother with a rebuild kit. Buy the gaskets separately, and get all the rest from a local hardware store. Rebuild kits are about $35 for stuff you can get for less than $15. Also, in my case the parts were of inferior quality (previously metal parts are now plastic or even paper!). This brings me to ...
• Buy a new power valve. I guarantee you, if it wasn't already broken you've broken it now. A broken power valve is what caused my engine to race and not idle below 3000 rpm (the power valve activates circuits supplying fuel for conditions requiring more fuel than at idle). It's an easy replacement I highly recommend, but is not included in most rebuild kits. It costs about $20.
• If your car comes with a vacuum advance vacuum port on the distributor, keep in mind that ported vacuum from a Weber acts differently than manifold vacuum, the system the distributor was designed for. The engine goes crazy when I connect the vacuum port from the Weber to my stock distributor (the original one with an amplifier in my '77). The engine runs fine without it, so I'm leaving it disconnected for now. UPDATE: This does not happen anymore. In fact, connecting the vacuum port to the distributor actually reduced misfiring. I think there were too many other problems at the time I wrote this article. Alternatively, you could buy a different intake manifold (a non-Weber manifold) which has more vacuum manifold ports (you'll see what I mean if you care about vacuum). As I'm replacing most things, I'm replacing the distributor entirely at some point.
  
• Do all fine-tuning before putting the carb back on. On my B the fine-tuning screws face the wrong way, towards the engine, making fine-tuning a pain in the butt. Also, with the carb out you can see what the fast idle screw is doing to the throttle, and how it's placed with respect to the fuel lines of the various circuits. I'll go into more details on this later.
• The float was another source of frustration. Too high, and fuel will spill over the holding tank and go everywhere, which could result in a mini aurora-borealis under the hood. Too low, and the car will never run on it's own. Weber manuals describe correct heights of floats, and I strongly recommend you stick to them. Unfortunately my float did not appear in any manual, and as it was a brass one (not plastic like the new ones) I wanted to keep it. So I just fiddled with it a few times till fuel wasn't overflowing or starving the engine. This took a while, and was very frustrating.
• British fuel pumps are garbage. One minute they're too weak, the next they're so powerful the intake needle won't close and fuel shoots out everywhere. I recommend buying a fuel pressure regulator and setting it for 3.5 psi.
  
• WEAR SAFETY GOGGLES!!!! Flames shooting out the intake almost cost me all my facial hair. I guess I'd recommend at the very least keeping the air filter and cover on while doing any work with an engine running... Also, I had to often remove the fuel line while it still had some pressure, resulting in fuel shooting out. Fuel in your eye can result in blindness! Wear goggles! (Well, atleast while doing stuff with fuel and oils.)
  

The trouble with carburettors is that they are very fine-tuned pieces of equipment. Screws slightly off can result in complete chaos. For example, if your mixture is too lean the engine won't start or stay on at all. Too rich, and you could blow off more than just your eye-brows. In either case, long term usage of bad tuning can result in severe engine damage (slightly off is OK).

General Weber tuning instructions can be found here. These are very good instructions that helped me get things started again. Remember to adjust the timing after this.

Some documents of interest:
Quick trouble shooting guide (did not help me)
Complete Weber installation guide

Note (from below): These files and links are posted for informational purposes only. No profit what so ever is generated from these files. These files can be found everywhere on the internet and are simply collected here for reference purposes.

UPDATE: I have done many more things to the carburettor. Click on the 'weber' label on the right for all updates.