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...

Good Timing

• Adjusted the timing

• Partially fixed misfiring problem
• Gained more engine power (timing was way off)
• Figured out how to set timing
  

Adjusting the timing is another one of those simple miracle cures that can really improve things. Just to name a few, the engine won't sound like a tractor anymore, emissions will improve, fuel economy will improve, no more misfiring, etc. It's also one of the easiest things you can do ... that is if you know what to do.

For a long time I didn't know what to do, and so kept postponing this. I finally took the plunge this weekend and figured out how to check and adjust timing. Sure enough, the timing was off (as if the misfiring hadn't told me that already).

First off, you'll need some equipment. You'll need a timing gun (see image below), a 12 V automotive source for the timing gun (car battery; from another car if you have an MGB), a 12 mm wrench, and some white-out (with a brush works best).


The 1977 MG B specs state that at idle (850 rpm) the timing mark should be at 5 degrees BTDC (before top dead centre), and for 1500 rpm at 10 degrees BTDC. This could be different on your car, so check before doing anything by reading a service manual. On my car I found the instructions on a lable under the hood (where other emissions info such as spark plug gaps etc is found).

Update: If you use a weber (like I do) you'll need to set idle to 14 BTDC. However this differs from car to car. Mine uses a free-flow Peco exhuast, so 14 is correct for me, but if you still have a catalytic converter you will probably need a lower offset.


Finding TDC is not the most obvious thing on my car. One can generally locate it as the 'odd one out' of the teeth that make up the timing alignment. Usually it's slightly bigger, however note that this differs on MGBs (based on the year they were built). The image above shows the situation on my car, the 'teeth' illustrated by #1 and the notch in the front wheel by #2. I used white-out to make them more clear. TDC is - I guessed correctly - the right most tooth. As per the specs of the car, at idle I need to be at 5 degrees BTDC (the 2nd tooth from the right), and at 1500 rpm at 10 degrees (centre tooth). All teeth have a 5 degree separation. On some MGBs these teeth are at the bottom of the engine.

What you do is simply adjust the distributor by turning it left and right till the notch on the wheel lines up with the 5 degree mark on the teeth (at idle). Do this by hooking up the timing gun to spark plug 1, the plug closest to the front of the car (the power cable on mine wasn't long enough to go the the battery, so I had to bring my other car next to it to connect it to a source), and aiming it at the teeth. To turn the distributor loosen the nut holding the sleeve of the distributor to the engine block (#3 in the image below).


And the result is... (idle, 1500 rpm, then idle again)

Note that all this should be done after driving the car around for atleast 5 minutes and everything is at running temperature. The timing marks on my car are on top of the bottom wheel (drive pulley?), but on older models it is often underneath.

Thanks to MG's great battery placement, my timing setup looks like this:
Get a friend to hold the gun.

UPDATE: I'd like to say that the top picture was a joke, but actually I was just being an idiot. A much easier timing gun setup is: connect the ground lead to a bare part of the frame (for example, that thing that sticks out of the engine where the air pump used to be), and the hot lead to the bottom fuse (ONLY TOUCH THE BOTTOM FUSE). The images below (taken with my phone... sorry) should explain.