power loss on full throttle

[jiskar]

Hey guys,

I have a 1981 Mk2 transit with a 2.0L ohc engine with a weber double barrel carb on it. Normally It runs just fine, only when I give it full throttle the engine starts making a loud, low humming noise and the power drops dramatically. The vacuum meter on my dash drops to zero when doing this. It's possible to drive by just never hitting full throttle, but that's kind of annoying when you need to accelerate fast. I'd like to understand what's causing this and fix it.

Here's the history of the problem:
When I bought it some months ago it ran fine (also on full throttle), only it wouldn't idle right (slowed down and stopped every now and then). I had it repaired in a shop: they said they cleaned the carb and adjusted ignition timing and idling speed. After that the idling was fine, but after some time the described problem appeared.

The spark plugs look pretty black, and the exhaust smells like unburnt petrol (it spatters a bit, actually). This got me thinking that the mixture gets too rich when hitting full throttle. I'm not sure how this double barrel carb works exactly, but I read something about the second barrel only opening in high demand situations. Could this second barrel be the problem? Or am I totally thinking the wrong way?

Thanks in advance

[Timbo]

Some Weber twin choke carbs have no direct linkage to the secondary barrel and operate the butterfly by vacuum from the primary barrel. If your carb has a diaphragm connected to the secondary spindle then check the small rubber pipe connected to it for splits and leaks. Also suck on the pipe and make sure the butterfly opens. If it doesn't move or returns quickly it could be a split diaphragm.

If the carb has been off then check the manifold nuts are tight and the gasket is O.K.