'B Vacuum advance problem

[ATE399J]

Setting up the timing I've found a problem with the vacuum advance. One usually sets it up at idle with the advance disconnected and then re-connecting (at idle) advances the timing very slightly.

Having set it up as above re-connecting the vacuum advances the timing about 20°. Is this normal for a 'B? Also the rpm rises so it's running at about 2000 rpm. Backing off the throttle stops makes no difference as they are no longer controlling the idle speed.

I've changed the distributor as I though that it was that which was causing the problem but it's exactly the same with the new one.

Any ideas?

[MaidstoneSports]

Hi,

I'd be looking for air leaks on the induction side.

[Mr2Mike]

ATE399J said:

Setting up the timing I've found a problem with the vacuum advance. One usually sets it up at idle with the advance disconnected and then re-connecting (at idle) advances the timing very slightly.

Having set it up as above re-connecting the vacuum advances the timing about 20°. Is this normal for a 'B? Also the rpm rises so it's running at about 2000 rpm. Backing off the throttle stops makes no difference as they are no longer controlling the idle speed.

I've changed the distributor as I though that it was that which was causing the problem but it's exactly the same with the new one.

Any ideas?

It's quite normal to see an increase in ignition advance and idle speed when connecting the vacuum capsule, your problem seems to be primarily that you are unable to drop the idle back to a reasonable value.

Common causes of this is that the throttle linkage that connects the two carbs is badly adjusted, or you have a damaged throttle cable (frayed inner) or insufficient slack in the cable which is preventing the butterflies from closing fully.

SU's fitted to later MGBs have an emissions control device on the butterfly, which is effectively a small vacuum relief valve which prevents high manifold vacuum on the overrun. These are known to cause problems, and many owners remove the spring and solder the valve closed.

Leaks from a worn butterfly spindle can also cause idle problems, and is fairly common on old, well used SU's.

It's also possible that a carb has a bent butterfly spindle or the butterfly is not properly centered within the carb body, though this is less likely unless the carbs have received the attentions of an unskilled DIYer in the past.