Possible Pinking at Idle

[ek96]

Hi All

So, after a fellow member on here very kindly sent me a secondhand carb after the small pipe that the vac advance pipe connects to somehow snapped off (thanks again!), I have now discovered that, although the vac advance is working, and I can feel suction from the pipe, it isn't enough to engage the vac advance. So I decided to fit a carb spacer with a vac advance takeoff to use manifold vacuum instead, but using that seems to cause a noise that I believe to be pinking at idle and when revving (unfortunately I can't drive it at the moment as it needs a new slave cylinder). I have tried retarding the ignition, but that only seems to lessen the noise, and no matter how far I turn it, it never goes away, at least, not before the engine dies.

So, a few questions:

1. Is this likely to be pinking that I am hearing? The noise has never happened before
2. If it is pinking, is it caused by the vacuum being pulled from the manifold?
3. If so, and manifold vacuum is deemed unsuitable, what could be the cause of- and solution to- inadequate vacuum from the location on the carb?

Thanks all!

Edd

[philthehill]

The vacuum connection on the carb is the right place for the vacuum advance pipe to be attached too.

If you are using manifold vacuum especially with the carb butterfly closed (idling) you will get an excess of vacuum so advancing the ignition.

What did the car drive like with the vacuum advance pipe connected to the carb as per ex factory?

At idling you will get very little vacuum so the vacuum advance will not move very much if at all.

Driving the car or using a timing advance light at known revs is the only way of determining if the advance both centrifugal and vacuum is working and/or correct.

Phil

[ek96]

Thanks for clarifying, though I thought the vacuum advance is an economy device for when the car is idling?

The car drove alright, but seemed somewhat sluggish, and got up to temperature alarmingly fast and then "bounce" between nearly 100C and 80C a fair bit, which I put down to ignition being too retarded, but advancing ignition didn't seem to cure this.

Unfortunately, I don't have a surefire of knowing revs, but I attempted my own test by asking my (less than willing) girlfriend to hold the accelerator about half open then, with a timing light, asking her to release the pedal quickly and watch for a "blip" in the timing, from the vacuum advance engaging before the mech advance disengages.

Though I appreciate this is a crude way of testing, but there doesn't seem to be a change in engine note when disconnecting the vac advance pipe (and, of course, blocking the hole) when I thought there would be?

[philthehill]

Have a read of this wiki article especially the paragraph relating to the vacuum advance.

https://en.wikipedia.org/wiki/Ignition_timing

As regards the timing of the Minor:-

Set the static timing.

Disconnect the vacuum advance.

Set the revs at the specific rpm (in wksp man) and with the timing light see what the timing advance is at those revs.

Increase the revs to the next specified rpm and with the timing light see what the timing advance is at those revs.

Reconnect the vacuum advance and see what the increase in timing advance now registers.

All the timing advance @ specific rpm details are in the BMC wksp manual.

Phil

[paul 300358]

Has the carb got the same jet and needle as the original carb?

[RobThomas]

Manifold vacuum is very different from the vacuum you get at the correct point just by the throttle butterfly. Wrong port = very wrong vacuum at idle.

[ek96]

That Wiki article was very informative, so thank you very much for that!

Unfortunately I don't have a tachometer so gauging the revs is a bit of a challenge, but I'm hoping to borrow one soon.

Hello again Paul! Yes, the needle and jet are the same as the old ones

Perhaps I am barking up the wrong tree and expecting too much... it certainly wouldn't be the first time! I'll confirm more once I've got my hands on a tachometer.

Thanks again!

[RobThomas]

Strange. When I added my post there weren't any from Phil already up, only the original first post. How odd.