loud pinging under acceleration intermittantly

[123-jn]

I have been having a rattle as I accelerate for a while now but only now and then usually after I have braked and gone back on the loud pedal. If I immediately take my foot off the gas then put it back on sometimes twice it will go away and normal service is resumed. I finally decided to look at the distributor and started by running with the vacuum disconnected and plugged. In this mode although acceleration is more sedate the rattle is almost not there, I say this as I have heard in once but a much less vicious sound than usual. I decided to strip the dizzy expecting sticky weights or broken vacuum etc I have found that only 1 spring is present on the larger of the advance weights no sign of a spring on the other although the design means that the one spring acts on both. So should my distributor have two springs ? Is this causing too much advance too soon? With the vacuum on top this would explain the rattling but why intermittant? Any help would be appreciated. Dizzy number is 025905205 Q Bosch Z415

[123-jn]

I've just been playing with the timing light. I set the timing to 7degrees btc with vac disconnected, as I advance the throttle to 2400 rpm the mechanical advance seems to advance by about 30 degrees, this seems a bit much, if I plug the vacuum in while looking at the marks with the strobe the timing immediately advances about 30 degrees at tick over!!! This is not right I'm sure what does anyone think?

[kevtherev]

No that's not right.

https://club8090.co.uk/forum/viewtopic.php?f=37&t=129595" onclick="window.open(this.href);return false;

[123-jn]

Thanks Kev, that is almost exactly what mine does in that thread without the pinking that I get, I guess the maximum depression in the inlet must be under hard acceleration so at tickover I shouldn't have and extra 20 degrees of advance caused by the vacuum as soon as I connect it. In that thread someone says just a couple of degrees on vacuum connection would be about right. I've checked the carb and hole in throttle looks fine so I have to assume that the advance is not working correctly. So I suppose it's a new dizzy and hope for the best? As it stands with the vacuum connected if I rev up to 2000 rpm the timing mark moves over 50 degrees this would perhaps cause a nasty pre ignition noise maybe? If mechanical advance is max 25 degrees at 3800rpm and vacuum is max 16 degrees 41 degrees should be the max possible advance at 3800 rpm?
Thanks for the link

[123-jn]

I have now looked at an article about ignition timing and have read that it can be quite normal for the ignition timing to advance at tickover to around initial setting + 15 or so degrees= 23 degrees in my case as the vacuum is high with a closed throttle. This apparently helps the fuel burn cleaner at low speeds. The article says that at high load with the throttle fully open vacuum reduces and timing retards to mechanical advance level only. I sort of get it!! when cruising with half throttle vacuum should be relatively high and advance is mechanical plus part vacuum. The vacuum advance is apparently there to improve fuel economy and smoothness when cruising and at tickover it allows a weaker mixture? well I'll work on trying to understand all that, either way my dizzy must be sticking or over doing it somehow.

[123-jn]

Fitted new dizzy from heritage, seems ok, slightly lighter than the original. Now the behaviour of this one is quite different. I set the timing to 7 deg btdc and tightened the clamp, upon revving to 2400 rpm this one advances just under 20 degrees. so far so good. Now when I connect the vacuum advance the timing jumps from 7 btdc to about 15 btdc. again so far so good. Now if I rev up to 2400 and hold it there the max advance is just over 20 degrees (in effect the vacuum advance is not doing much with this wider throttle position. If I increase revs to 3500 rpm the advance peaks at about 24 degrees. Previously it was running to almost 50 degrees.
On driving the van it is now much smoother at tickover and through the rev range and so far no mega pinking or even any pinking at all. long may this remain so!! shame Ive still got to replace the rt hand head due to a crack.

[Sir Brixalot]

Glad you got it sorted and thanks for coming back and finishing the thread.

[CovKid]

I found over many years that in the end, readings are great starting point but the ideal is back it off until it just doesn't pink then nip up the nut - job done, I say this because I've come across engines that defy all logic, at least with notches and dials. I once had a fella call me up after he'd had his crypton tuned and it wouldn't run right. I managed to get it bang on just by listening carefully. Sometimes marks don't take into account slight wear or differences in dizzy or carb and you have to find a compromise that works.