Timing Anomoly

[111Robin]

This car has a 23D4 fitted with an Aldon ignitor. Factory setting is 10 degree. BTDC at 1000rpm, before advance kicks in. Applying the strobe light with dial back turned off, the TDC mark looks bang on the10 degree timing casing mark. However when I try to dial it back to the TDC mark it's too erratic to get a proper reading, I can dial it back to 20 degrees. and it still looks like it could be around the TDC mark as it varies back and forth. Now I know the distributor could do with an overhaul, hence the scatter, but why does the timing look rock steady before I use the dial back ?. The timing marks are notoriously inaccurate so although it looks like it's on 10 before tdc it probably isn't. It idles very nicely, however on light throttle there's an audible popping in the exhaust that continues as the revs rise. I know it is advancing but would this popping suggest it's too advanced or retarded ?. I don't trust the marks on the casing so although it looks like 10 at idle it probably isn't. Is this likely due to the distributor wear and resulting timing scatter ?. I will be sending it to the Distributor Doctor eventually but any pointers on the above beforehand would be great. I don't want to go to this expense only to find it's just the same.

[iain1967s]

Does that Aldon Ignitor generate multiple sparks per firing cycle?

Electronic ignition which does that can confuse induction counters such as RVI tachos, and dial-back timing lights work on the same principle.

[111Robin]

No, it's hall effect with four magnets so just acts as a points set up (it's marketed as Petronix Ignitor in the states). I have the same on other cars without the dial back issue, I know there's play in the distributor shaft hence the timing scatter. I'm more curious to know the cause of the popping, too advanced or retarded ?.

[Oneball]

I have had this problem in the past with the strobe jumping and found it’s the timing light pickup moving around. I put a bit of rubber in the square hole with the plug lead and a spring clamp round the jaws. Sorts it right out.

Does your light have an rpm readout. If it’s not right then it’s just what mine did.

[111Robin]

No rpm readout on mine, I hook up a separate analogue meter to the coil that works fine. So if I can't rely on the strobe, what does the popping indicate ?. It's idling perfectly with no advance but as soon as the revs rise there's a continuous popping. It'll take a rapid throttle opening no problem but slowly increasing revs produces the popping.

[Oneball]

Just ensuring the ignition lead doesn’t move around in the timing light pick up fixes the erratic reading fine it’s worth a try. But your problem sounds like a carb issue to me. Maybe dashpot or dashpot spring or accel pump if you’re running a Weber.

[111Robin]

Just ensuring the ignition lead doesn’t move around in the timing light pick up fixes the erratic reading fine it’s worth a try. But your problem sounds like a carb issue to me. Maybe dashpot or dashpot spring or accel pump if you’re running a Weber.

Unfortunately it's twin Webers with which I have no familiarity aside from setting the mixtures. They have been fully rebuilt. To be honest, on the road it pulls very well and I'm not aware of the popping but I'd still like to resolve it.

[GraemeC]

I'd say popping was fuelling not ignition and if it is only slight could be perfectly normal.

[Oneball]

You have to change jets on a Weber to properly adjust the mixture. The screws only adjust mixture at idle. You can easily check the accelerator pump are working by pulling the one way valves.

[111Robin]

I think the accelerator pumps are ok, it starts from cold very easily after four pumps of the pedal. As soon as the throttle is opened very slightly it starts stumbling so signs are it's running weak, however the plugs are very sooty so must be too rich, very confusing.

[Oneball]

So it’s weak on the idle circuit and rich on the main jets. Get it on a rolling road would be my suggestion

[Fanfaniracing]

I would suggest to work with a wide Band lambda meter.

Had some issue's with my 8 Port recently and the Lambda Sensor helped a lot to isolate the problem...

[timmy201]

The advantage of a Weber is you can fine tune the mixture across the load range by adjusting each part separately

And air fuel ratio gauge is needed, or a good dyno session

[111Robin]

Thanks, having had nothing other than SU carburettors this is a steep learning curve.