Morning gents.

I'm trying the get the Gemini sorted for Stoneleigh, and I'm not currently happy with this problem - basically, after returning from a run i turn the ignition off as normal, but the car runs on for 10/20 secs before stopping. This will occasionally be followed by a backfire.

I'm running bike carbs with Aldon dizzy. It idles and runs well when on the road. It hasnt been set up on an RR (was hoping to before stoneleigh but the garage is booked up) which i know isnt ideal.

What might be causing this?

It was not unusual on old carburettor cars. Heat in the chamber is enough to keep igniting the fuel and keep it running.

Some manufacturers used to run a solenoid valve to bleed a large amount of extra air into the intake when ignition turned off to try and help.

[Edited on 23/4/15 by DW100]

Carbon build up, and/or retarded ignition timing can cause running on.

Retained heat in the carbon can cause combustion to continue, and retarded ignition causes excess heat in the combustion chambers due to inefficient burn, with more burning gasses being exhausted the exhaust valve gets over heated and will continue to ignite mixture post switch off.

ALL later carburettored Xflows were fitted with the "anti run on solenoid" which, as already said bled loads of air into the inlet manifold thus making it too weak to continue running. I once watched an early xflow keep going for over a minute till the guy finally got fed up, stuck in 4th gear & stalled it!

Running weak/too hot/timing out/carbon build up inside, all aggravate it, but you can have an engine in perfect tune & perfect condition & it would still do it in certain circumstances - hence why Ford fitted the solenoid!

[Edited on 23/4/15 by russbost]

Thanks guys - it was the potential weakness that worried me. Dont want to detonate the engine, especially afterall the hard work getting up and running.

I have a lambda gauge ready to plumb in, but havent had a chance to have a bung welded on. Perhpas this wshould be step one. I'll check the timing too, maybe i'm a bit retardedf (almost certainly on personal level) and thats the issue.

desperate to get to Stoneleigh, but trying to let the head rule, not the heart!

when you stay run on, do you mean running as if still at idle (normal running) or does it run rough and then stop?

Does the battery light come one during the above?

Have you made any electrical changes to the car recently?

As already said fairly normal on xflos, they were designed to run on highly leaded 4* fuel. Back in 1970's even before unleadedb was introduced the amount of lead in fuel was being lowered and Ford introduced the anti-run on solenoid valve which was usually just Tee'd into the brake servo vacuum hose. The valve used was the same part as used on MGB's of that era.

[Edited on 23/4/15 by britishtrident]

I had a customer contact me recently saying the same thing, i assumed it was running lean / timing / coked up.

It tuned out the alternator was keeping the ignition live causing it to run on for a short time, this problem only arose after fitted electronic ignition (points before)

But, as pointed out its probably pre ignition, if running bike carbs on standard idle jets its probably way too lean part throttle.