Why does it go "Pop!"

[thebartman]

Took the car out for a couple of short runs as a shakedown after some recent work.

Upon return, I do what I always have done - which is to leave it idling for about 3-5 mins just to let everything settle down before turning it off.

After I turn off the ignition, I can now count about 5 seconds, and there is a little "Pop" just like a small backfire.

This also happens even when the car has been running on the drive and getting up to temp.

Could someone explain why this happens ? - and if it is OK to leave as is, or is there another opportunity to fix something before it breaks ?

 

PS - Turning the stereo up doesn't fix the problem 😉 

 

[CarBuff]

Our friend's '85 produces this shutoff POP as well. Especially after I drive it...

 

For that matter, so does our snowblower...

[lotus-62]

• I had the same thing on a NA engine after I had wraped the exhaust in heat wrap, I had a high idle so after turning the engine off it still sucked in some air/fuel mixture, this then went into the hot exhaust manifold and 2 sec after turning it off BANG, (but only on a hot engine)  I lowered the (too) high idle back to 850 and this (for me) solved the problem.  
•  

[thebartman]

Interesting  - I too have an idle nearer 1,000 rpm, so will look at this.

Presumably it's not doing any damage to anything?

[910Esprit]

As mentioned, its almost certainly unburned fuel igniting in he exhaust.  But to do that, something needs to be pretty hot.  I'd probably double check that your ignition timing is to spec and your mixture is not too lean.   Maybe also check that spark plugs are correct grades?

[thebartman]

Thanks @910Esprit - looks like time to research setting carbs up properly. 👍 

[jonwat]

3 hours ago, thebartman said:

Presumably it's not doing any damage to anything?

It's a little explosion in the exhaust system so eventually it will blow a hole in a weak spot. 

[CarBuff]

5 hours ago, jonwat said:

It's a little explosion in the exhaust system so eventually it will blow a hole in a weak spot. 

So you are saying, Lotus planned it that way to advise owners of a dodgy exhaust?😆

 

[thebartman]

All pretty sorted now, so here's an update.

After getting carbs balanced first (Carbtune), and then locating a Colortune (14mm kit on loan) to set the mixture properly on all four carbs, the popping on overun has gone, the idle speed and tickover is sooo much smoother, and there is a more smooth progression through all the revs in all gears, and the smell of petrol has gone - as carbs 1&2 were running rich. #3 was fine, and #4 was lean. 

[jonwat]

2 hours ago, thebartman said:

then locating a Colortune (14mm kit on loan) to set the mixture properly on all four carbs, 

I have an old & well used but complete Colortune that you're welcome to for the cost of the postage, pm me if you're interested 

 

[thebartman]

Thanks @jonwat, tried to PM you, but system says that you can't receive messages  🤔

[jonwat]

2 hours ago, thebartman said:

Thanks @jonwat, tried to PM you, but system says that you can't receive messages  🤔

Curious 🤔

My email is crazyjonwatkins@gmail.com 

[thebartman]

👍