Engine ‘Popping’ on overrun

Technical advice Q&A
User avatar

Topic author
AshM
Posts: 413
Joined: Wed Dec 14, 2016 4:59 pm
Location: Edinburgh, Scotland
Great Britain

#1 Engine ‘Popping’ on overrun

Post by AshM » Tue Sep 03, 2019 5:49 pm

So my car is back on the road after an engine rebuild. Delighted, going really nicely and done about 800 miles so far.

It’s a frustration not to give it some stick as it is keen to run. It’s also lovey and quiet and gives the impression of a ‘balanced and calm’ engine at tickover (that may make me sound like a madman but you will know what I mean).

My only small issue is that it is popping slightly on overrun. If using the engine to slow the car you can hear it popping slightly. Not a massive good old fashioned backfire but definitely some late explosions.

Is it simply timing?

As always, value the thoughts of the collective wisdom.

Thanks
Ash
Series 2 FHC 1970
1R 20607

Link:
BBcode:
HTML:
Hide post links
Show post links

User avatar

Heuer
Administrator
Posts: 13515
Joined: Sat Mar 01, 2008 5:29 pm
Location: Nottinghamshire
Great Britain

#2 Re: Engine ‘Popping’ on overrun

Post by Heuer » Tue Sep 03, 2019 7:09 pm

Running too rich?
David Jones
S1 OTS OSB; S1 FHC ODB

Add your E-Type to our World Map: viewtopic.php?f=1&t=1810

Link:
BBcode:
HTML:
Hide post links
Show post links

User avatar

Tbob
Posts: 165
Joined: Mon Aug 10, 2015 1:44 pm
Location: Kenton co, ky
United States of America

#3 Re: Engine ‘Popping’ on overrun

Post by Tbob » Tue Sep 03, 2019 9:01 pm

Check this topic on JL:

https://forums.jag-lovers.com/t/throttl ... lve/352314

I found throttle bypass valves on Stromberg carbs can contribute to this.
Bob t

LHD '69 OTS. (Former) basket case

Link:
BBcode:
HTML:
Hide post links
Show post links

User avatar

abowie
Posts: 3017
Joined: Wed Sep 07, 2011 9:15 pm
Location: Australia
Contact:
Australia

#4 Re: Engine ‘Popping’ on overrun

Post by abowie » Tue Sep 03, 2019 10:18 pm

I would accept that as normal.

And at 800 miles I'd be starting to push the engine a bit too.
Andrew.
881824, 1E21538. 889457..oops. Jezza the V12 XJS race car.
http://www.projectetype.com/index.php/the-blog.html
Adelaide, Australia

Link:
BBcode:
HTML:
Hide post links
Show post links

User avatar

H7OB
Posts: 267
Joined: Wed May 10, 2017 3:42 pm
Location: Warwickshire
Great Britain

#5 Re: Engine ‘Popping’ on overrun

Post by H7OB » Wed Sep 04, 2019 10:41 am

Mine also does a bit of the same. I was concerned it might be too hot/lean but the recent time on the dyno proved otherwise. It might also be an air leak upstream on the exhaust allowing air in and igniting the unburnt fuel. My exhaust seems a tad louder / less restrictive so that might also be contributing but now I know it’s running well and fuelling correctly I quite like it!
:bigrin:
Paul
'68 Series 1.25 2+2

Link:
BBcode:
HTML:
Hide post links
Show post links

User avatar

Topic author
AshM
Posts: 413
Joined: Wed Dec 14, 2016 4:59 pm
Location: Edinburgh, Scotland
Great Britain

#6 Re: Engine ‘Popping’ on overrun

Post by AshM » Wed Sep 04, 2019 1:18 pm

Thanks all for the responses. Appreciated as always.

Will make checks based on ideas.

Paul, your reply made me think. The car is now running with a new ClassicFabs SS exhaust system and I wonder if that is giving me different noises to what I have been used to?

Thanks again.

Ash
Series 2 FHC 1970
1R 20607

Link:
BBcode:
HTML:
Hide post links
Show post links

User avatar

H7OB
Posts: 267
Joined: Wed May 10, 2017 3:42 pm
Location: Warwickshire
Great Britain

#7 Re: Engine ‘Popping’ on overrun

Post by H7OB » Wed Sep 04, 2019 1:53 pm

Hi Ash,
That would sound to be a likely factor. If you were previously mild steel theres also that difference.
Id just say make sure you re-jet / change needles to suit, to ensure its not too lean with the new exhaust.
A good rolling road session is always worth the money. I had just done 1000 mile when I braved it! Mine just runs beautifully now, makes great power & torque and ticks over smoother than anything!
Cheers
Paul
'68 Series 1.25 2+2

Link:
BBcode:
HTML:
Hide post links
Show post links

Post Reply Previous topicNext topic