Backfire!

[72BUG]

Hi guys.

My 2ltr Aircooled (LPG) has recently developed a backfire. Strange thing is it's running sweet as a nut and I had the gas setup checked at Gasure only last week so I know the mixture is spot on. It hasn't been messed with so the timings ok. Strangely it seems to happen when braking. Not immediately after lifting off the accelerator but actually as the revs are decreasing whilst slowing down.

Any ideas what could be causing it?

I have recently discovered a small hole in the silencer. Could this be a contributing factor???

Cheers

Martin

[albertramsbottom]

Hi guys.

My 2ltr Aircooled (LPG) has recently developed a backfire. Strange thing is it's running sweet as a nut and I had the gas setup checked at Gasure only last week so I know the mixture is spot on. It hasn't been messed with so the timings ok. Strangely it seems to happen when braking. Not immediately after lifting off the accelerator but actually as the revs are decreasing whilst slowing down.

Any ideas what could be causing it?

I have recently discovered a small hole in the silencer. Could this be a contributing factor???

Cheers

Martin

Yes holes in the silencer can cause popping and backfires.

Backfiring as you describe is called on overrun and is normally caused by holes in exhaust system or by leaking pipes air getting in to the vacume system

[72BUG]

Thanks Al.

Better borrow mi mates welder then.

[Peepshow Paddy]

A-ha! That'll be why mine is backfiring as the revs are decreasing too then, it's definitely noisy too so reckon there must be a hole in mine somewhere too. Will investigate - cheers!

[Red Westie]

Ayup matey.....sounds like there are a few things going on here.
Something I teach in class: Combustion/fire needs three ellements to be present....oxygen/fuel and ignition source...take any one of the three away and combustion cannot take place.
Now applying this to your problem:
1) Hole in zorst.....allows air (oxygen in) applying the brakes also allows air into the inlet manifold so more oxygen through the engine and into the exhaust.
2) A certain amount of unburnt fuel (HC) in the exhaust fumes, not all fuel is burnt during the combustion process and my guess is Steve sets them on the rich side so they don't burn valves etc. LPG does run alot cleaner than petrol anyway so isn't an issue for emissions.
3) Heat..enough in the exhaust manifold to ignite the unburnt fue/oxygen.Also...on engines that burn a bit of oil...hot glowing carbon build up on the back of the valves.

Take one away....easiest would be to seal the hole in the exhaust. (which will remove most of the oxygen.

Martin

[72BUG]

Ayup matey.....sounds like there are a few things going on here.
Something I teach in class: Combustion/fire needs three ellements to be present....oxygen/fuel and ignition source...take any one of the three away and combustion cannot take place.
Now applying this to your problem:
1) Hole in zorst.....allows air (oxygen in) applying the brakes also allows air into the inlet manifold so more oxygen through the engine and into the exhaust.
2) A certain amount of unburnt fuel (HC) in the exhaust fumes, not all fuel is burnt during the combustion process and my guess is Steve sets them on the rich side so they don't burn valves etc. LPG does run alot cleaner than petrol anyway so isn't an issue for emissions.
3) Heat..enough in the exhaust manifold to ignite the unburnt fue/oxygen.Also...on engines that burn a bit of oil...hot glowing carbon build up on the back of the valves.

Take one away....easiest would be to seal the hole in the exhaust. (which will remove most of the oxygen.

Martin

Cheers Mart.

So to paraphrase, what you're saying is "Bung t'hole up".

[kevtherev]

Do WBXers have wasted spark Martin?

[Red Westie]

No wasted spark.....just heat and the presence of enough oxygen and unburnt fuel to cause popping/backfiring.

Martin

[a1winchester]

2) A certain amount of unburnt fuel (HC) in the exhaust fumes, not all fuel is burnt during the combustion process and my guess is Steve sets them on the rich side so they don't burn valves etc. LPG does run alot cleaner than petrol anyway so isn't an issue for emissions.

Take one away....easiest would be to seal the hole in the exhaust. (which will remove most of the oxygen.

Martin

If I remember my chemistry lessons correctly (and it was a loooong time ago!) Carbon Monoxide will burn with Oxygen to produce Carbon Dioxide and a bit of a bang! Now, we all know that combustion engines produce lots of luvverly Carbon Monoxide. So could that be the (partially unburnt?) fuel in the exhaust fumes I wonder?

Whatever.... bung the hole up, Martin!

[iancc]

Hi,

I've got a small popping / backfire when I lift off the accelerator after 'hard(ish)' accelerating (on a 1.9 petrol/watercooled). Never did it before but just had it MOted and its now started. Sound like a hole? Or could it be something else. Everything else is running well.

I read in Haynes that its OK to put a rag up the exhaust and run the engine to look for leaks? Is this OK??

If is is a hole, how do I fill it?? If its smallish, is there any thing I can use to do it myself or would it need welding?

Thanks,

[Mocki]

chances are your mot man had to have a furtle about to pass the test, and leaned down your mixture a tad.....

[72BUG]

You can fill holes in the exhaust with Gun Gum exhaust filler but in my experience it's pretty much a waste of time and falls out after a few days.

Welding may be possible depending on where holes are and if the metal is thick enough to weld to, but the only real solution is to replace the exhaust.