Backfiring Going Downhill

[tahoe healey]

I recently read somewhere that backfiring going down hill is a sign of too rich or was it a sign of too weak mixture? Anyone know? Car runs well and Colortune shows Bunsen blue on cylinders 2 and 5 (I have two sets).

[steveg]

Too lean.
First I'd check for a loose joint between exhaust manifold and downpipes.

[tahoe healey]

Replaced gaskets on the exhaust pipe to manifold 4 months ago but it didn't make any change.

[Rob Glasgow]

My car has always made a sort of popping sound, off throttle, going downhill. Not a real loud backfire as I've heard other cars. More a pop, pop pop. Exhaust is tight and exhaust pipes shows black soot. No oil burning. Does that sound like too lean? It runs well.

[Bob_Spidell]

My car has always made a sort of popping sound, off throttle, going downhill. Not a real loud backfire as I've heard other cars. More a pop, pop pop. Exhaust is tight and exhaust pipes shows black soot. No oil burning. Does that sound like too lean? It runs well.

My BJ8 does too. My BN2, which I'm pretty sure has a leak-free exhaust system, does a little less.

'Aficionados' of modern cars--V8s in particular--go to great lengths to achieve rumbling/popping on lift. Apparently, the engineers of the Mustang GT350, with the flat plane crankshaft, were very proud of some overrun popping that sounded like machine-gun fire, after you wound it up and lifted (redline for that car was, IIRC, 8,200 or 8,400RPM). My '19 GT has an 'active exhaust,' which can emit substantial explosive noises in some modes.

[Madflyer]

Timing off a bit you have a lot of down hills at LT. RBCC member? Madflyer

[tahoe healey]

Timing is at 15-16 degrees. I don't know what RBCC is so I am not a member. (Grocho Marks said he would not be in any club that would allow him to join)
I just had a moment of clarity (I think) Reno British Car Club right?

[steveg]

I have a test hill up my street.

Experimenting with richness, I found it takes very little lean adjustment (maybe 1/2 turn) to induce mild backfiring going downhill. On my car a bit of a burble seems about right.

[DerekJ]

Popping is not the same as backfiring and is usually the result of raw fuel igniting in the exhaust. Happens more on performance engines as the valves lift higher and are open longer, fuel comes in and flies straight out of the exhaust valve when lifting off. I imagine too rich a mixture could contribute to this.

[mgtf328]

I've experienced this problem coming down hill. It's a pop, pop burbling sound not a backfire. I eventually discovered that the hose from the manifold to the brake servo had broken off at the manifold clamp letting air in when you take your foot off the throttle. Someone trimmed the hose off too short and it eventually gave way. Check all the connections to the manifold to see if they leak including vacuum advance.

Performance cars with turbos backfire on easing up after hard acceleration. It takes a little while for the turbo to slow and the excess fuel is dumped into the exhaust. This a different from the popping sound.

AJ

[steveg]

Tahoe,
Next time, pull the choke out a little. If the sound goes away, it was running lean. You just richened it.

Air leaks of the sort mentioned above would also make it lean.

[LarryK]

Lean backfires thru carbs, rich backfires thru exhaust.

[tahoe healey]

Thanks all. A lot of good information. It is popping. My concern is being accused of causing a fire as I live in the forest in California.

[Bob_Spidell]

Thanks all. A lot of good information. It is popping. My concern is being accused of causing a fire as I live in the forest in California.

Theoretically possible, but not likely (unless you're driving off-road in tall, dry grass). Much more likely is some idiot in an SUV pulling off to the side of the road in tall, dry grass--which I've witnessed--and his catalytic converter igniting the grass (which, fortunately, didn't happen).

[AUSMHLY]

TH, I sent you a PM

[tahoe healey]

Ausmhly, if the message is on zddp, the forum will not let me reply to your PM for some reason. I always add one 4 oz at oil change. One can go crazy with this. If the quantity were that critical, then you would need to add it when you top up your oil???? Probably not necessary to add any as it is only really necessary for break in of new tappets and cam. This has been debated for years on this and other forums with no real resolution but the consensuses seems to be Valvoline is good and has it already in it.

[DAVE WALSH]

AH 3000 same issue. I researched and found a article regarding back firing on the AH. It means your timing is perfect. Long story but he explained it and it made perfect sense.

[LarryK]

Is it real backfire or a residual fuel pop. On decelaration or downshifting a proper tuned carb car can pop. Supercharged and turbo does the same only louder. If the mifflers aren't expanding from backfire, you might be OK. I run a 3.5 Jag with straight pipes and it does sing and pop. The new MINI S for 2011 would do this and people would go into dealer and complain, so dealer detuned to eliminate along with performance and fuel economy. I have a 15% pully on my 03 MINI S and really pops when cool out. Sounds good going thru town.

[Healey Nut]

Pops n burbles are normal on Healeys on overrun be it flat down hill or whenever that’s what makes the classic sound of a Healey .
I recently uploaded a new tuning profile into my Aston to fix the lazy throttle response and one of the things the company that writes the program does is add pops n burble to the program if you want them .
They can make it mild to wild spitting flames from the exhaust on downshifts if that’s what turns your crank .

[Bob Hughes]

Have you checked the manifold to head connections, I had a similar situation and found that some of the nuts need pinching up.



Bob