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Thread: Backfiring Going Downhill

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    Yoda tahoe healey's Avatar
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    Backfiring Going Downhill

    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).
    Last edited by Editor_Reid; 11-21-2020 at 10:30 PM.

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    Yoda steveg's Avatar
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    Re: Backfiring goin down hill

    Too lean.
    First I'd check for a loose joint between exhaust manifold and downpipes.
    Steve Gerow
    Altadena, CA, USA
    1959 AH BN6 2-seat roadster / 1974 TR6 Emerald Green
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    Yoda tahoe healey's Avatar
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    Re: Backfiring goin down hill

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

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    Re: Backfiring goin down hill

    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.
    Rob Glasgow

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    Yoda
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    Re: Backfiring goin down hill

    Quote Originally Posted by Rob Glasgow View Post
    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.

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    Re: Backfiring goin down hill

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

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    Yoda tahoe healey's Avatar
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    Re: Backfiring goin down hill

    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?

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    Re: Backfiring goin down hill

    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.
    Steve Gerow
    Altadena, CA, USA
    1959 AH BN6 2-seat roadster / 1974 TR6 Emerald Green
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    Re: Backfiring goin down hill

    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.

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    Re: Backfiring goin down hill

    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

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    Re: Backfiring goin down hill

    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.
    Steve Gerow
    Altadena, CA, USA
    1959 AH BN6 2-seat roadster / 1974 TR6 Emerald Green
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    Re: Backfiring goin down hill

    Lean backfires thru carbs, rich backfires thru exhaust.
    Larry K
    58 Jag 3.4 MK 1 auto under restoration, 57 Jag 3.4 MK1 manual (parts car),
    03 Cooper S, 2011 Cooper S Countryman, 2011 Land Rover LR4 HSE LUX, 1964 Valiant V200
    Also had , 68 Cortina 1600E, 64 Spit 4 & 80 1500, 73 GT6 3, 71 XJ6, 79 XJ6, 86 XJS V-12, 53 XK120 OTS.

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    Re: Backfiring goin down hill

    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.

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    Yoda
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    Re: Backfiring goin down hill

    Quote Originally Posted by tahoe healey View Post
    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).

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    Re: Backfiring goin down hill

    TH, I sent you a PM
    1964 BJ8 phase II

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    Yoda tahoe healey's Avatar
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    Re: Backfiring Going Downhill

    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.

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    Re: Backfiring Going Downhill

    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.

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    Obi Wan LarryK's Avatar
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    Re: Backfiring Going Downhill

    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.
    Larry K
    58 Jag 3.4 MK 1 auto under restoration, 57 Jag 3.4 MK1 manual (parts car),
    03 Cooper S, 2011 Cooper S Countryman, 2011 Land Rover LR4 HSE LUX, 1964 Valiant V200
    Also had , 68 Cortina 1600E, 64 Spit 4 & 80 1500, 73 GT6 3, 71 XJ6, 79 XJ6, 86 XJS V-12, 53 XK120 OTS.

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    Jedi Knight Healey Nut's Avatar
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    Re: Backfiring Going Downhill

    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 .
    "If it aint broke ....dont fix it "
    " Thats not an oil leak ..........its a special automatic British rustproofing system "
    Best Healey in show ABCD Ottawa 2013
    Best Healey in Show Boot n Bonnett Kingston 2013

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    Luke Skywalker
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    Re: Backfiring Going Downhill

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



    Bob

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