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Old 07-14-2019, 11:55 PM   #1
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69 BB Backfire through exhaust

Hi All,

I have a 69 BB that was running fine with the exception that when the throttle was blipped and it idled down, the car would stall.

Turns out the car was running lean and the mixtures were adjusted to richen it up a tad using a gas analyzer.

As soon as I got the car back I noticed loud noises in the rear and hesitation under partial and full throttle, most likely backfire (after fire) out the exhaust. I think there was a backfire out the carburetor also but no where as loud as out the exhaust.

I have taken the car back to the engine tuner to have a look at, what should he be doing (I am not an expert but I want to make sure I ask a few questions to ensure the problem is sorted)?

I think he mentioned increasing primary jet size? When this is done does the car need the timing adjusted again?

The car has an aftermarket cam and does not run vacuum advance.

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Old 07-15-2019, 11:23 AM   #2
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Under normal conditions there is usually some fuel that isn't completely burned in the exhaust. The first half of a backfire out the exhaust is caused when this fuel gets ignited. The second half of the backfire is oxygen, this unburnt fuel has to have oxygen to ignite.

If running lean there is more air than fuel in the mix, the engine will burn all the fuel and expend left over oxygen into the exhaust. Don't overlook the possibility of a vacuum leak, very typical on these cars and can wreak havoc on the way the engine runs.

Ritchen the mix and you compensate for the leak but only at low engine rpm. They then usually run like crap until you get enough rpm to overcome the leak.

Just an old man's $0.02
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