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11-16-2014, 09:36 AM | #1 |
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Diagnostic Help Needed
All of a sudden:
1) engine makes a "tap tap tap" noise when under acceleration, but not at idle or at speed-- only under acceleration; 2) on deceleration there are frequent backfires. I assumed a points or timing problem and replaced points, retimed engine. No change. I adjusted the carburetor by leaning out the mixture considerably and got some improvement but the symptoms are still there. I'm down to the following thoughts: 1) Maybe I have water in the gas? I'm going to put in some dry gas and run out the tank. 2) Maybe the carburetor needs a cleaning? I didn't get a lot of response from the gas/air adjusting screw so perhaps there's a clog inside. I still get some "cinnamon " from the gas tank which was "de-rusted" three years ago. I empty the cinnamon from the sediment bowl but perhaps something got through? 3) Maybe the exhaust manifold gasket has developed a leak? I can't see without taking off the heater. Anyone have any thoughts on the matter? Thanks in advance. Smurph |
11-16-2014, 11:12 AM | #2 |
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Re: Diagnostic Help Needed
Just to get the ball rolling: Timing gear tooth/teeth?
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11-16-2014, 11:51 AM | #3 |
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Re: Diagnostic Help Needed
A blown exhaust gasket will give you that sound.
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11-16-2014, 12:14 PM | #4 |
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Re: Diagnostic Help Needed
X2 on exhaust gasket
Bob |
11-16-2014, 12:15 PM | #5 |
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Re: Diagnostic Help Needed
agree with rusted as a possibility.
"frequent backfires" is usually also timing or too lean. Open up GAV. It does sound like some crud is getting into your carb. Pull the plugs and read them for the burn. Even Edlebrock tells you to do this, despite there is a guy on here that says reading plugs is a waste of time
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11-16-2014, 12:33 PM | #6 |
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Re: Diagnostic Help Needed
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I did check and clean the plugs. When I took them out they had a black, wet look. I wasn't sure what to make of that particularly since I had flooded the carb in the course of getting the timing tight. But it did lead me to think the carb might need cleaning. As did the minimal response from the carburetor adjusting screw. Any further thoughts? Thanks. |
11-16-2014, 12:36 PM | #7 |
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Re: Diagnostic Help Needed
Can you isolate the source of the tapping noise using a mechanics stethoscope or a wooden dowel held to your ear?
Using this method, check the exhaust manifold. A blown gasket will produce a tapping noise. An air leak caused by a blown exhaust gasket will lean out the fuel mixture and may result in the backfire you're experiencing. |
11-16-2014, 01:02 PM | #8 |
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Re: Diagnostic Help Needed
Try cleaning out you carb and then think about putting a fuel filter in line after the shut off valve. This has worked for me. I also agree with checking the manafold gasket.
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11-16-2014, 01:07 PM | #9 |
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Re: Diagnostic Help Needed
...Difficult to do if the sound is only when the engine is under load! ... Like others I too would put leaking gasket as primary suspect- which may also explain weak mix.
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11-16-2014, 02:09 PM | #10 |
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Re: Diagnostic Help Needed
are the nuts holding the intake/exhaust snugged up? Also could the muffler clamp at the exhaust be leaking. I have also seen cracks behind the exhaust manifold out of sight.
Bob |
11-16-2014, 04:14 PM | #11 | |
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Re: Diagnostic Help Needed
Quote:
Since the symptom was reported as " a tap tap tap noise while under acceleration" but not while idling or at speed, use of the mechanics stethoscope is not difficult at all. Simply listen as the engine is accelerated. A single person can easily listen while simultaneously adjusting the throttle at the carb. Alternatively, one person can listen while a second person seated in the car revs the engine. Such a test can isolate the source of the sound and help diagnose the trouble. |
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11-16-2014, 04:28 PM | #12 |
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Re: Diagnostic Help Needed
perform the smoke test to check for an exhaust leak....
induce a little ATF into the wiper vacuum port and see if it smokes from around the manifold area. its recommended to do this test outside... |
11-16-2014, 05:49 PM | #13 |
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Re: Diagnostic Help Needed
As JBill stated, I just had the timing gear trouble. Check it out
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11-17-2014, 02:33 AM | #14 |
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Re: Diagnostic Help Needed
Tap tap only under acceleration in a modern engine can mean pinging.
Is it possible to hear this on an A engine ? If so, it can be caused by carbon buildup, overheating , low octane or bad gas, timing too far advanced, etc . |
11-17-2014, 04:54 AM | #15 | |
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Re: Diagnostic Help Needed
Quote:
I would start with a really good check of gaskets. loose nuts, bad connection from manifold to exhaust pipe, or maybe a cracked exhaust manifold. The backfiring sounds like a little air is getting into the system. If you hear the noise at speed, that would be the time when it would have the greatest manifold pressure and make the noise. Hard to tell without hearing for myself. Make sure you look all the way down to the muffler for cracks unless you are sure it is coming from the front of the car. Last edited by Marshall57; 11-17-2014 at 04:56 AM. Reason: reverse words |
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11-17-2014, 07:57 AM | #16 | |
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Re: Diagnostic Help Needed
Quote:
PS neat idea of the ATF Mitch - not come across that one before. Last edited by johnbuckley; 11-17-2014 at 08:18 AM. |
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11-17-2014, 01:49 PM | #17 |
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Re: Diagnostic Help Needed
Thanks for your replies. Just for fun, and to highlight the flexibility of the English language, the "tap, tap, tap" noise occurs both at rest when revving up and at speed when tootling along. Likewise the backfiring on deceleration.
So far the collected wisdom is either #1-the exhaust manifold gasket, or #2- the carburetor needs to be cleaned. I'm going to go after the carb first. Many thanks for your help. |
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