Diagnostic Help Needed

smurph

smurph

Senior Member
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
 
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
 
Thanks for your responses. I don't think I have a timing gear tooth/teeth problem. Car timed up really well and engine runs well.

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.
 
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.
 
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.
 
29er said:
Can you isolate the source of the tapping noise using a mechanics stethoscope or a wooden dowel held to your ear?
Click to expand...
...Difficult to do if the sound is only when the engine is under load! :D... Like others I too would put leaking gasket as primary suspect- which may also explain weak mix.
 
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
 
johnbuckley said:
...Difficult to do if the sound is only when the engine is under load! :D... Like others I too would put leaking gasket as primary suspect- which may also explain weak mix.
Click to expand...


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.

:D
 
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...
 
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 .
 
Mitch//pa said:
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...
Click to expand...

I like the idea Mitch has.
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:
29er said:
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. :D
Click to expand...
Ah - our two intrepretations of what is meant by "acceleration" are different:) - I assumed smurph used "acceleration" to mean increasing of the speed of the car - you presumably interepret his use of the word to mean increasing the speed of the engine (whilst the car is stationary). Isn't English language fun!
PS neat idea of the ATF Mitch - not come across that one before.
 
Last edited:
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.
 
You must log in or register to reply here.
Back
Top