Engine noise part 2


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By Professor Longroof - 16 Years Ago
OK folks, let's try this again. After this past weekend, a mechanic friend and I have determined the noise from my engine is not a bent pushrod (see earlier post). We did a compression test on all 8 cylinders. (readings were between 90 and 115 on all).

 The problem is a steady "popping" (burping?) coming out of the carb. When you place your hand over the opening it muffles the sound. Could this be a timing issue?

By Ol'ford nut - 16 Years Ago
Pull out the spark plugs (noting which cylinder they go to) and take a look at them. It would not be the first time I have seen a cracked insulator from installing the plugs. You will also be able to see how each is firing to see if one plug is a different color. You no doubt installed new plug wires but check them anyway, looking that the connectors are on the ends. You may want to start the engine in a dark location (night) and you can verify that you have no shorts in the wires.
By crenwelge - 16 Years Ago
A hairline crack in the distributor cap could cause it to hold moisture and cause something like this. Are you sure the wires are correct. There are some combinations of crossing wires that will cause this also.
By Speedbump - 16 Years Ago
The sound you discribe is the classic sound of a broken valve spring.  It can be masked by a compression test because the engine is turning so slowly the valve, even with a broken spring, has time to close.  A visual inspection will usually spot the culprit but it sounds like you've been in there and maybe missed it.  If you have a conventional ignition, you can carefully pull each plug wire with the engine running and find which one changes the sound.  Then you've isolated the offending cylinder.  Use good insulated plug pliers as the "plug bite" is something I don't care to feel again.  As an afterthought, this could also be a sticking valve or anything else that allows the valve to close at cranking speed but fail to do so at running rpm.
By Tom Compton - 16 Years Ago
Exhaust cam lobe could be worn down.
By Ol'ford nut - 16 Years Ago
  "If you have a conventional ignition, you can carefully pull each plug wire with the engine running and find which one changes the sound.  Then you've isolated the offending cylinder.  Use good insulated plug pliers as the "plug bite" is something I don't care to feel again."
 

This is the best place to start. That way you will know which cylinder is causing the noise. Then you can pull the valve cover and check.