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Kicked off the island..
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3,062 Posts
The rod bearings are brand new.. but they were installed by the previous owner's (a doctor) mechanic. So, i honestly dont know the quality of the installation. But the sound seems to be coming from the top end of the motor.. and is specific to certain rpms and load on the engine. Hence, it's nice and loud a 3/4 to WOT.
 

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Kicked off the island..
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3,062 Posts
Spark plugs are fine. I'm going to hypothesize on this one... the car was purchased from a guy who overheated the engine, his mechanic pulled the old head, milled the block and put a brand new head on it.. i'm thinking it may have been possible that he milled the head a little much?? BUT, the problem has progressively gotten worse.. any ideas responses to such a hypothesis :?: :?:
 

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Mufasa in Training
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35,145 Posts
Timing definitely isn't off. It's been a progressive problem.
Now that I think about it (at 4am :roll: ), since the Miata is non-interference, you couldn't have valves hitting pistons without valve float or overrevs...
 

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Kicked off the island..
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3,062 Posts
Ok, so i'm starting to swing more towards a rod bearing given that the noise seems to be coming more from inside the engine than just beneath the valve cover. Is it possible that there could be slack where the top of the rod connects to the bottom of the piston? I say this just because it doesnt sound like the noise is in the very bottom of the motor.
 

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Kicked off the island..
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3,062 Posts
I'm pretty convinced that's what it is especially after reading the "buying a used miata" post on miatanet that states: "Once the engine is warmed up, grab the throttle valve and rev the engine slowly up, listening for knocking sounds that come on at a certain rpm levels and fade away. Early signs of rod and main bearing wear will show up here." And this is exactly what is happening. I just tried to void it as a last option because the guy I bought it from said it had brand new rod bearings. I actually dropped him an email about it to see if his mechanic would check it out. He responded: "How much do you want for the car with the bad engine?" :evil:
 
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