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Old 01-07-2010, 01:26 AM
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Need your help diagnosing this problem

I am experiencing an issue with my 1950 Chevy Sport Coupe that has me stumped. Here is the history: I was driving when I heard a loud pop come from the engine. I let off the gas and immediately started hearing a grinding noise coming from under my feet. Additionally, it felt like something was slapping the floorboard from underneath. Lastly, I began to notice a much louder exhaust tone coming from under the hood. A check under the hood revealed that I had lost one of the brass plugs that are screwed into my Fenton headers (Choke hook-up I believe). I replaced the brass plug and the exhaust note went back to normal but the grinding noise remained. It can only be heard when I decelerate without my foot on the clutch or gas. The very second I put any pressure on the gas pedal the noise goes away. Lastly, I noticed that my speedometer needle began to jump around (jumping in a 30-60 mph swing). I did some research in an old Motor's manual and it pointed me to the speedometer cable. I replaced the cable with a new one and gave it a good dose of graphite lube prior to install. I then oiled up the wick in the back of the speedometer gauge. Well, doing all this I went for a test drive. The gauge was still bouncing around a bit (within a 5-8 mph swing) but the grinding noise still was present. It still feels like something is slapping the floor board under my feet. Again, as soon as I hit the gas pedal, the noise and the slapping goes away. The only things in the area where I feel the slapping are the arms to the pedals and the speedometer cable. Could this still be a speedometer cable issue (i.e. a bad "new" cable) or should I be looking elsewhere?

Sorry for the length of this post but I am at my wit's end. Thanks in advance for any help you can provide me.

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Old 01-07-2010, 09:22 AM
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I would first check for broken motor/tranny mounts. If you have a manual tranny, it could be a broken mount or a the throw out bearing squealing.
"When I was younger, I could remember anything, whether it had happened or not." - Mark Twain
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Old 01-10-2010, 09:51 AM
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First, by process of elimination, I would disconnect the speedo from the transmission and drive it. If the problem is gone....good. If not, at least you know it's not the speedo. If, for some reason, the speedo cable is bound up or twisted, it could 'jump' arouond, and possibly be hitting the floor or frame.
This will eliminate that possibility.

Have you checked the universal joints? How about the clutch throwout bearing? Does that car not have the enclosed driveline? Listen...

I think I'd want to find someone who has a frame lift, where you can raise the car, with it in gear, and see if you can hear it 'close-up'. Good luck.

Last edited by dalesy; 01-10-2010 at 04:30 PM.
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