knocking on acceleration

ModAfan

ModAfan

Senior Member
My ’28 Tudor starts easily, but stalls if I give it any gas until it warms up, and it has started to knock during acceleration. There is no sign of knocking at idle or at full throttle. The knocking starts about five minutes after I start driving. If I retard the spark lever to about 3/8” from the top the knocking doesn’t stop, but it is reduced. I have used three carburetors (Zenith, Tillotson and Marvel) and the same knocking condition exists. It may be that there is unmetered air getting into the a/f mixture. I have tried using propane around the intake manifold to no avail. There is spark at all plugs. Timing is correct. Any ideas?
Thanks
 
George is correct! Time to drop the pan and hopefully find a shim to remove in the center main and possibly the other 2 main bearings.
 
I was also wondering if you aren't hearing a mechanical knock, rather than an ignition knock. Ignition is more of a ping or sound like popcorn popping, while a loose center main is deeper or more of a heavy thud, or rattle. When I bought my 29 Tudor the center babbit was completely gone and the engine rattled and shook at any speed over about 15 MPH.
 
Can you make this monster make a similar noise while sitting still ? Try and locate it. If you can get some noise out of it, see if it comes from the front, middle or rear.
A front noise would tend to be timing gear, or fan [pump, generator] related. Rear could be bearing,flywheel, starter,etc. Middle is probably main bearing. That said, it can be hard to get a main to make noise without a load.
Isolating cylinders may help if you can get to make a noise.
By the time a center main is rattling, its pretty loose. It still may be able to be saved though. The front main probably is OK, they don't wear much. The center and rear wear the most.
 
I experienced similar symtoms after letting my Tudor sit over the winter with ethanol gas in it. It was missing and had a terrible knock. I checked my mains and rods, everything was good. I ran the crap gas out and put fresh in, and everything was copacetic. And still is, 2500 miles later. Now, if I plan to leave a car for over two months, I fill it with real gas. Try the easy things first.
 
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