Anyone have any ideas? Came back from a ride, ( finally running good after Strombergs installed ) shut it off and it was fine. After a few minutes, started it up, it was good, revved a little and Bam, there it was! Not gradual or subtle. Not there then zingo, it started. Took plug wires off one by one and removed drive belt, no change. It doesn’t miss at all and exhaust tone is sweet… any ideas?? This thing is my mental therapy, I’m a little distraught at the moment…Thanks fellas! Video short …link below… https://youtube.com/shorts/37LF5tBg6bc?feature=share
Rod knocks don't change exhaust notes or idling or even oil pressure. You can make them "talk" to you by revving the engine a bit. Can you drop the pan?
I was referring to the sound of a missfire with the exhaust tone…A miss would change the tone… I did rev…didn’t change much except the frequency increased with rpm. I’ll be pulling things apart tomorrow… Was still hot… Things will go better when it’s cooled off…
Sorry, just a little distraught at the moment and not thinking clearly. I’ll probably have a better plan tomorrow after I sleep on it…Thanks for the input…
Sounds like a valve broken or became loose would be my guess. That*****s. Do a compression check before tearing things apart. Good luck.
I feel your anguish, but can’t offer anything but sympathy. Just went through the same thing with a 283 I had spent time and money on for my 40. Ready to go in and a dull knock started while on the test stand and would not go away. Tried EVERY thing to nail it down to no avail. Still pissed and down about that. See if you can pinpoint the location with a screwdriver or stethoscope . Is it a hard rap, a dull thud or a little peck?
Sometimes the knock can go away when pulling a plug one by one while it’s running. That’s usually the sign of a bad rod. I had a Cadillac 429 that sounded fine cold but would start knocking when warmed up. Pulling the plugs one by one narrowed it down to which rod it was.
I was thinking that...I just put them on this weekend and have gone about 100mi. I would think it would be a more inconsistent noise if something was getting tortured by a piston...
I'll hit it again tomorrow after work. I'm just so dejected right now and need to clear my thoughts... I'll do a comp check and pull the intake tomorrow. If I don't find anything there, I'll remove the pan...
It's a medium consistent knock that increases in frequency with rpm...it's not erratic. It sounds like it coming from topside around the #3-4 cylinders... It doesn't sound like a rod knock to me, but this is my first flathead. Rod knocks usually give you that sharp metallic rattle. this isn't that. I've replaced my share of cranks and rods due one letting go. It just doesn't sound like one...
Doubt it’s a wrist pin. They can knock a little when the bushing wears or the pin wears but it wouldn’t be that bad that fast. I’m thinking something went into the cylinder from the top
What's the story on the engine? How it is built? High Compression heads? I ask because I had one with near zero piston to head clearance and carbon build up would make it knock See if it there cold
If the block has been bored out, it could be a piston hitting the fire ring on the head gasket. There are gaskets available for big bore flatheads that eliminate this problem. Do you know what year the engine was built new?
Thought of that too...but I peened over the throttle plate screws when I installed the 12deg plates. Unless the head broke off of one of the screws and is stuck in the piston.
It's bored .040" over, but I've put alot of miles on it since I put it on the road almost 2 years ago. Maybe one of the throttle plate screw heads broke off. I did just change the plates yesterday...
Based on the fact that you have oil pressure and the sound of the noise, I'm thinking something on top of a piston makes a lot of sense.
You’re on the right track with taking the intake off and looking at the valves. Move the engine over and look closely as the valves open. This sounds like valve train noise. You hear it increase and bounce more as rpm increased. You could start by pulling the plugs and see if you can see anything inside. You might see a valve stuck open or something in there.
Hopefully its a piece of carbon that broke off after the ride . I’ve had it happen and it sounds just like a rod but it doesn’t change sound when pulling plug wires . Good luck .
The more I think about it, the more I’m headed towards a throttle plate screw head that was compromised during the peening process. The head probably broke off and took time to work it’s way into the cylinder. Gotta pull the engine to get the heads off, but I built ease of removal into it when I built it. 1/2hr 45mins tops by myself to get it out… I’ll install the new pressure plate/clutch****embly while it’s out…I also have a set of timing gears I’ve been wanting to install…thanks for your input, greatly appreciated!
Thanks Gary! Will be pulling at least one head after work to investigate. I’m suspecting a throttle plate screw head that I compromised when peening the threaded ends. Thanks for your input!
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