My 4.9 in my 78 F150 clatters during acceleration from about 1500 RPM s to 2600 RPMs after that the clatter is gone. What might be the cause and cure of this. Pretty sure it is not detonation.

 

Have you checked the timing? Have you tried using a higher octane fuel?

 

Does it clatter when you rev the engine in the driveway?
Or is it only when driving?

Could it be something else associated with RPMs like the transmission?

If it's detonation, you should be able to tell by letting your foot off the gas and see if it goes away immediately.

 

Small exhaust leaks at the manifold from a deteriorating gasket, or cracked manifold can sound like valve clatter/clacking when the engine is under a load like that. Look around your exhaust manifold VERY well. Use a flashlight, and an inspection mirror for the spots that are tough to see. All it takes is a little crack...which may be hard to see if its an engine with some miles on it, so take your time.

 

during the winter my oil thickens up and makes my hydraulic lifters a little sticky, it only makes the sound in drive when i and almost trying to do a burnout, My clacking will go away after warm up. I would check your... Timing, a spark plug, oil, smell the exhaust to see if its rich or lean. if none of these show any signs of problems... start the truck, basicly try to do a burn out as slow as possible but don't spin the tires. will it make the noise when putting some torque on it?

 

Idling the truck doesn't make a sound....No ticks, tap,s clunks, or clatter. The only thing you can hear is the steering and water pumps and the alternator turn. It does hunt for a steady idle but never settles for long. A little about the truck. It has made this noise as long as I have had it 1.4 years. It has a 4.9, not sure of the year, not the original motor. The timing when I got it was at 0 BTDC. Don't know how it ran. Yeah I do...poorly. I changed it to 10 BTDC. Picked up some guts after that. I took #1 to TDC and made sure timing mark was good, it was. It has the crappy little 1 barrel Carter YF. Going to TBI it in the future, depends how it goes on my CJ5. It also has a Hedman Header. Will be swapping to EFI manifolds when able. I did do a large cap swap to it with a TFI coil. Still using the stock ign box. Will probably swap that out for a GM module here shortly. I can get on the truck and it does not matter from a full throttle stomp to slowly revving it up it makes this same noise between 2000 - 2600 RPM. After that it quiets down till about 35-3700 RPM which is as far as I have wound it out. Looses steam at about that point. It does not sound like any detonation I have ever heard before. Just sounds like the valves or rockers are just "loose" and slapping around. Just curious if I need to look closer or go with it. Runs fine other than that. Well it likes to die at stop signs when its cold.