1971 F-100, 302 with Edelbrock intake and carb, C4 trans, 170,000 miles (assuming).

I have a strange tapping noise that occurs at higher speeds with my foot on the throttle (40 mph and up). It appears to be coming from under the hood somewhere. I can not replicate it when I’m in park and revving the motor up, but it is a consistent tapping sound, like a wood dowel tapping on a metal table quickly. Its doesn't appear to fluctuate with the rpms of the engine. It also doesn’t occur at low speeds (40 mph and under). Normally begins when under load, and once it starts, any throttle depression will cause the tapping noise, but when I let off it goes away.

Sometimes if I hammer on the gas from a roll and get the secondaries to open up for a good pull (up to 65-70 mph) the noise will be less prominent for the rest of the drive. I have checked my points gap, and tried retarding my timing to 8 degrees but still get the same noise. I tried heavier oil with stabilizer in it, but that didn't change anything (I was thinking that if it was a rod knock heavier oil would change the sound).

Valve lash, piston slap, rod knock, exhaust gasket... seatbelt draggin out the door (joking)? What do you think? Thanks everyone!

 

Sounds like detonation

 

That is my thought as well... here is something I left out, I currently have my vacuum advance hooked up to a timed vacuum source. Should I have it connected to a manifold vacuum source?

I also noticed the hole in the arm of my vacuum advance (inside the distributor) is an oval shape, which allows the pin to move back and forth slightly before the arm of the advance pulls on it. Is that hole supposed to be an oval shape, or perfectly round where the pin attaches?

 

Have you checked with a timing light to see that the distributor is advancing and what the total timing is at RPM?

 

Not likely but it could be your speedometer cable. Speed related and not engine speed related.

 

I haven't ever checked for total timing, is that done at 2500 - 3000 rpm? I do know that it is advancing, when I had my timing light on it, the mark would move when I revved the motor. I will read up on finding the total timing and check that tonight. What is a good total timing number for a 302?

I will look into the speedo as well, however it sounds much more like its coming from the engine bay, rather than behind the dash. You are right though, it doesn't seem to follow any changes in engine rpms.

 

You were right about total timing! Thank you!

I put a light on it, and when I revved it to 2500 - 3000 rpm the total timing was at 55' - 60'.... way too high. When I unplugged the vacuum advance from the vacuum source (leaving it open), and plugged the ported carb source (where the vacuum advance was hooked up) my total timing was at 35'. I took the truck for a spin and the sound was gone! However, it doesn't seem to run quite as smoothly as before at idle. My two follow up questions are:

1) Should I go back and retune my carb (air mixture screws) after removing my vacuum advance, or should the idle circuit remain unchanged? I'm curious why this changed my idle characteristics if my initial timing is the same as before.

2) How is my timing advancing without the vacuum advance hooked up? It is at 10' initial and 35' total at the moment, not hooked up to anything. Is this just because of the centripetal weights in the distributor?

3) Should I find heavier springs to put in my distributor, and hook the advance back up, or am I okay leaving it off?

 

You will get better economy and better idle if you leave the vacuum advance installed - depending on your carb you setup you may want to experiment with the timed or full vacuum port. Many of the vacuum advance canisters are adjustable. If you find the right size allen wrench you can adjust the advance. I believe clockwise will reduce the vac. advance. 10 degrees from the vacuum should get you close. The mechanical centrifugal advance seems about right and I wouldn't mess with it. If your vacuum advance isn't adjustable you can also get a fixed advance replacement. I had an adjustable a few years ago that would slowly back out so I would have to occasionally screw it back in. After you get the vacuum sorted out you can then adjust your idle speed and mixture for the smoothest operation.

 

Thanks man! I will take a look at that tonight. So if my thinking is correct, without vacuum advance my total timing will be about 35' and with the advance hooked up it will be around 45' total? Or do I have that wrong?

 

That's a good target - about 10 degrees vacuum advance. If you can adjust it to 10, drive it and see if the knock is gone. You can test it in the driveway if you have a vacuum hand pump. Measure timing without vacuum and then measure after you take it to about 20 inches of mercury. If you have good vacuum at idle, check the timing and then attach the hose to a manifold vacuum source and check the vacuum contribution. There are many posts and threads on many sites on whether to use ported vacuum or manifold vacuum. I prefer manifold myself because you can advance the idle timing - usually runs smoother and cooler. Just try it both ways and pick your preference.

 

Awesome, thanks for the reply. I went home last night and adjusted my vacuum advance clockwise. It was 3 full turns out from being seated. I turned it in 2 turns (only one turn out). When I drove to work this morning I was still getting some light pinging and tapping, I'll turn it in all the way and see where that leave me.

If its turned all the way in, will I still be getting some extra timing advance, or does it block off that vacuum circuit completely?

 

I'm fairly sure that when you bottom it you won't get any vacuum. But when in doubt - test. I would try to see if you can measure the vacuum advance you have now and maybe go in 1/2 turn and test again. Other factors that can cause you issues are your fuel - low octane, plugs that may be too hot for the application, or high engine heat, so don't focus solely on the vacuum advance. If you want to get carried away, modify or change your reluctor to limit your mechanical to about 20 degrees, change your springs to bring the advance on earlier, and set your initial to 15 degrees for a total of 35 - but first things first....

 

Thanks for the input! I think I will start with a half turn in, and higher octane fuel. I have been running 87... If that doesn't help, I'll double check my plugs, and move on to changing springs in the distributor.