spark knock

[michael holcomb]

A friend of mine has a chevy with a 350 in it that spark knocks badly. It does it no matter where the timing is set. High octane gas helps a little but not much. The engine has been rebuilt and it has a new carburator on it (it did the same thing with the old carb. too). It has new plugs,wires on it. Any help will be very helpful.

[Mac_Muz]

Just for kicks try a octane booster with leaded additives.. Mac

[michael holcomb]

we did try octane additive and high octane together and it did help.

[wyldman]

What year,make and model ?

Are you sure the firing order is correct ? or if the noise isn't something else ?

[michael holcomb]

It's a 84 model 1/2 ton 4x4 with a 4 barrel. Its spark knock alright. It runs fine when it is not pulling up a hill or you aren't getting on it much.

[wyldman]

Check the centrifugal,and vacuum advance units for sticking.The rotor should turn freely back and forth about 5 degrees.

What is your base timing ? Are you sure your checking it correctly ?

[michael holcomb]

that is what I thought it is that the advance is not working right. I'll pass the info. on to my friend and we'll check it.

[Highpoint]

I got the exact same problem on our 63 GMC with the same motor. The old 305 ran perfect. Just had this 350 rebuilt and installed. Knocks bad under load. I'll check it out.

The guy put a "3/4 cam" in it but the engine is mostly stock. Could this be part of my problem?:confused:

[Backcountry]

On this motor; was the cam set with a degree wheel? Are the cam gears lined up?

Take the distrubutor out and take it into a ignition/carb shop and have them check it and set it up for this motor. When I remove my distrubutor; I remove the valve cover on the #1 piston side and make sure both valves are closed (TDC) and that the timing marks line up. I then remove distrubutor cap and make sure rotor is pointing to #1 spark plug terminal, then remove.

Also, if it has vacuum advance; make sure the port it is connected to on the carb is functioning. Once distrubutor is reinstalled you may want to try two different timing lights.

I would not run this motor any longer untill this problem is fixed. It could or may have already caused engine damage.

Good luck!

Guy :shades

[Highpoint]

Thanks. I'll try it.

PS. What kind of damage does this cause?

[Big Nate's Plowing]

it can and will burn holes in the top of the pistons, the pressure can blow spark plugs out and strip the holes.

[michael holcomb]

I looked at my friends truck and I think the distrubutor is backwards. To retard the timing you turn clockwise and advance turn counter clockwise. Am I wrong or is this incorrect? What would cause this? I really don't have a clue.

[wyldman]

While detonation will cause internal damage,it will not blow out spark plugs.

For all small block Chevy's,you turn the distributor clockwise to retard,and counter-clockwise to advance.

If your ever unsure,usually the vacuum advance points in the direction of advance.