Chevy 350

Hi All,

Ok so I have a rebuilt Chevy 350.

I’ve got a knocking sound and it seems to go away when I advance to timing.

When the timing is at 8-10 there is a knocking sound and as I advance to timing the knocking gets less as I move to more advance ??

The knock goes away when the advance is way too high but the engine will start fine ??

Not sure what the timing is when advanced until the knocking stops as the timing tab only goes to 14 but it is a long way past that ??

Any ideas what this knocking is a why it goes away by advancing timing ? ?

Thanks,
Mike.

 

Is it a standard or a right turn engine?

 

Hi Gonzo

Engine is standard rotation

Distributor turns anticlockwise to advance timing
Thanks,
Mike

 

Did you double check your ignition wire order?

 

Hi Gonzo,

Yeap i triple checked thinking this may be the cause

Distributor is a DUI HEI marine

 

It is a weird thing. Assuming that the groove in the harmonic balancer and the ignition degree metal indicator are a match. A very retarded timing can be actually burning on the exhaust and causing an explosion that could sound like knocking. If you keep on advancing it, does it start knocking?

 

Hi Gonzo,
Advancing it makes the knock go away

 

Get a 3/4" diameter dowel, about 18" (or so) long and use this as a probe. Stick it in your ear and place the other end all over the block, heads, exhaust manifolds, etc. This will narrow down the sound location. I'm betting on something simple like distributor weights. Usually you get a spark knock with more advance, not the other way around, which is why you need to identify the location.

Piston slap, pre and post ignition, bearings, timing issues, etc. all can create a knocking like sound. An experienced ear can tell the difference between a piston slap and a hot spot knock, but it takes a few engines to get a handle on this, so pin down where the sound is coming from. Simply put, if the sound is louder from the bottom of the engine, you got problems, like a rod or main (usually no. 1), but if it's coming from the head, it could just be pre-ignition, which might simply be timing related.

 

I got that advancing makes the knock go away. Does it start knocking again if you keep on advancing it? It will be very surprising if it doesn't.

 

Ok so i got hoje from work and tried the dowel trick
What I have found is the following
The knocking noise is coming from the area down around the number 6 and 8 plugs
I can also hear air puffing with the knock when I put my ear down in that area
Also a bit of an oil leak in the same area that I think is coming from the pocket cover bolt above this area
And yes the knock almost disappears when I advance the timing but when the timing is set at around 10 the knock is bad and the idel is not that great the idel get better when I increase the timing
It is a new DUI HEI marine distributorim wondering if I need to increase the initial timing with this as it doesn't seem to be good at the stand 8-10

Don't know if these comments have helped find the solution or just given more questions LOL
THANKS
MIKE

 

Hi gonzo
I can advance the distributor a fair way then it the distributor touches parts around it and won't turn anymore
It does go a fair way before it stops and no the knock doesn't start to come back
Not sure how much it does advance as the tab only goes up to 14 but the line on the balancer goes far enough that I can't see it on the top of the balancer but the strange thing is as I've said the knock gets a lot quiter and the engine idels nice
Not sure if it's something to do with the new DUI HEI distributor
I'm at a loss?
Cheers
Mike

 

There are several different positions for the mark on the balancer with respect to the keyway. They are set for different positioned markers in the timing chain cover. Set piston #1 at TDC, and check the timing mark. If it does not align with the TDC mark on the timing plate, make a new mark on the balancer and use that.

 

Hi Gonzo,
Thanks ill try that
Cheers,
Mike

 

Have you tried disconnecting a sparkplug (one at a time) to perhaps narrow things down to one cylinder?

 

Hi Westvanhan
Thanks yes I will try that to see if it isolates a cylinder
Cheers
Mike