I would first suspect bad fuel

 

I use 93 octane and always have. I have not changed what fuel I use. I don't always use the same station so they would all have to had started getting bad gas around the same time. I guess I could try octane booster and see if that fixes it.

 

How did you decide on 38 degrees of total timing?

 

Try another timing light, see what you have. I would be pulling the distributor, looking at the gear.

 

If you do pull the distributor, check and see how much endplay you have. I shim all mine to .015". I've seen some distributors with .035"+ endplay. That excessive play will cause your timing to fluctuate and the mark on the balancer to move back and forth when you're checking your timing. That's not good.

 

The dyno sheet from the builder showed that they set the timing to 38 degrees total when they tuned it on the dyno. I think that after setting my initial to 14 degrees I am less than 38 degrees total.

I may pull the distributor and look at the gear again to see if it looks ok and can check the end play. I have never done this. Is it just measuring how far the shaft moves in and out?

 

That's what I was wanting to know, if they determined optimal timing with the engine on the dyno. I would verify total timing to make sure it is still at 38. Even if you drop it to 36 it won't make too much difference in power.

 

Yes.

 

OP, your timing light and mark on damper should be DEAD NUTS steady (read not fluctuating). If it does this, as mentioned, you likely have to much end play so close that to .010-.015. You can buy shims. IMHO 38 degrees "all in" is to much for a built street motor with todays crap gas

Also your dizzy gear may be worn, especially if you are running a bronze gear. Those are sacrificial

 

Thanks, I will check the timing again with another timing light next time drive it. Unless it reads steady through the advance curve, I will pull the distributor and check the gear and end play.

I know that distributor does not have a bronze gear. The gear that the cam grinder sent the builder to pair with the cam was black. Here is a picture of the distributor gear that I took about 2 years ago when I had it out.

 

Looks like a melonized gear (good). I see shims above it. While some engines can tolerate 38 degrees, thats on the tall side of full timing. Engine dynos don't provide the same load as a chassis dyno does id the dyno was just engine and tune. Assuming your timing marks are on spec...and cam was not installed with +/-x degrees of advance, it may be to much so back off a few degrees. The fact detonation minimized when you pulled some out indicates you may need to pull 1-2 degrees more out.

 

If that gear looks bad when you pull it out this time, I would highly recommend spending the money on a GM melonized gear. You can mate a new distributor gear with a used cam but not vice versa.

 

Thanks, I know that the cam was installed straight up (no advance or anything) because I asked if the builder did that and he said that since he gets custom grinds for his cams, he doesn't need to adjust them when installing.

It's just weird that everything was fine until after I reinstalled the engine last year after replacing the rear main seal. I don't know why the timing would need to be retarded after that, but I am fine retarding it more. I'll report back after I recheck the timing and possibly pull the distributor.

 

was it a 2 piece RMS?

Did you remove distributor when pulling/installing engine?

 

Yes, 2 piece RMS. Yes, I pulled the distributor and then set the timing back where it was after the engine was back in.

Update: I just found an old timing light and rechecked the timing. It was 14-15 degrees initial (the timing marks on the balancer are light so I have sharpie marks every 5 degrees to be sure where I am). The light was rock steady as the timing advanced to maybe 36 degrees. I lowered the initial timing to 11-12 degrees so total was maybe 32 degrees and took it for a drive. After retarding the timing, I didn't notice any detonation. The windows were down so it's a little harder to hear but I think I may be good. I will keep monitoring it and if I hear it again, I'll retard some more.

I still may pull the distributor to check the gear at some point. I have 5300 miles on the engine now. When that picture of the distributor gear was taken, I had about 2000 miles on the engine. I'm not sure at what point I can assume I am good with the gears if it still looks the same but I think I am good for now at least.

 

My $.02 is you are good with the lower timing. Your dizzy gear is a iron melonized gear which will last forever and there were shims above it. Your timing does not fluctuate which happens when not enough shim and distributor shaft "walks" up due to the helical gear/shaft slop because not enough shim is in.

 

You said earlier you backed off your initial timing from 18-14*, My experience says it makes pinging worse, you need to retard it ...try 20*. I am having similar situation with my 383 mild, on this site read up near the end...

https://www.camaros.net/threads/l31-vortec-heads-sparkplugs.494971/?post_id=1783460000&nested_view=1&sortby=oldest#post-1783460000

All the logic I read on timing, doesnt work with my engine...??? noone has the answer... but it runs ok. my idle timing is 28*... to stop pinging..! If I go by theory engine runs totally ****..!! to fix it, I drive it and if it pinged I retarded till it nearly stopped, (5 speed).. and checking it was @28* idle.. go figure....??

 

You're misunderstanding. If you go from 18* to 20* you’re advancing the timing NOT retarding it.

 

if your distributor has vacuum advance, you want 20-24 degrees of timing at idle with VA (8-12 base + 16-12 VA) hooked up. As engine RPM increases when driving VA drops and mechanical advance comes in. Ideally 32-35 (aka all in) at about 3k RPM

 

retarding the dizzy.. CW, makes TDC timing mark on balancer move more to the left, away from zero tab mark...viewed from the front...?? thats what I see.. so timing tab goes higher in degrees from the zero tab mark.. ? I must be wrong with this thinking..? If my IT is 28* and I rotate dizzy anti-clockwise it will move balancer TDC mark closer to 27-6-5-4* etc closer to the tab, this is advancing...??? (therfore more detonation/pinging) help me understand. cheers.

 

When you rotate the distributor counter-clockwise you are retarding the timing. The mark as seen with the timing light would move towards the timing tab. Anything above zero is advanced. Anything to the right of zero is retarded. Typically less advanced equals less detonation.

Are you sure you have the timing light connected to #1 plug wire?

 

Thanks for the reply, yes #1 plug, We are both on the exact same track and correct with the mechanical process, we/they are both same. I think my difference is that I call Dizzy clockwise rotation 'retarding' and anti-clockwise 'advancing'.. I refer it this way because of the firing of the plug, turning dizzy clockwise makes the spark fire later in the cylinders rotation, hence retarding.. doing the opposite is advancing the "spark' , thats my understanding.. (some think advancing is the direction the dizzy is rotated when clockwise..its actually when the spark fires..it functions the opposite ) anyway I have photos in this post, check above for my post visual positions.. and the @28* mark at idle. Do I make sense with this..??? I guess it depends on peoples perception. Cheers.

 

this is the link to my post i should have put in..

https://www.camaros.net/threads/l31-vortec-heads-sparkplugs.494971/?post_id=1783460000&nested_view=1&sortby=oldest#post-1783460000

 

To clarify.

Most folks here know that advancing the timing causes the spark plug fire further in advance of the piston reaching top dead center and retarding makes it fire closer to piston TDC.

Firing further away from TDC increases possibility of detonation. So if you rotate the distributor from 28* so it fire later, for example 18*, the chance of detonation is lessened.

 

I will investigate and check further, no answer why yet at idle 28* run ok ….. 14-18* detonation and pinging. ??

 

Don't rule out carbon buildup and crappy gas. Pull the plugs and check. Seafoam + a good long highway run can do wonders for a motor that isn't used much. Running too rich too long = carbon buildup. I used to get ping if I applied full throttle below 2500 RPM. Now I add a bottle of VP Racing Maddative Octanium Octane Booster when filling up with 93 Octane, this has eliminated ping and kept my plugs clean.

 

To Clarify..

"So to advance the initial timing, you must move the distributor against rotation (counterclockwise). To retard the timing, you would twist the distributor with rotation (clockwise).1 Dec 2005"

I was correct.
To adjust timing from 28* to 18* is advancing, detonation increases it is not lessened. any firing before TDC is advanced and depending on the degrees can create hammer/detonation, ATDC it is retarded..also depending on degrees creates no detonation but a slight loss of power...

 

Incorrect. If you adjust timing from 28* to 18* you are retarding the timing which typically lessons the possibility of detonation.

28* Is advanced 28* BTDC. Anything before Zero is advanced. 18* is 18* BTDC. So it is advanced 10* less than 28*. By making this change you’ve retarded if from where you were at 28*. It is still advanced but not as much as before. Retarding it further 10, 6, 2, all the way to 0 TD it is still advanced If you keep going to ATDC it would be retarded 2, 4, 6 and so on.

Example. If the timing is retarded to 10* ATDC and you move it closer to 0 you are advancing the timing from where it was set.

Again if changing timing to lessen detonation, reducing the degrees of advance before TDC is the procedure.