Spark Knock

[sixt9x33rs]

My 69Z is experiencing slight spark knock under aggressive aceleration. I am burning 93 octane, my air/fuel mixture on the carb is dialed in, my timing is 10* at 900 RPM. The motor was rebuilt when I bought the car so I don't know a lot about it. I have recognized that the car seems to run with limited to no SK with the breather assembly off. Maybe just a coincidence but this did happen. Also, I have noticed at times when I get 93 octane as some stations the fuel quality will vary. If I put 100 octane in it runs great. Just wanted to see if anyone else is expericing anything similar. Eric Jackson at Vintage Muscle Car parts rebuilt the Carb.

Thanks,

[bertfam]

Not to hijack your thread, but where are you getting 93 octane (let alone 100)?? Here in So Cal, the highest octane we can get is 91, and they call that (laughable) "Premium"!! Forget about 100 octane unless you go with racing gas at $7.50 a gal.

Ed

[sixt9x33rs]

I am in Charlotte NC. 93 Octane is run of the mill. It is everywhere. We use to have 91 several years ago and it went up to 93 a long time ago.

Several stations in Charlotte have 100 octane but it is $6.99 a gallon. It is billed as racing gas but not hardly. If I want the real racing gas I go to nearby Gaston County (remember you are in NASCAR Country here and with NASCAR comes pure redneck) being from Charlotte I include myself in this group. There is a station right on Mecklenburg/Gaston County line that sells 110 Octane Can Am fuel for the dirt track racing boys. (Like I said pure redneck) This is where the real juice can be found. When I put that in any of my old cars they truely come to life. The problem is that it is about 20 miles away. So in the Z I will burn about 1/4 of a tank coming home. I have not been over there in awhile so I don't know what it cost a gallon now. I need to take my plastic tanks over there in my Suburban and fill those up, and fill up the Z and Vette. I know the spark knocking goes away then.

I figured you So. Cal boys would appreciate some good redneck humor.

[JohnZ]

my timing is 10* at 900 RPM.

What does the rest of your timing map look like? When does your centrifugal advance start, and how much advance does it add at what rpm? What vacuum advance unit do you have (stamping on the bracket) and is it connected to "ported" vacuum (stock) or to full manifold vacuum?

Spark knock (detonation) is a function of ignition timing, not carburetor calibration.

[bertfam]

Man, it must be nice to still be able to get fairly good gas. I grew up in the days of Sunoco 260. At .24 cents a gallon and 102 or 103 octane if I remember correctly!

Ed

[Gramps69Z]

Man, it must be nice to still be able to get fairly good gas. I grew up in the days of Sunoco 260. At .24 cents a gallon and 102 or 103 octane if I remember correctly!

Ed

Ed, I remember gas statons having "wars" in the sixtys and gas  going for 15-18 cents a gallon and selling good gas.  I think most of the suppliers today deliver to more than one station and that's why the prices are set.

[sixt9x33rs]

my timing is 10* at 900 RPM.

What does the rest of your timing map look like? When does your centrifugal advance start, and how much advance does it add at what rpm? What vacuum advance unit do you have (stamping on the bracket) and is it connected to "ported" vacuum (stock) or to full manifold vacuum?

Spark knock (detonation) is a function of ignition timing, not carburetor calibration.

John the stamping on my vacuum advance is B28
I don't have an adjustable light so I can not answer your other questions. Any way to do it with just a standard light?

[L78 steve]

Install a timing tape on your balancer. It's very important to know what you are dealing with as far as timing. All 3 stages of timing, Initial,centrifugal and vacuum. The only other thing I can think of that can cause a knock that has not been mentioned is a stuck closed heat riser valve. This causes the intake manifold to become very hot, super heating the intake charge.

[rich69rs]

Install a timing tape on your balancer. It's very important to know what you are dealing with as far as timing. All 3 stages of timing, Initial,centrifugal and vacuum. The only other thing I can think of that can cause a knock that has not been mentioned is a stuck closed heat riser valve. This causes the intake manifold to become very hot, super heating the intake charge.

As a follow up to Steve's good suggestions,

Timing tape or use a set back timing light which can read the full amount of timing.

As far as the heat riser valves, I removed mine (from my Camaro and Chevelle) and installed spacers in their place which are readily available from the aftermarket.  

Back in the day, the heat riser valves were problematic enough.  Today, it is tough to find replacements that won't bind up after little use.  By not having them, the preheat of the intake manifold is compromised, but it hasn't been a problem on either my 69 RS or my 66 Chevelle.  Once the engine warms the heat rise valve should be open and is just a restriction in the exhaust pipes.

Just my two cents.

Richard

Good luck.

[sixt9x33rs]

Install a timing tape on your balancer. It's very important to know what you are dealing with as far as timing. All 3 stages of timing, Initial,centrifugal and vacuum. The only other thing I can think of that can cause a knock that has not been mentioned is a stuck closed heat riser valve. This causes the intake manifold to become very hot, super heating the intake charge.

I will look into the tape or just buy an adjustable light. The tape and light do the samething right?

[sixt9x33rs]

Install a timing tape on your balancer. It's very important to know what you are dealing with as far as timing. All 3 stages of timing, Initial,centrifugal and vacuum. The only other thing I can think of that can cause a knock that has not been mentioned is a stuck closed heat riser valve. This causes the intake manifold to become very hot, super heating the intake charge.

Great point. When I put the car back together, everyone was telling me not to put that valve back in but I was determined to put the car back together as it was when new. I will check that and see if it is closed when it should be open.

Thanks

As a follow up to Steve's good suggestions,

Timing tape or use a set back timing light which can read the full amount of timing.

As far as the heat riser valves, I removed mine (from my Camaro and Chevelle) and installed spacers in their place which are readily available from the aftermarket. 

Back in the day, the heat riser valves were problematic enough.  Today, it is tough to find replacements that won't bind up after little use.  By not having them, the preheat of the intake manifold is compromised, but it hasn't been a problem on either my 69 RS or my 66 Chevelle.  Once the engine warms the heat rise valve should be open and is just a restriction in the exhaust pipes.

Just my two cents.

Richard

Good luck.

[JohnZ]

I will look into the tape or just buy an adjustable light. The tape and light do the samething right?

A dial-back timing light eliminates the need for timing tape installation and inaccuracy, and you can use it on any car; really simplifies "mapping" your advance curve. I've had an Actron CP-7519 for many years (about $70.00), and it works great.

[fireZ]

Has anyone done Johns vacuum setup without doing any other changes. If so did it work or did you have to do some other changes or did you have to do some re-timing on the engine. I do not have a timing light and do not want to get ahead of myself with the vacuum change if I also need to do timing adjustments.. Thanks ,

[L78 steve]

Has anyone done Johns vacuum setup without doing any other changes. If so did it work or did you have to do some other changes or did you have to do some re-timing on the engine. I do not have a timing light and do not want to get ahead of myself with the vacuum change if I also need to do timing adjustments.. Thanks ,

The vacuum change to a NON-ported source is important for best idle,drivability and economy. But is not your problem. If you are hearing ping at hard throttle the vacuum advance is not the cause. Because it is not a factor at low vac. Ping at hard throttle is initial timing and centrifugal advance totaled up too high,or some other issue. You must get a light and check your total with the Vac. can disconnected. It should be no more then 36*. Once you get this checked and or corrected  then see if you still hear any spark knock. A light ping at light throttle I don't worry about, but knock at hard throttle is not good.

[sixt9x33rs]

Has anyone done Johns vacuum setup without doing any other changes. If so did it work or did you have to do some other changes or did you have to do some re-timing on the engine. I do not have a timing light and do not want to get ahead of myself with the vacuum change if I also need to do timing adjustments.. Thanks ,

The vacuum change to a NON-ported source is important for best idle,drivability and economy. But is not your problem. If you are hearing ping at hard throttle the vacuum advance is not the cause. Because it is not a factor at low vac. Ping at hard throttle is initial timing and centrifugal advance totaled up too high,or some other issue. You must get a light and check your total with the Vac. can disconnected. It should be no more then 36*. Once you get this checked and or corrected  then see if you still hear any spark knock. A light ping at light throttle I don't worry about, but knock at hard throttle is not good.

Your comment about light ping at light throttle is interesting. My spark knock I would classify as a light ping at mild to aggressive acceleration. I don't have a hard knock at all.

I am still going to buy the adjustable light and map my timing. I need to do this for all my cars.
Thanks