ignition ping when vacuum added

37fatfender

37fatfender

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Here's my setup in my 40 Coupe, 8ba block bored 3 5/16, Merc 4" crank, 276ci, Schneider cam. Edlebrock 1125 early heads, the block is not relieved. I guessing a CR of 8 to 8.5 according to the Jim Lies/Joe Ablin spread sheets. Induction is a Holley 390 cfm carb with "50" main jets. I run 91 octane in it all the time. The engine has a steady 15-17 inches of vacuum at idle.

The ignition is a 49-53 Bubba Chevy dizzy WITH a vacuum advance can, it has 18 degrees mechanical advance.

I can set the timing to give or take 4 degrees initial without vacuum to the dizzy. Giving me a total of about 22 degrees. No problem. Drivability is good until under load pulling a hill then the engine will ping. Experimenting I pulled the vacuum from the dizzy and alas no ping?? No Vacuum advance, I must be missing something, I thought under load the vacuum advance would be very low or none. So why is it pinging? For now I am running it without Vacuum advance, as "Old Ron" would say its a race engine.

Should I increase the size of the main jets in the Carb?
 
The vacuum advance should be adjustable using a 3/32" allen wrench in the vacuum port on the can. Turn the screw clockwise to decrease the vacuum advance and counterclockwise to increase the advance.
Crank the screw in a couple turns, re-attach the vac. hose and drive to check. Re-adjust as necessary to a point where the ping is just gone.
 
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Using a chevy conversion the addition of vacuum is pretty hard to tune on a flathead.
Like someone said above the vac canister is adjustable using a allen wrench thru the vac tube (largest one that will fit inside the tube) and turning the screw against spring tension to make vac come in with higher vacuum.
It is more than likely set at the lowest setting..
 
I find that the mechanical advance has allot to do with this. I set the max adv at 20 degs at 2000 RPM. Bring the engine up to 2ooo set the adv to 20 degs. Then add vac. this should add 6-8 degs. Now if you have a stock cam then you can't add as much adv because the dynamic CR is too hi. As Bubba say you have to putz with it. Also the AF at cruse will also make a difference. Some of Bubba's distributors have an adjustable mechanical advance as well. One thing you have to remember is: your tuning for cruise not power. Done right you get both.
 
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Where is the vacuum source connection??? If the throttle is well open and the engine is under a good load the vacuum advance shouldn't be seeing much vacuum if it is connected directly to manifold vacuum.
 
JWL said:
Where is the vacuum source connection??? If the throttle is well open and the engine is under a good load the vacuum advance shouldn't be seeing much vacuum if it is connected directly to manifold vacuum.
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The original poster states that the 'ping' disappears when the vacuum is disconnected. So it must be seeing enough vacuum to cause it.
Sounds like an adjustment of the vacuum advance as stated in post #2 is in order.
 
I am using the ported vacuum, not manifold vacuum. I have a old vacuum gauge that is mounted on my dash. When under a load it really drops off. So I would think that there would be no vacuum advance under that driving condition.
What about bigger jets in the carb, If I am going lean wouldn't it cause the engine to ping?
 
He has a 390 4 bbl Holley carb. There is a port that is tapped off just above the throttle plates, so at idle there would be no vacuum advance. This is what is called ported vacuum on these carbs. Different than 2 bbl Holleys that have venturi vacuum.
 
flatjack9 said:
He has a 390 4 bbl Holley carb. There is a port that is tapped off just above the throttle plates, so at idle there would be no vacuum advance. This is what is called ported vacuum on these carbs. Different than 2 bbl Holleys that have venturi vacuum.
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So, isn't this backward to what you what? With manifold vacuum you would have a high vacuum at idle.
 
JSeery said:
So, isn't this backward to what you what? With manifold vacuum you would have a high vacuum at idle.
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Yep, this is the ported vac.source which I believe is being used and should not be.
 

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Yes, It goes something like this: When starting the engine the vac advance goes to max say6 degrees and the mechanical adv is at minimum so the engine sees the initial plus the vac or about 10 degs, which is about perfect for idle. Now as you accelerate the Mechanical will follow the RPM till it reaches 2000 RPM. Once the throttle is released the vac comes in and you have 28 degs total. Great for economy, and a lean mixture needs more advance.
 
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As I suspected, it appears as though the vacuum source is wrong. Low pressure created by air flow across an orifice is not the same as manifold vacuum.
 
It sure sounds like the detonation is from too much spark advance, but I would look at the spark plugs too, or use an 02 meter. I'm not sure what Holley 390's had for stock main jets, but 50's sound small. Lean mixture can contribute to detonation too, along with hot spots in the chamber.
I am not a flathead expert, but it sounds like the experts on this site say they run best with manifold vacuum to the dist. and not ported.

Sal
 
scicala said:
I'm not sure what Holley 390's had for stock main jets, but 50's sound small. Lean mixture can contribute to detonation too, along with hot spots in the chamber.


Sal
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The 390's come with #51 jets. I have run as small as #45's and never had detonation.
 
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