ignition ping when vacuum added

[37fatfender]

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?

[51 MERC-CT]

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.

[BUBBAS IGNITION]

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..

[Ol' Ron]

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.

[JWL]

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.

[34PKUP]

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Originally Posted by JWL

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.

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.

[willowbilly3]

Does Bubba say to connect it to manifold vacuum or ported? Ported I would think.

[36rgtop]

Originally Posted by willowbilly3

Does Bubba say to connect it to manifold vacuum or ported? Ported I would think.

Don't know what Bubba would say but I would say manifold vacuum.
It's a GM conversion not a Loadamatic distributor.

[37fatfender]

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?

[34PKUP]

Originally Posted by 37fatfender

I am using the ported vacuum, not manifold vacuum.

Unless the distributor was designed to run on venturi/ported vacuum, I think you are barking up the wrong tree??

[flatjack9]

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.

[JSeery]

Originally Posted by flatjack9

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.

So, isn't this backward to what you what? With manifold vacuum you would have a high vacuum at idle.

[51 MERC-CT]

Originally Posted by JSeery

So, isn't this backward to what you what? With manifold vacuum you would have a high vacuum at idle.

Yep, this is the ported vac.source which I believe is being used and should not be.

[Ol' Ron]

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.

[JWL]

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.

[scicala]

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

[51 MERC-CT]

Originally Posted by scicala

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

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

[Richard in Florida]

Here's a paper I wrote a few years back about Chevy conversions. I think it might shed some more light.

[JSeery]

Originally Posted by scicala

"they run best with manifold vacuum to the dist. and not ported".

Sal

It's not a matter of one being better than the other, they are practically the opposite of each other!

[BUBBAS IGNITION]

ATTA boy Richard !!!
ALL should downlload print and just follow the directions. Especailly when using a vacuum canister...

[37fatfender]

I'm confused the picture in Holleys installation instruction say the port in the metering block is "Timed spark vacuum source, Connects to Distributor advance system". That's were I have mine connected. I just read Richards download and will try it.

[51 MERC-CT]

Originally Posted by 37fatfender

I'm confused the picture in Holleys installation instruction say the port in the metering block is "Timed spark vacuum source, Connects to Distributor advance system". That's were I have mine connected. I just read Richards download and will try it.

Don't be confused just hook it up to 'Full manifold vacuum' before doing any of the procedures.

[JSeery]

Originally Posted by 37fatfender

I'm confused the picture in Holleys installation instruction say the port in the metering block is "Timed spark vacuum source, Connects to Distributor advance system". That's were I have mine connected. I just read Richards download and will try it.

The instructions are for a distributor that requires a "timed spark vacuum source".

[Richard in Florida]

Originally Posted by BUBBAS IGNITION

ATTA boy Richard !!!
ALL should download print and just follow the directions. Especially when using a vacuum canister...

Thanks for the flowers, Jim. I appreciated your mentoring and patience.

[Charlie ny]

Richard.......your article is worth a lot of $$$$$$ ,thank you for this great gesture !
I build a fair number of these distributors and dial them in to run well on
average but as you have so eloquently pointed out an average setting will
in all likelyhood leave room for fine tuning.
When we visited Ron last summer he told us about you .
Gracias,
Charlie ny

[51 MERC-CT]

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Originally Posted by 37fatfender

I'm confused the picture in Holleys installation instruction say the port in the metering block is "Timed spark vacuum source, Connects to Distributor advance system". That's were I have mine connected. I just read Richards download and will try it.

The 390/8007 Holley was meant originally as a replacement carb. on early emissions systems cars. The "Timed spark vacuum source" was connected to one of two ports on the dual port distributor can. The can provided both vacuum advance and vacuum retard in an effort to control emissions.

[37fatfender]

51- Merc, Thank You and everyone else who answered this thread. I never saw one of those two port distributors.

[51 MERC-CT]

Originally Posted by 37fatfender

51- Merc, Thank You and everyone else who answered this thread. I never saw one of those two port distributors.

Had one on my '73 Ranchero. Was about the first year Ford used them in an effort to meet the new emissions requirements.

[Richard in Florida]

Originally Posted by Charlie ny

Richard.......your article is worth a lot of $$$$$$; thank you for this great gesture!

Thanks for the kind words, Charlie.

All through the design and building of my car I enjoyed sharing everything I learned or dreamed up that could help the next guy: "aluma-peas," setscrew advance range adjustment, plug wire looms, four-barrel tuning, my "road rig," my homemade distributor machine, T5 selection and installation... MAN! I've had a lot of fun!