Crab distrubtor ignition advance

Rudolph

Rudolph

Senior Member
Hello, I set up a crab distrubtor on the Wasco coming fixture and set the initial advance like the instructions say but when its on the car it seems to like that sliding scale set to max and then I think slotting the slot more would help, I cant get the engine to ping even under max load, I am using 91 octane fuel, I want to keep advancing it to I hear the tinkel tinkel sound and then back it off a bit, I reckon the engine would go really well then

Do other people slot the distrubtor body? I think someone sed do it with the pertronix?
 
Rudolph said:
Hello, I set up a crab distributor on the Wasco coming fixture and set the initial advance like the instructions say but when its on the car it seems to like that sliding scale set to max and then I think slotting the slot more would help, I cant get the engine to ping even under max load, I am using 91 octane fuel, I want to keep advancing it to I hear the tinkel tinkel sound and then back it off a bit, I reckon the engine would go really well then

Do other people slot the distributor body? I think someone sed do it with the pertronix?
Click to expand...

For one of my crab distributors, with points, when I set it manually using a home made tool, it indicated that I needed more advance. I slotted the body to allow this. However, once on the motor I used a dial back timing light and found that it was too advanced. I'd check yours with a timing light and verify the setting.
 
it might be worthwhile looking into the definition and the physics behind pinging. as expectations may be impossible to meet under the circumstances in place while testing.
 
I have a crab with Pertronix and no need to advance more than the distributor allows.

If you’re trying to get it to detonate, try a lower octane gas? Your 91 prevents detonation/pinging in high compression engines. Our flatheads are low compression and don’t need the higher octane.
 
I've been told and normally use this method to check for pinging/advance settings like this.

With the car decelerating to a slow-ish speed in second gear, floor it and listen.

Any denotation will sound like a thudding coming from the back of the motor and not like a "pinging" sound modern cars make.

This method has worked for me.
 
You should also experiment with the vacuum brake settings. I usually back it out all the way and then turn it in until the leather pad is just rubbing against the advance mechanism. This will give you more advance, more of the time.
 
hueyhoolihan said:
it might be worthwhile looking into the definition and the physics behind pinging. as expectations may be impossible to meet under the circumstances in place while testing.
Click to expand...


It will ping if it gets enough advance, I was just going to advance it enough till it started to feel funny and then back it off till the sweet spot.
 
Fritz said:
I have a crab with Pertronix and no need to advance more than the distributor allows.

If you’re trying to get it to detonate, try a lower octane gas? Your 91 prevents detonation/pinging in high compression engines. Our flatheads are low compression and don’t need the higher octane.
Click to expand...

Detonation and pinging are 2 different things,
 
The vacuum brake is unique to the old Ford/Mallory designs. Ford changed the advance unit to the 11A configuration in 1941 and that flyweight set was used till the end of production in 1948. It was retro fitted as a replacement to all the older ones as a service part. The Ford instructions for adjusting the brake was to turn it far enough to eliminate pinging but they rarely ping even with minimal setting due to the volatility of the modern gas we use. Flatheads don't need a lot of advance due to the way the fuel mixture charge burns in the cylinder. Compression ratio is limited and the flow path is hectic. A person rarely needs much movement of the vacuum brake so the spring tension doesn't need much adjustment at all unless the motor is in a heavy truck pulling max load.

Just make sure the flyweights work smoothly all the way through their arc with good spring return and the advance will work as intended. If a person advances it too far, they could have overheating of the engine. That was the reason for the load control of the vacuum brake was to eliminate detonation under load by slowing down the advance. Pre-ignition is not the same as detonation, true, but it is rather rare with it's own distinctive cause. Detonation is caused by too much advance under high load conditions. Detonation is a spontaneous full burn of a fuel air charge. Another word for explosion. Engines are designed to have a burn start at ignition with full burn flame just a bit after TDC. If it burns too fast then it's an explosion before the piston starts travelling down which is not only counter productive but tends to do some damage. The flame front should fully transition into downward push of the piston.
 
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Hard to get the engine to ping on modern fuel.
The typical setting for the vacuum brake is to back it out, by hand, all the way and then turn it in about 1-1/2 turn so the spring add very little tension to the piston.
 
I made a new plate to hold the pertronix moduel from an old 6 cylinder singel point plate or something, I moved the moduel for more advanced, the cast alloy plate use to bind and and cant be adjusted like a steel one by bending the bush holder.

Now the engine runs smother and sweeter and has way more power than before and it runs cooler.

I promise you there is not enough advance in the Petronix unit with out slotting the distrubtor body or moving the moduel
 

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Rudolph said:
I promise you there is not enough advance in the Petronix unit with out slotting the distrubtor body or moving the moduel
Click to expand...

The Petronix is screwed down to a fixed location on the "contact plate". There is no movement within it for advance. Advancing is done with the "shaft, cam and weight assembly".

If you had trouble with timing, it may be that the Petronix is not positioned correctly.

581025
 
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glennpm said:
The Petronix is screwed down to a fixed location on the "contact plate". There is no movement within it for advance. Advancing is done with the "shaft, cam and weight assembly".

If you had trouble with timing, it may be that the Petronix is not positioned correctly.

581025
Click to expand...

Read what I wrote above more carefully and look at the photo
 
When i have the heads off a flathead I always make a timing pointer and mark the pulley at TDC 4, 8 and 12. That way with an "adjustable" timing light, I can check the initial and maximum advance. This is important to make sure the advance is working properly. I have a Sun distributor machine I found at a pawn shop, with the flathead adaptor, but I have never used it yet. It's being used as a storage closet for all my ignition parts. When you build a flathead or have the heads off make sure you install a timing pointer and mark the pulley.
 
So did you start off with the issues using the Petronix module, or did you try that afterwards as a solution?
 
I don't know anything about this electronic crap, but surely this module only picks up a pulse at a certain point , which must dictate the firing point, and the regular advance weights etc take care of the advance curve. Now, if the module was incorrectly located initially, that will affect the performance of the engine at all speeds. So, how does one correctly locate the module when they do this installation into the housing?
 
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