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Old 11-08-2019, 03:09 PM   #1
BUBBAS IGNITION
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Default Flathead "spark knock" ???????

Been building these ignitions for many, many years and have had very very few issues with spark knock !!!
At approx 9-1 compression ratios and todays fuel ( actually very good) its just pretty damn hard to make a flathead spark knock.
However i have had a few (4-5) customers wanting me to slow the advance rates don and limit the advance on a flathead ignition rebuild.

Heres what one customer said today??

Jim i am sending the rebuilt crab that I got recently from you along w/ my older Mallory that you also did a long time ago. See if you can tell the difference in the Mallory vs/ the crab, and if possible, set the crab like the Mallory. I left the vacuum brake almost all the way down, it would knock under partial throttle, especially in 2nd gear. High RPM at full throttle, no knock, but low power. Like I said, the Mallory ran very well after you set it up. Let me know what I will owe you and Iíll get a check to you as soon as possible. Thanks again.


Open for discussion ??? I will add the actual numbers as i have the time to put on machine.....


Kurt
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Old 11-08-2019, 03:33 PM   #2
mfirth
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Default Re: Flathead "spark knock" ???????

Bubba, don't know if this "fits" here, but my 8BA with Max 1 cam, !" bore 94 type carb, loadamatic dizzy & Offy heads is V E R Y sensative to timimg,, plug type, and fuel quality. It will spark knock pretty easily if everything isn't just right. .02 cents or nonsense........
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Old 11-09-2019, 09:09 AM   #3
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Default Re: Flathead "spark knock" ???????

It just sounds like it has too much advance - too soon . . . now the question is why? It will be very interesting to compare the advance curves on both ignitions.

The hard part is that you will have to guesstimate the amount of initial advance - given one must be a 49-53 (the Mallory) and the other a front-mount crab.

I would also want to know the total advance above 2000 RPM (initial, plus mechanical). I run pretty much 24 degrees total in my flatheads - and always premium gas. I have my front pully marked, so I can tell exactly how much advance I have.
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Old 11-09-2019, 11:15 AM   #4
Ol' Ron
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Default Re: Flathead "spark knock" ???????

Be interesting to see what the problems are. Haven't worked on many Crab distributors, most work just fine when cleaned up and there isn't much you can do to the advance, except lighten the weights. My friend George was the only one that used them. Some of his engines had pretty hi CR, Ran 87 most of the time.
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Old 11-09-2019, 11:18 AM   #5
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Default Re: Flathead "spark knock" ???????

I'll follow this with interest; I fitted '68' advance assembly into my crab; better, but still get knock like your customer. Thinking of drilling holes in the weights next....
https://www.fordbarn.com/forum/showt...advance+curves
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Last edited by Brian; 11-09-2019 at 10:08 PM.
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Old 11-25-2019, 02:51 AM   #6
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Default Re: Flathead "spark knock" ???????

Bubba, Have you had a chance to get back to this?
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Old 11-25-2019, 09:21 AM   #7
BUBBAS IGNITION
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Default Re: Flathead "spark knock" ???????

Quote:
Originally Posted by Brian View Post
Bubba, Have you had a chance to get back to this?
Just today will work some on a crab or two tomorrow.
I do think we are missing something in the discussion and thats fuel.
A lean air fuel will cause spark knock as well , often times just adding some fuel will fix the issue , try making the air/fuel ration a little richer ........
We change the heads , pistons , stroke and valve timing all at the same time time and then wonder why we have spark knock........
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Old 11-26-2019, 11:10 AM   #8
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Default Re: Flathead "spark knock" ???????

Been running my 8ba with the original distributor for 13 years with no knock using Marathon gas. this year noticed the same knock as above, changed to a different brand of gas and it stopped. I really think they've changed something in the mix.
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Old 11-26-2019, 11:20 AM   #9
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Default Re: Flathead "spark knock" ???????

Crab with Pertronix on 87 Octane. Factory ported 284 inches with 425 Offy heads equals compression ratio about 8.5:1 Adjustment on distributor set to full advance; vacuum brake set for a light touch. Was getting a little spark knock, most noticeable under moderate acceleration in second gear. Backed off 2 degrees on the advance and could still hear a little. Went in one turn on the brake on the distributor and that cleared it up.
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