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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
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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
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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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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
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Default Re: Flathead "spark knock" ???????

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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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Old 01-05-2020, 01:28 AM   #10
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Default Re: Flathead "spark knock" ???????

Jim, Have you compared the advance rates yet?
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Old 01-05-2020, 12:36 PM   #11
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Default Re: Flathead "spark knock" ???????

Brian, have some patience. Jim is coming back from some serious health issues and works when he can.
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Old 01-05-2020, 07:09 PM   #12
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Default Re: Flathead "spark knock" ???????

I know this is just terminology but I have always called the noise loose connecting rods make "knock", and this noise being talked about here "ping". I am I alone in this view?
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Old 01-05-2020, 07:28 PM   #13
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Default Re: Flathead "spark knock" ???????

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I know this is just terminology but I have always called the noise loose connecting rods make "knock", and this noise being talked about here "ping". I am I alone in this view?
Yep, rods knock. But "pinging" is also called spark knock; a whole 'nuther thing. "Pinging" is the more descriptive term. Sounds like a few small pebbles being shaken in a tin can.
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Old 01-05-2020, 07:34 PM   #14
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Default Re: Flathead "spark knock" ???????

Quote:
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Jim, Have you compared the advance rates yet?
Yes got both units tested on the machine. The crab (stock) was perfect with 22 degrees engine at 2500 rpm. Thats a good stock 11A unit.

The mallory ran well but only had 16 degrees at 2500 rpm. The mallory has a internal adjustable stop.

The customer wants the crab to be set up like the Mallory. Will look into it some more tomorrow and hopefully limit the crab 11A to be a 8 degree distributor
( maybe braze up the ring slots a bit )
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Old 01-05-2020, 09:12 PM   #15
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Default Re: Flathead "spark knock" ???????

Thanks Jim...you're the expert, however, may I suggest you try the 'guts' of a '68' distributor in the clients crab; I think you'll end up pretty damn close to what he wants?
I 'think' it's not the total advance degrees that's the problem, rather, there is minimal spring tension on the weights in an 11A assembly; consequently, they get pretty well full advance just off of idle. The 68 assembly definitely has heavier springs; this has the effect of slowing down the curve.
You've got all the testing gear so I'm keen on hearing the outcome; me, I'm a rank amateur at this stuff, just my experience in trying to eliminate/minimise this horrible noise!!
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Old 01-05-2020, 10:53 PM   #16
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Default Re: Flathead "spark knock" ???????

Brian , if you look in the service bulitens under electrical there is a great lot of info on all the dist,s, adv rate where and when etc,from model18 ,40 and 40b onwards , there you can see the diff in the 68 advance, let me know it you want me to get a copy to you.
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Old 01-06-2020, 04:04 PM   #17
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Default Re: Flathead "spark knock" ???????

Quote:
Originally Posted by Brian View Post
Thanks Jim...you're the expert, however, may I suggest you try the 'guts' of a '68' distributor in the clients crab; I think you'll end up pretty damn close to what he wants?
I 'think' it's not the total advance degrees that's the problem, rather, there is minimal spring tension on the weights in an 11A assembly; consequently, they get pretty well full advance just off of idle. The 68 assembly definitely has heavier springs; this has the effect of slowing down the curve.
You've got all the testing gear so I'm keen on hearing the outcome; me, I'm a rank amateur at this stuff, just my experience in trying to eliminate/minimise this horrible noise!!
Damn it Brian, i am out in the shop this afternoon and read your post . I had planned on brazing up the advance limits on the advance retard ring and grinding it out with a dremel.
Walked to our core cab and grabbed a Model 68 bag ( cleaned and ready to assemble ) . Looked over the entire unit , case has been overbored to allow more ( or less advance movement) will trash this one . Measured advance retard ring and there is some difference . Will use the model 68 ring and advance unit. Measured the spring tension (spring just begins to move) 4 oz on 11A and 12 oz on the model 68 !! Limit slots are close ( approx .0005 difference ). Assembled and tested on machine , damn it its 8 degrees just like the customer wanted !!!! Normally we would charge a few hundred dollars for this service but this one is free will ups back to customer tomorrow.........rbg >>>>>

Thanks Brian..................
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Old 01-06-2020, 04:56 PM   #18
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Default Re: Flathead "spark knock" ???????

Jim,
I'm surprised you haven't run into a crab with 11A weights and a 68 index ring (or spark ring in your terminology) I've torn apart several that way. I dig that setup as it gives the quick initial advance of a 11A but limits it to the 8 degrees of a 68.

The most recent one I had that was had the housing hogged out to move the adjuster down to allow extreme initial advance, but then had the 68 index ring.. That one stumped me and maybe you could explain that.
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Old 01-06-2020, 05:38 PM   #19
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Default Re: Flathead "spark knock" ???????

Maybe it was to improve the idle and initial throttle response with a big cam. At low RPM a long overlap cam will allow a lot of exhaust gasses to remain in the cylinder, which dilutes the incoming charge. This slows down the burn rate so more initial advance is needed to get a better idle. With added initial timing, you obviously need to reduce the overall amount of advance so that total timing is not excessive.
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Old 01-06-2020, 06:10 PM   #20
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Default Re: Flathead "spark knock" ???????

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Jim,
I'm surprised you haven't run into a crab with 11A weights and a 68 index ring (or spark ring in your terminology) I've torn apart several that way. I dig that setup as it gives the quick initial advance of a 11A but limits it to the 8 degrees of a 68.

The most recent one I had that was had the housing hogged out to move the adjuster down to allow extreme initial advance, but then had the 68 index ring.. That one stumped me and maybe you could explain that.
Might have had a Camaro parked in the driveway OR a Chevelle., those chevrolet guys are pretty brutal...
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