The Ford Barn

J Witt · 04-26-2016, 01:05 PM

What would be the amount of advance change expected per turn of the vacuum brake adjustment screw. I know it will vary with throttle opening, but in getting the timing adjusted, is one full turn a lot or a little? How much pinging should one expect with the the screw backed out to start?

I would assume one looks for an audible ping or rattle at low rpm/full throttle condition (WOT). Given that, how much adjustment is made at each effort to get the ping to just go away?

This is with stock 85 hp, iron heads engine. I think it's not too far off, but the fuel mileage is only about 12. It runs pretty good though, but is a little lacking in hill climb power.

And, a corollary is, if the static advance is centered, how much advance would be appropriate for modern 87 unleaded, no-ethanol fuel.

Thanks,

John

ford38v8 · 04-26-2016, 01:24 PM

I don't know if you'll get any pinging with modern fuel on a flathead engine. The old fuel had low octane, modern fuel not so. The vac brake adjustment today is pretty much useful only to prevent erratic bouncing of the advance plate. If your engine runs smoothly, that's about all you can expect out of it.

Bruce Lancaster · 04-26-2016, 01:24 PM

The brake could be described as changing the rate of advance...it essentially puts a drag on the operation of the centrifugal advance at low vac that slows down the advance.
Probably easiest way is to back it off almost entirely, almost because some drag stabilizes the brake disc that can otherwise float a bit. Back it off with fingers, because if you go too far with a wrench you will pop the spring seat off of the bolt almost effortlessly. Put it back down maybe 1 1/2-2 turns and see if you have pinging as you drive. Generally modern gas is high enough octane to need no further adjustment.
If your 85 is running the '37-41 type distributor, the base of the shaft at drive end will show a 68 or an 11...find an 11 shaft and advance as used in 1941 and you will get a slightly quicker and faster advance that should pick up mileage and power. Your mileage is way off!

J Witt · 04-26-2016, 04:09 PM

It's this distributor, if I understand what you mean:

Bruce Lancaster · 04-26-2016, 04:19 PM

Yes...all the '32-48 use the same adjustment, the bolt with locknut at bottom center of your picture.

J Witt · 04-26-2016, 05:58 PM

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OK, I set it as described and also bumped up the static setting by 4 degrees, will give it a road test tomorrow.

Thanks very much for taking the time to describe this. I couldn't find a good answer on how that vacuum brake worked and it's not shown in any of the stuff I have.

John

04-26-2016, 01:05 PM	#1
J Witt Senior Member Join Date: Mar 2015 Location: Acworth GA Posts: 534	Vacuum advance adjustment What would be the amount of advance change expected per turn of the vacuum brake adjustment screw. I know it will vary with throttle opening, but in getting the timing adjusted, is one full turn a lot or a little? How much pinging should one expect with the the screw backed out to start? I would assume one looks for an audible ping or rattle at low rpm/full throttle condition (WOT). Given that, how much adjustment is made at each effort to get the ping to just go away? This is with stock 85 hp, iron heads engine. I think it's not too far off, but the fuel mileage is only about 12. It runs pretty good though, but is a little lacking in hill climb power. And, a corollary is, if the static advance is centered, how much advance would be appropriate for modern 87 unleaded, no-ethanol fuel. Thanks, John __________________ Houston, Tranquility base here. The Eagle has landed.

04-26-2016, 01:24 PM	#2
ford38v8 Senior Member Join Date: Jul 2011 Location: SF Bay Area Posts: 6,661	Re: Vacuum advance adjustment I don't know if you'll get any pinging with modern fuel on a flathead engine. The old fuel had low octane, modern fuel not so. The vac brake adjustment today is pretty much useful only to prevent erratic bouncing of the advance plate. If your engine runs smoothly, that's about all you can expect out of it. __________________ Alan

04-26-2016, 01:24 PM	#3
Bruce Lancaster Member Emeritus Join Date: May 2010 Location: Madison, NJ Posts: 5,230	Re: Vacuum advance adjustment The brake could be described as changing the rate of advance...it essentially puts a drag on the operation of the centrifugal advance at low vac that slows down the advance. Probably easiest way is to back it off almost entirely, almost because some drag stabilizes the brake disc that can otherwise float a bit. Back it off with fingers, because if you go too far with a wrench you will pop the spring seat off of the bolt almost effortlessly. Put it back down maybe 1 1/2-2 turns and see if you have pinging as you drive. Generally modern gas is high enough octane to need no further adjustment. If your 85 is running the '37-41 type distributor, the base of the shaft at drive end will show a 68 or an 11...find an 11 shaft and advance as used in 1941 and you will get a slightly quicker and faster advance that should pick up mileage and power. Your mileage is way off!

04-26-2016, 04:19 PM	#5
Bruce Lancaster Member Emeritus Join Date: May 2010 Location: Madison, NJ Posts: 5,230	Re: Vacuum advance adjustment Yes...all the '32-48 use the same adjustment, the bolt with locknut at bottom center of your picture.

04-26-2016, 05:58 PM	#6
J Witt Senior Member Join Date: Mar 2015 Location: Acworth GA Posts: 534	Re: Vacuum advance adjustment Sponsored Links (Register now to hide all advertisements) OK, I set it as described and also bumped up the static setting by 4 degrees, will give it a road test tomorrow. Thanks very much for taking the time to describe this. I couldn't find a good answer on how that vacuum brake worked and it's not shown in any of the stuff I have. John __________________ Houston, Tranquility base here. The Eagle has landed.