Uncle Don - Off-Road Forums & Discussion Groups
Ford 67-96 F-Series, 78-96 Bronco All discussions of 67-96 F-Series Trucks and 78-96 Broncos

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post #1 of (permalink) Old 10-17-1999, 06:50 PM
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Uncle Don

Bog'n sent me this on his vacumm hook up to the distributer.

< mine is an 83 also but From the factory it have the red and yellow vacumm line the t into one just before the distributor.the red
< line has some kinda dohicky on it(looks like a very small filter whicn goes to manifold vacumm-t and the yellow goes directly to
< the carb timed port. this is before I went to a 4bbl and offy intake... I'm also running Autolite 45 plugs and they work good for
< me, but I think I like AC/delco plugs best.

Also, when I was a mechanic for a living, longer that I want to say, a fellow mechanic told me that vacumm advance is only
used when crusing. The mechanical advance is used during exceleration. So what I did was reconnect the vacumm to the
manifold and it stopped pinging and runs well. I just wonder what Bog'ns rig does. My guess the gadget he speaks of is a
check valve. What do you think?

Also I was asked when ordering my 85 I6 if I wanted EFI, and my neighbor ordered an 84 302 with it. he now has 250K on it.

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post #2 of (permalink) Old 10-18-1999, 02:30 PM
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Re: Uncle Don

Yeah, if it doesn't ping on light acceleration, then full manifold on the vac advance should work, as the manifold vac will go down at high RPM and heavy load (wide throttle opening). I think they usually set them up to have mostly vac advance at idle/light load, then as RPM would increase the mechanical advance would start to come into play, and at high RPM would be mostly mechanical as the vac advance would drop off.

Playing around years ago, I temporarily turned an Olds 455 into a real slow-rev'er by plugging the hose to the vac advance (so only the mechanical was left). When you would get on it, it would rev up slow. The vac advance can give a quick response to throttle opening when it's on the venturi port, but mechanical advance only increases as RPM increases (the two tied together mechanically through the advance weights and springs turning the breaker plate ahead in time).

It's been almost 30 years since I played around with changing weights and springs on mechanical advance!

On the Bronco, maybe they were trying to get some sort of mixture between ported and manifold vac. Check valves of the early 80's Fords were about an inch in diameter, but a "Vent" gizmo was black rubber, about 1/2 inch in diameter, maybe 1 inch or so long, with a little screen in the end of the cylinder. I had an '82 302 full-size Ford car that had an unbelievable assortment of check valves, restrictors, 3-way vacuum gizmo's, miles of vacuum tubing, etc. on it. All that stuff and the V-V carb disappeared with Fuel Injection.

So is your Bronc OK now? Or did you mean that on manifold vac it behaved the best, but still pinged at higher RPM?


post #3 of (permalink) Old 10-20-1999, 06:44 PM
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Re: Uncle Don

It seems to run very well now. I have great response on exceleration - no pining !

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post #4 of (permalink) Old 10-25-1999, 12:31 AM
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Re: Uncle Don

Agree with Uncle Don: I was running my '79 351M on port vacuum, and had pinging at high revs (light load, as in highway acceleration) with the factory advance at 10deg BTDC. Went to manifold vacuum, and found total advance at 53 degrees at 3K revs, well above the recommended spec, but it ran like a banshee (but I couldn't hear the detonation at high revs due to the exhaust noise and engine). Since then, I have disconnected/plugged the vacuum, and just let the centrifugal weights work at 8deg BTDC initial timing, and there is better economy (13-14mpg highway), throttle response, low end and no knock. The top end has no end. I recommend no vacuum advance if the distributor is properly weighted. Maybe this goes back to changing weights and experimenting, like Uncle Don said.

'79 Bronco
'89 Trooper
'86 Trooper II
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