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first post on this site. 79 f250 with towing issues. checked timing and have a problem with mehanical adv coming in late. question is, what is too much vacume.
600-800 rpm 0"
800-1000 rpm increases to 12"
1500 rpm 17"
2000 rpm 18"
2500 20+"

checked in park not on road.

79 F250 SC CS
04 F350 dually 6.0 PSD
 

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Scientificly speaking, 21" of mercury is a perfect vacume. It doesn't get any better than that.....even in space! LOL What you're seeing is a ported vacume signal from the carb, designed to be 0" at idle because the throttle plates are below the port in the venturi. As soon as the throttle plates start to move and there is air movement past the port it sends a vacume signal to the VA @ the dist.. With the high vacume readings that you've posted the VA should be all in by then, and if I remember correctly it takes in the neighborhood of 8" or 10" (depending on the VA/year of the dist/year motor/emission parameters) to pull the VA all in. High vacume readings also are the result of alot of timing advance at idle (maybe too much). Are you experiencing detonation at those lower RPM's? If not then all is well with the timing, if you are then back the inital back to the stock setting with the VA line disconnected and pluged......bottom line......you've got great vacume and that tells me that the rest of the motor is probably in great shape too. High vacume means that you should be experiencing crisp/responsive throttle response. Hope that helps.
Rob
 

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Mech set at 8' no vac. Still bogs down at low rpm. Checked fuel pressure and only have 3.8 PSI to carb. After shutting down the presue drops to 0 after 10min. Probably a bad fuel pump. Only run engine to 2800 RPM max. So I always have vac advance. Am going to get new fuel tank and line and a new pump. New plumbing all the way to carb. Can I use braided fuel line from the rear tank to the pump? Stock has combination of rubber and steel. Truck set for a long time and I can't seem to get the tanks clean.
 

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8 sounds awful low for initial, this will cause your "BOG" problem, try 12 to 14
 

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With factory iron heads total advance should be somewhere in the 34 to 36 range for best power. In a towing combo a bit less can be utilised. I usually place max mechanical advance in a few hundered rpm above your cruising rpm at 60 to 70 MPH.

Vacuum advance is at part throttle and with out egr should be limited to a range of 8 to 14 degrees additional at light load part throttle amount can be tailored via a 1/8" allen wrench in the vac canister nipple. CCW lessens and delays. CW allows more earlier.


:D
 
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