vacuum advance
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Junior User
Joined: Jun 1999
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vacuum advance
Hi Guys,
Well I've finished my 300/6 to 302ho swap in my 85 f-150. I was told by a number of "Ford Experts" not to run my dist. off the manifold vacuum, "there should be NO vacuum at idle." When I have the AFB hooked up like the "experts" say I get a hesatation when drivin off from a dead stop. For the last 2 weeks I've pulled both gas tanks, I have 5psi at the carb., I've been swappin vacuum lines, installing vacuum check valves, pulled the carb. 3 times and checked the floats, adj. accel. pump, etc.,etc.,etc., and could not lose the hesatation, till I ran a vacuum line from the manifold distribution block to the dist., hesatation is gone, motor runs smoother, had to lower the idle speed. Can any harm come to the motor by runnin it this way? Why would the Ford "experts" tell me not to do this?
Ahh.....the mysteries of life........
Any Ideas or sugestions?
Moe
Well I've finished my 300/6 to 302ho swap in my 85 f-150. I was told by a number of "Ford Experts" not to run my dist. off the manifold vacuum, "there should be NO vacuum at idle." When I have the AFB hooked up like the "experts" say I get a hesatation when drivin off from a dead stop. For the last 2 weeks I've pulled both gas tanks, I have 5psi at the carb., I've been swappin vacuum lines, installing vacuum check valves, pulled the carb. 3 times and checked the floats, adj. accel. pump, etc.,etc.,etc., and could not lose the hesatation, till I ran a vacuum line from the manifold distribution block to the dist., hesatation is gone, motor runs smoother, had to lower the idle speed. Can any harm come to the motor by runnin it this way? Why would the Ford "experts" tell me not to do this?
Ahh.....the mysteries of life........
Any Ideas or sugestions?
Moe
Senior User
Joined: Jun 1999
Posts: 349
Likes: 1
vacuum advance
The distributor vacuum should be drawn from the carb venturi. As engine speed increases so does the venturi vacuum and advances the timing. Connecting directly to manifold vacuum puts 18 inches of Hg vacuum to the distributor at idle and immediatly results in maximum vacuum advance. Recommend advancing the timing at idle to about 10 degrees with the vacuum disconnected, then reconnecting to the carb and trying it. This assumes you have a single vacuum tube going to the distributor. Good Luck.
Posting Guru
Joined: Jun 1999
Posts: 1,100
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From: Durham
vacuum advance
I asked the same questions to tech@affordableperformance.com and they wrote back to advise the following:
>From: "Vernon Pitts" >
>Reply-To:
>To: "Steve Likness"
>Subject: Re: Questions
>Date: Sat, 3 Apr 1999 08:15:27 -0800
>
>Steve:
>We have found that MOST small block Fords respond better to manifold vacume to the dist. What this does is provide vacume to the dist at idle and at cruising speeds which helps with throttle response and milage. At wide open throttle you will lose most of your manifold vacume and thus lose the vacume advance which helps keep the engine out of detonation.>
> Timed vacume (ported) has no vacume to the dist at idle and gives you all the vacume advance off idle and even at wide open throttle which can promote detonation. Your best bet is to try it hooked up both ways with the base timming set at around 10-12 degrees. Manifold vacume can be picked up off the intake manifod or off the base plate of the carb. On an Edelbrock carb the vacume nipple on the drivers side of the front of the carb is manifold vacume or normal. The pass. side vacume port is ported or timed vacume.>
I'm currently running off the manifold vacuum, but I do get pinging when my engine is hot and under full acceleration. I think I'll try the timed port just to get a comparison.
'82 F150 w/351W (orig 300 I6)
Steve
>From: "Vernon Pitts" >
>Reply-To:
>To: "Steve Likness"
>Subject: Re: Questions
>Date: Sat, 3 Apr 1999 08:15:27 -0800
>
>Steve:
>We have found that MOST small block Fords respond better to manifold vacume to the dist. What this does is provide vacume to the dist at idle and at cruising speeds which helps with throttle response and milage. At wide open throttle you will lose most of your manifold vacume and thus lose the vacume advance which helps keep the engine out of detonation.>
> Timed vacume (ported) has no vacume to the dist at idle and gives you all the vacume advance off idle and even at wide open throttle which can promote detonation. Your best bet is to try it hooked up both ways with the base timming set at around 10-12 degrees. Manifold vacume can be picked up off the intake manifod or off the base plate of the carb. On an Edelbrock carb the vacume nipple on the drivers side of the front of the carb is manifold vacume or normal. The pass. side vacume port is ported or timed vacume.>
I'm currently running off the manifold vacuum, but I do get pinging when my engine is hot and under full acceleration. I think I'll try the timed port just to get a comparison.
'82 F150 w/351W (orig 300 I6)
Steve
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