Ported vacuum no longer working after throttle shaft re-bush

254 Views 8 Replies 3 Participants Last post by surfbox, 14 d ago

surfbox

Discussion Starter · 20 d ago

Hey all, this is my first and certainly not last post. I have a 1978 F250 with a 460 and all of the factory original smog equipment (I'm in California after all). I've also posted this question at ford-trucks.com, and I'm curious to see what kind of answers I get here vs. there. Now onto my issue:

I recently installed bushings in both throttle shafts on my 4350 Motorcraft/Autolite carb. I did this on the base plate from a spare carb in case something went wrong, but it seemed to come out great. I clamped the base plate vertically in my mill and was very careful to take my time and get it right.

My old base plate had a lot of wear in all directions. Not only did the shaft have axial play, but the primary butterflies had noticeable gaps all the way around with the throttle closed. Just eyeballing it I would say you could see a .005-010" gap all the way around each butterfly, as if they were too small in diameter. However even with this amount of play, the ported vacuum with that setup worked fine.

The butterflies in the spare base plate I modified fit much better in the bores, with barely any light visible around their edges when closed. Before reassembling everything I decided to dress up the butterflies by tapering the leading and trailing edges slightly, with careful attention to leaving the very outer edge undisturbed so as not to induce any air leaks. I didn't touch the top leading edge that uncovers the port, as it already had a slight taper from new. As I mentioned before, on reassembly the fit was very precise, with hardly any light visible form the back side when closed. I also thinned down and contoured the back side of the throttle shaft to improve air flow at WOT.

Now after reassembling everything I am getting constant vacuum from the port even at idle. I'm really baffled by this and wondering if the work I did on the butterflies could be the cause. It just doesn't make sense considering all I stated above. I can always swap in the original butterflies as a test, but I'm wondering if there's some other possible cause that I'm not thinking of.

By the way, as far as I can recall when the spare carb was on my truck the ported vacuum worked fine... I think. Or at least I think I think... ;-)

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Stevem

· #2 · 20 d ago

For any ported vacuum port to functioning either the throttle plate has to be open enough to expose the slot that feeds it, or you have a throttle plate to center section gasket issue .

These are the only two things in my mind that I can see to cause that issue.

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surfbox

Discussion Starter · #3 · 20 d ago

Stevem said:
For any ported vacuum port to functioning either the throttle plate has to be open enough to expose the slot that feeds it, or you have a throttle plate to center section gasket issue .

These are the only two things in my mind that I can see to cause that issue.

Thanks for that reply. Over at the ford-trucks.com forum somebody suggested that my thinning of the underside of the leading edge of the plates has uncovered the port below the plate. I'm going to try the stock plates and see if the issue goes away. I will also check for any gasket issue like you mentioned.

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Stevem

· #4 · 19 d ago

If your looking for air flow gains then thin the throttle shaft, not the blades.

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surfbox

Discussion Starter · #5 · 19 d ago

Stevem said:
If your looking for air flow gains then thin the throttle shaft, not the blades.

That's exactly what I'll have if I put the stock blades back in, at which point I'll be quite curious as to whether or not that fixes the problem.

It seems to me that as long as the top edge of the blade (which I didn't touch) is covering the port, it shouldn't matter what's happening underneath. I suppose I could have framed my original question without mention of the blade mods, since there may be other potential reasons for the issue I'm having that are being overlooked because my mention of the blade mod is leading the discussion down the wrong path. Or not. I will find out and report back.

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19Fo79rdF250$

· #6 · 18 d ago

surfbox said:
That's exactly what I'll have if I put the stock blades back in, at which point I'll be quite curious as to whether or not that fixes the problem.

It seems to me that as long as the top edge of the blade (which I didn't touch) is covering the port, it shouldn't matter what's happening underneath. I suppose I could have framed my original question without mention of the blade mods, since there may be other potential reasons for the issue I'm having that are being overlooked because my mention of the blade mod is leading the discussion down the wrong path. Or not. I will find out and report back.

Maybe some debris stuck in the port? I noticed you said something about machining.

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surfbox

Discussion Starter · #7 · 18 d ago

19Fo79rdF250$ said:
Maybe some debris stuck in the port? I noticed you said something about machining.

Yeah I was thinking the same thing but also thinking that wouldn't debris block air flow?

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19Fo79rdF250$

· #8 · 15 d ago

surfbox said:
Yeah I was thinking the same thing but also thinking that wouldn't debris block air flow?

I misunderstood your post. I thought no vacuum. But you have constant vacuum. Does the throttle blade close above or below the port? If below, that would be correct. If above, incorrect.

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surfbox

Discussion Starter · #9 · 14 d ago

19Fo79rdF250$ said:
I misunderstood your post. I thought no vacuum. But you have constant vacuum. Does the throttle blade close above or below the port? If below, that would be correct. If above, incorrect.

I think what is going on is that because I thinned the underside of the blades the ports are now open on the underside even when the blades are closed. I'll be putting the stock blades back in soon and will report back.

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