Ported vacuum at idle?

[xsdg]

So, I have to pull the carb for an unrelated reason, but I finally had a chance to dig into a couple issues that my 720 has been having.  If you have suggestions of things to double-check while the carb is off, lemme have 'em.

 

I have the stock Hitachi Z24 carb.  I rebuilt it last year after I got the truck, and given that I just fixed some symptoms that showed up after the rebuild, I'm pretty sure it's had this issue since the rebuild, if not before.  I can't say anything conclusively about before the rebuild because it had a bunch of vacuum leaks and was continually running in limp-home mode.

 

Anyway, looking at the carb from the right side (passenger side in US), there are two vacuum ports at the base of the carb.  I found that the left one shows no vacuum at idle, and the right one shows 5.5 in Hg at idle.  Wha?  From previous tests, I know that I run 18-20 in Hg at idle from the manifold, and my measurement was made without the vacuum restrictor thingie that's in a few of the vacuum lines.

 

After switching ports and re-adjusting the idle screw, I'm running 3deg BTDC at idle, and I'm idling right at 850RPM.  Based on [1], 5.5 in Hg will activate the EGR at idle if I plug it into that port, so that's not right either.

 

As mentioned above, I'm about to pull the carb, so it'll be a good opportunity to take a closer look at everything.

 

 

[1] http://community.ratsun.net/topic/15144-z24-engine-advice-wanted/?do=findComment&comment=204932

 

 

[xsdg]

So, after posting this, it occurred to me that I've got a spare carb that I can investigate.  I (accidentally :o) hooked up a blow gun on my air compressor to the port where I was seeing no vacuum at idle.  I put the throttle in the fully-warm idle position (so off the fast idle cam).  Then I dropped some water down the carb (just to get things wet) and hit the trigger on the blow gun.  Here's what I saw:

 

You can see that the air has evaporated the water that was sitting by the port that's directly upward (in the photo) of the brass nipple.  So this port is clearly connected to some fraction of manifold vacuum at idle.  That said, this is the up-turned port, which is the one where I measured zero vacuum at idle.

 

 

I took another picture with the throttle butterfly open.

It appears that both of the ports on the right side (both directly upward of the brass nipple in the photo) are connected together.  That is, when I continued blowing air through that single external port, the water clears away from both of these internal ports.

 

That said, the ports to the left (in the picture) of the brass nipple (and downward of the left butterfly screw) also appear to be connected to each other.  When I connected the blow gun to the straight port, where I was seeing 5.5 in Hg, I saw air out of both of those ports.  Clearly, the bottom one is going to see manifold vacuum at idle, unless the butterfly is 100% closed.  Given the size of that port, I'm guessing the partial-vacuum is intentional, and I'm guessing that it ramps up from partial vacuum to full manifold vacuum as the butterfly opens.