New springs for the carb? Also, ID the weird vacuum port on the manifold...

Well...the K5 is officially back on the road...sort of.
The last change I made to the carb (rejetting) before the fire apparently *did* lean it out a bit. However, there's a bad lean spot
right off idle...right where you hold it to cruise. So today, my driving method involved "idle...accelerate...repeat" to maintain speed. I gotta get under there and check a couple of other things. I may have discovered yet another vacuum leak. Actually the tranny guys did. It may have been plugged off before the fire, and the "plug" destroyed in the fire. I'm just not sure.
It's between the carb and distributor, right on top of the intake--a big vacuum port. Big as in, "brake booster big". It has a line coming of it with a couple of "T's" but it's all plugged off. It could've been a source of vacuum leak.
Question #1: What's that port for?
Anyway, carb's an Edelbrock Performer, 600cfm... Uses metering rods and springs.
Question #2: Would putting a slightly stiffer spring in there help it to not go lean and bog down right off idle?
If I get on it a little bit, it quickly tries to get out of its own way. There's just no "cruise mode". I can tell the potential for good power exists. And the new, lower 3.73 gears in the rear seem to pull well on the interstate.
Thanks,
~jp
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Jon, try a step richer metering rod, the springs will probably send it to full rich prematurely. Like I think I said before make sure the rods move freely in the jets. The vacuum port can be used as a feed off point for any or all vacuum accessories, usually though the booster is a stand alone port.

-
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

You may want to recheck the timing and the vacuum source to the distributor. Off idle stumbles are often traced to the distributor.
Dave
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Thanks Shep and Dave... I'll pick up a calibration kit for it. I believe that'll come with new rods. If not I'll pick them up too. The local performance shop stocks that stuff.
I'll double check the vacuum and reset timing.
If it means anything, when it's near the "stumbling" stage, I can hear a whistling, or whooshing of air moving. That's with the throttle cracked open slightly while attempting to "cruise". That sound is unfamiliar to me and isn't present at idle or WOT.
~jp
Hairy wrote:

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Jon R. Pickens wrote:

figure....if you look closely it is either tiny hole(s) or slit(s) just above the throttle plate-they could be plugged. also look for a vac port that draws vac at just off idle to make sure its not leaking. I'm not sure this is exactly what you are discribing though....this is for *just* off idle not really "cruise" off idle.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Jon,
Did this originally have a 700R4 in it? If not could the vacuum port have been running to the vacuum modulator valve on the tranny? I know the th400s/350s had the valves on them and a decent amount of vacuum going to them. If it had a 700 originally, then completely disregard :)
ShoeSalesman wrote:

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Yep... 700R4 is what is has. The new tranny is setup to work without any electronic or vacuum feedback from the engine.
My vacuum lines are as follows: port on the manifold is plugged. brake booster is connected. manifold vacuum port on the carb plugged. PCV port connected to a new PCV valve with new hose. ported vacuum plugged directly to the vacuum advance with new hose.
~jp
89GMC wrote:

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
I read this post this morning before heading off to work. I paid special attention to the feel of the accelerator and engine. It does have a "*just* off idle" response. But just past that you hear the hissing of air and it bogs really bad. Sometimes it'll slightly backfire (sounds like it's through the carb) if I try to hold it there--especially bad when cold.
~jp
ShoeSalesman wrote:

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Classic lean symptoms, make sure the carb base gasket is good, also try to close the choke partially, keep it locked there and see if it improves the problem, then you know it's lean from carb issues or a vacuum leak.

-
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Thanks... I feel very confident about the gaskets and don't believe there's a vacuum leak. I've been down that road already and I believe it's as leak-free as possible.
I'll be picking up a calibration kit and maybe a few extra metering rods to experiment with either tomorrow or Saturday morning.
~jp
Shep wrote:

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Related Threads

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.