You don't simply take the carb off to set the transfer slots.  With the engine idling, use a mirror to look down the primary bores.  If you see fuel coming out of the boosters, you are idling on the main circuit, which means the primary throttle is too far open.  You should also find that the idle screws have no or very limited effect if it is idling on the main circuit.  Also look down the secondary bores to verify that no fuel is being provided at idle.

If no fuel is provided by the primary or secondary boosters at idle, then the throttle position should be ok.

If the primaries boosters are active but not the secondaries, close the primaries slightly and open the secondaries slightly.  There is a screw on the bottom of the carb to do this and you have to remove the carb to gain access.
If the primaries are not active but the secondaries are, close the secondaries slightly and open the primaries slightly.
Repeat the above until it is idling only with the idle circuit.

If both primary and secondary boosters are active, then you can drill holes in the throttle plates - google it - or get a bigger carb.

Regarding secondary spring tuning, Holley have a spring kit as well as a kit which includes the quick change housing.  Follow the instructions.  If you can feel the secondaries open, then it is incorrect.



Lawrenceville, GA

Thanks Charlie. I've done a ton of research and no one has suggested to adjust the carb that way. It seems alot easier to do without taking the carb off. I can get to the secondary set screw without taking the carb off. I have a 1" spacer to get the carb away from the heat . I'll see what damage I can get done this weekend. Thanks again.

Durham Missouri

Just a couple thoughts. My roadster runs one of Mummert's 284 cams, within a couple degrees @.050 of yours. Lots of other variables, like lobe centers, timing , etc. Mine's a stroker, at 340cid, but I get close to 14" at idle, depending on tune and temperature.

Someone just has some other post similar to Charlie's about the secondary idle opening. I think it was here, and by Greenbird56. Might have been the ford barn. Worth a search.

miker
55 bird, 32 cabrio F code
Kent, WA
Tucson, AZ

Chassis dynos are expensive. If you are capable of doing the work yourself, that's the direction I would go.
I would buy and install a threaded bung for one or both of the exhaust head pipes and get an AF meter.
I would check for vacuum leaks using a propane torch (unlit, of course).
Your cam is NOT too big . Mine is 246 @ .050. (Not a Y block).
My idle is 900 rpm with 10" vacuum.
Your carb is just fine. There should be NO reason to have to mess with the secondarys for idle stability.
This may only come up when using (2) 4 bbls.
First, check for vacuum leaks

You don’t mention what your cubic inches are but the camshaft being 236° @ 0.050” isn’t that big a camshaft in the grand scheme of things.  The 312 engine I use as a dyno mule for testing has a Crower 238° @ 0.050” cam in it and actually idles quite well at 850 rpms with a 750 Holley on it.  Your manifold vacuum is on the low side but I failed to see where you mentioned the actual rpms the engine idles at.   The dyno mule I use for testing has 12-13” Hg vacuum at 850 rpms.  Changing out the carb to a smaller size does not affect the manifold vacuum at idle on this engine.  Changing the ignition timing and valve lash does have an effect though.
  
Where your camshaft is installed in regards to intake lobe centerline and TDC will have an effect on the manifold vacuum at idle.  If the camshaft was not degreed in upon installation, then you may have a camshaft that’s not installed at the optimum position and based on the manifold vacuum at idle, then it could be in a retarded position.  The valve lash being set on the ‘tight’ side will also have a negative effect on manifold vacuum at idle.
 
Assuming that the distributor vacuum advance chamber is hooked up to the staged signal port on the carb, then there should be no vacuum signal to the distributor at idle.  As soon as the throttle is cracked open, then the vacuum advance portion of the distributor will throw some extra advance into the ignition curve; this occurs due to the staged vacuum port hole within the carburetor throttle bore now being below the throttle blades instead of above them.  By its original design, the vacuum advance works specifically during cruise situations to add additional ignition advance for a leaner fuel mixture during low load conditions (cruising).  At idle and any primary throttle position past 1/3 open, there is no staged port vacuum signal available.  This is due to the lack of air velocity going across the hole located within the throttle bore when the throttles are either at idle (closed) or open to the point that air velocity drops in speed enough that the vacuum signal cannot be generated.
 
If the vacuum is hooked up to a direct vacuum source, then the tuning parameters must be altered to compensate for a full vacuum advance when the throttle blades are in the idle (closed) position.  This is a different set of tuning parameters that will not be found in the factory service manual for your Y-Block.

Lorena, Texas (South of Waco)

Ok,my engine is a stock bore 292,has mild head porting 1.85 and 1.6 valves,1.43 rockers,cam is 280°duration 110 LSA 236@.050,9.1 comp,valve springs are 70lbs seat 190 open, ported Blue Thunder intake with a Summit 600 carb.
I checked the carb as Charlie suggested and all is good there. No fuel from either booster. I put the vacuum gauge back on and played with the idle mixture and got it up to 12 hg. It is still a tad rich though. I did notice the vacuum gauge would bounce about 2-3 hg at idle. I checked for vacuum leaks and didn't find any. Summit has a idle feed restrictor kit that is supposed to cure a rich idle. How does it do this if no fuel comes from the booster at idle? I was planning on going down one jet size on the booster and see if that helps.

Durham Missouri

Forgot to add that I don't have a tach but I would say it is idling around 900 rpm.

Durham Missouri

If you want to get serious about tuning, you need to have a tach of some kind.  No guessing allowed.
If you don't want to have one permanently in the car, there are tachs designed specifically for tuning, which attach, temporarily, to the coil using alligator clips.

I didn't realize that summit carbs had that tuning ability.  That is pretty sweet.  Might cure your problem.



Lawrenceville, GA

The Summit carbs also have a accelerator restrictor kit available. I looked for a cheap tach that would hook up to the 1 wire hook up that is on my MSD dizzy. No dice yet. What exactly does the idle feed restrictor restrict? The jets are in the booster but no fuel is coming out at idle.

Durham Missouri

The restrictors are probably the same thing as air bleeds, which affect the fuel curve.

There should be loads of tachs that will work.  It is not a one wire tach.  It is simply the output for a tach.  The tach will still use 2 or 3 wires.



Lawrenceville, GA