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

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

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

Right now the initial is at 14°. My plan was to bump it up to 18° and change the advance springs for full advance at 3000 rpm. That should make total at 3000 rpm 38°. The power valve has been changed to a 3.5. I was planning on taking the carb if to set the primary transfer slot then put a vacuum gauge on it to set the idle mixture for max vacuum. Then see how rich it was at idle then start reducing the idle feed jets until it was ok. Then start on the primary jets until I got max vacuum at cruising speed. The secondaries are only going to be open enough to achieve a good idle if needed other wise I plan on setting them to just cover the t slot. I'm open to all suggestions as I'm a novice at this. Oh and the carb has vacuum secondaries if that matters. Thoughts?

Durham Missouri

I certainly don't see where the cam is too big to run a vacuum advance. Charlie does have some good points. What are you timing the engine at? are you adjusting the idle mixture screws for best idle? How big is the engine? Might try dropping the primary jets down two numbers too lean it out a bit.


-Gary Burnette-

With a cam like that, it is important that the power valve, idle throttle blade position, and secondary actuation and throttle position are correct for the application.

The power valve could be opening at idle.  The throttle blades could be open to far so that it is running on the primary circuit at idle.  Since bigger cams typically require a higher idle speed, this is very likely.  The secondaries might be opening sooner than they need to.  If the secondary throttle blade position is incorrect, the secondaries will be active at idle and cruise.


Lawrenceville, GA

I am running a MSD Pro Billet dizzy. My manifold vacuum at idle is 9-10 hg. As far as chassis dynos there are none close to me. Maybe a few hours drive from me but nothing close. I was just trying to see if the shops theory held any water. I didn't think it did but wanted to ask. I'm sure my engine just needs a good tune and tweaking. It runs good,no hesitation or flat spots and no problems starting. It is just a tad rich at idle and cruising speed. I just simply don't have the free time right now to work on it.

Durham Missouri

62bigwindow.  I'm late getting on board with what you've got under the hood of your '56.  IF you are running a pre-'57 LoadoMatic ignition distributor with the ignition advance curve controlled ONLY by engine vacuum signal, I can see why you may be having problems.

IF you are using a '57 or later FoMoCo distributor that has the combination of both centrifugal AND vacuum advance, that's a different situation entirely.  IF you are using a '57 or later ignition distributor, consider disconnecting the vacuum line from the carburetor to the distributor at the carburetor and plug the connection at the carburetor.  My suspicion is that the vacuum diaphragm on the distributor OR perhaps a connecting rubber vacuum hose is leaking.

The vacuum advance mechanism on the '57-'64 FoMoCo distributors only affects the distributor ignition advance when your engine is running at a constant speed such as idle or driving down the road at a constant speed with little load resistance.  Put another way, when the engine vacuum is high, the distributor vacuum advance mechanism adjusts the ignition timing to optimize fuel economy.  The rest of the time, unless the vacuum advance diaphragm is leaking, the distributor vacuum advance system has nothing to do with engine performance.

NOTE:  See Ted Eaton's entry further on in this thread for a better AND more complete explanation of the tie between carburetor staged vacuum signal and distributor ignition advance.  Ted, thanks!  

Hope that this or others suggestions help.    


NoShortcuts
a.k.a. Charlie Brown
near Syracuse, New York

Do you know what the vacumn is at idle and cruise? You don't give all the cam spec's, but I'm running one close to that in the roadster, and it's not a problem for either the vacumn advance or the vacumn secondaries. Did take some tuning, though.

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

Your best bet will probably be some place that has a chassis dyno and offer dyno tuning.  It will be expensive, but they usually know how to tune carbs.

Otherwise, you probably will have to figure it out yourself.

That carb may not really be appropriate for your combo.  Contact Holley and see what they recommend.


Lawrenceville, GA