I'm in the process if looking for a shop to do a little tweaking on my carb. It's still running a little rich and I just don't have the spare time right now to fiddle with it. So, in my search to find a shop that knows there way around a carbureted engine,alot harder than it should be, I talked to a local machine shop and they suggested that my problem was the fact my cam,236 @.050, is too big to run vacuum advance. I have never heard this before. Is there any truth to it?

Durham Missouri

I don't know if there is a point where the cam gets too big for vacuum advance but it is an easy theory to test.  Disconnect the vacuum advance and plug the ports.  Same situation or not?

It is also possible that your carb is too small for a cam like that, particularly if it is a lower priced, entry level type of carburetor.  The bigger cam affects the fuel curve negatively, making it a real challenge to tune via conventional methods.

It is not necessarily that your engine needs more cfm, rather a larger carb is typically set up for a higher performing engine so it can fulfill the fuel demands.


Lawrenceville, GA

Thanks Charlie. The carb is a 600 vacuum secondary. It's just a little rich across the board. It doesn't smoke black but the exhaust is a little strong. I think maybe a idle feed restrictor kit and smaller primary jets will correct it. The problem is finding someone around here that knows there way around a carb.

Durham Missouri

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

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

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

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

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 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-

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