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By 62bigwindow - 9 Years Ago
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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?
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By charliemccraney - 9 Years Ago
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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.
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By 62bigwindow - 9 Years Ago
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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.
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By charliemccraney - 9 Years Ago
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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.
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By miker - 9 Years Ago
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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.
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By NoShortcuts - 9 Years Ago
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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. 
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By 62bigwindow - 9 Years Ago
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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.
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By charliemccraney - 9 Years Ago
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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.
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By speedpro56 - 9 Years Ago
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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.
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By 62bigwindow - 9 Years Ago
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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?
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By charliemccraney - 9 Years Ago
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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.
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By 62bigwindow - 9 Years Ago
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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.
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By miker - 9 Years Ago
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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.
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By Kahuna - 9 Years Ago
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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
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By Ted - 9 Years Ago
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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.
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By 62bigwindow - 9 Years Ago
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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.
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By 62bigwindow - 9 Years Ago
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Forgot to add that I don't have a tach but I would say it is idling around 900 rpm.
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By charliemccraney - 9 Years Ago
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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.
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By 62bigwindow - 9 Years Ago
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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.
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By charliemccraney - 9 Years Ago
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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.
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By 62bigwindow - 9 Years Ago
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Thanks. I appreciate your guys being so patient with me.
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By Kahuna - 9 Years Ago
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62,
If all is good as you say, try advancing the timing a bit. The richness you describe may be unburned fuel in the exhaust.
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By 62bigwindow - 9 Years Ago
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Thanks. That was next on my list. Right now I have it at 14° initial. Going to try 18°. Also a valve lash adjustment. The cam manufacturer recommend. 022 hot so that may be off also.
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