Secondary opening rpm

whatbumper wrote:I hope you don't put that on your 302. If that is going on a big block then that would be better but your little 302 will not like the secondaries opening that fast. How much vacuum does your motor pull at idle?

whatbumper wrote:that carb came with a 72 primary jet and a power valve that opens at 6.5 inches of vacuum and the plain spring. I would wait until it is running to make too many changes. those are very easy to tune.

bb429power wrote:

whatbumper wrote:that carb came with a 72 primary jet and a power valve that opens at 6.5 inches of vacuum and the plain spring. I would wait until it is running to make too many changes. those are very easy to tune.

Well I know that it came with the black spring in it. But I switched it out to the white spring but that's all I did. I wasn't planning on doing any more until its done. I was thinking that I could run a bigger carb and I might switch it out to a bigger one down the road, but this one is fine for what I'm going to do. I was thinking about trying to radius the veturies like how the 4150 hp carbs are, but I don't think that's necessary.NOT NEEDED

Thanks.

EDIT- I was also thinking about using a shorter secondary linkage so it would open up really fast, but I don't think I'll do that.DON'T, Letting it work as it was designed is your best bet.

whatbumper wrote:

bb429power wrote:

whatbumper wrote:that carb came with a 72 primary jet and a power valve that opens at 6.5 inches of vacuum and the plain spring. I would wait until it is running to make too many changes. those are very easy to tune.

Well I know that it came with the black spring in it. But I switched it out to the white spring but that's all I did. I wasn't planning on doing any more until its done. I was thinking that I could run a bigger carb and I might switch it out to a bigger one down the road, but this one is fine for what I'm going to do. I was thinking about trying to radius the veturies like how the 4150 hp carbs are, but I don't think that's necessary.NOT NEEDED

Thanks.

EDIT- I was also thinking about using a shorter secondary linkage so it would open up really fast, but I don't think I'll do that.DON'T, Letting it work as it was designed is your best bet.

My chart is old so I guess they revised their carb a little since. Either way, I like that carb for general purpose duty.

EDIT: Dang, I checked and the black spring is the heaviest.

whatbumper wrote:I hope you don't put that on your 302. If that is going on a big block then that would be better but your little 302 will not like the secondaries opening that fast. How much vacuum does your motor pull at idle?

rmcomprandy wrote:Beside the spring, the secondary opening point and rate of opening of a vaccuum secondary Holley type carburetor are controlled by VENTURI vaccuum, (NOT manifold vaccuum), which is dictated by air speed mostly through the primary side at the venturi.

DaveMcLain wrote:Remember, manifold vacuum has ZERO to do with secondary opening rates/spring selection and what's optimum for the carburetor package and engine.

whatbumper wrote:

DaveMcLain wrote:Remember, manifold vacuum has ZERO to do with secondary opening rates/spring selection and what's optimum for the carburetor package and engine.

Right. nowhere above does it say that. we are helping him tune the carb. I was using the manifold vacuum question as a sense of how good the motor runs at idle to help with the different tuning techniques.



Now for a tuning lesson for bb429power. Carbs work off of Bernoulli's principle. As a fluid moves through a tube, the areas of low velocity produce high pressure, and the areas of high velocity produce low pressure (vacuum). So when the tube necks down and becomes narrow, the fluid flowing through the narrow section has to move faster, and pressure in the narrow section is lower than the pressure in the larger section. As air enters the carb, it passes through the “necked-down” area of the venturi. As it passes through the venturi, the air accelerates. This fast-moving air creates a low pressure area right in the middle of the venturi, and this low pressure area is used to discharge fuel from the float bowl of the carb into the air stream. The low pressure area in the venturi allows atmospheric pressure on top of the fuel in the float bowl to “push” the fuel up and out of the venturi discharge nozzles. The venturi vacuum changes only by the total amount of airflow passing through the venturi and its velocity. The faster the air is passing through (more air), the higher the venturi vacuum will be. As the other guys stated, which I did NOT contradict, this is completely independent of the manifold vacuum.

rmcomprandy wrote:I was under the impression that the thread was titled SECONDARY OPENING RPM and had nothing to do with the outright tuning itself, just what would control secondary opening RPM.
Also, to correct a small information glitch, the point in the venturi which will exhibit the greatest amount of LOW pressure is not in the middle, (that is where the low pressure area is most even), but, lowest pressure is located at the point of fastest air speed which is about .070" away fron the outside edge. THAT is also usually where the secondary opening port finds its signal.