Charlie is right, I had too many problems with an electric pump on a every day driver having no lasting ability as compared to a mechanical pump for longevity, dependability is what I needed.


-Gary Burnette-

HMMMM interesting. thats what I thought tooo i thought they were supposed to be very rugged. Mine was going from 1 to 6 psi about 50 times a second. Can the arm jump off the internal bar somehow? I will look when I get home and update.

I don't know, but given that a mechanical pump's function is dependent, to a degree, on engine RPM, some fluctuation at idle may be expected, particularly if it has a bigger cam which produces a rougher idle.  I'd rev it slightly to see if it stabilizes.

Lots of people still run mechanical pumps on pretty high performance motors, so that should not be an issue.  Gary, speedpro56 here has a ~500hp tbird with a mechanical pump.


Lawrenceville, GA

Right on Ted thank you so much. I got my holley 600 vac secondary carb really dial now and can feel the additional performance over the 500 cfm eddy I was running. I drove the truck to work today and felt some funnyness in the gas pedal of sorts around low rpm cruising. I checked the engine at lunch while idling and my fuel pressure gauge is all over the place. Its a carter mechanical fuel pump thats been on for about 6 months. I think its going out. I thought these things lasted for years. Maybe it is susceptible to the engine mods and a bit higher revs that a stock 292. The question is now do i go electrical fuel pump? LOVing the holley now with the 5.5 power valve and 64 jets. If anything I might go to 62 jets. Thanks a ton for all the help. I hope to drive my truck for the rest of my life. if no one buys it for lots and lots of $

The engine can be idling with 1-2” Hg manifold vacuum and the secondaries can still open the same as an engine idling with 15-18” Hg manifold vacuum.  The secondary spring tension is not related to an engines’ manifold vacuum.  When the vacuum secondaries are opening correctly, you will not be able to feel them as a jolt or hiccup when they open.  It should be a very smooth transition and all you should be feeling is the extra pull or acceleration afforded by the increase in horsepower that accompanies the extra cfm being provided by the carburetor.
  
Manifold vacuum at idle has no effect on the secondary spring selection for a vacuum secondary carburetor.  Keep in mind that most engines will have almost no or minimal manifold vacuum when the throttle is in the wide open position.  The air flow speed through the primary venturi of the carburetor is the controlling factor of permitting the vacuum secondary side of the carb to open.  It’s the secondary diaphragm spring tension in this case that dictates at what rpm the secondaries actually start to open.  Everything being equal, smaller sized carbs will prefer a stiffer spring while larger carburetors will sometimes like a secondary spring with reduced tension.  Always start with the secondary spring that comes with a carburetor and never try to second guess the spring rate without first testing the original.

Lorena, Texas (South of Waco)

With the isky cam in my engine only producing about 11-12 inches of vacuum at idle what has been your experience with the holley secondary springs? I have the kit. With low vacuum do you need a light or stiffer spring? thanks

The problem with double pumper carbs (mechanical secondaries) is that they are perfect sized only in the rarest of instances.  The slightest changes in weather conditions will have them too big or too small. Much of the time they are simply too big due to being fully opened while the engine is in an rpm range that’s simply too low to adequately handle all that cfm.  When picking a vacuum secondary carb, always err to the large side as the vacuum secondaries are opening based on demand and as such will only open the amount required and be perfect sized.  I have very good results with the 750 cfm vacuum secondary carbs on the Y engines as the secondary side of the carb is only opening the amount required by the engine.  The primary side of these carbs is still on the order of size of the original two barrel carbs so drivability is expected to be good when driving with a light foot.
 
Having too small a carb on an engine can potentially have the engine in a lean condition when pushing the rpms outside of normal ranges such as when using the passing gear.  This occurs due to the increase in manifold vacuum that takes place at the higher rpms which in turn allows the power valve to close and subsequently lean the fuel mixture.

Lorena, Texas (South of Waco)

Ok so I have another carb that has caught my interest. I am reading that many people really like having a carb with mechanical secondaries if they have a manual transmission. Any of you guys have thoughts or experience on this topic? I also read that with a manual trans and vacuum secondaries you can get away with a slightly higher cfm carb. Thoughts. Quikfuel have this 450 cfm carb i am highly considering. https://www.quickfueltechnology.com/carburetors/street/hr-series/hr-series-carburetor-450cfm.html

I'm saying try both if you can.  If you can't, then go with the bigger one, which is the Summit carb,


Lawrenceville, GA

Charlie you are saying go with the bigger one the summit?