I have a Holley 870 Street Avenger with vacuum secondaries on a 460/C6. Due to the cam I run, my vacuum is 10 inches at idle in neutral, 7 inches at idle in gear. I'm using a 3.5 power valve and have idle circuits and jetting handled.

My problem is the vacuum secondary spring(s). I've used all of them, including the so called lightest one, and still experience slow opening of the secondaries under hard acceleration when I know the engine is ready for more carb. With my relatively low engine vacuum, I'd think it would be the opposite. Holley's tech line still hasn't gotten back to me.

Any suggestions appreciated.

 

Hi,

While I may not be able to help you directly with your problem maybe this will help for you to figure out your problem.

The way I understand it, you can't use manifold vacuum numbers when trying to figure out vacuum secondary opening points. For the secondaries to open the velocity of the air flow in the primary ventures create a vacuum at a metered port in the primary. This vacuum signal is routed to the diaphragm and starts the vacuum secondary diaphragm to collapse thus opening the secondary throttle plates. This occurs when the vac signal overcomes the strength of the diaphragm spring. So, it is the "speed" of the air flow in the primaries that open secondaries, not manifold vacuum directly.

To support this: if I am in 4th gear and putting along at say 15mph and floor the gas, I will have near 0 manifold pressure but also low air velocity in the primaries(because I am putting along in 4th) and no opening of the secondarie's until the RPM's come up alittle. Then the secondaries will start to open. If this delay was not there then the engine would bog and flood.

Are you using the quick change cover on the diaphragm? Reason I ask is because if your not and you are making lots of spring changes that diaphragm is VERY easy to tear. Thus causing a vacuum leak at the diaphragm and would result in your problem. In fact this is what it sounds like to me is the problem. A vacuum leak. Even if you are using the quick change cover I would look for a vacuum leak all around that housing. This is going to be easier said than done as you can't just spray carb cleaner around it like you would normally do due to the fact that the vac signal is not there at idle. I have tested them before by removing the diaphragm assembly from the carb body, then compress the plunger rod underneath it. With the plunger compressed cover the port on the side with your thumb and release the plunger. The diaphragm should stay compressed. If the plunger slowly comes back out, you have a leak.

Sorry this is so such a long post and good luck,

 

Thanks for the reply. It does have the quick change cover and I'll check that out.

 

Good post from Lee! Also if you have had the choke body off the carb there are on or two very small cork gasket between it and the carb.

 

I think that the yellow is the lightest. It will not open very fast, beacuse if it does, it will bog due to the lack of an acceleration circuit on the secondaries.
A 460 @ 100% VE can only use 425 cfm @ 3200rpm. 1/2 of 870=435. You are only needing the front barrels until around 3000 rpm anyway so they really do not need to open any sooner.
you will have to turn about 6600rpm to use 870cfm.
Jimmy

 

That's pretty standard on stock cam ramps. This one is a 235/242 @ .050 .572/.586 solid lifter cam. The steep ramp and overlap will usually burn fuel faster than velocity alone can provide it once it reaches the beginning of the power curve, which normally calls for a double pumper. Its a street truck and I was trying to keep my mileage above 5mpg and not foul the plugs with the vacuum secondaries.

 

Yeah, I see. The yellow spring is the lightest and will cause the secondaries to open the soonest. Other than that, I don't know of any other way to open them faster without a cheater screw in the secondary linkage and that will cause it to bog because of the lack of an accelerator pump.
Jimmy