I just dropped a ZZ502 with Holley 4150 carb into my 1980 and my vacuum secondaries are not opening at all and it feels like I have less then 100HP (can't even spin the tires). I can't even rev it past 3500 rpm.

I did the paperclip test and I am sure that the secondaries are not opening at all even after driving around for a while with varying throttles. I can open them fully manually by hand so I don't think it is a clearance issue.

I am running BB hooker headers/sidepipes with the BB reverse flow inserts. One of my mechanic buddies thinks that the exhaust is way to restrictive and this might be what is causing the problems. Does this make sense that a restrictive exhaust could cause this much power loss and the secondaries to not open?

I might buy the holley vac sec spring kit to see if some of the lighter springs help. The carb is brand new and came from GM with the ZZ502 package so I don't really think that the carb is broken.

I am going to remove the muffler inserts tonight to see if that helps, but If anyone has any other suggestions of what my problem might be it would be greatly appreciated. Thanks in advance.

 

Your car has got to just be out of tune.
With a 502, you should be able to burn the tires off without ever getting into the secondaries.
Also, while the inserts you are running are restrictive, they will cause you more trouble at higher RPMs.
You should still make good low rpm torque. Where do you have the timing set?

 

Agreed - the tune is off. With a 502 you should be able to melt the tires with 3 plug wires off.

 

Is there possibly a fuel delivery problem? 3500 RPM is very low...

 

Is this the 850 cfm vaccum secondary carb that comes with the 502? If it were me, I'd trade it for a 800 DP.

But agree, with the above. Something is off. Set your timing to 36 total at 3000 without the vacuum advance hooked up, then let the initial fall where it falls. A simple way to find 36 degrees if you don't have timing tape on your harmonic, is measure the circumference of the harmonic then divide by 10. Take that length and measure from the TDC timing mark to somewhere just past 1:00 (if the TDC mark were at 12:00) and scratch a mark at 36 degrees (360 degrees / 10 = 36 degrees).

Make sure your timing and balancer are matched to each other. Chevy moved the timing tab around through the years.

See if that rips the asphalt. If it doesn't we'll start diagosing the carb.

 

I don't think your exhaust is the problem at all. It has to be in your timing or next in the carb. You may just have a bad seal in you secondary housing. Also you may have not put enough demand on the engine for the secondaries to even open. It took me to really romp on my 383 to get the secondaries to open with a 750cfm carb with the lightest springs.

 

Before checking timing, I'd double check that distributor. sounds to me you're off by 180*.

 

First off, thanks everyone for all the replies, all input is much appreciated.

I had set the initial timing at 12* BTDC at 800rpm as per the Chevy instruction manual and the marks on the balancer that came with the 502. The carb is the 850 CFM holley vac sec4150 that also comes with the 502.

I am running a Holley red elec fuel pump that is feeding a constant 7psi to the guage at the carb so I don't think fuel delivery is the issue.

During initial break-in and for the first 50 miles, the car idled/ran fine except for the lack of power issue. Last night I changed the vacuum secondary spring to the lightest one in the kit and when I had it open I inspected the diaphragm which seemed to have no holes or tears.

Now after driving it last night to test the new vac secondary spring, it was idling nice for the first 20 minutes or so, then all of a sudden it stalled. After I restarted it, I cannot get it to hold a good idle and it will always stall now if I don't constantly give it some gas to keep the RPM's up. I pulled a few of the plugs and they are all carbon fouled with a dry black soot. Do you think if I swap out the plugs it will idle nice again in order to start tuning the carb and re-checking the timing?

Now how should I go about tuning the carb and setting the timing properly?

I have heard conflicting information of what total timing should be and at what RPM (i.e. i keep heard 36* total, but is this at 3000RPM or 5000RPM?)

As for the carb, I had assumed it would run nice right out of the box since it was a brand new turnkey crate package. I checked the float level in both bowls and they seem fine. Should i try to adjust the idle mixture screws? I am new to carb tuning and don't really know where to start.

Thanks again for all your help.

 

You have a bad power valve.