Secondaries not opening?

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Dustin The PcP

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At first I thought it was the vacuum diaphragm so I took it apart, and it was torn so I replaced it. My secondaries still do not open up at WOT though. I have a holley (#1850) 600cfm with electric choke. I also tried changing it out to a holley (#3310) 750DP with electric choke, and the secondaries would not open up with that one either. The 750DP does have other problems with it, but I was simply trying to see if I could get the secondaries to open with it. I am not completely sure of how they work, but I didn't think it could be a vacuum leak because my power brakes still work.

So I have a couple of questions.

1. What could be the cause of the secondaries not opening up?

2. My 750DP will not idle down past about 1200RPM without dying, I have adjusted and adjusted it, I even replaced the power valve in it to see if that would help, but it did not. I am at a loss with what to do, all of the gaskets are fine on it. If it was adjusted to lean would this cause the problem? I just need help with this one, if you have any questions or want more information please ask.
 
1. Check to see if you can manually open the throttle plates by hand. You do this by manipulating the linkage on the side and looking down the top of the carb (WITH THE ENGINE OFF).

If you can not open the throttle plates by hand you have a mechanical issue.

2. How do you know they are not opening?
 
1968FormulaS340 said:
1. Check to see if you can manually open the throttle plates by hand. You do this by manipulating the linkage on the side and looking down the top of the carb (WITH THE ENGINE OFF).

If you can not open the throttle plates by hand you have a mechanical issue.

2. How do you know they are not opening?
Click to expand...

1.Yes, I can open them manually. And yes they do squirt fuel when I do.

2. With the engine running, I pull the linkage back on the carb manually to WOT, and the secondaries do not open.
 
They will NOT open unless the engine is under a load. That is how the carbuertor is designed. There is not enough vacuum signal with no load just free revving the engine. If the carburetor is in good shape, I can promise you under load at full throttle they are opening.
 
Dustin The PcP said:
2. With the engine running, I pull the linkage back on the carb manually to WOT, and the secondaries do not open.
Click to expand...


This is what I suspected. With a vacuum secondary carb the secondary throttle plates open under load.

You will never see them open by winging the throttle.

When properly set up you shouldn't feel the secondary come in on a vacuum carb.

They are probably working fine.

This is a very common mistake made by people unfamiliar with how they work.
 
Different color springs can be added for opening the secondaries sooner.The 3310 is a great carb and came on many brand BB cars back in the day.
 
This makes sense to me, thank you both very much. I am pretty new to messing with carburetters, and I was told that they should open regardless.

Any suggestions on what to do, to get the holley (#3310) 750DP running correctly?
 
First time I messed with a 3310 Holley, was around 1970. LOTS of us made the same mistake you did in this matter.
 
Make sure the giant vacuum fitting pointing out the rear of baseplate is plugged if you don't have PCV valve.
 
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