Question for Carb Gurus

[Guest Z-rific]

I recently took off an old Q-jet and Perofrmer intake and bolted up a Weind X-celerator and Holley 4160 750cfm carb on top of my 350 (medoerately built). I know 750 is a lot, but after an upcoming head swap, I figure it'll be okay. I've gone down from 72 primary jets to 68 and tuned the whole system.

 

Here's the problem. I don't think my secondaries are opening (vacuum secondaries). I know they won't open without a load. On the road, the car doesnt have nearly the pull it did before. I know that low end torque has been sacrificed, but the engine feels much weaker.

 

I read that you can put a paper clip on the shaft below the secondary diaphragm housing and take the car for a hard run. I did this, pushing the clip all the way against the housing, took her for a hard run, and the clip didn't move at all.

 

I have taken the carb completely off and tried to force the secondaries open and they don't budge.

 

I know you can get different size springs to drop in there to open the secondaries sooner or later, but mine appear to not be opening at all.

 

All I have hooked up to the carb is the dual inlet feed and the vacuum advance. The little vacuum port in the base of the carb on the front passengers side is plugged. It's a mech choke, which I dont have hooked up either.

 

Any suggestions?

[blueovalz]

Make sure the primaries are open before attempting to open the secondaries. Closed primaries lock down the secondary linkage. I have seen the base of the manifold interfere with this linkage as well.

[Mike C]

What Terry said. Make sure that you are getting COMPLETE opening of the primaries with your throttle linkage setup. You can pull the vacuum pot and test with a mity vac or similar. ALso, that Xcelerator is a piece of crap. Go back to the performer, or SOMETHING else regardless of what carb you run. Let me revise that, without the plenum divider it is terrible. It is acceptable with it, but still a lousy intake all in all compared to some newer designs. Almost all of the dual plane aftermarket intakes will make not only more power, but a wider power band as well. If you have a bog with the Holley, work on the accelerator pump cam and shooter. Bog means too much air and needs more pump shot.

[Guest Z-rific]

Thanks guys.

 

It looks like the primaries open up all the way. No problems there.

 

I think I will open up the vacuum assembly and make sure the shaft works from inside. Then I might try buying a set of springs and trying different springs to see if anything changes.

 

I got the manifold cheap and it is a low riser, which gives me the hood clearance I was after. When I swap heads, I may look for a different low rise single plane manifold.

[Guest zbot43]

Ok, I use to work in a carburetor shop. I still do a lot of carburetor work. I happen to have 760 cfm vacuum secondary holley on my 350. I also have the accelerator manifold for hood clearance. first, I have to say, I disagree with with the recommendation to go with the performer. A performer RPM would be a better choice. But not a basic performer. I have had customers complain of loss of fuel economy, with no power gains to loss in power over the stock manifold. The only reason to go for the performer over the stock would be intial cost.

Now let's get to the carburetor. You are going to have one hell of a hard time getting that carburetor to open the secondaries, without changing the springs, and pulling out the check ball. The Z just doesn't weigh enough! On truck carburetors they make it much harder for the secondaries to open, otherwise you end up with spinning tires and loss of traction. ON Q-jets the spring for the secondaries wound be tighter. When you want those secondaries to open for wheel spinning fun, you just loosen the spring tension up. You have to do the same thing. Just make really sure you've got good traction. Otherwise you are going to have a car that is going to want to have the rear end where the front end was. It will be like hitting ice in a turn.

For what you are doing I would recommend going with a mechanical secondary. You are the one controlling it! When you loosen up the secondaries with vacuum control they tend to come in quickly, which breaks traction - bad idea. I am running street legal drag tires. Before I was using them, I couldn't get the secondaries to open either. I didn't want to change the springs, knowing it would make the traction a problem when the secondaries came in. Don't forget the tires!

Good luck.

 

Marcello

[Guest Z-rific]

Thanks Marcello. I was hoping that a spring change might cure the problem. Hopefully I can find a spring light enough to open but not too soon.

 

I wouldn't mind mechanical secondaries. But I read on the Holley website that you can't really make a vacuum operated secondary into a mechanical one. I guess it's easy enough to attach a screw and nut into the secondary linkage, but they say that on a vacuum operated secondary, there is a potential for misfiring.

 

If you have a trick way of making it mechanical without suffering side effects, let me know.

 

By the way, when I say the secondaries don't open, I mean I can't even force them open by hand. Even with the carb off, they don't budge. I hope that is just because they have to have load with vacuum to move. I may try softer springs today.

[Guest zbot43]

Sorry it took me a while to get back to you. I've been busy like crazy doing a project of my own. In regards to changing a V secondary to a M secondary - I wouldn't recommend it. Better to simply get a M secondary carb.

Now in regards to not being able to move them. There is a lockout for the secondaries. Is the choke fully off when you are checking them? Part of the choke linkage doesn't allow the secondaries to open. Now after it is warm you should be able to move with your fingers no problem. If not you need to pull the throttle plate off the carb. and check for what is binding up.

 

Regards,

 

Marcello

[Guest Z-rific]

Thanks for the reply.

 

I don't have the choke hooked up at all and haven't even really paid any attention to it.

 

Now that today is dryer and warmer, i'll play around with it and let you know what I find.