Carb Rebuild - Another Question

[Zilverado]

So, I'm finally back at 'er - - gotta get the truck out of the garage for the weekend (hopefully).

I'm just doing the final cleaning on all the parts. In the picture below, what is this that is circled, and how do I remove it (or should I). None of my exploded views seem to show this piece. I can feel a spring underneath it, but I have no idea how to pull this thing out.

Any suggestions?

[swervin ervin]

There is nothing under this screw. This is called the APT (adjustable part throttle) screw. All it does is tell the power piston how far down it can be pulled under vacuum conditions. If you have tried to remove it, they are usually around 2 turns out from bottom, but that is not set in stone. I would leave it alone as like I say, there is nothing under it that needs cleaning.

If you get this thing out adjustment, it will cause it to suck gas like no tomorrow. Been there done that. It will also cause a very rough running truck.

[Zilverado]

Thanks Man. I don't think I've really turned it a whole lot - just back and forth a bit. How sensitive is this thing. Since I've "played" with it, should I maybe think of turning it all the way in and then backing it off 2 turns - like you suggest?

[Zilverado]

Well, I decided to check the position of this adjusting screw, and as usual, Swervin was right!

It was about 1 & 1/2 turns from the bottom (probably from me playing with it). So I backed it off 2 turns from the bottom, and that'll be where she stays.

It's coming along...

[swervin ervin]

If you want to you can turn it in but count the turns from where you are now so you can put it back. But it you think you haven't moved it much, you can just let it alone. It's not real sensitive. I have mine set at 2 but once a long time ago, I thought 2.5 would be better. Wrong. Believe me, I have tried all kinds of settings with this thing, even to the point of adding a removable plug on top of the carb so I could get it out without having to remove the airhorn. 2 turns out is the best setting for me.

Like I said all it does is raise or lower the power piston resting position. Under high vacuum conditions, such as part throttle or cruise, the power piston is pulled down. If this screw is too low, it will have a lean condition at cruise. Too high and it will have a rich condition and cause stumbling and rough idle. There is no set in stone setting, but from all I have read, 2 turns is about right. My Edelbrock Q-jet was at 2 from the factory, where I should have left it to begin with.

[Zilverado]

Thanks again Swervin.

Another question. This reminds me of the old Sesame Street sketch "which one of these things doesn't belong?..."

The photo below shows the existing float needle/seat assembly (on the right) and the new one that came with the rebuild kit (on the left). While the needles look the same, the seats are obviously different - holes/no holes. I guess I'll need to reuse the existing seat, I'll use the new needle - - is that going to be an issue?

[swervin ervin]

No, use the new needle and seat. They are a matched set. Not having the holes don't matter.

[Zilverado]

Cool - thanks. This is the first part in the kit that was not the same as the original. But if you say that the holes don't matter, I'll take your word for it.

[Ivan D.]

IIRC pretty much all new needle and seat sets are of the high-flow type, which is what you want anyways - heck, even GM's own rebuild kits for the Q-jets included them. Why that is necessary? Q-jets runs on fuel volume, not fuel pressure, which means they'll work just fine with like 2-3psi off the pump (try that with a Holley of you dare ), but the flow must be as high as possible, which the stock needle and seat is not too great at.

[Zilverado]

Originally Posted by Ivan D.

IIRC pretty much all new needle and seat sets are of the high-flow type, which is what you want anyways - heck, even GM's own rebuild kits for the Q-jets included them. Why that is necessary? Q-jets runs on fuel volume, not fuel pressure, which means they'll work just fine with like 2-3psi off the pump (try that with a Holley of you dare ), but the flow must be as high as possible, which the stock needle and seat is not too great at.

Thanks Ivan,

Looking at the configuration of the 2 types of seats, I can't see how the new one (without the holes) would allow higher flow - if I understand what you're saying - since the needle assemblies appear to be the same. Unless the hole at the base of the seat is larger.

[Zilverado]

OK - here's another question...

In the photo below, are the tubes that are circled supposed to be hollow "tubes" or rods? They are open for the most part from the other (top of the air horn assmebly) side. But from the side you're seeing (bottom) they are plugged. I can't push a small pin through them. Now, I haven't tried too hard yet, 'cause I didn't want to damage something, so I thought I'd ask. The tubes on either side of these are completely open.

Anyone?

[swervin ervin]

Hollow tubes. Clean them out.

Are you soaking the parts in a good carb dip? If not you may never get all the tiny holes and passages clean.

[Zilverado]

Thanks again Man!!!

[Ivan D.]

Yep, open then back up. As for the seat and needle assembly - if needles look the same, use the new one with the seat that looks like it'll flow more gas, probably the one with the holes as the one I have in my Q-jet is stock and has no holes.

[swervin ervin]

I would not mix and match parts, old and new. Use all new parts no matter what they look like.

[Zilverado]

Thanks alot guys - I appreciate the input ALOT!

I took a look at the seats and it appears as if they both have the same size inlet at the bottom. I think that would likely be the restriction for the amount of fuel into the bowl - probably not the holes along the side. So I think I'll stick with replacing the needle and seat with new.

Any further thoughts on sealing the plugs on the underside of the float bowl (people had commented on my earlier post http://67-72chevytrucks.com/vboard/s...ight=zilverado )? I was going to apply some JB Weld - about all I could find - to seal these up. I assume I wouldn't need alot, just around the edges?

[crm318]

when i rebuilt my quadrajet i ran into the same problem with the seats. i went with the new seat that had holes in it and when i got the truck running, it ran like crap and fuel was actually coming out of the top if the carb. i switched back to the old seat and new needle (cleaned the old seat thourghly) and it runs great. that is just my situation though. by the way i the past few carbs i have rebuilt i have used the gel aircraft paint remover and paint it on with a small paint brush and i works better than any carb cleaner or dip i have ever used. it cuts right through the carbon. i use it with cleaning all engine parts now.

[Zilverado]

Thanks for the comments CRM.

Just to clarify, the existing seat in the carb is the one with the holes. The new one does not have the holes.

The truck has run beautifully for years. Just before I started the rebuild, the truck would not even stay running at all. The fuel was coming out of the top of the carb just as you describe. So I am hoping that this clean-up and rebuild will restore this girl to her former beauty.

Thanks again.

[Ivan D.]

My double-pumper flooded the secondaries twice in three days in a row, both times during cold start at near freezing temperatures. Pulled took the secondary bowl, bend the float down a little so it cuts off the fuel a little earlier, slapped the bowl back on, truck fired right up - didn't even have to take whole carb off. Point is, maybe yer float level was set a little on the high side, or it moved itself up.

[Zilverado]

Originally Posted by Ivan D.

... or it moved itself up.

How the heck would that happen?

[Ivan D.]

Originally Posted by Zilverado

How the heck would that happen?

Beats me, but it's a carb, so anything is possible

[Zilverado]

So I'm hoping to get 'er all back together tomorrow - exciting. The one thing I can't seem to find is a torque spec for the screws that hold the 3 carb sections together (I believe one of my manuals has the carb-to-manifold spec). I realize we're not talking about multiple foot-pounds, but are they simply "screw-driver" tight? Is this enough to properly compress the gaskets?

Now, the one place I haven't looked for this info is in the instructions that came with the re-build kit, so I'll check there when I get home, but...

[Ivan D.]

I used screwdriver with torx bits for both the Q-jet and the Holley, no leaks at all and not overtightened either.