Vaccuum Secondaries...not opening????

[Paso76GMCstepper]

Ok Guys the truck runs awesome I just dont feel that surge of power when you really get on it. Could my secondaries not be working. Its got a quadrajet carb. There are no vaccuum leaks and it runs perfect. No pinging or anything. What can I check to see if they are working correctly?? Any info or easy test would be super.

Thanks again guys.



Tony

[TwinTurbo]

Only a mech. secondary carb will give that feeling of the secondaries kciking in. The Q jet is a variable flow type of carb, meaning that the pos. of the secondaries varies with engine demand (that's why they fit so many engine applications)

[Paso76GMCstepper]

Hey Twin, Thanks for the speedy reply I just looked at it it looks like its got mechanical secondaries??

I see a rod next to the throttle linkage and when I push the throttle linkage all the way it doesnt seem to move. Haw can I check if its manual or vaccuum. It has an electronic choke. I dont think the is the original carb.



Tony

[TwinTurbo]

Is it a seperate rod (as something for a cruise control) or do you mean the pump link rod??? Kind of hard to figure out what you mean. Can you snap a pic and point out what you mean?

[Paso76GMCstepper]

I understand Here are a few pics. If you need anything else let me know. If I open the throttle all the way manually should the secondaries open??



Tony

[skillet]

Chevy High Performance March 2004 has a good article on Q-jet entitled "Special Q." It mentioned that "the air-door tension (on the back barrels) must be properly adjusted. If it's too tight, the door won't open under load and the engine will strain. If it's too loose and the door opens too quickly, it will bog. If you feel some light-to-moderate resistance when pushing the door open, the adjustment should be about right." If the front of the carb was facing north, the screw would be at the southeast corner of the carb, corner passenger side as it's mounted. On mine(1969), an allen set screw should be loosened before the other screw is adjusted. I tinkered with mine, but never really noticed much difference. Still learning.

[TwinTurbo]

The little rod just operates the butterflies (throttle plates) on the secondary part of the carb, the plate above them in the airhorn assembly is controlled by a valve (pressure difference between primary and sec. sections) so the secondary butterflies are mechanical of course but the plate in the airhorn is what determines the amount of flow possible through the secondary stage.

[Paso76GMCstepper]

Ok I kinda get it. What do I check to see if they are truly working or not.

[NeCrOmAnCeR]

Keep in mind also that if the choke is closed (engine cold) the q-jet secondary butterflies are prevented from opening . If you look on the pasenger side of the carb when you open the throttle by hand, you can see the lever which blocks the secondaries from opening. Make sure that when the choke is fully open that that lever is held out of the way by the choke mechanism. The secondary air valve uses both spring pressure and engine vacuume (choke dashpot / pulloff) to prevent it from opening too soon. You can see this linkage running from the air-valve to the vacuume dashpot, also on the passenger side of the carb.

[Paso76GMCstepper]

Ok I am really stumped. After taking the air cleaner off and looking at it. Here are some pics. I know this is a nwebie question but I need some help. When I move the throttle linkage by hand it goes all the way back but the secondary doesnt move. I CAN move it both if I use the secondary arm then they both move. Here are some pics. Will I be able to see this sitting still or will this be one of those u gotta drive it deals.

Any and all opinions are appreciated.

Also here is the numbers off the carb 17066575 2302W

I tried decoding it it seems to be a Pontiac California Emmissions carb on it. The truck is registered federally




Tony

[Destructo]

Originally Posted by TwinTurbo

The little rod just operates the butterflies (throttle plates) on the secondary part of the carb, the plate above them in the airhorn assembly is controlled by a valve (pressure difference between primary and sec. sections) so the secondary butterflies are mechanical of course but the plate in the airhorn is what determines the amount of flow possible through the secondary stage.

This rod that controls that goes from the secondary linkage at the top to the same vacuum pot that controls the choke right? What happens if it is missing? Mine is not there and I suspect I have a carb problem but I did rebuild it but don't think I did a good job and didn't get it totaly clean.

[NeCrOmAnCeR]

To Paso... When you operate the throttle by hand, you may open the secondaries depending on wether the choke is warmed up (see my previous post) Even if it is opening, the secondary air valve (the plates you see on the top of the carb which lift the secondary metering rods) will remain closed. In your most recent post, the second picture shows the linkage I talked about earlier (holds the secondaries closed until the engine is warm), and in this position it is keeping the secondaries closed. Warm up the engine and check to see if this linkage is swinging out of the way to allow the secondaries to open. I have attached the image with the part circled (hopefully).

[NeCrOmAnCeR]

To Destructo...

If this linkage is missing it is almost certain that the secondaries are allowed to come in to soon. A tempory linkage could probibly be made out of some welding rod, bend it to look like the one shown above. With the vacuume pot fully withdrawn, the linkage needs to just hold the secondary air valve in the closed position.

[Paso76GMCstepper]

Well I had my boss look at it and there is a black piece of metel on the drivers side that when the throttle linkage goes back the black metal thing was all bent in towards the carb. We bent that straight and now the secondaries open. I got on it from a stand still and it scared the Krap out of me. Definatly working now. Thanks for all the help. What I wonder is how that metal tab got bent in the first place. The piece Im talking about gets pushed by the throttle linkage when it goes back it now hits the black metal making it possible that the secondary linkage now works.

NECROMANCER:: The thing you circuled has the black metal piece on the throttle linkage side. on the other side of the carb is what was bent so the thing you circuled didnt get out of the way. I bent it back and works like a dream

Tony

[apstguy]

Do you have a PCV valve on that truck?

[Paso76GMCstepper]

Yes I do would that make a differance. I put a filter breather on the valve cover where the thermostatic air cleaner used to be hooked up

[apstguy]

Not the breather but the actual PCV vlave. You have the PCV valve line blocked off at the carb. No it probably wouldn't make a difference. The yellow cap is where my PCV valve is connected.

[Russell]

Yeah, I'd reccomend getting a proper PVC valve for that truck... You'll gum your motor up something nasty if you don't get all that stuff taken out and burnt

[swervin ervin]

The port with the yellow cap is what normally went to the charcoal canister. It could be considered the bowl vent I guess, but it's not meant to be vented to the air. It does need to be open though. If you keep it capped off, it could cause problems with not being able to vent gas vapors.

I see his hose going to the PCV port. It's under the fuel inlet where they have always been. Look again at the second group of pics he posted. Look real close at the first pic in this group.

[TwinTurbo]

how does not having a PCV gum up your motor? Just breathers work fine, no brethers won't work because you will blow out your dipsstick because of blowby. PCV is nice to keep fumes from turning the oil acid but it also makes a mess of the intake manifold and the carb. EGR makes a mess of the intake too.

I like brethers or an exh. evac. system like I have on my other engine.

[Paso76GMCstepper]

My PCV valve is brand new and working properly. I even put a new Grommet in. I have a question about the line with the yellow cap. If not plugged and it should be able to vent what would I put there. Thanks for all the help I always learn something from you guys.

[Destructo]

Originally Posted by NeCrOmAnCeR

To Destructo...

If this linkage is missing it is almost certain that the secondaries are allowed to come in to soon. A tempory linkage could probibly be made out of some welding rod, bend it to look like the one shown above. With the vacuume pot fully withdrawn, the linkage needs to just hold the secondary air valve in the closed position.

That would certainly explain the bog I have when I go from closed throttle to WOT, I figured it was supposed to have a linkage there and the air coming through would allow them to open to quickly, i'll have to see if I can fab something up or get a rod from somewhere.

[swervin ervin]

The air valve dampener/choke pulloff is a critical part of the secondaries operating properly. The spring is a helper and controls them somewhat. I have my spring adjusted to 1/8 turn from zero tension, to where stock it is set to 7/8's of a turn. 1/8 is just enough to keep the AV's closed. Power comes from the dampener. If this is not working properly or even hooked up, they will slam open and you will have the infamous bog. The dampener lets them come in in a more controlled or slower manner to help catch up with the fuel. Something the spring won't do. Of course you have to tune the secondary hanger and rods too. In order to have a lot of air, you have to increase the amount of fuel.

On the vent port, I think just leaving it open would be OK. I've never tried it, but I doubt any gas would come out. Maybe if you off roaded or made a lot of real hard stops. But, it does need some kind of bowl vent or else vapors will probably increase fuel level and may seep raw gas into the intake. It might even go as far as to cause vapor lock. We all know a hot carb/fuel mix is not a good thing. When the fuel percolates it needs a way to let the rising pressure to escape.

[Destructo]

It seems to be pretty dang common that these carbs don't have those linkages on them, I wonder why they are removed or maybe they just fall off? I know I have 3 Q-Jets laying around and none of them have the rod on them, I don't understand why somebody would want to remove them. Oh well.

[swervin ervin]

I believe why you see so many missing is clueless folks thinking they cause problems. Not knowing any better they think if they take the linkage off it will let the secondaries open faster. This is true, but at the cost of a big bog. Then they find out there is an adjustable AV spring and overtighten it to overcome the mess they've made by removing the linkage. Another thing is going to the junkyard to get used Q-jets. A ton of these things have had the linkage rods removed because folks like us need them to replace the ones somebody else took off.