Quadrajet operation question

[qystan]

Hi all,<br /><br />How does the dashpot and secondary throttle air valve work?<br /><br />I've just cleaned out the carb and it worked fine for some weeks now. Yesterday the engine seems to be starved for air at speeds beyond 3000rpm, pushing the throttle it loses rpm. There is a constant throathy sounding roar throughout its rpm above idle, much louder than normal.<br /><br />Couldn't figure out the problem, but disconnected the link rod between the dashpot/idle break diaphragm assy and the secondary throttle air valve and it worked fine and able to reach WOT with a 3/4 load. Have checked the manual, the linkage was correctly installed.<br /><br />Examining the secondary air valve linkage and dashpot operation. With vacuum, it is retracted and effectively locks out the secondary air valve from opening. How will it ever be opened for secondary throttle to come in? Is the vacuum to the dashpot available at certain throttle positions only or is there all the time? It is quite stiff to overcome the dashpot, it is unlikely airflow can override this.<br /><br />Thanks in advance. <br /><br />Qystan

 

[Mahoney]

Re: Quadrajet operation question

The secondary air valves will not open until the vacuum releases inside the carb as you throttle up. You will see the vaccum diaphram let loose slowly as you throttle up.<br /><br />To adjust the airvalve tension you need to adjust the spring on the starbord side of the secondary valve. This is usually done with a small allen haed wrench and a screwdriver. <br /><br />I have been battling this issue as well, and it's pretty tricky. Remember, if you make the airvalves open too easily, it will let air in, but the cam that operates the secondary needles raises to allow fuel to flow to the secondaries when this happens, which can bog it down as well.

 

[qystan]

Re: Quadrajet operation question

Thanks Mahoney<br /><br />"vaccum diaphram let loose slowly as you throttle up". <br /><br />You mean the vacuum is lesser at higher throttle settings? <br /><br />Did not want to disturb the tension spring as I don't the spring scale and don't want to risk messing the whole thing. <br /><br />A few more questions. <br /><br />After examining the linkages and dashpot, was thinking that the lockout is to prevent air bypassing the choke valve via the secondary circuit - right?? If yes, when warmed up, does the thermostat re-position the linkages such that the dashpot is no longer able to be retracted? <br /><br />I can' try this out because the thermostat is missing. I'll try to examine the linkage to figure this.<br /><br />Thermostat wasn't there when I bought the boat, didn't get a replacement as the weather here is warm and choking isn't necessary. <br /><br />To keep the choke valve open I tied the choke linkage. Wonder if it worked previously because the wire to keep the choke opened all the time prevented the dashpot from locking out the secondary valve. During the recent rebuilt, re-tied the choke linkage and this time made sure the secondary valve linkage is free...and it dosen't work anymore... re-tie to lock out the whole thing??

 

[Mahoney]

Re: Quadrajet operation question

When the engine revs and loads up, pressure in the carb raises to equal the outside pressure letting the secondary lockout release. The airvalves on the secondary should then be free to open and let in more air as the engine needs it.<br /><br />Nothing on the choke should prevent the vacuum from retracting the secondary lockout arm. You really need to have that thermostat linkage on there I thnk to have it work correctly. The tension on the choke lever is the only thing preventing the choke valve from slapping shut.<br /><br />Its hard to visualize what you have going on there, but a possiblitiy is that when you are hitting the secondaries and the vaccum starts to drop, the choke valve is slipping closed and richening up the mixture causing your bog.<br /><br />You will need to watch the carb operation when you are out on the water. hook everything back up as it should be. When you engage the secondaries, do the airvalves open? Not at all? If not at all, check the tension with your fingure when you shut the engine down. If they are pretty hard to open, you should adjust them. With the engine off the secondary vacuum break should be released.

 

[cny boater]

Re: Quadrajet operation question

Mahoney, you are a quick study on Quadrajets! However, I'd like to add that there is a mechanical lock-out feature for the secondary air valve. I think you may be confusing the fact that the choke pull-off will dampen the air valve opening with the fact that there is supposed to be a true, mechanical lock-out feature incorporated with the choke linkage to keep the air valve closed until the choke comes off. <br /><br />Qystan, you lost me with your last sentence. Can you explain it again?

 

[qystan]

Re: Quadrajet operation question

CNY<br /><br />Restated: How do I operate the carb without the auto-choke thermostat installed. <br /><br />1. Lock the primary choke valve open.<br />2. Remove the secondary air valve link from the diaphragm.<br /><br />As Mahoney described, looks like I can leave the link on or off. On is better as it can regulate the max secondary valve opening. Without it, it is dependent on the spring tension only.<br /><br />Thanks all. I'll restore the linkages and put it thru its paces.<br /><br />Qystan

 

[Mahoney]

Re: Quadrajet operation question

Thanks CNY, <br />I have been learning a ton about those carbs, if anything they will keep you busy.<br /><br />On my carb, I only have one arm on the vaccum diaphram connecting to the secondary air valves. I know there are 2 connections to this diaphram on others. I think he is saying that he does not have the mechanical linkage to prevent the choke from opening up all the way and is having to tie it down.<br /><br />On mine I have the remote coil choke on the manifold to perform this for me.<br /><br />Qystan. <br />If your engine is warm, yes you should be able to rig something up that keeps the choke airvalve open, if the engine is not warm, your mixture will be way to lean if you do this.<br /><br />The problem with removing the secondary lockout linkage is that once you hit the secondaries your airvalves will be free to open regardless of your manifold vaccum. Any book you find will tell you proper operation of quadrajets, and the fundamental operation of all carbs is performed exclusively by varying pressure in and outside the carburetor/manifold. You are esentially disabling a primary check of this for your carb.<br /><br />I would keep the linkage on and throttle up while watching the diaphram and see if it releases. if it releases under load and heavy throttle, then its operating correctly and your adjustment is somewhere else.

 

[Mahoney]

Re: Quadrajet operation question

Before changing anything else try this. I had the exact same problems as you and made a recent adjustment and it seemed to have had a very promising result.<br /><br />With engine off. try readjusting the airvalve tension. You will need a small screwdriver and small allen wrench. <br />1.Disconnect the airvalve lockout and with allen wrench on starbord side you will find a litle allen screw, there is also the small screwdriver slot on a seperate screw right on the side.<br />2.Slowly loosen the allen screw until your airvalves loose tension and fall open. <br />3.With small screwdriver begin tightenting the screw on the side until tension builds on the spring and the airvalves just close. <br />4.From this position turn the tension down somewhere between 1/4 and 1/2 extra turn, then tighten the allen screw back up.<br />5.reconnect linkage and give it another whirl.

 

[intoodeep]

Re: Quadrajet operation question

The vac. break on the primary side is for cull off opperation. It has a secondary function of slowing down sec. air door opperation. Thesepull off's have different bleed rates be shure you have the right one.! The Q Jet uses the sec. air doors for three purposes, to act as a accl.<br /> pump by way of two fuel transfer ports located on the air horn, as the air doors open the ports are uncovered and enrich the mixture as the low pressure in the manifold appears at the leading edge of the air door. Also the secondaries do not use a venturi proper, only a feed tube which is a from the sec. jet wells. The metering rods limit the amount of fuel avail to the tube. so a slow opening door will prime the tube.<br /> With a large throttle bore dia. and too light a spring tension this will cause a large bog or hesisation or accl. if the doors open too fast.<br />no fuel= large sag. Set spring tension as mentioned above or as much as possable WITHOUT BINDING. THE Q JET IS A MANUAL SECONDARY CARB.<br />slow opening is nessary to get things working.<br />The pull off is on the air doors to pull them closed after throttle is closed.<br />If you have gm # I can Email carb specs.

 

[qystan]

Re: Quadrajet operation question

Mahoney, InTooDeep, cny boater<br /><br />Thanks for all your time and writeups, it surely sheds light on the operation. Great info. <br /><br />Have restored the linkage and the carb worked fine. <br /><br />Looking at it again, the linkages were stuck and because of the way I tied the primary choke valve, the secondaries could not open, causing air starvation.<br /><br />Yes, Mahoney, the weather is always warm here (85F average), starting without choke is not a problem, I am tying the primary choke open - this time making sure it does not jam.<br /><br />Thanks again.<br /><br />Qystan