Rochester Q-Jet operation question
Thread Starter
Instructor
Joined: Jan 2005
Posts: 241
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From: Sebastian Florida
Rochester Q-Jet operation question
I recently rebuilt my Q-Jet (#17054919). I did do any "adjustments" because I really don't understand the instructions regarding the various adjustments. I've assumed that it was adjusted properly before my rebuild so I didn't try to mess with it. After the rebuild the car started fine and I adjusted the fuel mixture. It seems to run ok except that the idel doesn't remain constant. It seems to run low in the beginning, the run ok when warmed up, but then sometimes it will run a little low again after warmup. But the reason I'm writing is that I don't think my secondary throttle is openning up at full throttle. The linkage moves freely by hand but doesn't open after warmup at full throttle. Additionally, the valves at the top don't open up either. There seems to be too much vacuum holding it closed. When I take the hose off the vaccum and manually open both the primary and secondary throttles, the valve on the secondary at the top opens like I'd expect (letting more air in). Attached are two pictures of my carb (if I do it right). I cannot figure out exactly how the secondary throttle is supposed to opened. Any suppestions will be appreciated.
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Melting Slicks
Joined: Oct 2005
Posts: 2,056
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From: Redruth Cornwall
The secondaries will only open on the move.
If you havent done any adjustments, it may be that your secondary spring requires adjustment.
Do you have any of the tech papers written by Lars? A must for rebuilding the qjet correctly.
Le Mans Master
Joined: Oct 2006
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From: Gladstone MO
C3 of Year Finalist (appearance mods) 2019
Your first picture is good.The vacuum pot is the choke pull-off. When you start cold, set the choke by pushing on the gas. As soon as it starts, it needs more air, so the pull-off opens the choke butterfly a bit, I think 17*. It also does dual duty, the link running back to the secondary air valve keeps you from WOT until everything is warmed up and the choke is completely off. If the choke pull-off holds vacuum, (pull the hose, push the arm back into the pot, cover the hose connection with your finger) it's ok. Then adjust how much the choke is opened by tweaking the link with pliers.
And the secondary air valve is vacuum operated. You probably will not see it open by "winging" the throttle. With the engine off, open the throttle by hand and look down the carb. All the throttle plates should be full open. When you open the throttle by hand, you should be able to open the secondary air valve and see.
And the secondary air valve is vacuum operated. You probably will not see it open by "winging" the throttle. With the engine off, open the throttle by hand and look down the carb. All the throttle plates should be full open. When you open the throttle by hand, you should be able to open the secondary air valve and see.
Thread Starter
Instructor
Joined: Jan 2005
Posts: 241
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From: Sebastian Florida
#17054919 is not a vette carb, but it is a carb on my vette, and I'm trying to get it running ok until I replace the engine (in process). The choke seems to open properly during warnup (again I haven't taken any measurements or adjusted anything). I was hoping to get someone to look at the linkage pictures and say it looks ok or not its not set up right. It sounds like there is nothing obvious that isn't set up properly. I've tried to do some of the fast idel adjustments but I cannot seem to figure out how to set the linkage up on the different steps. (i.e. the second fast idel step). That's why I posted the pictures. I thought maybe it wasn't put back together correctly.
The choke pull-off holds does hold a vacuum and the secondary opens fine by hand.
The choke pull-off holds does hold a vacuum and the secondary opens fine by hand.
Last edited by pkgtaxcpa; Feb 5, 2007 at 08:58 AM.
Race Director
Joined: Mar 2006
Posts: 14,112
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From: Florida
#17054919 is not a vette carb, but it is a carb on my vette, and I'm trying to get it running ok until I replace the engine (in process). The choke seems to open properly during warnup (again I haven't taken any measurements or adjusted anything). I was hoping to get someone to look at the linkage pictures and say it looks ok or not its not set up right. It sounds like there is nothing obvious that isn't set up properly. I've tried to do some of the fast idel adjustments but I cannot seem to figure out how to set the linkage up on the different steps. (i.e. the second fast idel step). That's why I posted the pictures. I thought maybe it wasn't put back together correctly.
The choke pull-off holds does hold a vacuum and the secondary opens fine by hand.
The choke pull-off holds does hold a vacuum and the secondary opens fine by hand.
Good pics BTW.
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Tech Contributor
Joined: Aug 1999
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From: At my Bar drinking and wrenching in Lafayette Colorado
Your carb is operating as it should. The choke pulloff should hold the secondary airvalve firmly closed. You cannot get it to open by "winging" the throttle in neutral - there is not enough airflow or loss of manifold vacuum in a no-load condition to make it open. Your carb is fine as long as you have enough throttle cable travel to get the carb to WOT - from the photo, I don't think you do. Have someone press the pedal to the metal (with engine off...) and see if the cable is pulling your secondary throttle shaft fully open.
Thread Starter
Instructor
Joined: Jan 2005
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From: Sebastian Florida
Ok, here are the pictures. The engine was cold so I used a small socket to simulate the heat expanded coil. The first pix is of the linkage with the pedal to the floor. 
The next is of the secondary throttle opening (or mostly not opening) at full pedal
The next is the choke linkage side at full throttle
The next is a view of the accelerator linkage under the dash and the slack in the cable attached to the linkage
Lastly, is a pix of me manually applying full throttle. It shows that the linkage would travel all of the way and the secondary throttle opened all of the way when I did this.
I await your responses. Patrick
The next is of the secondary throttle opening (or mostly not opening) at full pedal
The next is the choke linkage side at full throttle
The next is a view of the accelerator linkage under the dash and the slack in the cable attached to the linkage
Lastly, is a pix of me manually applying full throttle. It shows that the linkage would travel all of the way and the secondary throttle opened all of the way when I did this.
I await your responses. Patrick
Drifting
Joined: Sep 2004
Posts: 1,959
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From: North Bend Ohio
You're going to have to straighten out the secondary actuating rod and get it adjusted properly. Then, adjust the secondary opening tang to get the throttles moving a little sooner. This is at the primary throttle shaft. Adjust the throttle cable to get the right amount of travel at the carburetor.
BigBlockk
Later.....
BigBlockk
Later.....
Race Director
Joined: Mar 2006
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From: Florida
Lars saw limited cable travel from your first post.
As you know the secondary throttle plates are not opening fully.
A lot of the pedal arms get bent over the years and don't have enough travel in them before they hit the floor or carpet or pads. Also people put too much padding or carpet etc under the pedal. Check those 2 conditions. Your secondary airvalve will not try to open unless your secondary throttle plates can open.
Your secondary air valve looks like it should work freely once the choke is open. Check the spring tension for the airvalve and set it to 3/4 turn tension.
Nice clear pics. Good way to troubleshoot over the internet.
Get some real rubber vacuum caps instead of using old tubing and screws. It may smooth out your idle a little if leaking. Suggest to make a solid fuel line to the carb too. A little safer than the old rubber hose. Check the wire you used instead of a hairpin clip on the choke element rod looks like it could jam.
Missed that, your primaries are fully open by the looks of the tang.
I'm sure Lars will straighten us both out.
As you know the secondary throttle plates are not opening fully.
A lot of the pedal arms get bent over the years and don't have enough travel in them before they hit the floor or carpet or pads. Also people put too much padding or carpet etc under the pedal. Check those 2 conditions. Your secondary airvalve will not try to open unless your secondary throttle plates can open.
Your secondary air valve looks like it should work freely once the choke is open. Check the spring tension for the airvalve and set it to 3/4 turn tension.
Nice clear pics. Good way to troubleshoot over the internet.
Get some real rubber vacuum caps instead of using old tubing and screws. It may smooth out your idle a little if leaking. Suggest to make a solid fuel line to the carb too. A little safer than the old rubber hose. Check the wire you used instead of a hairpin clip on the choke element rod looks like it could jam.
Missed that, your primaries are fully open by the looks of the tang.
I'm sure Lars will straighten us both out.
Tech Contributor
Joined: Aug 1999
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From: At my Bar drinking and wrenching in Lafayette Colorado
The photos show exactly what I was predicting above with insufficient throttle travel. You need to get under your dash and bend your gas pedal lever arm back to produce more travel.
Last edited by lars; Feb 6, 2007 at 02:24 PM.
Instructor
Joined: Dec 2006
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From: Naples Florida
YES... i just had the same problem with mine. I was not getting WOT.. The peddle was taken off and bent and now im sound as a pound!!! goodluck!!!
Thread Starter
Instructor
Joined: Jan 2005
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From: Sebastian Florida
I will apply all of the above suggestions. I'll report back as soon as I have made some of the major recommendations. I've always thought my engine (1968 350/350hp) was just old and tired and that's why I wasn't getting as much power as I might otherwise expect. Now it looks like I'll be able to improve that a little. Thanks to all of you. I wouldn't be able to solve most of my corvette problems without Forum members like yourselves.
Thread Starter
Instructor
Joined: Jan 2005
Posts: 241
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From: Sebastian Florida
I've bent the gas pedal to get more cable travel, replaced the vacuum tube covers with proper covers and strightened the secondary linkage. Still have to replace the ruber fuel inlet line but will do that when I replace the 350/350hp with a 454/390hp currently at the machine shop. Took the car out for a test drive and it has a lot more power than before. Go figure, now I'm actually running the engine with a 4 barrel carb rather than the 2.4 barrel carb (.4 represents the partially opening secondaries before fixing the cable travel). Thanks guys. It has made a world of difference.
Final carb question. I was trying to adjust the fast idel to the specified RPM's for the engine and am not sure about something. I'm supposed to put the fast idel cam on the second step to set the fast idel, but I really don't see how to do that on this carb. The cam seems to rotate freely but doesn't want to stay in any particular position while cold. Once again what an I missing??? Do I have to hold it in position? Do I have to tighten or bend something. My engine seems to idel low (around 400 to 500RPM's) until it is warmed up. Doesn't the fast idel mechinism help with that. If you need more pictures please let me know.
Thanks, Patrick
Final carb question. I was trying to adjust the fast idel to the specified RPM's for the engine and am not sure about something. I'm supposed to put the fast idel cam on the second step to set the fast idel, but I really don't see how to do that on this carb. The cam seems to rotate freely but doesn't want to stay in any particular position while cold. Once again what an I missing??? Do I have to hold it in position? Do I have to tighten or bend something. My engine seems to idel low (around 400 to 500RPM's) until it is warmed up. Doesn't the fast idel mechinism help with that. If you need more pictures please let me know.
Thanks, Patrick
Tech Contributor
Joined: Aug 1999
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From: At my Bar drinking and wrenching in Lafayette Colorado
When cold, the choke rod should pull the choke linkage firmly downwards, closing the choke blade fully and raising the fast idle cam into position. There is a lever arm under the fast idle cam attached to the choke linkage that will raise the cam when the choke rod pulls downwards.
Thread Starter
Instructor
Joined: Jan 2005
Posts: 241
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From: Sebastian Florida
If I understand this correctly, the fast idel cam will already be in the proper position when the engine is cold (see very first photo above). Then all I have to do is adjust the fast idel screw to the recommened RPM's (approx'ly 1,300). So when I stat the car cold the fast idel cam will be in the upright position and the car will idel at the fast idel speed (approximately 1,300) until the engine is warmed up, at which time, if I tap the gas the fast idel cam will come off (the step) and the normal idel (600RPM's) should take control. Please let me know if this is correct. Thanks, Patrick
Tech Contributor
Joined: Aug 1999
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From: At my Bar drinking and wrenching in Lafayette Colorado
When the intake manifold is cold, the remote choke should be pulling firmly down on the choke lever/rod. When you open the throttle lever (push the gas pedal down), the choke should snap fully closed, and the fast idle cam will rotate to the high step position (the forward weighted end of the cam will be lifted up by the lever under it). This will only happen when you open the throttle lever on the cold engine. As the engine warms, the fast idle cam will be allowed to drop by the choke rod being pushed upwards. The throttle must be opened slightly to allow the fast idle cam to drop. Yes, once the cam has been pushed up by the choke, you can set fast idle speed to the desired setting - usually 1000 - 1200 rpm.
