My car has bog when secondaries open after swapping intakes at the track

I took my car to the drag strip with my Performer intake and Osborn built Quad. It ran great and I ran a best of 13.04 at 106 mph. I swapped the Performer out for an SP1 and I lost three tenths of a second and 3mph. I have a four speed and the car bogs down on every shift.
I developed a big bog when opening the secondaries. Otherwise it idles and runs fine. I also checked for vacuum leaks and there doesn't appear to be any.

What can I look for?
Thanks
Tim

 

It may be the spring tension adjustment on the secondary air flaps is out of adjustment. Thinking more...could be a choke prob or lockout prob, also. Maybe a gasket prob? It's hard to say, but I would be looking at that stuff if it ran good before you swapped intakes.

 

If the secondary air valve spring tension hasn't been changed, try opening the carb to WOT and verify that the secondary metering rods are lifting. If not, the little plastic cam could be broken.

 

Thanks guys, like I said I have put 8,000 care free miles on this car since the rebuild 5 years ago and I have not even had to adjust the idle. It pulled strong without any hesitation up to 5,500 rpm's with the Performer intake and a 1" four hole spacer. All I did was swap intakes, I used all new gaskets for the carb and for the intake and now it bogs badly under hard acceleration. I am by any means a carb guy so this is why I am looking for help.
The choke is on the carb but I do not have the linkage hooked up to the intake.

Thanks
Tim

 

To piggyback on Steve's post, did you have someone sit in the car with the gas pedal to the floor and verify the throttle plates are fully open, engine off of course. the manifold change could have messed with the throttle cable. good luck

 

You say you don't have the choke hooked up. Perhaps the choke is not fully open and the secondary lock out lever is still engaged. At least wire the choke open and look to see if the lock out lever is now away thus allowing the secondarys to open.

 

I just checked the throttle on the floor and the throttle plates open all the way however with the engine running the top plates will not open by hand as the choke pull off is preventing them from opening even as I revitalize up. Is this normal?
I disconnected the vacuum line and plugged it. I will drive it tomorrow
To check it at full throttle on the road.

 

No...not normal if the choke is in the wide open (hot) position. You should be able to push on the secondary flaps and open them even if the cam is broken. It sounds like a lockout lever prob. There are 2 styles if it's a divorced choke. One is at the top on the left right behind the choke flap. Typically, they are at the bottom of the choke assembly and require a certain amount of clearance to open. IT depends on your carb, and if it's original, or a rebuilt "hybrid" carb as to the type of lever you may have. With the car warm + running, you should be able to flash the throttle by hand under the hood and see the secondaries open.

 

When John Osborne rebuilt a Q-Jet for me about 10 years ago he asked if it was going onto a single or dual plane intake. I remember him saying something about q-jets on single plane manifolds don't get along real well without some extra mods.... I've never heard that before or since but John sure did seem to know what he was talking about most of the time.

 

If the Performer intake is a dual plane, then you switched to a single plane, I experienced a similar issue with my 350 when I swapped my TA Stage 1 intake (dual plane) to the SP3 (single plane) I had a stumble, hesitation, bog, when going to WOT.
I adjusted the secondary air door spring tighter an 1/8 turn at a time till the stumble went away.
I "think" its from the single planes huge open plenum, it goes lean when you go WOT and the secondary air doors open too quickly.
I may be way off base in my theoryo No:

 

Probably is spring tension.
You aren't using a spacer any more?
The front pull off, at idle should hold the air valves closed. Then it controls the release at WOT. If your pull off is cracked or not holding vacuum, it will cause a bog.

I have a Quadrajet adjustment sheet I could email you showing how to adjust the air valve tension spring. Just get me your email address if you would like me to send it.

 

Right Larry, my error! I was trying to figure out the prob in my head. I just was checking the spring tension on a carb OFF the car a few days ago. It didn't have a choke assembly on it, and I forgot about the link!

 

Just send Mr. Osborne a PM......

 

The signal the carb sees and operates on, is much weaker with a single plane manifold. This reduction of signal will result in less fuel being pulled out of the metering circuits.

How changing from a single plane to a dual plane affects different combos and different cars, will depend on how well the carb is tuned for that combo. If it's tuned dead on with the dual plane, it will very possibly be lean enough with a single plane intake to cause issues. On the other hand, if your a touch fat with the dual plane, you won't notice any issue when you put the bigger manifold on.

Tim, if you can't find the issue, then come on up, we will weld a bung on on exhaust pipe and put my air/fuel gauge on it, and go for a ride, to check your carb now with the new manifold.

But most likely it is simply the air valve adjustment.

JW

 

I agree with others that probably tightening up the secondary spring will help, a litttle richer carb because of the weaker signal, or possibly the secondary pulloff has been opened up a bit too much allowing the secondaries to open back up too fast after each shift. Maybe shifting at a higher rpm would help so that more air is flowing through the intake after the shift which would help get the fuel flowing faster because a lower shift point takes the intake out of its' sweet spot as far as torque goes. The dual plane would clearly make more torque down low and get the fuel/air moving faster than the single plane.

The thing nobody is talking about and what sets it apart from other's experiences is that it is a stickshift. Going by Tim's strong MPH but weak 60' time I'd say he is probably taking it easy off the line by slipping the clutch a bit or not revving it up much. Therefore I'd say he is probably not flat shifting it at WOT. It is the plight of the stick owner, you have to drive it like you stole it for best times. If he was I can't imagine there'd be any bog because the secondaries never close at all. Once you back off the throttle a bit to shift the secondary pulloff has full vacuum for a second and then the secondary air plate slams shut and it has to go through the whole process of opening up again and has to get the air and fuel moving.

This same problem probably cropped up more often back in the day with so many more Chevys because large numbers of them had stickshifts. That is probably what lead them to remove the secondary pulloff completely and just use the secondary windup spring. More often than not the Q-Jet was probably complained about and tossed for a Holley Double Pumper which really is the way to go for a stick car IMO especially if you don't plan on flat shifting it. Everytime you shift with the Q-Jet the pulloff has to go through the time it takes to release the secondary air plate. The DP doesn't have to wait to get back to fully open secondaries. I also read somewhere a long time ago that you should be conservative on the carb sizing with a stick so it can recover quickly between shifts. It goes to show how many ills a good torque converter can cover up. I just wanted to ramble a bit on the subject.

 

tcoyne1@embarqmail.com

 

Sent to you Tim.