Because of an earlier post, I believe my sec. on my qjet are opening slightly at cruise due to low vacuum. I will be adding a vacuum gauge temperarily inside the car to check vacuum driving under load.

Is there a way to lock out the sec. so that won't open under low vacuum so that I can perform a test and see if that is indeed one of my fuel mileage issues.

Kerry

Sure. Just tighten the secondary air valve spring. If you look on the passenger's side of the carb, you'll notice a small screw that's flush mounted in the bowl cover and is aligned with shaft of the secondary air valve. On the bottom side is an allen head lock screw. Loosen the allen screw while holding the spring tensioning screw with a small screwdriver. You'll feel it try to turn when you loosen the allen head screw. That would be the spring trying to unwind. Turn it in the opposite direction to tighten the spring tension for a turn, turn-and-a-half. (I'm not sure what the maximum is. You may be able to tighten it even more.) While this won't lock it completely, it will require a substantial amount of foot in the throttle to get it to open. That should help you determine if this is contributing to your problem.

Don't forget to retighten the allen head lock screw before you let go of the tension screw. (Been there, did it!). And don't overtighten the allen screw. They will break off if you're not careful. (Been there also!)

F.Y.I.,it's the pwr valve that can open prematurly when cruise or idle vacuum is low,not your secondaries on the q-jet. If you have a fairly long duration cam with less than approx 14-15 iches at idle/part throttle cruise then this could be happening and you need to get the car/carb to someone that really knows q-jets to get it squared away. The q-jet can work well in fairly hot street setups with good mpg and hp but it takes a person that understands them to get it right and your avg street perf guy does not want to be bothered with them,thier loss.

Again your key to success here is to get to a good q-jet doctor,once your there things will get fixed very easily with the expert.

I remember i answered your 1st post and you should not be hitting your secondaires with the 373's at 3000rpm at 60 mph unless some one bent the mechanical lickage on the bottom of the carb. They also may have loosened the spring adj on the top vacuum actuated secondary butterfly valve on the top rear of carb as Randy mentioned above. Hook up the vacuume advance,bump up the timing to the detination point and then back off untill the detination stops,and then get to a good q-jet doctor/expert to dial it in,it's all not that hard to do,just be patient but the q-jet expert is a must. (the carb shop in c.a. could help you out or others listed in the rear of super chevy/hotrod,etc. Check out carb sites and carb perf rbld sites on the web for help.

Scott

[This message has been edited by SWHEATON (edited 10-03-2002).]

[This message has been edited by SWHEATON (edited 10-03-2002).]

Thanks for the help guys. A little info on what I got tonight. I set the idle rpm in park to 850-900 rpm and hooked up my vacuum gauge to the small port on the back, I believe it is the one that goes to the vacuum advance.

I got 8 in/hg at idle and turned the idle adj. screws both in and out half revolution at a time. I was at 2.25 rev. from bottoming out before I started so I used that as my baseline. I turned the screws all the way in, one half rev. turn at a time and went 5 full rev. out with both screws. It made no difference at all. Still got 8 in/hg at idle.

I then got my buddy to speed the idle to 2000 rpm and checked the vacuum there. It raised to a steady 15 in/hg and did not move. Upon letting off and it going back to idle it settled back to 8 in/hg.

Shouldn't the vacuum move somewhere when turning the idle screws either in or out. Even as far as I did? So are these numbers way off or about right.

I will tighten the screw on the sec. flaps tomorrow and try that out. Thanks for the tip. Also filled car up yesterday morning on way to work and filled up again today on way home. It went 95.4 miles on 15.43 gallons. Thats 6.4 mpg. Just seems something is off to be that bad. And the idle is so bad it chops in drive form 400-600. I expected that but in park it idles 800-1000. The only way I can speed up the idle in drive is the adjustment screw, but idle at 800 rpm = park at 1300 and then runs on when turned off.

Kerry
66 Chevelle

Did you hook up your vac. advance yet? If you hook that up to manifold vac. it`ll help the idle it will speed the idle up & you`ll be able to close the throttle a bit & see more vacuum.


Jim

sounds to me like you've got a big vacuum leak somewhere or the idle circuits in the carb are plugged with something.

how big is the cam in that thing?

Tom

I am checking now for a vacuum leak. I tried the paper on pcv valve grommet. If a leak is present, the vacuum in the crankcase will try to suck the paper in. It would blow the paper off. Or at least that is the trick I was told about for checking for vacuum leaks.


Specs are
454 .030 over
2399 Speed Pro
10.6 static/ 7.98 dynamic comp ratio. (93oct)
14 degress initial, 34 total
063 2.19/1.88, ported, 101cc
Ultradyne 288/296 .550/.575
Comp. Pro Mag Rocker Arms
Balanced bottom end
Stock points dist with msd blaster 2 coil
Edel RPM Q-jet
750cfm Q-jet off 78 suburban(stock jets/rods/etc.)
1 3/4 Hedman Hedders
3in Dr.Gas X-pipe with 3in Dynomax UltraFlo's

That's the hydraulic cam, the one that says " no power accesories"?? If so, you're vacuum reading might be right, albiet 2-3" lower than it should be since YOUR POWER PISTON IS BEING RAISED AT IDLE AND MOST OFF IDLE CRUISING>

First thing I would do is to find an Edelbrock tuning kit and use the lightest power piston spring in the box. If they have vacuum readings, you want the one that opens at 6". I would guess msot open at 10", so yours is nearly always on.

Weaton speak s of using a pro. I know that Dr. QJ ( a retired GM design engineer that designed the thing) charges $150 to go through one. He'll make it exactly right. Or try a lighter spring.

Remember that the QJ sort of works opposite, the power piston holds the rods down in the hole while the spring tries to push it up. The lighter the spring, the less vacuum it tkae to hold them down in the hole.

BEFORE YOU DO ANYTHING, if your QJ allows you to see the hanger on top of the pwoer piston through the vent hole, stick something like a QTip stick in there ( all the cotton fuzz off obviously ) and make sure the power piston goes up and down freely. Maybe its just stuck open?? Or one of the rods is caught on the edge of the jet and is causing this. Anyway, a 0 dollar 10 second check before you buy a spring kit.

Also, was this a new or used carb? I don't recall offhand. Here's where you get into trouble with used Q-jets. Around the early to mid 1980s, GM came out with the predecessor to EFI called Computer Command Control. It used all of the engine monitoring components as later EFI engines, but it used a solenoid controlled metering system in place of the power piston in the Q-jet. When there is no power applied to it, it goes to full open, which makes for a very rich running engine. If by some chance you grabbed one of these CCC carbs at a swap meet and stuck it on your car, it will not work, period. CCC Q-jets require a computer just like later EFI systems.