giannibigler

Hello everyone,
My son just bought at 87 T/A with 305 carb. Automatic.
The problem is:
When you hit the throttle cable under the hood the the rear openings in the carb open just fine. But when you hit the gas pedal inside the car, they don't open and the car bogs and drowns.

I'm thinking maybe vacuum? The lines have definitely been messed with and they're sitting in a jumble on top of the engine. Most look like they're hooked up but some are capped off.

Or would it be something electronic? Sensors?

Any advice would be appreciated.

Joe Tag

iTrader: (4)

This makes no sense. You're saying it runs fine by opening the throttle by hand, but not by the pedal? Qjets have mechanical secondaries. If the secondaries are not opening, its possible the cable does not have enough throw to open them. Have someone hold the pedal to the floor (engine not running of course) and see if you can open the throttle more by hand under the hood. Also, the choke will lock out the secondaries until it is completely pulled off. The bogging? Is the difference that you have the air cleaner on when trying from pushing the accelerator peddal? Dirty air filter causing more vacuum and running way rich?

giannibigler

Thanks for the reply. FYI I'm no mechanic so my apologies for not knowing terminology.
Your interpretation of my problem is correct. The secondaries operate by hand but not by the pedal. The air cleaner wouldn't be the issue because we've tried it with the car sitting in the driveway, air cleaner off ( although it also doesn't open up driving down the road).
I'll try your cable test tomorrow when I have a helper.

Some additional info that might be helpful:
The car starts and idles just fine.
It takes off and runs down the road just fine.
No check engine lights.
We did just replace the transmission with one from a 86 camaro. They are both 700R4.

Joe Tag

iTrader: (4)

Ensure that you have the TV cable properly set. It's very detrimental to the transmission if it is not. https://www.thirdgen.org/forums/tran...djustment.html

naf

iTrader: (7)

The flaps you see on top of the secondaries are the air valves. They are not opened by the throttle but open up under demand (air flow). Their opening rate is determined by the air valve tension spring and the choke pull off. Opening rate is tuned for smooth throttle transition to the secondaries.

You won't normally see them open with the motor idling in the driveway. They should only open under large demand, under heavy acceleration.

The throttle blades are below and can be seen to open by peeking beneath the airvalves. There is, however, a lockout mechanism for them that is tied into the choke. This prevents them from opening until the engine is warmed up, which could potentially damage the motor.

There's a sticky above this forum that has a manual for this carb and diagrams as to the throttle and choke functions. A glance at that and a few minutes dickering with the rods and stuff can usually make their functions evident.

First thing I always do with a throttle/choke issue is to liberally clean the mechanisms with some carb cleaner, work them through their operations then go from there.

If you still have an issue you may have to explain what's happening a little clearer, but everyone here would be glad to offer insight, I'm sure.

naf

iTrader: (7)

Oh and welcome to TGO.

Damon

Naf gave you a good description of how the secondary system on the QJet works. There is no way to test it just revving the engine in the driveway. Those secondary air valves (up top) only open under a good bit of engine load. The secondary throttle plates, however, are mechanically actuated with a qualifier that the choke must be FULLY open.

Long story short.... you can free-rev the engine in your driveway and it might rev clean because you're only ever using the primaries. But out on the road (where you MUST use the gas pedal inside the car) it might stumble all over itself if the secondary system isn't working properly.

giannibigler

Thanks to everyone who has replied. After reading your responses along with info on the forum I have a slightly better understanding of the problem but not necessarily how to fix it.
So it seems that (while driving) the air valves aren't opening on the secondaries. It's probably bogging down because it's dumping more gas in but not getting enough air (?).

I read about adjusting the spring tension on the A/V's. I can see the flat adjustment screw thru the linkage but I can't find the set screw. Do I have to take the carb off and flip it over to find it?
FYI- the plugs and wires were changed last week with no affect on the problem.

Thanks!

Damon

The lock screw for the air valve tension spring adjustment is underneath the set screw, with the head pointing straight down. You can get to it easily without removing and flipping the carb (having a small mirror to see it from underneath helps, though). Depending on the year of the QJet it will either take a small allen key (1/8"??) or a small Torx-head key to turn it.

HOLD THE TENSION SCREW IN PLACE AS YOU LOOSEN THE LOCK SCREW. The minute it breaks loose the tension adjustment screw will try to unwind to zero tension and you won't be able to count back how far from zero tension it was when you started (3/4 of a turn from the zero tension point is where you start if you loose you place).

I doubt that's your problem, though. If you can push the air valve open easily with your finger, it's probably in the ballpark. Air valve tension is a fine-tuning aid, not a "holy cow" kind of difference if it's a little off.

You didn't mention what area of the country you're from but if you're anywhere reasonable driving distance from Philly, I'm pretty sure I could nail this down for you pretty quick. There's only so many things it can be, but it takes a long time to type them all out. (And I know what it means when Dad is looked upon for answers by his son! Been there, done that.)

naf

iTrader: (7)

choke pull off failure also seems to be fairly common as these things age. check the pulloff by removing the vac line, pressing in the plunger and holding your finger over the nipple. the plunger should stay in until you remove the plunger.

giannibigler

Thanks for the advice! I'm 20 miles west of Pittsburgh. So a little too far to go but I appreciate the offer.
I did find the set screw and loosened it. I could only back the adjustment screw out so far because it was still spring loaded even with the set screw almost all the way out. So I turned the adjustment screw back a little (under spring load) and then tightened the set screw. It did make a difference. I would say the AV's open about 75% of the time when you hit the pedal out on the road. I wouldn't exactly say it's fixed but it's better than it was.

giannibigler

I will check the choke pull off tomorrow. Although a friend of mine thought that part might be new because it was still a clean, shiny brass. If it is new, could it need an adjustment? Maybe whoever put it on didn't know what they were doing?

Thanks!

naf

iTrader: (7)

if the secondaries are working some, the choke pull off is probably good, but a good idea to check anyway. No real adjustment to the choke pulloff, unless you get into some fine, fine tuning which can require re-sizing the vacuum orifice. Shouldn't be needed for normal driving stuff.

If the secondary operation is intermittent, ensure that the flaps move freely and aren't getting held up somehow. If they've been removed at some point they can be re-installed such as to cause a bind. Overtightening the airhorn can also cause them to bind. The secondary rods should also lift up as they open. I've seen the little cam that lifts them crumble to dust before.

Damon

The adjustment screw for the air valve tension sometimes gets rusty and doesn't want to turn in its hole (you know about rust if you are from the P-burg area!). Spray with a little PB Blaster, let sit, try again. It'll move more than it did in your first attempt. It should turn with almost no effort, until the spring begins to pre-load it (and don't take it tighter than 1 full turn of spring pre-load from the zero-tension point, or you could distort the spring permanently).

And if THAT part is rusty, it makes me wonder what other linkages might be hanging up for similar reasons. Like the secondary lock-out arm not wanting to kick open with the choke, perhaps.

BASSETT IROC 85

iTrader: (2)

Are you test driving the car without a air cleaner?

2firebirdpals

Would it matter- as long as the hose is plugged- should it make a difference. ?

BASSETT IROC 85

iTrader: (2)

You will suck the hood insulation into the carb.

BASSETT IROC 85

iTrader: (2)

five7kid

iTrader: (14)

It's actually the other way around. The air valves are opening too quickly, which dumps a bunch of air into the engine before sufficient fuel can flow to mix with the air.

Slowly opening the air valves allows the fuel to be drawn through the secondary metering passages so you don't get the bog.

If the secondaries aren't opening from time to time, then the throttle blades are probably being held closed by the secondary lockout lever. See this thread: https://www.thirdgen.org/forums/carb...e-rebuild.html and this figure https://www.thirdgen.org/tech/images/e4me016.jpg , item #34. When the choke is on, this lever drops down and locks out the secondary throttle blades. When the choke opens, the fast idle cam, item #37, weight drops down, and the other end pushes up on the lockout lever, disengaging it from being able to lock out the secondary throttle blades. If this isn't working correctly, it's usually either caused by the choke not fully opening, or the mechanism being gummed up.

If that's actually what's going on...

Personally, I could never tune out the bog with just adjusting the secondary air valve tension. The choke pull-off needed to be operating properly, and the only way to check that is to do a vacuum lock-on test - pull a vacuum on the pull-off, lock the vacuum on, and see if the pull-off stays pulled in. If it does, it's good. If it doesn't, replace it as it is probably not properly doing its thing with the air valves (even if it pulls the choke open while the engine idles).

Chuck84TA

Probably been mentioned above, but remember to not NOT over tighten the secondary air valve spring.... Bad, been there.. Done that.