When I stomp it it makes a air sucking sound and just about stalls, then it recovers and is fine unless I punch it. I switched from a 76 350-2 to a 81 350-4, but I didn't have a 4 bb carb. So I switched the 2 bb intake onto the newer motor, it ran fine. I got a early cheby carb rebuilt (q-jet) by a experienced q-guy. I switched intakes and bolted on the carb, it runs smooth and there is no smoke. I adjusted my total timing to 32 deg @4000 and set idle to 650 rpm. The fuel pump is a 2bb version, I know I need to get a 4bb one. It runs fine untill the secondaries open and then it boggs. I don't know were all the vacume lines go so I plugged em all except the advance. Can someone point me in the right direction? Sean.
Sounds like the brake (not sure what it's really called) for the secondary air valves is missing or plugged also off. A picture is worth 1000 words, but I may not have one. I'll look around.
Yup, check the little vacuum dashpot (vacuum break?) on the right front corner, make sure it pulls in with vacuum. If it's shot, it'll let the secondaries slam open, bogging things. Also, check the air valve windup spring, under the passenger side of the air valve shaft. You'll need a small allen wrench to loosen the locknut, then you want the spring preloaded against the shaft 2 turns. How does it act if you ease into the secondaries?
The front break isn't plugged in, I thought it was to do woth the choke so I left it alone. I've been reeeding other posts and I think I have the rest of the vacume lines sorted out. So were do I run the vacume from to the break? There is no rear break or threads for one. Thanks for helping out a inexperienced buick nut!! Sean.
Yup, you need to get that vacuum pot hooked up. Should be a vacuum nipple right beside it, and a hose about an inch long should do it. Here's a pic: "A" is the hose, on this Ch*vy-style carb it's vacuum source is from the airhorn. "B" is where the vacuum nipple is located on most old Buick Q-jets "C" is where the secondary air valve spring is located, there's an allenhead lockbolt under there.
Not all Q-Jets use the link from the front vacuum break to the secondary air valve. Some do, some don't. You could try increasing the tension on the air valve spring, as suggested by jameyers. By the way, 32 degrees advance at 4000 RPM is at the borderline of too little, and definately too late. Try bringing it in at 2500-3000 RPM, and also try adding some additional advance--perhaps you could benefit from 3-5 more degrees of advance. Why do you need a "4-barrel" fuel pump? I bet the capacity is the same as the one you have, but it may be plumbed for a return line or something. Have you compared a "4-barrel" pump to the one you have?
Note to all: any Quadrajet had the vacuum control controlling the secondary air valves. It does two things: it assures that the air valves close at idle, and two, it controls how quickly they respond to the throttle. This and the afore mentioned air valve sping setting will control the opening rate. Too fast an opening rate will cause a bog. One other thing to check is the secondary accelerator pump. Most think that the Q jet did not have one, but they did. Just above or below the leading edge of the secondary air valve is an orifice responding to the opening of the air valves. Once the air valves passed about 10 deg of opening, these orifices stop feeding fuel. If they are blocked, a bog will occur as well. Ray
I did put in the crane adjustable vac advance kit with the springs to quicken the mechanical adv, but I need to limit the vacume advance (I know how now) but thats why it's backed off. So I realized another problem, I'm low on gas and the clear fuel filter shows a lack of flow. This alone could cause the secondary bog I bet. I'll post the results after I get some gas in ther tank and try again. Thanks for everyones help, I apprecaite it!! Sean
Fuel Pump If you have a gauge you should check your fuel pump pressure. There appear to be two schools of thought but I know if you are low on pressure you'll get a bog, and might also have drain back.
Usually the fuel filter is full when running, but as the tank there is less fuel flowing through the filter. I dropped my tank a while ago and cleaned it really well, that made a difference. I am going to get a gauge but i think pressure is ok unless tank is low.
That's odd...fuel tank level shouldn't have much, if anything to do with fuel pressure or flow. I've got a see-through filter, and sometimes it's full, sometimes it looks like there's nothing in it, but the car runs fine. Hard to tell anything from looking at the filter... o No: Have you checked out the vacuum diaphragm? That's where I'm putting my bet. :Smarty:
The vacume is holding the vacume break fine. I think it's the air flap on the top of the secondaries. Is that the air valve? Is it suppost to open slowly as the secondaries open or what? It seems to open a little when I hold the throttle wide open but it still bogs hard. I tried advancing the timing and that made no difference. Thanks.
Something I have frequently seen on these units is the plastic metering rod cam disintegrating. What this creates is a situation where the air valves open, but the metering rods do not move, or move insufficiently, causing a very lean mixture. With the engine off, try opening the air valve by hand, and observe the cam. Located in between the air valves, it operates the linkage moving the metering rods. Careful observation will indicate whether the metering rods respond to the air valve operation. There should be no play here; when the air valves move, the metering rods should follow, immediately. Ray
Yes, those are the air valves, they should open as the engine demand pulls them open, kinda slowly, not slam open. Ray's got a good point with the cam.
I'm still not having any luck with this carb. What can I do to troubleshoot while it's running. When I slowly hold it wide open it revs right up no prob, maybe thats still just the primaries? When I quickly floor it it still makes this weird sucking sound and stumbles hard. It almost stalls and slowly recovers. Could it be vacume related? The break is holding fine. In another post Larry talked about the air valve, and how I shouldn't be able to push it in while it's running. Well I can, when the fron break is sucked in is that suppost to hold it closed? Thanks for your help everyone, Sean.
If the secondary air valves are moveable with the engine running, it is likely the vacuum break, or choke pull-off not holding. If that is the case, you will likely have flooding issues with a cold engine as well. Have you checked the secondary metering rod cam? It should rise when the air valves are operated. The choke pull off and the secondary cam are the two things to check here, to begin with. Also make sure that the metering rods are properly inserted into the hanger. We'll start with this. One other thing to check is whether the power piston is free to respond to changes in manifold vacuum. Try inserting a small screw driver into the forward bowl vent, and depressing the power piston hanger, with the engine off. It should move freely, up and down. Frequently, with backfiring, carbon can be driven into the power piston cylinder, making it stick. Let us know...
Yes, this is your problem. When the front break is sucked in, the rod from it to the secondary air valve should be all the way to the front of the slot at the secondary air valve, keeping it from openning until the vacuum break dumps. When the vacuum break is sucked in you should not be able to push the secondary air valve down with your finger. Sounds like the rod is bent, maybe?
