Question regarding GM Quadrajet carburetor
rykrosgt
11-11-2004, 10:41 AM
Hey everyone. I've got a '76 Pontiac Ventura with a 2bbl 260ci V8.
I'm relatively new to carburetor tuning, but I don't want to get too indepth into it cause the car is being sold soon anyhow... but I just wanted to know about the mixture screws.
It's obviously a 2 barrel... which it has a mixture screw on the left and right front of the carb. My books that I have tell me to adjust them equally, and so the engine runs smoothly. MY question however, is whether these are equal jets, or if one is an Economizer, and the other one for Power; however the terminiology is I just want to know if they both do the same thing, or what.
By some stroke of luck I had adjusted it beautifully, but then I had to change the fuel filter so obviously that flubbered it all up. As it currently sits, it runs fine without any hiccups, hesitation, or dead spots (actually runs great) when cold... but once it warms up, I hit a deadzone anywhere past 1/4 throttle. No backfiring, spitting, or anything... and has a lack of power. It also stumbles a little bit even if I ease onto it nice and slow. I have a feeling it's still a bit too lean, but I don't feel like enrichening it too much more. It's bad enough I only get 13mpg on the highway.. and that's only doing 60-65mph.
Part of the way I've gauged power is how much pedal effort is required to maintain speed up the minor hills I have goig to-from work. If it's too rich, it hits a dead zone but doesn't gain much speed. If it's too lean, it gains speed slowly, but it required more pedal effort than what I had with that "sweet spot" I found before the filter replacement.
Thanks
I'm relatively new to carburetor tuning, but I don't want to get too indepth into it cause the car is being sold soon anyhow... but I just wanted to know about the mixture screws.
It's obviously a 2 barrel... which it has a mixture screw on the left and right front of the carb. My books that I have tell me to adjust them equally, and so the engine runs smoothly. MY question however, is whether these are equal jets, or if one is an Economizer, and the other one for Power; however the terminiology is I just want to know if they both do the same thing, or what.
By some stroke of luck I had adjusted it beautifully, but then I had to change the fuel filter so obviously that flubbered it all up. As it currently sits, it runs fine without any hiccups, hesitation, or dead spots (actually runs great) when cold... but once it warms up, I hit a deadzone anywhere past 1/4 throttle. No backfiring, spitting, or anything... and has a lack of power. It also stumbles a little bit even if I ease onto it nice and slow. I have a feeling it's still a bit too lean, but I don't feel like enrichening it too much more. It's bad enough I only get 13mpg on the highway.. and that's only doing 60-65mph.
Part of the way I've gauged power is how much pedal effort is required to maintain speed up the minor hills I have goig to-from work. If it's too rich, it hits a dead zone but doesn't gain much speed. If it's too lean, it gains speed slowly, but it required more pedal effort than what I had with that "sweet spot" I found before the filter replacement.
Thanks
curtis73
11-11-2004, 11:49 AM
Hey everyone. I've got a '76 Pontiac Ventura with a 2bbl 260ci V8.
It's obviously a 2 barrel... which it has a mixture screw on the left and right front of the carb. My books that I have tell me to adjust them equally, and so the engine runs smoothly. MY question however, is whether these are equal jets, or if one is an Economizer, and the other one for Power; however the terminiology is I just want to know if they both do the same thing, or what.
They are equal jets. Same thing, mirror image.
(actually runs great) when cold... but once it warms up, I hit a deadzone anywhere past 1/4 throttle. No backfiring, spitting, or anything... and has a lack of power. It also stumbles a little bit even if I ease onto it nice and slow. I have a feeling it's still a bit too lean,
The mixture screws to which you refer have nothing to do with the car running on the road. They are idle mixture only. Once your foot touches the accelerator they are no longer part of the metering. This means two things; 1) changing the mixture requires removal and disassebly of the carb, and 2) since it is fixed and doesn't change without doing this, the question is, why is the car running funny if the mixture didn't change?
My first step would be to re-set your idle mixture. With the car off, turn the screws in until they bottom out. Be gentle, they don't need to be torqued, just so you know where they are. Then back them out each 1.5 turns (it might be 2.5 but someone here will remind us). That will get you very close; enough to start it and tune it from there. At this point there are several methods to get the right idle mix, but the way I always use is this; Go out (richer) until you notice a change. Then go in (leaner) until you notice a change. Move slowly and be accurate. Then I leave it set half way between those two. If its a catalytic converter car, I usually go in just a touch on the lean side of halfway.
After you set idle mix, try tapping on the carb with a screwdriver handle. I know it sounds funny but it often works. Sometimes the float partially sticks to the seal in the valve and won't properly open. Sometimes tapping on it will let it drop.
The next best ideas I have are dirt in a jet, a cracked spring in the metering rod step up piston, or an air leak in the carb's bowl gasket. All of which are usually fixed by removal, disassembly, and re-assembling with new service guts. If you haven't changed the engine's cam, heads, intake, or other major part, the metering jets don't need to be changed and should be perfectly metered as they are. Its also possibly a misadjusted or broken choke. If it runs fine cold but doesn't hot, maybe the choke sticks on.
I might suggest a good old fashioned carb rebuild. You can do it yourself or find a local shop to do it, but you don't need to change the metering.
Are you sure its a 2BBL? You named it as a Q-jet which is a 4 BBL. 2s were called 2-jets and dualjets so I was just curious. You also mentioned no power after a certain throttle opening which sounds Q-jet-ish to me.
Otherwise, you're looking at a million possibilities ranging from fuel pump, to ring seal, to valve seal. This should keep you occupied for a while, so sometime (if you can) give us some more info on symptoms. Did it happen all at once or gradually? Any other associated noises show up too? How much vacuum does it make at idle and is it steady or jumpy?
OOOhhh. Just thought of another one. Make sure the exhaust crossover valve is still functioning. On one of your manifolds there is a butterfly operated by a thermo coil. When cold, its closed forcing most of the exhaust through the crossover. If it never opens it would be seriously restrictive causing similar symptoms.
Have fun and keep us posted
It's obviously a 2 barrel... which it has a mixture screw on the left and right front of the carb. My books that I have tell me to adjust them equally, and so the engine runs smoothly. MY question however, is whether these are equal jets, or if one is an Economizer, and the other one for Power; however the terminiology is I just want to know if they both do the same thing, or what.
They are equal jets. Same thing, mirror image.
(actually runs great) when cold... but once it warms up, I hit a deadzone anywhere past 1/4 throttle. No backfiring, spitting, or anything... and has a lack of power. It also stumbles a little bit even if I ease onto it nice and slow. I have a feeling it's still a bit too lean,
The mixture screws to which you refer have nothing to do with the car running on the road. They are idle mixture only. Once your foot touches the accelerator they are no longer part of the metering. This means two things; 1) changing the mixture requires removal and disassebly of the carb, and 2) since it is fixed and doesn't change without doing this, the question is, why is the car running funny if the mixture didn't change?
My first step would be to re-set your idle mixture. With the car off, turn the screws in until they bottom out. Be gentle, they don't need to be torqued, just so you know where they are. Then back them out each 1.5 turns (it might be 2.5 but someone here will remind us). That will get you very close; enough to start it and tune it from there. At this point there are several methods to get the right idle mix, but the way I always use is this; Go out (richer) until you notice a change. Then go in (leaner) until you notice a change. Move slowly and be accurate. Then I leave it set half way between those two. If its a catalytic converter car, I usually go in just a touch on the lean side of halfway.
After you set idle mix, try tapping on the carb with a screwdriver handle. I know it sounds funny but it often works. Sometimes the float partially sticks to the seal in the valve and won't properly open. Sometimes tapping on it will let it drop.
The next best ideas I have are dirt in a jet, a cracked spring in the metering rod step up piston, or an air leak in the carb's bowl gasket. All of which are usually fixed by removal, disassembly, and re-assembling with new service guts. If you haven't changed the engine's cam, heads, intake, or other major part, the metering jets don't need to be changed and should be perfectly metered as they are. Its also possibly a misadjusted or broken choke. If it runs fine cold but doesn't hot, maybe the choke sticks on.
I might suggest a good old fashioned carb rebuild. You can do it yourself or find a local shop to do it, but you don't need to change the metering.
Are you sure its a 2BBL? You named it as a Q-jet which is a 4 BBL. 2s were called 2-jets and dualjets so I was just curious. You also mentioned no power after a certain throttle opening which sounds Q-jet-ish to me.
Otherwise, you're looking at a million possibilities ranging from fuel pump, to ring seal, to valve seal. This should keep you occupied for a while, so sometime (if you can) give us some more info on symptoms. Did it happen all at once or gradually? Any other associated noises show up too? How much vacuum does it make at idle and is it steady or jumpy?
OOOhhh. Just thought of another one. Make sure the exhaust crossover valve is still functioning. On one of your manifolds there is a butterfly operated by a thermo coil. When cold, its closed forcing most of the exhaust through the crossover. If it never opens it would be seriously restrictive causing similar symptoms.
Have fun and keep us posted
rykrosgt
11-11-2004, 12:12 PM
If all they affect is the idle mix, and nothing to do with throttle.. then ...ugh... I'm not bothering with all of that. I'll set the idle, and call it a day. It smogged OK 2 years ago, and was driven by an old lady who didn't drive that car too much. If it doesn't smog ok this time then I'll just have to have someone fix it. It's definetely a 2bbl... it only has the two mix screws and only one intake opening with the one butterly. Only the 350 and the 400 or whatever... came with a 4bbl option that year. I guess it's kind of obvious that a Quadra-jet would mean FOUR... I call it a brain fart! hah. Maybe it is a dual jet or something. I don't know...
I might play around with it a little bit, especially regarding the exhaust redirect and the choke. I'm not bothering with my vacuum gauge and such unless I'm really bored.
In which.. if I am, and curiosity gets the best of me, then your info (which I appreciate VERY much) will be put to some use.
I might play around with it a little bit, especially regarding the exhaust redirect and the choke. I'm not bothering with my vacuum gauge and such unless I'm really bored.
In which.. if I am, and curiosity gets the best of me, then your info (which I appreciate VERY much) will be put to some use.
curtis73
11-11-2004, 01:02 PM
Have fun! Mmmmm... gasoline :)
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