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Hey guys, I have a 1979 4-speed. I just installed Doug's Headers and Side pipes with STS baffles and rejetted my carb from a 72 main jet to a 74.
When I rejetted the carb last week, I just made one first gear pull on an open area of road and it seemed like it pulled well. I've been busy for the last week so I hadn't had time to mess with the idle and mixture.
Well tonight I decided I would try to fine tune the idle and mixture screws (it was idling kinda rough after the rejet and header install). I adjusted both mixture screws to 3 turns out as a starting point and adjusted the idle to about 900 RPM. I went out on the road to see how the car was pulling, and it just hits a brick wall around 3000 RPM. What gives? I thought the mixture screws only adjusted mixture at idle?
Now that you've started modifying your Corvette, I highly recommend both a quality fuel pressure regulator and fuel pressure gauge, like the Aeromotive unit pictured below. The $29 Holley's will fail eventually and leave you stranded (as I've witnessed a few times already, including personally). You need to know what you're fuel system is doing.
Secondly, I can't stress enough, the superb advantage of having an Air/Fuel Ratio gauge, like the wideband unit, sold by AEM. You will know instantly at all times, exactly what A/F ratio your carb is at, whether it's at idle, 3,000 RPM or Wide Open. Easy install, and they even include the O2 sensor and the bung to weld onto your headers. Way better than guessing and scratching your head at the results. I also run a dedicated vacuum gauge as well, because there's a lot that can be told from that too. I replaced my clock with the vacuum gauge.
when you had the carburetor open to install the jets did you do anything that would have effected the needle and seat or float? Did you check or adjust the float while you had it open? Float should be set for .250. Did you check if your fuel filter is clogged? I'm assuming you still have the stock Quadrajet.
Now that you've started modifying your Corvette, I highly recommend both a quality fuel pressure regulator and fuel pressure gauge, like the Aeromotive unit pictured below. The $29 Holley's will fail eventually and leave you stranded (as I've witnessed a few times already, including personally). You need to know what you're fuel system is doing.
Secondly, I can't stress enough, the superb advantage of having an Air/Fuel Ratio gauge, like the wideband unit, sold by AEM. You will know instantly at all times, exactly what A/F ratio your carb is at, whether it's at idle, 3,000 RPM or Wide Open. Easy install, and they even include the O2 sensor and the bung to weld onto your headers. Way better than guessing and scratching your head at the results. I also run a dedicated vacuum gauge as well, because there's a lot that can be told from that too. I replaced my clock with the vacuum gauge.
Funny you should say that, I have an AEM wideband 02 sensor and gauge sitting on my desk at home! Planning to install it this weekend. I completely agree that having accurate readings is key. I will look into installing a fuel pressure gauge as well. Thanks!
when you had the carburetor open to install the jets did you do anything that would have effected the needle and seat or float? Did you check or adjust the float while you had it open? Float should be set for .250. Did you check if your fuel filter is clogged? I'm assuming you still have the stock Quadrajet.
I checked my float and it is set at about .370. Isn't .250 a bit high? I don't have Lars papers handy at the moment, but that seems like a high float height. Correct me if I'm wrong!
I haven't checked the fuel filter. Is there one in the carb? Or is it inline somewhere between the tank and carb?
There is a lot of room to improve A/F ratios with metering rod changes. It may be as simple as dropping in richer secondary rods. If the metering rods are stock, there is a lot of room for improvement.
Funny you should say that, I have an AEM wideband 02 sensor and gauge sitting on my desk at home! Planning to install it this weekend. I completely agree that having accurate readings is key. I will look into installing a fuel pressure gauge as well. Thanks!
I checked my float and it is set at about .370. Isn't .250 a bit high? I don't have Lars papers handy at the moment, but that seems like a high float height. Correct me if I'm wrong!
I haven't checked the fuel filter. Is there one in the carb? Or is it inline somewhere between the tank and carb?
The float level is from Cliff Ruggles book on the Quadrajet. Factory would have installed filter in carburetor inlet.
There is a lot of room to improve A/F ratios with metering rod changes. It may be as simple as dropping in richer secondary rods. If the metering rods are stock, there is a lot of room for improvement.
I'll post up my a/f ratios at idle, cruise, and WOT as soon as I have my 02 sensor installed (hopefully this weekend)
If it hits a brick wall....it would have to be way off on A/F to do that...especially if it was OK before. Idle screws aren't going to do that.
Ck the secondary lock out from the choke linkage. If it has heat stove choke on the intake, the headers pretty much keep it from working like original and it may not pull far away to un lock the secondary plates. Either wire off he lock or remove it.
If it hits a brick wall....it would have to be way off on A/F to do that...especially if it was OK before. Idle screws aren't going to do that.
Ck the secondary lock out from the choke linkage. If it has heat stove choke on the intake, the headers pretty much keep it from working like original and it may not pull far away to un lock the secondary plates. Either wire off he lock or remove it.
JIM
Could you explain that better? Sorry. I"m still learning my way around Carbs. So the choke has some sort of lockout that stops the secondaries from opening?
427HOTROD hit it on the head. Your secondaries are not opening. The jump from 72 to 74 may be too much, but it's not causing your problem.
Ok, so what causes the secondaries to not open? I just took my air cleaner off and checked and they move freely. Sorry, I'm pretty new to carbs, but I'm learning.
Look on passenger side of carb down where the throttle shafts come through. You'll see a little lock lever between the two that is hooked to the choke linkage. It has to be pulled out of the way to unlock the rear throttle shaft. There's a plastic diaphragm choke pull off on right front of carb hooked into all of this linkage too. But probably the lock out tab
Look on passenger side of carb down where the throttle shafts come through. You'll see a little lock lever between the two that is hooked to the choke linkage. It has to be pulled out of the way to unlock the rear throttle shaft. There's a plastic diaphragm choke pull off on right front of carb hooked into all of this linkage too. But probably the lock out tab
It's interesting that the choke linkage seems to be the culprit. When I was last in the carb, the choke arm became unattached from the linkage inside the carb and I had to reinsert it. I'm pretty confident I put it back on correctly, but maybe something came out of alignment?
It's interesting that the choke linkage seems to be the culprit. When I was last in the carb, the choke arm became unattached from the linkage inside the carb and I had to reinsert it. I'm pretty confident I put it back on correctly, but maybe something came out of alignment?
May not be the issue but I'd start there. You can open air door and look down in carb to see if secondary plates are actually opening. With lock holding it you can push pedal all the way and open primaries but linkage will wind a spring instead of opening the plates
It's interesting that the choke linkage seems to be the culprit. When I was last in the carb, the choke arm became unattached from the linkage inside the carb and I had to reinsert it. I'm pretty confident I put it back on correctly, but maybe something came out of alignment?
May not be the issue but I'd start there. You can open air door and look down in carb to see if secondary plates are actually opening. With lock holding it you can push pedal all the way and open primaries but linkage will wind a spring instead of opening the plates
05-25-2016, 08:51 PM
Bogging around 3000 RPM
