General Category > Quadrajet Carb Talk and Tips
Main air bleed question
Jim H:
Hi guys,
I have a '77 Corvette, California car with the original carb 17057504. The engine has been ugraded and the carb rebuilt and modified to Cliff's Recipe 2. The car is running well with good performance but I am wondering about something, 76 jets were installed but the upper and lower main air bleeds were left alone so they are in the neighborhood of .110 - .120" diameter. Was that a mistake? The car is getting poor gas mileage, less than 10 mpg, could that be why?
Thanks,
Jim
Jim H:
I forgot to mention, 44 rods...
Kenth:
Factory jets in 17057504 is #72 with 41K rods. Upper and lower main airbleeds are .120".
If economy is the main consern i would use the factory jetting and modify only the idle/low speed circuit. Start with APT at 4.0 turns up from seated.
If #76 jets gives best WOT performance i would use 47K-49K primary rods and use the "tip-in" method for APT adjustment.
Cliff Ruggles:
Unlikely that is the cause of fuel mileage that poor. Those carbs are horribly lean as delivered and I NEVER use the larger "K" series rods in them. They don't offer enough room for adjustment with the APT system for one. IF you look into the carb with the PP in place you'll see that the larger section of the tapered portion of the rods don't go all the way into the actual metering area of the jets. With "K" rods you only have .005" of taper right to start with, then they "step" to .026". You really you have very little rood for adjustment.
With a full tapered rod you'll have a LOT more control with the APT system. We have full tapered 45 rods for them and that's what I use in all of those carburetors here with perfect results.
Every single time I've put the stock 76 or 77 jets back in one of those and the larger "K" rods they are too lean on the test engine and we get customer complaints if I let them out of here that way.
I don't leave the idle system stock in them either for the same reason, especially with the L-82 engines.
A few years ago I had a customer who DEMANDED that I return every single part of the carb to the exact stock settings. It was a 77 or 78 L-82 engine from a Corvette and factory original carb. I told him that the idle system was too lean and it needed a slightly larger idle tubes and DCR's to make it happy. He told me to leave it stock, so I did. It ran poorly on the test engine and sure enough he didn't have the carb on his engine 10 minutes before he was calling the shop bellyaching about it being too lean at idle and speeding up/smoothing out whey they shot some carb cleaner around the throttle shafts or closed the choke flap some.
His engine builder got involved, a guru of some sort and he demanded that I install secondary shaft bushings as it was his opinion that it was sucking too much air around them making it lean.
So I get the carb back and opened up the idle tubes .002" and DCR's slightly, and replaced the huge "K" metering rods with our smaller full tapered rods. I did NOT bush the secondary throttle shaft as they had demanded.
The carb ran flawlessly on the test engine after those mods and guess what, it worked perfect on the customers engine as well.
What folks need to realize here is that these carburetors were emission calibrated and they had better fuel back then. This new fuel takes about 5-7 percent more of it and if you leave one of these carbs completely stock more times than not the engine is NOT going to be happy about it......FWIW.......Cliff
Jim H:
Kenth: Fuel economy is not the biggest concern but it needs to be better than it is now.
Cliff: So are you saying replace the rods with full taper 45 rods (your Custom 50M Style rods?), keep the 76 jets and leave the main air bleeds as they are?
The engine was originally an L-48 but now has 9.2:1 CR, ZZ4 intake, AFR 195 heads, Hedman long tube headers, 700R4 tranny with 2400 stall converter.
I also have another carb, 17058504 and all options are on the table with that one.
Thanks guys
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