Gas mileage???

[RITTER]

Far from being a Quadrajet mechanic :!:
I have already looked the carbs up to see what they originally came from. How do I tell what jets and rods are in the carb? Do I just measure them? Is it stamped on them? I'm not going to tear it back apart unless I know exactly what I am looking for

 

[Bonnewagon]

Sure you are. You did it once, now it's not so scary. The jets are stamped with tiny numbers around the top. Sometimes they overlap into the slot, so they apply 2 sets of numbers and at least one set is legible. If not, a set of decimal drill bits can be passed through the jet until you get the right one. The primary rods are also stamped with tiny numbers and I usually need a magnifying glass to see them. If not, a micrometer can be used to measure the top of the taper. The tips are all .026", and it's the tapered part that sits in the jet at idle. The link I gave you from NEW AGE METAL has factory carb sheets. Go to Chevy, look up your carb number and in the breakdown will be the actual GM part number. The last 2 digits of the jets and rods are the sizes. Rather than guess at what sheet has the carb you are looking for, I found it easier to download the PDF and then rapidly scroll through it all. They pretty much go in ascending order by year and size.

 

[dogsht]

2.29 and 2.14 gears! Yuck I bet semi trucks leave you guys at the lights. But at least once you get up to speed you should get good mileage on the interstate.

 

[Bonnewagon]

Well I may have 2:78 rears but my 4 speed Saginaw has a 3:50 first gear. That gives an overall ratio of 9.73 to 1. ( I consider 10 to 1 to be ideal ). By comparison, a close ratio Muncie with 2:20 first gear and a 4:11 rear only has a 9.04 overall ratio, yet it's considered "muscle car" stuff. So I accellerate quite nicely, yet I loaf along at highway speeds spinning only 2000 rpm at 60 mph. This is not something new but rather what the factory was doing in the late 70's- early 80's before overdrive transmissions with lockup converters became common.

 

[gto78]

Well I may have 2:78 rears but my 4 speed Saginaw has a 3:50 first gear. That gives an overall ratio of 9.73 to 1. ( I consider 10 to 1 to be ideal ). By comparison, a close ratio Muncie with 2:20 first gear and a 4:11 rear only has a 9.04 overall ratio, yet it's considered "muscle car" stuff. So I accellerate quite nicely, yet I loaf along at highway speeds spinning only 2000 rpm at 60 mph. This is not something new but rather what the factory was doing in the late 70's- early 80's before overdrive transmissions with lockup converters became common.

That's good info. How are the other gears spaced out in overall ratios? For example, what speeds does each gear top out? I have no idea what mine does with the Super T-10 and 3.73 rear. I haven't gotten it on the road legally with this setup yet.

I know that on my 350Z they spaced them so that you get just about exactly 30 mph for each gear- 1st=30, 2nd=60, 3rd=90 etc. and when you shift into each gear it's still in the power band of the next gear.

 

[RITTER]

To make the gas mileage mystery even better, my speedometer stopped working, so no tripometer either

 

[Bonnewagon]

I never checked the other gears but what I like to see is no more than a 500 to 1000 rpm drop between shifts. That is considered "wide ratio". I've had a Chevy V-6 on a three speed Saginaw with a 3:50 first and a 2:41 rear. What a DOG! It dropped about 2000 rpm between shifts and struggled to recover each time.