A Little Better Mileage ??

[Roy W.]

Following my rebuild on my 17085213 Q-Jet using Cliffs kit which included .073 primary jets and 50C primary rods, I set the float level at 1/4 inch. The car (1981 Corvette) runs absolutely great. Following the change in jets and rods I was hoping for a little better gas mileage. The car is used mainly for short trips averaging maybe 45-50 mph. Present mileage is 13-14 mpg. My question is: My APT is backed out exactly 3 turns. Would turning it in 1/2 -3/4 turn increase mileage any? I'm looking for maybe 2 more mpg.

[Cliff Ruggles]

It may take several tanks of fuel to see the exact results but fine tuning the APT once you get close takes some time.  Since the rods are full taper, make small adjustments one way or the other.

Don't forget about the vacuum advance, it is equally as important when you are trying to get maximum efficiency in the "normal" driving range.......Cliff

[Roy W.]

Cliff.....Thanks for the response. Yes, I have 12 degrees of vacuum advance at cruise, and mechanical plus initial (36 degrees total) is all in at 2500 rpm due to my 2.87 gears which is my 65 mph cruise rpm. I'll run a couple more tanks, and try to check it at 65 mph which is probably a more realistic indication than local driving.

[77cruiser]

I'd try turning it down about 1/2 turn at a time until if feels like it's surging or really lacking power on light throttle. When you find that go back 1/2.

[Cliff Ruggles]

I've done a lot of testing in this area and have found that setting the carb noticeably lean superficially will yield slightly better results but only if you drive very gently.  I used to make 600 mile runs between Ohio and Virginia when I was stationed there and my family still here in Ohio.  At that time I had several vehicles equipped with Quadrajets to make that trip.  Since there are quite a few steep grades involved in that run with a decent amount of flat ground I'd spend some time fine tuning the APT system and even played around with vacuum advance a bit.

What I found was that on flat ground and very light load, combined with a very conservative right foot I could pick up 1-1.5 MPG's setting the APT on the lean side.  Even doing so I lost MPG's when I started into the hills and had to pull a lot of steeper grades as it took more throttle angle to maintain speed.  This simply happens because the engine is not able to pull a heavy load with the leaner A/F ratio and you had to push on the throttle harder to bring in more fuel to compensate.  I also didn't like the slight "anemic" feel of the throttle when I started to lean things up a bit.  I tried to compensate with more timing from the VA but quickly found there is a point of no return with these things.

Even so I was able to get some pretty impressive fuel economy numbers from the vehicles I was using at that time.  I continued testing in that area for quite a few years, and all this started decades before I wrote the Q-jet book or went full time into the carburetor rebuilding business.

I've also noticed that modern vehicles with all sorts of computer controls and TB or fuel injection don't really do any better pound for pound as some of the vehicles I was driving way back then.  I guess the laws of physics also apply here....it's going to take a given amount of BTU's or energy to accomplish the task at hand, and difficult to get past that deal no matter if you are using a computer to tune or doing the tuning manually......FWIW.......Cliff

[Roy W.]

Thanks for both of your responses. With Florida primarily flat land, I think I'll still leave well enough alone and concentrate on improving mileage by driving with a more sensitive right foot. Any corvette owner that can do this should get some sort of award LOL.

[Cliff Ruggles]

Of course driving style has a major impact on fuel economy. 

I find it difficult if not near impossible to go thru an entire tank of fuel with the Ventura and not do a few full throttle blasts.

So fuel economy testing is difficult for me.  I much prefer drag strip testing and try to make it to one of our local track a couple of times every summer......Cliff

https://www.youtube.com/watch?v=6zVdoLR-VzM

[Roy W.]

Nice run.....right on your dial in !!.......Gotta love that leaving on the last yellow with a shallow stage.