Fast305

My 1983 305 HO is getting terrible mileage such as 10 m.p.g no matter what I do (drive like a granny or hammer it makes no change!). I have checked almost everything. I shift into overdrive, timing set as it should be, have vacuum advance. This is a non computer engine. I am curious what the Adjustable Part Throttle screw in the non computer q-jets should be set at.

Here is my setup-
160,000 mile 9.2:1 305 shortblock w/ 7% leakdown in worst cylinder, recurved factory distributer 10* initial, 22*@3,000 centrifical, 20*@10 in/hg vacuum advance, the carb is a 1983 795 cfm Q-jet (72 jets, 50m primary rods, 7 in/hg spring, DR rods, F hanger, float & air valve tension set, APT 3.5 turns out), 2 1/4 dual exhaust without cats from factory manifolds. All factory smog controls function, I added a Federal Mogul CS 1014R cam (204*-214* @.050", .423" & .442" of lift), rebuilt the "601" HO heads with Z/28 springs, swirl polished, undercut 1.84 & 1.50 stainless valves, heavy duty keepers & retainers, as well as valve stem seals. The valve guides were redone along with milling for gasket surface and 3 angle valve job. I burn no oil. Idle with 18 in/hg vacuum, pull 13-15 @ 70 mph. The transmission is 700r4 (goes through all gears & lockup) with 3.42:1 gears. What is up? I don't seem to have any added drag, fuel leaks, etc. I think it is tuning related but don't know???? Can someone verify that it is the carb or ignition tuning? Also I have gotten the same gas mileage since I started mods. I used to get 20 MPG on the highway @ 70 mph when it was new and almost new

Streetiron85

Have you tested your vaccum advance for leaks?

Fast305

I checked it with a hand vacuum pump and timing light. It worked by bumping timing up 20* @ 10 in of vacuum. It held vacuum for a while. I have checked all of the emissions vacuum systems for leaks and have found none.

Streetiron85

Sometimes those power pistons in a Q-jet can get stuck in the power position and it's similar to a blown power valve on a holley.
That's something to look at.

CALIROCZ28

iTrader: (3)

Do you have a EGR on your car? If so check it for proper operation. Any manifold leaks? Did you used to get good milage with the same carb setup or was it something else at the time you were getting 20 mpg? I would suspect your carb setup. How do your plugs look? Something to consider checking.

Streetiron85

I just noticed that you said you were getting good mileage before your mods.
It seems like eliminating the power piston as the trouble is a good step. There's a way to check that without taking the carb off the car by using a small bent piece of wire in the shape of an L. You can place that wire down the vent hole at the front of the choke horn and feel the operation of the power piston as the engine is running.
If you want more info on that, I can find out more.

Fast305

I was getting terrible mileage BEFORE the mods. That is the reason I did the mods. I originally thought I had a plugged cat. I have alot more power after the mods but NO increase in mileage. In fact I can drive faster and get better mileage??? I have checked the EGR valve etc. I am too the point of blaming the quadrajet. My plugs seem to loadup with crud after less than 5,000 miles. A quick run at WOT seems to blow it out though. I am thinking of going to the Edelbrock 1904 or the Performer RPM quadrajet? Anyone have experience with these? I got good gas mileage with the same carb setup as this is the factory carb setup. The only question on the carb setup is that someone else rebuilt it years before I did and I think they played with the APT screw. It is 3.5 turns out, edelbrocks 1904 is set @ 2 turns. Could this be my mileage problem? The APT is a screw which limits the insertion of the primary metering rods into the jets (I believe more turns out is richer but need this verified).

CALIROCZ28

iTrader: (3)

I would start with your QuadraJet for certain. I'm sure some of the guys on the site can tell you exactly what you should do. Today is Saturday and I'm sure you will receive numerous replies.

Good Luck!

Streetiron85

The apt screw controls the height of the power piston that I referred to earlier, at the bottom of it's stroke. You can turn that apt screw all day and all night long and it won't have any effect if your power piston is stuck at the top of it's stroke. That's why I mentioned the trick with the L shaped wire to check the operation of the pp. It's the most common cause of poor mpg with qjets. And it explains your sooty plugs also.
Turning the apt screw in, lifts the rods a tiny bit.
A stuck power piston lifts the rods too... WAY up.
Your repair could be a shot of gumout away.

Could you send that Qjet my way if you decide to get a new carb.

Fast305

I the power piston with a small wire down the bowl vent. It felt nice and smooth and went down nice and easy. Just like it does when you play with it, with the top off. This next weekend I am thinking about rebuilding this Q-jet again as it has been about 20,000 miles since I did it. I am thinking about cutting 1/4" X 3/8" transition slots in the secondary air valves for a smoother WOT transition just like the old pontiacs used. I screwed the APT screw in until it was only out 2 turns (lets metering rods go farther into jets). It seems to run fine with the change. I have a very light part throttle surge @ 30 mph or so but goes away as you drive faster. It seems to run smoother around town with no bog/hesitation. I need to drive it farther to know if mileage has gone up. I am crossing my fingers that this works.

Streetiron85

Did you check the PP operation with the engine idling too?
It sounds like if what you did doesn't help then a rebuild is a good idea.
I'm still not sure whether I love or hate Qjets, there's potential .... but the primaries are a huge pain. I've always given up before I ever got one dialed in 100%, but they seem to be pretty good if you can get them 90% anyhow.
It gets to the point where you know there's still room for some improvement and then you realize that you could spend the entire summer experimenting with different metering rods, jets and PP springs.
When I'm done with all my mods I'm gonna send mine to a pro and have them dial it in on a dyno.

Joshua

Try disconnecting the vacuum advance and see if that improves or decreases gas mileage.

Have you checked the primary rods recently? A rod that is REALLY bent would also kill gas mileage.

five7kid

iTrader: (14)

The APT holds the rods out of the jets. If set too high, the rods will never lean out the mixture. You want it set just high enough to eliminate the off-idle flat spot.

I have an Edelbrock 1901 and 1903 (hot air choke which I converted to electric). I've been pretty disappointed with both of them. The 1901 was on the 396, I could never get the secondaries to open consistently (the AV has a gritty feel to it - inconsistent secondary opening isn't a good thing in bracket racing). The 1903 is on my full-size van 350, I expected a big improvement in power & gas mileage over the stock carb, but it didn't happen. It starts & runs fine, passes emissions testing (barely, but same with the stock carb), but in hindsight I shouldn't have spent the money on it.

Another place you might look for problems is the galley plugs under the fuel bowl. They stick through the throttle plate fully exposed to the manifold plenum. They are known leakers, allowing fuel to go directly from the fuel bowl to the intake. So, epoxying them over is a good idea.

Make sure those lead plugs aren't leaking. You kind of need to play with jetting/rods & APT together to get the cruise & off-idle right.

Fast305

The rods are not bent. I checked not too long ago when I couldn't pass inspection due to CO.

five7kid- what are you getting in your van for gas mileage. I have a 1984 that gets 17 on the road and is making me mad. It is setup just exactly like this other DUMB camaro (same cam, heads, porting, exhaust, intake, carb, distributer, etc. ) I am thinking of switching carbs between them for a while just to see if that is my problem.