350 gas mileage, same 10mpg city AND highway

I've got a new (1500 miles) GM crate 350 in my 1960 Impala 2-door (and a 20 yr old 350 trans). It's the 190 horsepower version which may be a bit wimpy for my big car. I wasn't too surprised to get only 10 mpg around town but I was hoping for better with a brand new engine. I just took my first prolonged highway trip of 115 miles. I used exactly 11.5 gallons! 10 mpg. I was hoping to get up in the teens. I think that's not unrealistic for a new 350. I've gotten almost 20mpg with an old 283 and a beat 327 in big old cars. Does anyone have any ideas why my highway mileage would be lousy and why it would be exactly the same for city and highway? Seems like there should be some difference. Could it be this bad just from the carb set up? There is no smell or residue of gas when I open the hood.

I'll put some extra info here after some kindly requests seen below:
I've got MSD (multi-spark) electronic ignition. It's got a stock intake with a rebuilt quadrajet (that never gets the secondaries opened). 3.08 gears in the rear. Tall-ish tires on 14" rims...basically stock size. Torque converter? I dunno. Oh, and these mileage numbers are while I'm driving granny-style...trying to get the numbers up. I don't even want to know the cost of stomping on the pedal all the time.

 

1500 Miles is still new and it still might be going through the break in period. Also it would help to know other stuff like carb, ignition, tires rear end and ratio and also if you drive anything like me then be happy you got that much. Damn heavy foot.

 

What carb? What distributor? What torque converter? What rear Gear? Hold it up to the monitor so I can solve your problem without seeing wtf you have.

 

ahahaha...

 

Yeah, more info is in order. It's got a stock intake with a rebuilt quadrajet (that never gets the secondaries opened). 3.08 gears in the rear. Tall-ish tires on 14" rims...basically stock size. Torque converter? I dunno. Oh, and these mileage numbers are while I'm driving granny-style...trying to get the numbers up. I don't even want to know the cost of stomping on the pedal all the time.

 

Put a HOT ignition in it. High output coil, high quality plug wires, brass cap, high end spark plugs. THEN some must set that carb like a pro. Syntech oils in the entire drive train. Timing SPOT on! By then you should see 2-3 mpg easy. Finesse that super fine tuning.

 

I'd look at that Q-Jet, bet you a mile or two is in there somewhere.

 

Hell, back in the early 70's, I had a stock 1970 Rallye 350. Hi compression 4-bolt main block, 325hp, 650 Quadrajet, Muncie 4 speed, 3:91 rear with G-70/14's.
I got 8-12 in town and 16 on the highway. I even drove it from DC to California and got 15-16 mpg.

I would say the distributor/timing/fuel jets/floats need closer examination.
Timing would really make your MPG go south. If it's a cruiser, you only need a small 4 bbl carb. Better yet, a bolt on throttle body would get your mileage up. check your exhaust tips. I bet they are real black and sooty!

good Luck!

 

W/the 3.08, you've got a great highway gear for that combination.
Set acording to the tips you've gotten, I'd expect ~16 TO 18 mpg on the open road.
Syn. oil helps, front end align, tire pressures, exhaust free-flowing, you'll find what it can do.

 

You're supposed to pull the anchor up before you leave the dock









Seriously though, are the brakes dragging? Something has quite the load on that motor. You'd probably smell the brakes when you stop, but even a small drag or resistance will eat mileage real fast. Just a thought...

 

You can use a small stirring straw to place down the vent tube and check that the power piston moves up and down freely.

If you hve a vacuum gauge you can check when the power piston moves, should be around 8-10 inches.

If the metering rods are not in the jets, it happens, or the piston is stuck up (rare) or has too strong a spring and opens early you will spend a ton of fuel.

An easy way to check is to put it on the fast idle cam and pull off the pcv hose.

If the engine races it has way too much fuel.

The engine should miss with that big a vacuum leak.

Besides the power piston, there were problems with sinking floats and leaking main wells.

If the engine races with the pcv hose off you know what, (way rich) then you can look why.

Vacuum advance can be worth a couple MPG too combined with correct mechanical.


Watch the vacuum (gauges are cheap) and raise the engine to 2500 RPM, if the vacuum falls off you most likely have a restricted exhaust, less likely cam is out of time.

Hoop

 

70`s smog 350`s are notorious for bad mileage,any equipment from that era trash it.

 

That makes me feel pretty good about the 12 mpg my 454 dually with 4.11's gets towing a trailer...

Brian

 

Check the timing and the carb.

 

Try an Edelbrock Performer intake and a 600cfm Holley? May help add a little torque for the around town mpg. Just a thought.

 

Charlottesville Va is in the mountains. Lots of steep hills. Combine that with 190hp, 3.08 gears, and a quadrajet carb. Not a great combo for fuel economy.

 

I can't imagine which carb would get significantly better mileage than a well setup Q-Jet. Small primaries with a triple venturi booster were only matched by the dual-jet.

The annular discharge autolites are in the same class IMO, but most were far worse.

I drove my 351 CJ with a Motorcraft Spreadbore with a Q-Jet during the week fpr MPG and put the motorcraft on to be legal at the strip.

Of course that 4160 pound Torino with a 2500 stock converter got 21 MPG, 9 in town, 12 on the highway.

Hoop

 

Pitmans right, that engine-car combination should be getting 16-18 or better. If I were you I would try borrowing a carb that is proven good and run that a day or two, if your mileage is the same at least you ruled out a half dozen isssues that could be carb related.

Tfeverfred has my next (or maybe first ) guess , check timing, that's probably easier to do first than swap carbs.

 

You are feeding 350 cubes
You are only getting 190 hp. ( at the flywheel before drivetrain loss)
That's a whole lot of dead weight , about 40% worth of dead weight - wasted fuel.
You are feeding 350 cubes and they ain't paying you any rent.

 

These engines have low 7.9 to 1 compression they need lots of timing and a fast timing curve .Is the vacuum advance hooked to manifold vacuum.

 

Wow. I've got the same engine with alot more miles and I'm getting between 12 and 15 mpg in an OT brick that weighs 3600+/- lbs. and has a 3.42:1 rear end, 15x7s, a rebuilt stock equivalent TH350C with the lockup unhooked (have yet to get a lockup controller), stock HEI, 6 degrees advance, QuadraJet that runs rich, and little 2-in duals with cheap turbo mufflers, etc.

 

What's the easiest test? Are we gross rich, like in power enrichment.

Raise the RPM to 2500.

Give it a large leak like a PCV hose. If there is a lot of extra fuel it will pick up 3 - 6 hundred rpm. If it picks up less than 100 or slows down you are near 14.3/14/9 AF ratio and this is not the issue. You could find an extra 1 MPG with fine tuning but you shouls be 13 city- 16 hi way based on the power required to propel the car.

If you are not gross rich, then timing etc is next but ck the PCV hose off test first, it will eliminate the carburetor or make it the focus.

Pull the PCV

Hoop

 

change out that smog cam and boost that lame compression ratio to 9.5 to one and lets check it again

 

Qjets are great for gas mileage when set right but first thing I would go for is the initial timing and the curve. I had a distributor loosen up once on me and retard the timing, instant killed gas mileage, otherwise ran fine.

 

i think torque is your issue? what kind of lb ft you running? i can speak for cadillacs only, but my 60 sedan gets 18mpg and my 68 coupe got 20 going to the beach at 75 mph....,,but im running between 425 lb ft and 525......if shutting down your secondaries kills your torque output, you are shooting yourself in the foot. try running down the street with someone holding your mouth closed....you wont perform as best you could, stop take a deep breathe and your back in the race. down forget a engine is a glorified air pump, the better it breathes the happier it is!!!

 

OPEN THOSE PLUGS up to at least .065 and lean it down
takes a big fire to lite off scatered molecules of fuel
crank the timeing till it kicks back and retard a little and go to the full vacume adance not the ported
most of the rest is just to have it in good shape and drive decent too
i got a 4200 pd buick wagon w 350 and turbo 350 with something like 2.41 and leaned down edel. and it pulled 18 mpg consistantly on the road
it can be done
a 2 gas analyzer and get the readings down and it will make milleage just on a correct mixture on idel jets
check avaiable firing voltage with a scope

good luck
it is a lot of work to learn this stuff but it will pay off and you can do it on any car
learned a lot on stock cars in 18 years
and took a lot of specialty schooling

 

A lot of good direction and advice posted already, but it sounds like you're already are not sure where to go at this point. My suggestion would be to try to find somebody close to you that has some reputation for tuning carb engines,before you get buried trying to follow too much advice as good as it might be.
I have been doing this for about 40 years and have found that you need to start with the very basics befor getting to far into changing parts etc. I installed the same motor last year and it just was not right, checked the compression and found 2 cyl's with leakage, so don't assume that just cause its new that its right. I would verify first that the compression is even in all cyl's and no vacuum leaks before I did anything else,then press on from there.
I have the same basic setup in in a Pickup I tow with and it gets 14-16 with about 6k on the motor since installed. Hope you get it dialed in.

 

A lot of good direction and advice posted already, but it sounds like you're already are not sure where to go at this point. My suggestion would be to try to find somebody close to you that has some reputation for tuning carb engines,before you get buried trying to follow too much advice as good as it might be.
I have been doing this for about 40 years and have found that you need to start with the very basics befor getting to far into changing parts etc. I installed the same motor last year and it just was not right, checked the compression and found 2 cyl's with leakage, so don't assume that just cause its new that its right. I would verify first that the compression is even in all cyl's and no vacuum leaks before I did anything else,then press on from there.
I have the same basic setup in in a Pickup I tow with and it gets 14-16 with about 6k on the motor since installed. Hope you get it dialed in.

 

That 342 gear help the thing get moving !

I had one of these 190 hp pigs in a 1988 k5 blazer .
700r4 and 342 gears. 11 if I was lucky but more like 9-10 average city or highway same/same. That's Factory rated mileage too.

I swapped in a little smaller (327) and more powerful (315 hp) engine and get 22 average mpg. Oh yea it will plant you into the seat now

 

Initially my 55 when first put together with 400sbc, quad, performer, 200R, was sounding good but not getting the mpg.12-14 I replaced the junk HEI I initially put in, played with the timing, changed the rod-cobbled linkage to lokar cable, added a elec fan and picked up an easy 3-4mpg. I got 20 on way back from LSR last wk end with wind at my back. You just need to trial and error some. My 63 bel air 283, 3speed, worn out gets 16mpg.
cruise it on hwy at 2500rpm or down to 2000 and you'll see mileage