gas tank evaporation and MPG

[sqrlnts]

I calculated my MPG for my last tank and came up with 8.5 MPG!

Truck has a 350 2bbl carb, Th350 trans, and 3.73 gears. All in city driving no hot rodding around, no real high freeway speeds.

I have noticed that I can see a stain developing on the top of my gas tank (externally mounted '73, 25gal tank). I think the stain is from a very small leak on the sending unit vent tube. In the hot ash Arizona sun how much fuel could I be losing? To be fair I only noticed the small pool of gas around the tube connection tonight when i took the photo. Normally it only showed as the black stain you see, no real leak or drip of gas on the floor.

The gas cap seals fine and I have the return port plugged cause I don't have one. Could this be some of my poor MPG woes?

I just find it hard to believe that a 2bbl 350 can drink this much gas

[1project2many]

A 350 with no OD on 3.73's isn't going to do well but 8.5 is extremely low imo. That small leak isn't the problem.

Check timing, distributor mechanical and vacuum advance working, carb condition. Are you using a Rochester 2G carb? Check that power valve is working properly and that engine vacuum at cruise matches valve rating. Check to make sure choke is opening fully. Are you using a Holley 2bbl? Good luck with the economy game there. Switching to Rochester 4bbl Quadrajet can net better fuel economy than either 2bbl. The small primary side is more efficient than the Holley or older 2G carbs. Did you start with a smog engine and remove smog parts? Carb and distributor should be "corrected" to match non-smog operation. Do you have a good cam for the rpm range of the truck? Might consider a swap. Is the heat under the hood great enough so fuel is "boiling" out the top of the carb and being sucked into the engine? Check the plugs to see if they're black from running too rich.

An overdrive equipped 700R4 or 2004R trans with operating lockup converter will make a noticeable difference as well.

[flyingtim01]

What 1project said about the 2 bbl is correct. Swap to a Quadrajet if you're looking for better economy and performance. The primaries are small, so as long as you keep your foot out of it, you'll get better mileage. Of course, its a q-jet, so when you DO put your foot into it...nothing like it! FWIW, I drove my '71 K20 with a wheezy old 350, Performer intake and q-jet and 4.56s about 120 miles round trip a couple weeks ago to move some furniture, with about a 50/50 mix of highway and street driving, and averaged 11.7mpg. Probably could have done even better if I ran 87 octane that day, but it was about 95 in the shade, so I thought the old girl would be a little better off with 89 in the tank.

[sqrlnts]

Thanks for the tips guys. The carb has not been rebuilt and is a Rochester I believe? I am running a HEI ignition with 28* total timing if I remember correctly. I will check the power valve, choke and plugs. The engine was from a '73 Caprice so I think smog was OE equipment but I have none of it installed. Cam is stock.

How would I check if I have gas boiling out of the carb? GoPro camera? I have no doubt that the under hood temps are very high. My truck runs ~205-215* with the AC on. Outside temps here are easily 110 right now.

Another thing I have noticed. When accelerating from a stop, not WOT but probably 3/4 throttle, the truck has a tendency to stumble and fall on its face. If I let up on the throttle and get back in it, it picks back up.

I was hoping someone would recommend a QuadJet upgrade!!
Any particular models better than others?

[dubie]

i ran a 350 with a 4 barrel rochester with 3:73's for years in my 72 Chevy and averaged 15-20 MPG depending on my foot. So you should be able to pull a lot better than what you're doing right now

[1project2many]

Hmm... 28 degrees total advance seems low. Most of the older engines like 30-36 degrees. Any idea what the compression ratio is?

Oh my... that is an older manifold. It's painted blue like a 76-81 engine but I believe the choke coil was on the carb by the time GM was using light blue. Many of those older manifolds with divorced choke had an exhaust passage in the intake directly under the front of the carb. You need to run the correct heat shield to keep the heat away from the bowl or it will cause fuel to boil out. If you pull the carb off you should see the shield sandwiched between two gaskets.

What's the "best" Q-jet? Hmm... The "best" one for a stock application is usually the original one so you can get parts easily in the future. For an application like the truck it's probably better to get one with a choke coil attached to the carb If I were installing one today on a mild street truck I'd probably look at a 79-81 car carb. Those models used a choke coil housing bolted to the side of the carb instead of a divorced style like your 2bbl. You can use that housing and install an electric choke conversion kit to heat the choke when the engine's running. Out of the box a 98 Q-jet like that is probably jetted lean for your engine but installing larger jets usually isn't a big deal. Make sure you get a Chevy carb, though, so you can use a pre-bent line down to the fuel pump. The fuel inlet is in different locations on the BOP carbs.

How to check for fuel boiling out? Hmmm... With engine running at idle, and hot, look for signs of vapor coming out the bowl vent (small tube that goes off to an angle looking into top of carb).

If you want to play with the carb you can use this manual:
http://oldcarmanualproject.com/manua...ual/index.html

[sqrlnts]

Originally Posted by dubie

i ran a 350 with a 4 barrel rochester with 3:73's for years in my 72 Chevy and averaged 15-20 MPG depending on my foot. So you should be able to pull a lot better than what you're doing right now

Thats the kind of information that sells these upgrades! It will pay for itself right. LOL! Seriously if I could increase my MPG anywhere near that much this would pay for itself in ~4 tanks!!!

Originally Posted by 1project2many

Hmm... 28 degrees total advance seems low. Most of the older engines like 30-36 degrees. Any idea what the compression ratio is?

Oh my... that is an older manifold. It's painted blue like a 76-81 engine but I believe the choke coil was on the carb by the time GM was using light blue. Many of those older manifolds with divorced choke had an exhaust passage in the intake directly under the front of the carb. You need to run the correct heat shield to keep the heat away from the bowl or it will cause fuel to boil out. If you pull the carb off you should see the shield sandwiched between two gaskets.

What's the "best" Q-jet? Hmm... The "best" one for a stock application is usually the original one so you can get parts easily in the future. For an application like the truck it's probably better to get one with a choke coil attached to the carb If I were installing one today on a mild street truck I'd probably look at a 79-81 car carb. Those models used a choke coil housing bolted to the side of the carb instead of a divorced style like your 2bbl. You can use that housing and install an electric choke conversion kit to heat the choke when the engine's running. Out of the box a 98 Q-jet like that is probably jetted lean for your engine but installing larger jets usually isn't a big deal. Make sure you get a Chevy carb, though, so you can use a pre-bent line down to the fuel pump. The fuel inlet is in different locations on the BOP carbs.

How to check for fuel boiling out? Hmmm... With engine running at idle, and hot, look for signs of vapor coming out the bowl vent (small tube that goes off to an angle looking into top of carb).

If you want to play with the carb you can use this manual:
http://oldcarmanualproject.com/manua...ual/index.html

Thanks a bunch for the information, i appreciate it. I found a quadrajet buyers guide here which details some good to know information.

I am thinking a rebuildable quadrajet and a factory aluminum intake manifold are in order. I refuse to sink any money in this 2bbl, who in their right mind would do that?! Any advice on cheap factory aluminum intakes? I have read early ones may have a hard time sealing vacuum due to the early inferior casting process. Anything else to watch for?

[mechanixman]

Originally Posted by sqrlnts

Any advice on cheap factory aluminum intakes? I have read early ones may have a hard time sealing vacuum due to the early inferior casting process. Anything else to watch for?

Can I ask why you would want a factory aluminum intake?
I got an Edelbrock Torker II Manifold for $45.
I couldn't find any aluminum intake cheaper than that.

[sqrlnts]

Originally Posted by mechanixman

Can I ask why you would want a factory aluminum intake?
I got an Edelbrock Torker II Manifold for $45.
I couldn't find any aluminum intake cheaper than that.

Sure. 1. My engine is just a factory '73 350 with headers, there is nothing performance about it and I won't kid myself about that fact. 2. I am a cheap SOB. 3. I am searching for an aftermarket unit but if I came across an OEM intake I want to know that I can inspect it and use it.

Also for whatever reason, I will call it the "Barrett Jackson effect" every guy that has a part for sale thinks its rare, vintage, and/or one of a kind.

That being said I just located an Edelbrock intake with a rebuilt Quadrajet for $100! Hope to pick it up Monday.

[1project2many]

I refuse to sink any money in this 2bbl, who in their right mind would do that?!

Class racers. You'd be amazed at how much money can be spent on 2bbl carbs and intakes. Brzinski sells ported cheater intakes that start as Canadian castings and receive enough work as to allow nearly 40 hp increase over untouched manifolds for $895. Add a carb spacer and you're $15 short of an even grand. Oh, and you need a carb, too. Add a Jet 500 cfm IMCA carb and you're at $1234 for parts, gaskets not included. And 2bbl racing is supposed to be budget minded, entry level stuff!!

Any advice on cheap factory aluminum intakes? I have read early ones may have a hard time sealing vacuum due to the early inferior casting process. Anything else to watch for?

It's amazing what people come up with. I've never heard anyone say there's a problem with OE castings for a smallblock. Big block stuff in the '60s had some issues but SB OEM aluminum intakes were made in the 80s so they usually fit without trouble. By the time GM switched to aluminum intakes they'd pretty much switched thinking and put emissions as a top priority. Coolant passes through the intake from the front ports of the head and out the rear heater hose connection at all times to keep the carburetor warm to help fuel vaporization. While it does cut down on emissions it can also reduce overall power. Your truck will probably have a shutoff valve of some sort on the heater (do you even need a heater?) so this might not be an issue. The ports might be a bit smaller, too, as many of the aluminum intakes were used on 305 and 267 powered cars. On the plus side if you get the carb and intake together then the choke setup will already be there. But honestly, I wouldn't be afraid to pick up an aftermarket intake if the price was right.

BTW, big important correction. There is no 98 Q-jet. That should have read 79.

[Divine 59]

Regarding the fuel line.....Sorry, but if you have a 2GC the fuel line will need to be modified or replaced when using a Qjet. As an aside Chevy passenger cars typically used a side inlet, where the Chevy trucks depending on year would use a front inlet. I would not get too hung up on the fuel inlet. Look for a Qjet with the APT circuit, it will provide the most flexibility in setting it up for your application. Also highly recommend Cliff Ruggles or Doug Roe's book on the Qjet. Lots of good information. Doug's has more theory while Cliff's book has more practical applications (he provides a couple of "recipes" to set up a Qjet for certain engine types. If all else fails and you don't want to tackle the rebuild yourself there are some really good rebuilders around.

[Divine 59]

One more thing. When you are looking for a Qjet be sure to check the throttle shafts for play and the air horn for warpage. These are the two most common faults in used carbs.

Good Luck Sir.

As far as the 2GC, many dirt trackers have to use 2 barrels. Might be able to sell it to a racer? Just a thought.

[NEWFISHER]

Join the kool school and toss in an inliner...17-20 mpg's 200hp and a 390 Holley

ok, maybe not the best advise I would lean towards the carb and agree that the tank stain is residual seepage, not leakage.

[AZ59apacheguy]

I know there is a local guy ( West Side ) on ebay that sells intakes. At least if it's local, you can check it out before buying. Good luck

[Coupeguy2001]

John, If the deal falls through, I have a factory alum. intake and a rebuildable Q jet looking to find its way into your truck.
Chris

[sqrlnts]

Originally Posted by Coupeguy2001

John, If the deal falls through, I have a factory alum. intake and a rebuildable Q jet looking to find its way into your truck.
Chris

thanks Chris! I always forget you have alot of good parts. Not next time, calling you first.
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