Uses Lots Of Gas

I just installed at 1973 350 engine in a 72 skylark using the Q jet from a 1969 400 #7028240 E MD and It seems to use alot of gas. The EGR valve has been removed and a plate installed (not by me) I don't have any info on the vacuume hose routing so I ran a line from ported vacuum to the HEI vacuum advance, ran line to PCV valve and capped everything else.
I set the timing to 6 BTC with vacuum advance line plugged. I installed new air filter, plugs and wires when engine was installed. It seems to run fine and start right up and be responsive. The choke opens fine. Do I need to have a 1973 Q jet or what? I adjusted everything but get no gas millage. I have only driven this car about 70 miles and my daughter ran out of gas. Iam guessing it gets about 10 to 12 mpg. I could overhaul it but, maybe someone here has the correct carb and willing to trade me?

 

I'm not sure the 12MPG sounds so out-of-place. How are tire pressures? What kind of driving (stop and go highway)? What style of driving (spirited or grandpa pace)? Any sticking brake cylinders or excessive weight in the trunk?

Lots of variables to ponder. I'm not sure what others will say or what mileage others get with a range of driving styles. I would guess any increase in MPG will be minimal with carb upgrades, etc.

Good luck!

 

Grandpa driving. 60 mph on divided highway. Long ago when I had a 350 in my suncoupe I got 18 mpg on the road. So I guess I just expected more. I use to be a cert mechanic in the 70s. so yes I checked the tire pressures and brakes for dragging but everything checks out. It has 273 gears with 14 inch tires. Just seems that I should get more but, it is what is is.

Thanks for reply
Rick Kennedy

 

Too rich an Air/Fuel ratio at cruise RPM can KILL your MPG. Check with 02 sensor or experiment with primary Jets and primary rods. You can get way better than 12 MPG. Whatr jets/rods are in the carb now?

 

I don't have access to o2 sensor but I recall adjusting screws about 3 and a half turns out or adjusting until idle drops slightley. I am a shade tree mech now and have forgotten alot since the 70s. I can take it apart and find out what jets/rods are in it.

 

What is your total timing set at? The vacuum advane is hooked up? Both advance mechanisms are in good working order right?

 

Glad you mentioned that advance. I just rememberd the body shop guy put a cap on the ported vacuum and thean put the vacuum hose over the cap said it looked good!ou:

Thank you Iam going to check it out.

 

That's why I love this forum, lots of good help. I've overlooked the easy stuff now and then myself. Good luck!

 

Well, well, Yes the cap was on the vacuum port so I removed it and hooked the line back up. I will ckeck the total advance. you are correct that this is a 1968 430 carb. Anyone want to trade for a 350 carb?? I could ship it to my friend in Minnesota and have it rebuilt and the correct jets/rods replaced to the correct ones but, I would perfer the correct carb.

 

"I could ship it to my friend in Minnesota and have it rebuilt and the correct jets/rods replaced to the correct ones but, I would perfer the correct carb."

Not a good idea at all to start swapping around jets and metering rods in those older units to match something found in a carburetor specifically set up for the factory 350 engine. They used different calibrations, ie, main airbleed "packages" to make each carburetor work correctly with the jets and metering rods that it came with.

The other big myth here is that big block engines required larger jetting than small blocks, etc. In any and all cases, the carburetor being used must be set up exactly for the application. This includes setting up the entire caburetor, idle tubes, idle airbleeds, bypass air, main airbleeds, main jets, metering rods, height of the power piston, etc.

Before even messing with or worrying about all that, completely/correctly rebuild the carburetor with modern ethanol compatable components. An incorrect float setting or "heavy" float could be 99 percent of the problem here, for example. Best advice I can give is to do the basic stuff first, making sure the carb is in perfect working order, then evaluate mileage, driveability, performance, etc, and make changes if/as needed.....Cliff

 

That was my concern also. I will have it rebuilt unless I can find a 350 carb.

 

I agree with all the above... Post a wanted add in the parts wanted section for a 350 carb. Untill you find a 350 carb set the idle mixture screws to get highest vacuum readings on a gauge with car in gear. I am betting you are way rich at idle and that will kill the milage because even when your foot is off the gas you are burning extra fuel.

 

I noted nothing involving checking the advances once they were properly connected. I have frequently seen centrifugal advances stick due to gum on the pins and pivots. Gently rotate the rotor on the distributor, and see if it returns to same place everytime, without sticking. You can also check it with a timing light, gently revving the engine, and obaserve the marks for smooth movement, and returning to the same place evert time.
The vacuum advance can be checked, also using a timing light. With vacuum applied, the marks should move to the high limit, or fully advanced, and when vacuum is disconnected, the advance should allow the marks to return to the fully retarded position. You have a timing light, right? They are cheap, and invaluable in tuning and in troubleshooting.