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Old 02-23-2007, 03:23 AM
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Only 8 MPG!!!! Why?

Hi everyone. My 47 chevy coupe is killing me. It as a .030 over 350, a Weiand Sthealt manifold, Holley 650 (I think) carb, th 350 trans, 3,06-1 12 bolt open dif, and 305-50-15 tires. Why the hell I am getting only 8 mpg??

Please help!!!

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Old 02-23-2007, 04:34 AM
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Do you ever let the gas pedal off the floor?
Seriously driving style has a lot to do with milage might try that.
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Old 02-23-2007, 05:16 AM
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gas mileage

I once had a plymouth fury with a stock 318 that had similar numbers. I had it for several months before I noticed a very small but very steady drip coming from the fuel line just forward of the gastank. A new section of line and I was back in double digits. Had it been parked closer to the house than the barn down back I would have noticed it sooner.
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Old 02-23-2007, 05:51 AM
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Whats your timing set at, and is the advance working properly?

If you can, try a different carb than that Holley....they are known for bad mileage, but then again, I have seen them get excellent mileage. Borrow a carb and see if it changes....maybe it needs rebuilt.

Also, what cam are you running?

Noticed that you are in Uruguay.....Excuse my ignorance, but what is the quality of the gas there?
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Old 02-23-2007, 09:41 AM
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Yes, ignition timing has big effects on mpg. You need to use the vacuum advance. Make sure the mechanical and vacuum advance is working.

50 to 55 degrees of timing going down the highway is best for mpg. When the throttle is opened, vacuum will drop which will take out the timing advance from the vacuum advance and yield about 38 total mechanical degrees at 4K rpms. Initial timing should be 15 to 20.

Have you jetted the carb for your application?

A q-jet would be better for mpg.

What cam is in the engine?

What heads are you using?

What compression ratio?
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Old 02-23-2007, 12:08 PM
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Quote:
Originally Posted by south65ss
Hi everyone. My 47 chevy coupe is killing me. It as a .030 over 350, a Weiand Sthealt manifold, Holley 650 (I think) carb, th 350 trans, 3,06-1 12 bolt open dif, and 305-50-15 tires. Why the hell I am getting only 8 mpg??

Please help!!!
You don't say what kind of camshaft you are running?

If the carb has vacuum secondaries you could put a heavier spring in to make them open later as opposed to sooner.

The biggest problem...unless you go with a late model engine with all the electronic and fuel system advances...getting anything more than 10 mpg out of a 350 is not going to happen.

I watched my grandfather for years try everything under the sun to get his 72 350 Chev PU to get better mileage - he spent hundreds if not thoudands of dollars and got nothing better than 10mpg 6 while towing!
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Old 02-23-2007, 12:15 PM
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is it your foot or is it the motor (tune up).....

mount a ($34?) vacuum guage on the steering column hooked to a manifold vacuum port......

gives you "real time" diagnostics info for what the motor is doing while you are driving

for best mpg you want to tune to the highest Hg you can at all rpms.....and...keep it ABOVE about 14Hg while accelerating!!!!!

every motor combo will be different but for examples:
25Hg+ on a mild downshift with the motor warmed up=rings and valves are sealing (motor is sound)

18Hg+ cruisin at 70= cam and carb and timing are working extemely well together....A/F mix is good

<13Hg to accelerate from a stop sign=carb and/or timing is off (carb wants 15Hg+ from 1,000rpm+)

needle is shaking at idle on a warmed up mild build motor= possible loose/bad wire, small vac leak, fouled plug, cracked dist cap, etc etc (you won't feel it, the guage will show it and any will kill mpg)

lots more that a simple vacuum guage can tell you......
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Old 02-23-2007, 12:15 PM
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Quote:
Originally Posted by Rambo_The_Dog
Y

The biggest problem...unless you go with a late model engine with all the electronic and fuel system advances...getting anything more than 10 mpg out of a 350 is not going to happen.

I watched my grandfather for years try everything under the sun to get his 72 350 Chev PU to get better mileage - he spent hundreds if not thoudands of dollars and got nothing better than 10mpg 6 while towing!

I have a 350 in my 51 Ford pickup that got 16-19 mpg all day long, with the 2.78 gears..........a bit less since I put in 3.55s, but no where near 10.

A 72 pickup is a different story...Its a BRICK.........but should have done a little better than 10.
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Old 02-23-2007, 12:39 PM
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My 70 chevy 1/2 ton LWB truck with a BBC (396 or 454) and 3.08 gear gets around 8 in the city and 12 to 14 on the highway. Weights 4600 pounds with driver and rolls around on 275-60-15's on all 4 corners.

The same truck got 10 to 11 in the city and 15 to 17 on the highway with a mild 350. I ran a holley 750 (3310) on both the bbc and sbc. And yes, my speedo accurate. Just drove it 100 miles down the highway a few days ago and the odometer matches the highway mile markers perfectly.

My datsun 240 with a 350 and T5 5 speed gets over 20mpg on the highway. And that is with a pretty lumpy cam (244 at 0.050). But with a curb weight of less than 2800 pounds with driver and a overall gear ratio around 2.65:1 in 5th.

So you can get decent mileage with a 350 with a carb.

Ignition timing and proper air/fuel ratio is the key.
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Old 02-23-2007, 02:13 PM
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Rambo,
"you can't make a silk purse out of a cows ear"=grandpa's 72' vintage 350

all early 70's motors were an abortion/aberation to run the new 87 octane unleaded and a smog system......very low CR, retarded cranks, retarded cams, crummy timing and 10kv spark, low hp and tq etc etc

not fair to slam all 350's based on any 72' motor performance.....they all were a pile of dog crap
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Old 02-23-2007, 05:59 PM
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Mmmm...... Cheking timing and carb.
No idea what the specs of my cam are. It came on one of those cheap rebuild kits.
We have good gas down here in the south.
How can I identify my Holley?
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Old 02-23-2007, 06:28 PM
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only 8 mpg

keep checking , there's more gas mileage in there somewhere. I have a 1971
chevy 1/2 ton 350 quadrajet , gets 17/18 mpg on the highway
standard trans with 373 rear gear. bone stock engine runs strong
always has ( it's been in the family since new.)
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Old 02-23-2007, 06:42 PM
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Quote:
Originally Posted by south65ss
Hi everyone. My 47 chevy coupe is killing me. It as a .030 over 350, a Weiand Sthealt manifold, Holley 650 (I think) carb, th 350 trans, 3,06-1 12 bolt open dif, and 305-50-15 tires. Why the hell I am getting only 8 mpg??

Please help!!!
Exactly what type driving conditions are you describing?
What is your rpm at 60 mph, and the speed you usually drive and mph?

My new cars have instant mpg meters and average mpg meters,
and in town mpg is half the highway mpg.

It is suprising how such a small amount of slow/in town driving knocks the overall mileage.
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Old 02-24-2007, 02:30 AM
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If you spend most of your time in traffic, try rear tyres with a shorter diameter. If you see an improvement and wish to retain tall tyres then a rear gear change is in order. Remember that a 10% change in gearing/tyre height results in a 10% speedo error.
But if you spend more time on the freeway, go through the tune one step at a time. What is your idle vacuum? what altitude?
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Old 02-24-2007, 08:14 AM
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ouch.....

if it is a stock grind 350 cam in a "cheap rebuilders kit"......it will be a "challenge" to tune for mpg due to the much larger runners and plenum area in the Stealth intake......the much lower air velocity while driving normal kills the TQ.....takes more pedal to accelerate with 3.08 gears

I wouldn't be surprised to see only about 5-8Hg when accelerating moderately from a stop sign.....that kills all the vacuum advance additional timing for a complete burn and makes the carb meter almost "full rich"=0 mpg

borrow a vacuum guage and road test????
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