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350 getting 5mpg


Crom Cruach

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I know a Monte is not a Prius but 5 miles per gallon fuel economy seems excessively poor! Bone stock with AC. I can’t see any fuel leaking out, but I only drive it occasionally so it could be sitting for a week or so. Still getting lots of black smoke on cold start, but I have a lot of the basics sorted:

22 vacuum at idle

30 dwell 

37 total timing at 4200 rpm / 20 initial 

750 rpm in drive 

1500 fast idle 

new wix fuel filter in carb

choke seems to be operating normal

clean spark plugs

Where should I start guys?

 

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Could be carbon build up in the engine.... Might need to get that flushed.

What shape is the transmission in ?

Have you changed the transmission filter and oil yet ?

If it's the original TH 350, there could be some slippage, but, depending on the rear gear ratio, It's still going to run about 2700-2900 rpm at 60 miles an hour. 

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Hey Scott. I don’t know how to tell if the trans is in good shape but it seems to behave well. I haven’t changed the oil myself but the previous owner said he did. There is a small leak of red fluid, possibly the O ring for the trans dipstick?

It’s the Original TH350 with 2.73 ratio. There is no tach to tell rpm when driving. 

would a quart of dexron trans fluid with 4 quarts  oil change help to flush the carbon deposits?

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Original carb? . Sounds like it’s choking for air , I agree w Thomas . But I had a guy adjust valve lifters on an old car I had in the 70s slant 6.. boy he tightened em up really good!. No power , high gas consumption ....

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Fast Idle 1500 ?  

I set mine at about 850 rpm.   Idle in drive, with AC on it drops to about 700 rpm.  Vacuum at idle is 18 inches……. With vacuum advance,, Initial timing is set at 8 degrees.

If running with mechanical advance only,, I'd set her at about 12 to 13 initial.. 

Make sure the vacuum advance module on the distributor works and isn't leaking…. 

Ed C.

sb350, 300hp

700r4

Edlebrock 2701 intake with 1406 Carb. 

 

I recently replaced my Edlebrock 2101 intake with the square bore 2701 Edlebrock intake… For me, It made no sense to use a spread bore intake with the 1406 square bore carb….It made a big difference in performance ,fuel flow, and economy. I also blocked off the heat crossover under the carb… 

 

 

 

 

 

 

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Yeah original quadrajet on it.

1500 high idle comes from my engine bay decal (auto hydra-matic ). Is there a reason not to follow that? See photo. 

The decal also calls for 8° initial at 600 rpm, but I believe that modern fuel needs more advance. It is using manifold vacuum. I set total first at 36ish and that left me with 20 initial. Maybe I should dial it back a bit. 
 

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Paul , the only time my car idles at 1500rpm is on initial start up… I usually tap the accelerator and drop it to 800/850. Personally I don't want it to idle at that higher speeds when the oil hasn't flowed thru the engine to my satisfaction …

The original carbs and ignition worked well back in the day, but I use my car for some serious road duty and found that tinkering with this stuff wasn't what I wanted to do in the middle of the desert or Tucumcari New Mexico with my wife and dog in the car… 

Over the years, I've modified my cars with things like HEI distributors, better carbs and the 700r4 trans. Anything I did to my car had to improve the safety, reliability, fuel economy and performance. My Monte has been in only one show and my 63 Bonneville in two, so being bone stock never worked well for me. 

The biggest problem I've had over the years is the usage and interpretation of the word "performance" when dealing with some mechanics. 

Ed C. 

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20 hours ago, Crom Cruach said:

Hey Scott. I don’t know how to tell if the trans is in good shape but it seems to behave well. I haven’t changed the oil myself but the previous owner said he did. There is a small leak of red fluid, possibly the O ring for the trans dipstick?

It’s the Original TH350 with 2.73 ratio. There is no tach to tell rpm when driving. 

would a quart of dexron trans fluid with 4 quarts  oil change help to flush the carbon deposits?

Not really...

It's better to drop the pan, chance the filter, and take a look at what kind of residue you have.

Depending on how long the car sat, everything probably settled and dried up a bit. (should take about 7 liters)

As for the carbon in the fuel system, there are additives you just put in the fuel tank. Watched and interesting auto restoration show where they hooked up a machine to the fuel injection system and pressure flushed the whole system. There must be something similar for carbed engines. 

Just curious, but have you had M.O.T. inspect the emission levels yet ???

Other ideas, you may want to look into the "jetting" of your carb..... 

When I got mine, it was jetted way to rich. (there are guides based on altitude) Could barely keep it running without holding the "go pedal" down. 

Montelishi started out with a similar set up... TH350 & 2.73 gears. But once I got the jetting close, I was getting about 14 mpg (imp.) on the highway. Adding the 200r4 now gets me up around 20 mpg (imp.)

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Just now, Scott S. said:

Not really...

It's better to drop the pan, chance the filter, and take a look at what kind of residue you have.

Depending on how long the car sat, everything probably settled and dried up a bit. (should take about 7 liters)

As for the carbon in the fuel system, there are additives you just put in the fuel tank. Watched and interesting auto restoration show where they hooked up a machine to the fuel injection system and pressure flushed the whole system. There must be something similar for carbed engines. 

Just curious, but have you had M.O.T. inspect the emission levels yet ???

Other ideas, you may want to look into the "jetting" of your carb..... 

When I got mine, it was jetted way to rich. (there are guides based on altitude) Could barely keep it running without holding the "go pedal" down. 

Montelishi started out with a similar set up... TH350 & 2.73 gears. But once I got the jetting close, I was getting about 14 mpg (imp.) on the highway. Adding the 200r4 now gets me up around 20 mpg (imp.)

When I mentioned dropping the pan, I meant the transmission. ;)

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6 hours ago, 71MonteCarloMD said:

I think that’s too many turns out as I have mine 1.5 or 2 turns out on both ends.

I set it again to where it starts to hit 20 hgi and it’s 1.25 turns now. So it’s on the lean side of peak vacuum. I used the port in the passenger side, that hooks up to the air cleaner, is that correct? (It’s covered with tape here)

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Also the solenoid on the far left of the photo, should it have a vacuum connection? The old tube had a 3 way connector that was wrapped up badly in electrical tape. Was that supposed to connect to something, or did the last owner bodge something together because he didn’t have enough vacuum tubes to hand? 😂

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5 MPG ,  Sounds like our gas mileage. Oh well, What better excuse to go parking with your best girl !!! 

 Honey , with the price of gas , we have to go parking !  We can't afford to ride around.  

Can always find a way to turn a negative situation into a positive outcome. 😂

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Again I may be out of bounds here but here it goes. Try not to over complicate the issue. Set your timing as the sticker states 8° before top dead center at 600 RPM with the vacuum advance line removed and plugged and then drive it. I have a 1970 350 4  barrel four-speed transmission which I have set at 2° advanced from specs, and I get far better mileage than five mpg. If you have a 1972 350 engine it should get better than five mpg. A little exaggeration here but you can almost pour it out of a gas can quicker than you can burn it.

John S

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All this milage talk had me wondering what my 71 stock 350 4 BBL has been averaging.

I have a spreadsheet with all my vehicles miles and gas station fill up data since 2006. 

Here is what my Monte has averaged each year with a total average of 16.17 M.P.G. mostly regular 87 unleaded gas.

  16.17
      YEAR  MPG
2006 15.5
2007 16.4
2008 16.1
2009 15.4
2010 16.2
2011 16.5
2012 17.0
2013 16.4
2014 15.7
2015 15.9
2016 16.0
2017 15.6
2018 16.9
2019 16.5
2020 16.4
2021 16.2
2022 16.2

 

 

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@Crom Cruach Hard to diagnose something like this with so many variables but something I keyed in on was you saying that it's blowing black smoke. That obviously means it's running rich so the question is whether it's running rich because it's not getting enough air or because it's getting too much gas. Assuming the choke is indeed fully open and the air filter is clean, I would check for a stuck float or dirt in the float bowl that's getting on the surface of the needle and seat which prevents it from shutting off the flow of fuel into the carb body. Is it a brass float or one of those foam type floats? If it's brass, make sure it doesn't have leaks and if it's the foam type, make sure it's not gas soaked. Both of those scenarios would cause the float to sink/sit too low letting too much fuel in.

Those are the things I would check. When setup properly, the Quadrajet is a remarkably efficient and reliable carburetor.

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