Author Topic: 86 Chevy 305 MPG  (Read 2639 times)

Offline Ed4all10

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86 Chevy 305 MPG
« on: March 16, 2018, 07:27:25 PM »
I am sorry if this has been asked elsewhere, but I have a question about mpg: as mentioned I have a 86 305 carbed with a TH700R4 trans W/overdrive. I am getting about 10 mpg. I just started taking the truck out again a few times a week, so I really began to notice the mpgs.  The truck is completely stock. It has brand new oil, plugs, wires, cap, rotor, diff fluid etc. Also tire pressure has been checked. I even ran a can of sea foam through it haha.  I eventually am going to swap the 305 out for a 383 TBI, but since the 305 only has 136k on it, I would like to run it a little more. I understand that the best way to save fuel is with the right foot, but even when I am driving conservative at constant speeds, the best I can get is 11. Any advice on how to squeeze a few more mpgs would be appreciated. I know these squarebodys arenít supposed to be a Prius! Thank you.

Offline Rapid Roy

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Re: 86 Chevy 305 MPG
« Reply #1 on: March 16, 2018, 07:46:01 PM »
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Online Irish_Alley

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Re: 86 Chevy 305 MPG
« Reply #2 on: March 16, 2018, 08:12:48 PM »
i cant really help you with tuning but something doesnt seam right i was getting 12 with my 91 350 4l80 crew cab. im not saying you should be getting 15 but i would imagine your number should be higher.
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Offline Ed4all10

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Re: 86 Chevy 305 MPG
« Reply #3 on: March 16, 2018, 08:36:45 PM »
I agree. My buddy has a 84 with a 350, 3 in lift, 31s, and a loaded toolbox in the back, and he is getting 14. I have been through this truck several times in the years that I have owned it, and I canít seem to find anything out of the ordinary. I thought I had a fuel leak at first but I Havenít found anything. No smell, nothing on garage floor, nothing. To calculate mileage, I run the entire tank down, record the mileage on the Odometer, and switch to the aux tank. For a while I even thought my odometer was incorrect, but it isnít.
« Last Edit: March 16, 2018, 08:43:30 PM by Ed4all10 »

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Re: 86 Chevy 305 MPG
« Reply #4 on: March 17, 2018, 03:21:55 AM »
You don't need to run the tank down even a 1/4 tank of fuel can tell you your millage. All you have to do is set the trip meter or record the odometer, fill up and stop when the pump turns of automatically. Then drive to a destination and back to the same fuel pump (could just be a 30 mile trip) and fill back up and let the pump turn itself off.

If the tbi is in poor condition it could cause bad mpgs and there's a few sensors that will give poor millage also.
« Last Edit: March 17, 2018, 03:04:53 PM by Irish_Alley »
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Offline Ed4all10

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Re: 86 Chevy 305 MPG
« Reply #6 on: March 17, 2018, 06:05:02 AM »
Thank you. I will try that this weekend.

Offline Ed4all10

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Re: 86 Chevy 305 MPG
« Reply #7 on: March 20, 2018, 03:43:50 PM »
Just an update: This past weekend, I changed my fuel filter, and took my old Rochester 4bbl out and thoroughly cleaned everything. It was ďrebuiltĒ about 4K miles ago, so the seals and rings shouldnít be an issue. I ran 168 miles over the next few days and I filled my main tank up with 15.75 gallons. That equals 10.67 per gallon. Filled up at same pump. Again this truck is bone stock. Would there be a possible stock tuning issue on the carb? I was consciously trying to take off easy, maintain constant speed, and not idle any more than needed. Any help would be appreciated.

Offline Big Chip

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Re: 86 Chevy 305 MPG
« Reply #8 on: March 21, 2018, 07:41:52 AM »
Is the choke opening all the way?  Does it seem like it's running rich when you're idling?  I had an 86 with the same setup, although it wasn't that old when I had it but I was getting between 15-20 mpg depending on the conditions.  It went down to nothing when I was pulling a travel trailer but that's another issue entirely!

Offline Ed4all10

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Re: 86 Chevy 305 MPG
« Reply #9 on: March 21, 2018, 08:11:24 PM »
Hi Chip, I canít say for sure about the choke, but I donít believe it is idling rich. Timing is set to stock as well. Did you do anything to your 86 to improve the mpg? At this point, with the weather getting warmer,  I might just go ahead with the 383 TBI swap. I figure I will get the same mpg with double the performance. All about those smiles per gallon, instead of mpg. Thank  you for your reply.

Offline Big Chip

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Re: 86 Chevy 305 MPG
« Reply #10 on: March 21, 2018, 09:39:22 PM »
Hi Chip, I canít say for sure about the choke, but I donít believe it is idling rich. Timing is set to stock as well. Did you do anything to your 86 to improve the mpg? At this point, with the weather getting warmer,  I might just go ahead with the 383 TBI swap. I figure I will get the same mpg with double the performance. All about those smiles per gallon, instead of mpg. Thank  you for your reply.
I didnít do anything to my 86 but like I said, it wasnít that old when I owned it.

If you take your air cleaner off when itís warm you can check to see if the choke plate is all the way open. I have a problem with mileage on my current truck but I think itís a combination of things and my solution is flourishing be fuel injection and possibly another gear via the 4l80e.


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Offline 75gmck25

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Re: 86 Chevy 305 MPG
« Reply #11 on: March 23, 2018, 06:25:20 AM »
One suggestion is to try a little more than stock timing.  Most of these engines are set for 8 degrees BTDC from the factory.  However, I have been running 12 degrees BTDC for years without any issues, and some folks run as much as 16 degrees BTDC.  These are low compression engines, so a little more base timing can help with power and gas mileage. 

At the same time, make sure your centrifugal advance is giving you about another 18-20 degrees of advance, and that its coming in by about 2500 RPM (or even lower).  This will help acceleration, which will help you keep your foot off the accelerator. Total timing (base + centrifugal/mechanical) should be no more than about 32-36 degrees.

Also verify that your vacuum advance is working, since the advanced timing at cruise is what gives you better cruise MPG.   Many stock advance units will give you up to 20 degrees, but that may be a little too much if you already advanced the base timing.  For example, 20 degrees vacuum advance will not be an issue with the factory 8 degrees BTDC base timing, but you may have to use a can with less vacuum advance (16, 18 etc.) if you have advanced the base timing.  General rule of thumb is that base+centrifugal+vacuum should not exceed about 52-54 degrees.

If you are willing to spend a little more money, you can get a decent wideband air fuel ratio (AFR) gauge for about $200-210.  Measuring AFR at idle is not very effective, since the reading tends to jump around.  However, measuring AFR at cruise and during acceleration works really well.  An AFR of 14.7 is considered the ideal ratio for emissions, but it is not the best for power.  In most cases you want to run about 12.8-13.2 (based on this article http://www.hotrod.com/articles/innovate-air-fuel-ratio-meter/), and during full throttle acceleration you will see it go even lower, maybe closer to 10.0.   

If you ever had a part throttle lean bog due to a misadjusted carb (very common on Edelbrock 1400 series; also typical for Quadrajets), you can usually see the AFR change (go lean) at the same time you feel the bog.  Then you know you need to play with step-up springs and accelerator pump shots.  You can also see the AFR go rich as the secondaries kick in.

I've been running the Innovate gauge on my '75 GMC for about 7-8 years, and works very well.  Just make sure you are careful with the initial wiring, since AFR is based on measuring very small voltage changes at the O2 sensor, and you don't want bad grounds or other flaky wiring to affect the gauge reading.

Bruce

Offline zieg85

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Re: 86 Chevy 305 MPG
« Reply #12 on: March 23, 2018, 09:23:59 AM »
my 85 that I sold, same set up got close to 16 mpg.  I drove it real easy though, mostly rural setting
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Offline kennyw

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Re: 86 Chevy 305 MPG
« Reply #13 on: March 25, 2018, 10:46:43 PM »
What gear ratio do you have? GM was notorious for putting extra steep gears in the axles on under powered engines thinking it would be good for gas mileage but it has the opposite effect when the engine is under powered.

Offline Stewart G Griffin

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Re: 86 Chevy 305 MPG
« Reply #14 on: April 11, 2018, 06:10:26 PM »
Describe exactly how you are calculating mpg?    You should be able to get at least 16mpg highway with overdrive.

See my threads entitled mpg pt.1 and mpg pt.2