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Unread 01-22-2013, 10:58 PM   #1
kgb_m3
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Poor gas mileage

Hey guys, I'm a bit confused as to what is going on with my gas mileage lately. I have been working on the jeep for the past year or so and have replaced just about everything under the hood.

I've been getting around 18 to 20mpg until recently. Bought a hard top and had my mc2150 tuned to eliminate rough idle and slight misfire. Since then, the Jeep runs a lot better but uses up a lot more gas. I would say I'm down to 12mpg or so.

Would the hard top and properly tuned carb really cause this much of a drop? What else should I consider looking into?

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Unread 01-23-2013, 06:49 AM   #2
Mark05059
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lots of drag/extra weight with that hard top. If the tune of the carb added fuel to the mix then yeah you would loose some efficency there too. Just throwing things out there. Take the hard top back off and see what happens with it.
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Unread 01-23-2013, 07:14 AM   #3
EABiker
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If your carb was running a bit too lean before the tuneup, you may have been on the edge of the balance between running lean with good efficiency, and not quite enough fuel to keep away the misfire. After the tune, the F/A ratio was probably fattened up a bit more than needed to eliminate the poor running, and caused a drop in MPG.

Of course, your mileage may vary!
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Unread 01-23-2013, 07:31 AM   #4
Rufe
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I lose about two miles per gallon when I switch from a safari to to hardtop, but my hardtop has a roof rack on it. I suspect that the roof rack is the main offender. Roof rack was installed by previous owner and I would be afraid to put more than a fifty pound load on it.
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Unread 01-23-2013, 07:57 AM   #5
EABiker
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Your hard top weighs less than a normal passenger; do you see the same drop with 2 in the Jeep? Also, the aerodynamics of a Jeep are so bad anyway I cannot see that the minor change in the top shape would make such a big difference. The outdoor temperature change when running a soft/no top versus a hard top, (colder out), probably has alot more to do with the MPG losses than the aerodynamics. My bet is still on the carb.
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Unread 01-23-2013, 10:01 AM   #6
Beachcruiser
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What's your setup on the MC carb?

IOW, do you know what was changed?

They are VERY easy to tune yourself......
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Unread 01-23-2013, 12:22 PM   #7
Rproject
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I had the same problem when we did the nutter bypass. Since it's winter, and the Wrangler doesn't get driven much this time of year, I'm holding off until spring to really dig into it.
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Unread 01-23-2013, 03:42 PM   #8
kgb_m3
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Thanks for ideas guys. The carb was bought from the "famous" ebay guy as a "pre-tuned, plug and play" setup. I installed the carb, nuttered the jeep and the gas mileage has been great. However, the choke was set around 1200rpm on cold start and at warm the idle jumped from 450-600rpm barely keeping the jeep running. Since I have never owned a carbed vehicle, I took it to a shop for them to tune the 2150 months after running it with ebay guy settings.

I picked up the Jeep and couldn't believe how nice they got it running. Smooth idle, healthy sound, in Park around 900rpm and when drop it in Drive 650 -700 or so. Cold start choke is much higher close to 1800rpm. I have electric choke.
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Unread 01-23-2013, 05:27 PM   #9
Rproject
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Quote:
Originally Posted by kgb_m3 View Post
Smooth idle, healthy sound, in Park around 900rpm and when drop it in Drive 650 -700 or so. Cold start choke is much higher close to 1800rpm. I have electric choke.
Those sound dead on for factory idle settings.

Mine was adjusted about a month or two ago to those specs and that's when I saw my fuel economy plummet as well.
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Unread 01-23-2013, 08:27 PM   #10
kgb_m3
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Quote:
Originally Posted by Rproject View Post
Those sound dead on for factory idle settings.

Mine was adjusted about a month or two ago to those specs and that's when I saw my fuel economy plummet as well.
Makes sense then. I would rather waste more gas and have the jeep run the way it does now over how it ran before. I couldn't even go through a trail without killing my battery. The idle speed was so low and due to slight misfire, the alternator wasn't spinning enough to keep everything at 14V.

I'm going to replace the fuel filter, clean my K&N air filter (haven't done so in about a year now with over 15 offroad trips), check my alignment (toe will kill your gas mileage if it's off).
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