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Old 10-16-2012, 09:12 PM   #1
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replaced carb and lost mpg

I have an 87 wrangler 4.2 automatic that I've had for a couple months now. It has 4 inch suspension lift and 2 inch body lift. Im running 35 12.50 15 Goodyear tires with Kevlar. When I first got the Jeep it ran rough but every time I checked fuel mileage I was at 15. It began to run rough and was puffing smoke so I put it in the shop and found out the carb had been filled with jb weld putty to fill a leak. I ordered a new Weber from quadratec to replace the Weber that was on there, now it runs great but im only getting 11. Any ideas of what caused it?

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Old 10-16-2012, 09:34 PM   #2
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One possibility, its not tuned for your rig.

Jetting, tuning, CFM need to match properly.

Another idea, could be a vacuum leak or something not hooked up that once was with the old carb.

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Old 10-16-2012, 09:52 PM   #3
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Im lost when it comes to all of that and there's no place around here that even wants to mess with them. I am changing the plugs tomorrow since it was running so rich in hopes that will help some
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Old 10-16-2012, 11:19 PM   #4
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If its running rich, you will need to find someone who knows what to do. Changing the plugs may help it run better for a bit, but will not fix the problem.

Its not difficult really, carbs are really a dumb air fuel mixer; but finding someone who works on them is getting harder. I for one like them because they are easy, and if you are willing the internet and I could help you walk through it.

Basically, assuming you are right in it running rich, then you basically need to get new jets. If you do a search you should be able to find the sizes available and a starting point. Once you find out what you have, step down in sizes, one size at a time then go run your jeep. Once the plugs come out the proper color (milk chocolate brown, to light brown) then you basically have the mixture right.
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Old 10-16-2012, 11:25 PM   #5
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Here is an excellent write up for tuning carbs... Very good suggestions and things to verify before you even start to tune the carb.

Weber Carburetors Carbs 40 DCOE 45 DCOE 32/36 32/34 Jeep. Top-End Performance 818 764-1901
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Old 10-16-2012, 11:29 PM   #6
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also just read that part of the running rich could be too much fuel pressure; you could try to put in a fuel pressure regulator and set it to 3 lbs and see if that helps.
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Old 10-17-2012, 01:17 PM   #7
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Ill get to work on it this weekend. I hope to get the mpg back up to at least 15. I was hoping that after the carb replacement it would do better than that
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Old 10-17-2012, 01:24 PM   #8
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No most Jeeps don't get much mpg better than that. Some will post as much as 20 but my 2.5 TBI with 33s is dead on 15 everything is new.
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Old 10-17-2012, 01:35 PM   #9
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X2 on the fuel pressure regulator.

Most carbs need to be dialed in to their intended use, altitude, etc.
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Old 10-17-2012, 03:53 PM   #10
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Silly question but could I just use the jets from the other carb?
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Old 10-17-2012, 07:02 PM   #11
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no, Jets are typically carb specific.

Start with the regulator, and with new plugs check for the color. If you get a light brown/tan then you're as good as that gets. But if they are black then you need to re-jet. At that point is where you would verify what you have, and buy a few sets smaller. They should be fairly cheap, between $4-$6 a pair.
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Old 10-17-2012, 07:40 PM   #12
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The first thing is getting the jets set so that it runs well under full throttle, then tuning the metering rods and any power pistons to run well under part throttle conditions. would read up a lot on carbs and order a tuning kit. It takes some time to get right, but a proper carb will run very well.
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Old 10-17-2012, 08:31 PM   #13
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Ok ill get some info on tuning the carb. Its a Weber k551 from quadratec. It runs awesome down the road as is, about 1300 rpm at 55. But I hope with this great information and a Nutter bypass I should be even better
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Old 10-17-2012, 08:46 PM   #14
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Ok ill get some info on tuning the carb. Its a Weber k551 from quadratec. It runs awesome down the road as is, about 1300 rpm at 55. But I hope with this great information and a Nutter bypass I should be even better
Im assuming that is a 3speed auto?

If so, I believe 1300 at 55 is a bit low; the 4.2 is a torque motor, but honestly it likes a higher rpm at speed. More like 1800-2000 would be better at 55; not that it cant do it, but lugging the motor like that can also drag down your mpg.

BTW, did you change into those tires before your last mileage check? I would suspect your odometer is off unless you changed the speedo drive gear.

I just ran a gear ratio calculator and it suggests your speedo is off. At 55, assuming stock gear ratio and the 35" tires, your tach should read about 1800 at 55; otherwise your gear ratio is 2.5 which was not an option on the Wranglers.
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Old 10-18-2012, 07:28 AM   #15
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It is a 3 speed automatic. It had the wheels and tires on it when I got it about 2 months ago. I was told it had been geared and i checked the speedometer with my gps and it was correct. It did run at w higher rpm with the other carb because it kept trying to die so I had to rev it higher to keep it going
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Old 10-18-2012, 08:14 AM   #16
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Maybe your tach is not right, 1300 is not possible with what you describe. But we'll focus on getting your carb tuned up first.
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Old 10-18-2012, 09:51 AM   #17
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Yeah. Im headed to the parts store for a pressure regulator and some jets and a new inline fuel filter now. Im hoping to see something over the 15 I was getting while it was blowing black smoke and leaking all over the place
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Old 10-18-2012, 11:41 AM   #18
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Yeah. Im headed to the parts store for a pressure regulator and some jets and a new inline fuel filter now. Im hoping to see something over the 15 I was getting while it was blowing black smoke and leaking all over the place
Im thinking the regulator will help a lot; and then if you want to check for the jets.

You should verify what you have for jets before you go buy them, if not you have a chance of getting the same ones again. Not to mention, the parts store will probably not have them in stock.
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Old 10-18-2012, 12:07 PM   #19
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I just got back from the parts store with the regulator plugs and filter. When I went to leave the Jeep wouldn't fire. First time I've ever had that happen. I tried for about 5 minutes with no luck. Finally put it in neutral and rolled it down the hill and it fired up, blew a big cloud of smoke and the whole way home the tach said 1800-2000 @ 55
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Old 10-18-2012, 12:33 PM   #20
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I just got back from the parts store with the regulator plugs and filter. When I went to leave the Jeep wouldn't fire. First time I've ever had that happen. I tried for about 5 minutes with no luck. Finally put it in neutral and rolled it down the hill and it fired up, blew a big cloud of smoke and the whole way home the tach said 1800-2000 @ 55
Thats what the calculator said using the standard equipment axle ratio of 3.55

But your starting issue leads me to believe there is too much pressure or a leak causing gas to dribble into your cylinders. Basically, it flooded out while sitting there.
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Old 10-18-2012, 01:23 PM   #21
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Its never done it before..even when the other carb was on and it was leaking fuel out of the spacers. But after letting it sit in the garage for a while I get a small hint of a fuel smell
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Old 10-19-2012, 10:18 PM   #22
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You either have too much pressure while running or the carb is leaking down through plugs in the fuel bowl if that no start condition happened
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Old 10-20-2012, 01:36 PM   #23
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I changed out the plugs and put on a new fuel filter and it fired right up and ran smooth again. However, I noticed this morning a small bead of oil around the plastic elbow on that hooks into the bottom of my air filter. What would cause this to happen and is it bad news?
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Old 10-20-2012, 02:04 PM   #24
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crankcase pressure, not to worry, it happens. But to be sure all is good you can check your PCV valve and associated hoses and make sure they are clear.

Now that its running good, take it out for about 15-20 min drive then check the plugs for their color.
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Old 10-20-2012, 02:14 PM   #25
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Im going to pick up the pcv valve after work. There's a bunch of.vacuum lines that are missing from what I can tell. Just glad its nothing huge. There was a bunch of stuff replaced right before I got it though, like distributor, cap, wires, head gasket, exhaust gasket. Not sure what else to look for.
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Old 10-22-2012, 08:55 PM   #26
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Checked the plugs again and they looked in good shape. It runs down the road great but died on me again while driving down the road. If I put it in neutral it will fire right back up. Im changing the fuel pump tomorrow but anything else I should look at replacing?
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Old 10-23-2012, 09:50 AM   #27
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Well, honestly until you find out why its dying, just replacing things can end up expensive and still not correct your issue.

At this point it looks like youve gotten better, but it still could be carb, or ignition, or?

What color did the plugs end up?
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Old 10-23-2012, 10:29 AM   #28
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The plugs looked to be fine ..just a little brown on the end. The carb is a brand new one and I had it installed and tuned but im not sure where to go next because it just dies randomly. I really don't want to waste money replacing stuff that doesn't need to be. The MPG has gone up to 12 though
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Old 10-23-2012, 10:44 AM   #29
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Check your fuel tank for leaks around the straps holding it on--if not leaking was probably replaced (87s eventually leak!) with a used one and your filter screen on the pick up in the tank is plugging up--happened to me!
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Old 10-23-2012, 10:46 AM   #30
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I'll check that tonight because I get a slight gas smell after it sits in my garage for the night

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