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Old 01-16-2006, 04:52 PM   #1
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Underpowered 4.2 auto

I have an 88 yj and I just moved to Colorado. I had it rebuilt less than 2000 miles ago, put in an auto trans, have a weber 400 cfm carb, TFI ignition, crane cam and I still cant make it over 45 mph on inclines on the hwy and only get 11 mpg. It idles smooth and oil pressure is fine, what could it be?

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Old 01-16-2006, 05:27 PM   #2
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What size tires are you running? If you have 33 or 35" tires then I am sure your axle gearing is what is sucking the power out of your engine.

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Old 01-16-2006, 09:06 PM   #3
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my 90 yj seems very slow too. it is standard. i get up to about 55 or 60 and thats it. i always thought thats just how it was supposed to be but other jeeps dont have this problem so idk.
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Old 01-16-2006, 09:12 PM   #4
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i'd agree witht he post about tire size and gear ratio, what is your setup?
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Old 01-17-2006, 06:47 AM   #5
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Had mine up to 80 before rolling almost 3000 RPMs. I don't really like to push it that hard, but I know it'll do it.
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Old 01-17-2006, 05:55 PM   #6
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only 31x10.5

Quote:
Originally Posted by 1BLKJP
What size tires are you running? If you have 33 or 35" tires then I am sure your axle gearing is what is sucking the power out of your engine.
Only 31's
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Old 01-17-2006, 07:39 PM   #7
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The 31's is all it will take to throw your gearing out of whack, considering that your stock tires were about 27". Especially if they are mud terrains, they have a ton of wind drag. I would think 4.11's would help a bunch. Also, the biggest performance gain that I have seen on my Jeep is the exhaust upgrades. I have a header, high flow cat, and a Flowmaster muffler and they really made a difference. I still have the stock 3.07's with 31's too. It runs great on flat ground, but just like you it really can't pull the hills. Gears are the answer.
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Old 01-18-2006, 03:50 PM   #8
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how to determine gears

Where would I look to find out what gears it has?
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Old 01-18-2006, 05:13 PM   #9
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Quote:
Originally Posted by mtngoat888
Where would I look to find out what gears it has?
Easiest would be to look at your rear differential. There might still be a metal tag on it that would say the gear ratio. You would be seeing 307, 354, 373, 410. Those are the most common ratios in your stock axles.

Some of you YJ guys might know better, but I think 4.10's were only put in the 4cyl models right? The 4.2's all had either 3.07 or 3.73 right?
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Old 01-18-2006, 05:36 PM   #10
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feel your pain

Quote:
Originally Posted by mtngoat888
I have an 88 yj and I just moved to Colorado. I had it rebuilt less than 2000 miles ago, put in an auto trans, have a weber 400 cfm carb, TFI ignition, crane cam and I still cant make it over 45 mph on inclines on the hwy and only get 11 mpg. It idles smooth and oil pressure is fine, what could it be?
I have an 88yj also and there is a large incline between
town and home. However I believe that I may have a vacume advance problem because my jeep backfires and pops.
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Old 01-19-2006, 08:34 PM   #11
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Quote:
Originally Posted by 1BLKJP
Easiest would be to look at your rear differential. There might still be a metal tag on it that would say the gear ratio. You would be seeing 307, 354, 373, 410. Those are the most common ratios in your stock axles.

Some of you YJ guys might know better, but I think 4.10's were only put in the 4cyl models right? The 4.2's all had either 3.07 or 3.73 right?
You are right about the 4.10' for the 4cyl, and also the 3.06 or 07's( can't remember which), but I am not sure about the 3.73's for sure in the D35. These may have been for the D44's. I have all this info somewhere, just not sure where.
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Old 02-11-2006, 08:57 PM   #12
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no power a plugged up cat. converter will cause all kinds of problems top end speed and overheating are only 2of them tire size and gears also an uphill power issue 31by10.50s can still go 70 uphill with a run also some kind of high altitude adjustments for your carb that i dont no jack about good luck keep jepn
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Old 02-17-2006, 11:36 AM   #13
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I may just be one of those dumb 19 year olds with a jeep, but my four cylinder manages to turn those 33" pro comp MTs on the highway at almost seventy. The only thing it has is high flow intake and a muffler. I would say to take it to a diagnostic or engine guy and make sure your compression is good. I would think that your jeep should haul @$$ with that bigger motor and only 31"s. But gears may be your only option.

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