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Old 07-11-2013, 08:52 PM   #1
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Need help

Okay, 88 4.2, new engine, Howell TBI, new ignition set to 8 btdc. 2.5 inch Rc lift, 3.07 gearing, with 31/10.5/15s. Problem is that I can't get over 1500 rpm in 5th gear, and I have a lot of vibration in the entire drive train. No power going up hill, have to down shift going over a pebble. Been fighting this for about 6 months.

A year thoughts?

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Old 07-11-2013, 09:07 PM   #2
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Originally Posted by sfhusser View Post
Okay, 88 4.2, new engine, Howell TBI, new ignition set to 8 btdc. 2.5 inch Rc lift, 3.07 gearing, with 31/10.5/15s. Problem is that I can't get over 1500 rpm in 5th gear, and I have a lot of vibration in the entire drive train. No power going up hill, have to down shift going over a pebble. Been fighting this for about 6 months.

A year thoughts?
I have had a couple of yj's with the exact running gear as yours except ours had carbs..they still pulled good in 5th gear..you could try bumping the timing up a little and see if that helps..vibration could be u joints or loose mounts.I wouldn't think that 2.5 inch lift would affect it but if you can't find anything else bad I guess you could try dropping your transfer case a little

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Old 07-13-2013, 07:00 PM   #3
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I installed a 1" training drop and that didn't change anything. I'll check the mounts and adjust the timing to10 and see what that does.
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Old 07-13-2013, 07:52 PM   #4
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Isn't that really high gearing? I'm still learning how gearing affects power so I'm not really sure.

What RPM did you set your timing at?
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Old 07-15-2013, 06:35 PM   #5
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3.07 was standard for 4.2 I6. I believe it was standard until the 4.0 came along but not 100% sure.

However I did figure out what I think my problem is. The drive shaft comes out of the transfer case at 20 degrees from the horizontal in line with the slip yoke but has 0 degrees difference with the pinion angle at the differential. I typically have no problem hitting 3000 or more rpm in 2nd and 3rd gear but in 4th I can't get over 2500 and in 5th I'm lucky to get 1900. Plus, I get a bad vibration and a resonating humming noise. But only at or above 50 mph. I didn't have this before the lift was installed. So because of the mismatch in pinion angles and my rudimentary knowledge of pinion angles and how they work, I'm going to see about an SYE modification and a new drive shaft.

Any opinions against this line of thought?
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Old 07-15-2013, 11:34 PM   #6
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Yeah I think you need to bump that timing up a little more and I believe you are going to have to re-gear to at least 4.11's. with those tall gears and bigger tires you already are probably down in the 2.70's so no top end power in the top gears
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Old 07-16-2013, 09:11 AM   #7
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be aware with 3.07 gears you will need a new carrier the break is some where around 3.74 as I recall so if you go to junk yard for parts make sure gears are higher than 3.74 with those gears I am assuming this is a standard Dana 35 rear?

the big decision is do you want to re-gear it? or replace it?
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Old 07-16-2013, 10:17 PM   #8
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probably cheaper to replace rear ends from junk yard , gears seem to be pretty pricey and if you can't install that will cost ya too
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Old 07-16-2013, 11:07 PM   #9
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Which transmission are you running? Depending on the gear ratios 3rd gear at 3000 rpm and 5th gear at 1900 rpm may be spinning the drive shaft at the same rpm (therefore the same ground speed). If your drive shaft misalignment is preventing the shaft from spinning any faster than that without ripping the U-Joints apart. If you remove the lift what does that do to the drive shaft angles?
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Old 07-17-2013, 06:55 PM   #10
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I have a sye and driveshaft coming. I have a set of 4.10 axles in my garage, but I'm only running 31s. I'd like to get 3.73s, so if anyone in the Richmond area would like to trade?

I have an ax15 (I think) it definitely is not a Ba10. The po installed it.

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