84XJSS

Uhm so... yeah, I have a 1984 XJ with the AMC 2.5L I4 Carbureted... Just lifted it 4.5" and put some nice little 31x10.5x15's on it. My highway top speed is around 60 in 4th.. my OD gear (5th, I have the manual) seems to only make my Jeep lose speed.... Maybe a carburetor tuning issue? Any ideas are greatly appreciated before I make my normal 6.5hr drive become a 7.5hr drive lol.

Thanks!

Atmos

you may need to speak with a paleontologist, because that thing is OLD

XJEvo

you need to regear to at least 4.10s, probably even 4.56s

84XJSS

haha my specialty for restoration projects are pre 73 vehicles... all these computers make my head hurt why couldn't I have just lifted a Type 57 Bugatti? There would be less issues

As for gearing- I looked and I do have 410's in my Dana 35... it just seems like a hit a brick wall in 4th at 60.. its odd, maybe a bad computer? and I HATE VACUUM SYSTEMS!!!! BLAH! Maybe a vacuum leak?

mentalbreakdown00

yes because you went from a stock 27-28" inch tire with that 4cyl, to 31" tires therefore making your gear higher (lower numerically).... you need a 4.56 to bring it back to stock, a 4.88 might not hurt since your a 4 banger either...

Outlaw Star

Ding, ding, ding! Yes, you need to gear to atleast 4.56 to compensate for the larger tires.

93_xjcherokee

I am running 4.5 inch lift on 33s with 3:55 gears and run 65 and 2k rpm

JamDeeper

89 on 33s with 4.5 lift, stock gears 5th gear (manual as well) slows down too. Its a cruise gear, not the gear I pass Toyotas with.

84XJSS

So I was talking with a buddy while working on one of his TT cars and we got on the subject of heat soak causing power loss... I swapped thermostats (original was in there) and it ran 72mph in 4th while staying cool and not revving too high... Crazy huh? I'm still looking into gearing even though I can get home this weekend. Never thought 20 degrees could make that much of a difference,

84XJSS

YES!!!!! Hahaha thanks for a good laugh! So true, my friends' FJ40 barely hits 50

Fred/N0AZZ

One of my best upgrades was going from a stock set of gears (in my case 3.55's) to 4.56's when I installed lockers. My highway speeds improved a bunch no more up/down shifting all the time even on level ground just a slight breeze. I also increased my MPG by 5 with the swap, my XJ is quite heavy from all the gear I carry.

My stats 99, 4.0, AW4, 4 1/2" Lift, 33x12:50x15" tires, 4.56 gears, you can see most of my mods in my Signature.

Outlaw Star

OP is running a 4cyl not a 4.0L so 3.55's won't cut it for that rig.

JamDeeper

I know. His 4cyl would still pass Toyotas. I was just throwing out there that 5th gear isn't a power gear. Wasn't meant to be. 4 or 6 cyl.

mentalbreakdown00

not meant for power, but still shouldnt have to down shift to go up slight inclines

macgyver35

First rule of working on cars (or computers)...
Use deductive reasoning. It ran fine on the highway, and now it doesn't.

What changed?
In your case, suspension height and tires. Logic dictates that the likeliest cause of your issue is tied to one of those two changes. A lift by itself won't create any significant performance problems.
Going up what amounts to three tires sizes will have an immediate impact. Now you've effectively changed your final gearing.

So, in summary, as many others have stated, it's a gearing issue. That means you can stop looking for other causes to your performance problem. That being said, by not re-gearing you are causing your engine to have to work harder, thus creating more heat. So, if you suddenly have a cooling/heat soak issue, it is a symptom of your root problem, not the root problem itself.

A 4-banger that old with 31's would benefit from 4.56's. And a new motor...

In all seriousness, congrats on keeping one of the oldest CJ's on the road.