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spshultz 12-12-2008 10:58 AM

Wrangler really slow on hills
I've got an '87 Laredo with 4.2L/5-spd BA-10/Nuttered/MC2100 (1.08) #46 jets/Team Rush upgrades including coil. Gears are 3.54 (3.55) and tires are 235x75x15. The exhaust system is all new from the down pipe all the way back to the tail pipe. Timing is currently set to 8 BTDC. And I live just above 5000 ft. in altitude.

I've had an issue since I first got the Jeep and that is it's been a real dog on hills in 5th gear. With my foot to the floor it won't go above 60 mph and it even feels like I have to use a lot of pedal to keep it cruising on flat road.

A while back I hooked up my multimeter to the O2 sensor and took it for a spin. While cruising the O2 was reading around .8v - .9v which is really rich. And when I'd get to a hill it would go to .9v and hold steady. Only rarely could I get the O2 sensor to cycle during cruise like it should. But know that the Jeep doesn't smoke nor does it smell like gas.

Does this sound like the MC2100 is jetted a bit rich? It's got #46 jets in it now but I'm thinking maybe I should try some #45's or #44's. The float level is set to 7/16". However I'm not sure what my cruise vacuum is or what power valve came with the carb. I've been looking for jets but they are hard to find (aka non-existant) or they want almost $30 for a pair. And the junk yard doesn't have any 2100's available. I can't spend $90 on 3 pairs of jets because that just seems absurd.

Any help or tips is definitely appreciated. :cool:

Dare2BSquare 12-12-2008 11:35 AM


jpdocdave 12-12-2008 08:07 PM

ya, sounds like you got a good handle on carbs, but hills + 5th gear + 4.2 @ 60 mph is gonna be sluggish.

Felsengleiskette 12-12-2008 08:29 PM

in inches whats the 235x75x15? anything over 3o" and those gears are too dinky.

jpdocdave 12-12-2008 08:52 PM

they're like 28 or so.

87YJWrangler 12-12-2008 09:41 PM

I got the factory 4.2 5Spd and I have no problem running those tires up hil

spshultz 12-12-2008 10:04 PM

I just measured them and they are about 28".

One other thing is that on the hills the temp goes up.
Make it over the hill and the temp goes up even higher (about 220) then goes back just below 210. And I can idle around town all day long and the temp never moves above 210. It has a new radiator, new fan clutch, new hoses, two new 195 degree thermostats, couple of flushes and a new radiator cap.

It's got to be running rich or there's just a ton of carbon build up on the pistons. I may SeaFoam it again to see if it makes any difference.

ty95yj 12-12-2008 10:49 PM

put a high output 4.0head on it. that should solve your problem

John 12-13-2008 12:23 AM

fifth gear is overdrive not made for climbing hills
more for cruising level road over 60
hows the compression in the motor?
how many miles? hows the power in the lower gears?

spazinweasel 12-13-2008 07:46 AM

i think this is more of "a jeep thing" they are built for tourqe not speed thats why they wheel so well. the 4.0 head is one option another is gears or motor build. yet another option is learn to accept this quirk and downshift on hills.

racer-xerols 12-13-2008 09:18 AM

I never use 5th on my 4.0, it falls flat on its face if I do. Stock truck with 31's, but even with stock tires it was a dog in top gear. 3rd on backroads, 4th on highways, 18-20mpg - not bad for a brick.

John 12-13-2008 04:38 PM

Hey Spazinweasel
You did that 4.0 HEAD ON A 4.2 ?
What did you do for the fuel injection

spshultz 12-13-2008 09:28 PM

Engine has just over 150,000 miles on it.
I haven't done a compression test on it yet but that's coming up soon.

The power in 1st, 2nd and 3rd is pretty decent. But 4th is just OK and 5th is horrible.
I have real trouble trying to get it to go above 65mph in 5th gear and that's on flat road and it simply will not do 70 mph no matter how flat the road. The speed limit on the highway here is 75 mph and everyone does 80 mph. I won't even think about taking it on the highway. And I don't want to drive it in 4th because at 65 mph the engine is screaming at around 3100 RPM's.

Not sure if I'll end up doing the 4.0 head swap, swapping out an entire 4.0 or going the 4.3 or V8 route. I just want to get it running good enough now to be able to hold 75mph on the highway.

flattietj 12-13-2008 10:30 PM


Originally Posted by spazinweasel (Post 292180)
i think this is more of "a jeep thing" they are built for tourqe not speed thats why they wheel so well. the 4.0 head is one option another is gears or motor build. yet another option is learn to accept this quirk and downshift on hills.

i run 5:13 gears and 39" tires and my three speed auto can go 70 mph so i think this 4.2 is a turd which is typical of that motor, i have owned about 4 258s and they have all ben turds.....get a 4.0 if your gonna drive on the street

spazinweasel 12-14-2008 06:15 AM


Originally Posted by John (Post 292333)
Hey Spazinweasel
You did that 4.0 HEAD ON A 4.2 ?
What did you do for the fuel injection

nope just did complete 4.0 with the mpfi out of a 95 cherokee

John 12-14-2008 04:17 PM


Grandpa Jeeper 12-14-2008 06:52 PM

I have close to the setup you are running. I have 42's in my MC2100 but it still won't run up the hills in 5th. I am at 6500 ft.

Have you had a backfire or dieseling? You might want to check the power valve in the carb.

spshultz 12-15-2008 09:57 AM

No backfire or dieseling thankfully.

By the way, when measuring vacuum for the power valve do you use the idle vacuum or the cruise vacuum? Is it "take your vacuum reading, divide by 2 then subtract 2 from that"?

Oh and thanks everyone for the help.

On a side note, my son and I took it out for some fun in the snow yesterday. He's just learning how to drive so I decided we needed to find some empty space so he can practice doing donuts and etc so he can get a feel for what it's like when the rear end decides to come on out and play. And he did really well. I also had him try it in 4wd but the tires were just biting to hard to get it to swing around properly. It was -11 F but we had heat and we had fun.

spshultz 12-24-2008 09:05 PM

Just an update but I took off the MC2100 and rebuilt the old Carter BBD carb and put it back on. After working out a flooding issue due to some stupidity on my part we finally got it running today.

The BBD has a lot more power and I was able to hold 70 mph on hills. It's still a smidge flat in 5th gear but nothing like it was with the MC2100 on. The Carter BBD uses .092" jets. I'm going to put in a set of Edelbrock .089" and see how it feels. I may even try a set of .086" jets and see how they do.

AzTJ 12-24-2008 09:25 PM

If it's getting close to overheating on long hills, check the fan clutch.

spshultz 12-25-2008 10:34 AM

It has a new fan clutch, new reverse fan, new 2-core radiator, new radiator cap (13lb), new reverse water pump, new VDO temp gauage and sender and has had two new 195 degree thermostats. Oh, and new radiator hoses and the spring is in the bottom hose. It's also been flushed out a couple of times and uses a 50/50 mix. And it's been burped a bunch of times. Oh and I checked for exhaust gases in the coolant but nothing.

With all that I just don't think it's the cooling system unless there is blockage inside the block or head that isn't budging but I'm not willing to dismantle the engine for that. I want to swap it out for a 4.0, 4.3 or V8 this coming summer. It's just got to last that long.

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