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Yj 4.2 lacks power

975 views 7 replies 5 participants last post by  Nubby55 
#1 ·
I have a 87 yj with the 4.2 5 speed with 307 gears. My jeep has a hard time keeping up on the highway at 65 is this normal? First second and third pull ok but fourth and fifth are a dog big time.
 
#2 ·
What size tires
What size lift
I run 33s on my 90 had 307s could not use fifth
gear. Have now went to 410s massive
difference now i can use fifth and gain speed
one of the best things i have did LUV IT
 
#3 ·
What size tires are you running ?

Do you still next a stock carb or distributor ?

I'm running 3.07's and 35's until the spring then I will have 4.56's and my built 8.8 installed but I have no issue maintaining 65-70 in 4th gear on the hiway. I also have a weber 38/38 carb, and hei distributor , and 2.5" hiflow cat and cat back. Which really woke up the 258
 
#4 ·
You might want to check your cat. I have a 90 4.2 with the stock Carter carb, 4" lift and 33" tires and have no problem doing 70-75 on the freeway. But I never use 5th gear unless I'm on a flat stretch of highway. It does bog down a bit on the hills but I can still keep up with the cars in the middle lane.
 
#5 ·
I'm only on 29" tires and stock every thing just Nuttered ,but my old Heep will run way faster than you have Guts to drive it, over 80 you get Real Busy! It's Short!
 
#8 ·
Check your timing, mine is set at 12deg. BTC, when I had it at 8deg. as some one said to start with it ran good but low power then I went to 10deg. and better, the last year or so at 12deg. and runs great and better MPG, about 14 MPG and I don't baby it.
not all engines are the same, wear on different parts make them all run better at different timing. good luck, play with it back and forth.
 
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