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post #1 of (permalink) Old 12-29-2002, 04:01 PM Thread Starter
 
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Help on 4.2 power&mileage!

My '88 YJ is getting about 10 mpg and has very little power.I've put a motocraft 2 barrel carb and bypassed the computer for the ignition,but my problem is still there.My mechanical and vacuum advance seem to work.I have an automatic, 31-10.50 tires,3.54 gears,and I am easy on the gas.It seems to get the same mpg on freeway or around town.I think the engine is a 1980 [img]images/graemlins/cussing.gif[/img]or earlier.Any ideas?Thanks.
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post #2 of (permalink) Old 12-29-2002, 04:13 PM
 
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Re: Help on 4.2 power&mileage!

Is the engine tuned properly? Does it have the proper compression on all cyls. Is the tail pipe black? Test for total advance on the distributor. Test for proper vacume at idle. Also test for vacume leaks. Do you points or electronic ignition?
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post #3 of (permalink) Old 12-29-2002, 04:30 PM Thread Starter
 
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Re: Help on 4.2 power&mileage!

I have the '88 electronic ignition and all tune up components are new. The plugs are light brown and it runs real smooth.I haven't checked the vacuum or compression though.It does have a ton of oil pressure, and doesn't seem to have much blow by,so I assumed it had good compression.I have the timeing at the top of the scale(on the timeing cover)and when you give it gas it goes up under the water pump somewhere.It seems weird to have the timeing so high but it does worse when I lower it. I kind of wondered if the '80 and earlier 4.2 had an inefficient cam profile.
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post #4 of (permalink) Old 12-29-2002, 06:06 PM
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Re: Help on 4.2 power&mileage!

</font><blockquote><font class="small">In reply to:</font><hr />
I have the timeing at the top of the scale(on the timeing cover)and when you give it gas it goes up under the water pump somewhere.It seems weird to have the timeing so high but it does worse when I lower it.

[/ QUOTE ]

Are you not disconnecting and plugging the vacuum line to the distributor before setting the timing?
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post #5 of (permalink) Old 12-29-2002, 07:26 PM Thread Starter
 
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Re: Help on 4.2 power&mileage!

Yes I disconnected the vacuum line first.
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post #6 of (permalink) Old 12-30-2002, 08:37 PM
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Re: Help on 4.2 power&mileage!

Are you sure you haven't jumped time? That is, you can't get the timing work in the "normal" range because the chain's off a tooth or two? This could explain much.
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post #7 of (permalink) Old 12-30-2002, 08:46 PM
 
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Re: Help on 4.2 power&mileage!

The plugs should be a light tan color. Like lighty browned rye bread toast. Throttle response should be good, and timing should be advanced till it pings, then backed off just enough that it doesnt'. With 33's, 4.56 gears and a T5, I get 17 mpg for a combo city and highway using either the Motorcraft 2100 or the old bbd.

Are you heavy footed? I drive a bit like a granny, so that may be my milleage secret.
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post #8 of (permalink) Old 12-31-2002, 11:35 PM
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Re: Help on 4.2 power&mileage!

make sure that when you check your timing you set it with vac. hose disconnected and engine at 2000 rpms i believe. check emmissions sticker
post #9 of (permalink) Old 01-01-2003, 02:34 PM Thread Starter
 
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Re: Help on 4.2 power&mileage!

I just ran the numbers on my engine and it is a '76 232. So this may explain my troubles with power and mileage.
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