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post #1 of (permalink) Old 06-07-2004, 07:02 PM Thread Starter
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Loss of power

I'm losing speed on the highway lately. Here's what I have.

Completely rebuilt 4.2.
RV Cam
TFI with plugs set to .045 gap
5 speed tranny
Stock gears
Weber 32/36
Stock exhaust w/ no cat
Timing is right.

I can accelerate fine in all gears until I hit 5th. As long as there are no hills, I can hold speed. I have to downshift to keep speed up on hills or to accelerate. I checked fuel pressure with the old pump and after installing the new fuel pump and filter at rebuild. It is at 4 lbs idling and drops to about 2 lbs at around 2000 rpm under no load. I don't know what it is under load on the highway. It doesn't miss or try to stall at any time. I'm missing something.
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post #2 of (permalink) Old 06-07-2004, 07:06 PM
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Re: Loss of power

What size tires?

If you've gone to a taller tire, they could be eating up your highway power.
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post #3 of (permalink) Old 06-07-2004, 09:13 PM Thread Starter
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Re: Loss of power

Sorry. Forgot that. 31/10.50
Same size as when I got the jeep
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post #4 of (permalink) Old 06-08-2004, 12:33 AM
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Re: Loss of power

"""It is at 4 lbs idling and drops to about 2 lbs at around 2000 rpm under no load. I don't know what it is under load on the highway"""

Pressure should not drop with speed.
Either the pump's defective or the supply line to the pump is restricted. Try removing the gas cap and try it - if air can't get in the tank because of a bad cap, it'll create a vacuum.
Have a close look at the steel lines - tank to pump - one might have got smashed somehow.
Put a vacuum gauge on the suction side of the tank to see if it's restricted. Should be about the same no matter the speed - if yes, replace pump.
New doesn't always mean good.
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post #5 of (permalink) Old 06-08-2004, 06:45 AM
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Re: Loss of power

Checking the pump would be a good idea, but it kind of seems to me that with a carb set up, the motor doesn't really care what the fuel pump is doing. As long as the fuel bowl is full the motor stays running. If the fuel pump can't keep up, and the bowl goes dry the motor will start to caugh, and miss. (run out of gas) So unless you are running out of gas under load, I would guess that the problem is not fuel pump related. But if you have varying pressures, you should probably check it to make sure it is ok.
I would carefully check all the vaccume lines. The problem could be vac advance.
Does your carb have an electric choke, Maybe you are getting choke. That happend to me on my CJ. I had a bad electric choke, I would lose power for no real reason, and the motor would not run right. Pull your air cleaner, and start her up from cold. Make sure your choke is working correctly.
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post #6 of (permalink) Old 06-08-2004, 11:27 AM
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Re: Loss of power

Err ..what kinda rpm are you turning in 5th gear, what speed are you doing, and what do you define as "hills" ....and are you meaning that even at WOT you can't maintain speed.

Just trying to determine your "perception" base here (you don't seem to know your gears and we have no idea what speed you're travelling).
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post #7 of (permalink) Old 06-08-2004, 01:43 PM
 
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Re: Loss of power

Man, I don't think that's incredibly odd, not being able to accelerate up a hill in 5th w/a 258. Even with your fuel upgrades, and the TFI upgrade, I still wouldn't expect the 258 to be a top-end powerhouse.

You want to pass a truck in 5th going up a grade? Do a 4.0 head swap. After I did mine, i was absolutely blown away at the increase of topend power.
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post #8 of (permalink) Old 06-08-2004, 02:52 PM
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Re: Loss of power

If the pump pressure drops during load, it can still supply enough fuel to keep some fuel in the bowl, but the level would be low.
A low fuel level causes it to go lean, robbing power.
It's a matter of how much it's not getting. Yes, if the bowl goes dry, it sputters and stumbles. It's a matter of being "in between" good supply and starvation.

Plugs can also be the culprit -- overheating plugs don't always create a misfire like a thump thump. They just don't ignite the mixture well, losing power. Slow down a bit, power comes back. Have a look at them. Bosch, Splitfires, Multis, and Plats have a reputation for doing what you describe. Use the plugs the engineers designed the combustion chambers around - good old fashioned spark plugs, not advertising scams.

And as suggested - check the timing advance.
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post #9 of (permalink) Old 06-08-2004, 06:41 PM Thread Starter
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Re: Loss of power

I'll check fuel lines and vacuum on the inlet the next free weekend I get.
The plugs are good old Champions. I don't trust the high dollar ones to deliver that much better performance. I will pull them and check on how they look.
The vacuum advance appears to work freely and I haven't found a vacuum leak yet. I'll check for leaks better when I have the time again.
As far as rpms, speed, and knowing my gears, I think after 30 years of various standards I know what 5th gear is. I know it isn't one for acceleration or speed. I am running about 1600 rpm at 55 mph. The hills I am talking about are probably no more than 5 degree rises about 1/2 mile long. Just standard highway fare. I'm not looking for a race car. I just know that before my rebuild and with the good old Carter carb I was doing better on the highway.
Right after I installed the Weber it started running better. Then right after the engine rebuild it was running great. Now with about 600 miles on it, something has changed.
Thanks for the input. Looks like I have a good full Saturday of stuff to check now. The yard will have to wait.
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post #10 of (permalink) Old 06-11-2004, 07:25 AM Thread Starter
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Re: Loss of power

I think I found the problem. The hose clamps on the inlet hose to the fuel pump were loosened. I tightened them and also found one intake manifold bolt slightly loose. I think it was sucking air through the fuel pump under load. Thanks for the help.
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