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Old 01-27-2014, 03:00 PM   #1
Join Date: May 2013
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crossed fuel lines??

Good afternoon everyone.

I just yesterday finished replacing my 258 4.2. It drives fine and runs ok, except when turning sharp or braking. Then it acts like it wants to stall. Is it possible that I've got my supply and return lines backwards? I was stupid and didn't label before the swap. It might be worth noting that I don't run an FPR, as I didn't need one before with the MC2100, which is installed now. I'm thinking either the lines are backwards, or I need to retune the carb to this engine. Any help is greatly appreciated.
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Old 01-27-2014, 11:55 PM   #2
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Was that carb ever laying on its side while you were doing the motor swap. If it was anything but horizontal (same as when its on the motor) at any time, you might have a float sticking issue in the carb.

I have an 87, and it has original lines on it (they need replaced this spring) but they cant be backwards my supply line crosses from the drivers side to the passenger side at the back of the motor, and the return continues up the drivers side to the shock tower, then turns upwards toward the filter.

'87 YJ, 4.2L built and bored, 8" of lift, 8.8 rear, AX15/NP 231J, Tom Woods drive shafts, Flat fenders, 12,000LB winch, Custom bumpers, Rattle can paint job. Never completed...
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Old 01-28-2014, 08:16 AM   #3
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The carb might've been on its side at some point, though I'm pretty sure it wasn't. My fuel lines are both on the drivers side. The supply crosses over to the passenger side to get to the pump. PO installed new lines it looks like, and made them the same length.

I think I figured out the issue. I remembered sometimes bumping wires near the dizzy would cause the engine to stumble. When it wouldn't start last night, I looked there. The connector from the dizzy to ICM is broken, so the terminals were just barely touching. I plugged it in all the way and zip tied it closed for now. Fired right up, no stumbling at turns.

Still runs kinda sluggish, but that could be due to timing/carb tuning or the fact that the new engine sat for about a year without running.
I'll look into the float to be safe.

For anybody skeptical of which line is which, I found after turn the engine over and pumping the gas that only one line shot fuel out of it when the soft line was pulled away. Figured that has to be the supply.

Thanks OIIIO. You've been a big help recently.
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Old 01-30-2014, 08:19 AM   #4
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I've adjusted the timing and idle. Still lags and fuel economy sucks. Is it possible for the jeep to even run with the fuel lines crossed? Could it be that the pump is working extra hard because its hooked to return instead of supply?
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