confusing 390
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confusing 390
I have a 390 in an old half ton that we just threw together one night (shortblock from one 390 and heads from another). It seems to run fine for about 5 minutes after started but then it completely bogs out and wont rev up or anything. The original fuel tank was rusted out and kept plugging the fuel filter, so we put a different tank in and now the gas is fine coming out of the filter. After it dies we wait about 3 minutes and it starts up fine again and runs for about 5 minutes again. It is about 105 degrees but it does it whenever we run it, not just today.
We're completely stumped and any help would be greatly appreciated.
Thanks
Matt
We're completely stumped and any help would be greatly appreciated.
Thanks
Matt
Lead Driver
Joined: Jan 2003
Posts: 9,893
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From: Ohio
No, actually if the diaphram of fuel pump is damaged in anyway...it will leak fuel INSIDE the motor.
As long as you have any steady pressure at carb..it should stay running. If it seems like it's running out of fuel(when it quits),then I would think it is fuel related problem.
After it quits...try pumping carb and see if you see fuel come from accelerator pump. If nothing...then it's a fuel problem.
As long as you have any steady pressure at carb..it should stay running. If it seems like it's running out of fuel(when it quits),then I would think it is fuel related problem.
After it quits...try pumping carb and see if you see fuel come from accelerator pump. If nothing...then it's a fuel problem.
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Fuel comes out after it quits, I don't think theres anything wrong with the fuel pump. I Know it seems like a fuel problem but the gas is perfectly clean and if it was a fuel problem it wouldnt start and run good right away, it would be choppy right when you started. We were playing with the choke and when we hold the choke closed the motor runs alright. This makes it seem like the motor isn't getting enough fuel but it is we can see it in the carb.
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This could be very well true most of the vacuum lines arent hooked up, but it runs fine for the first few minutes, why wouldnt it run bad right away?
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Elder User
Joined: Jun 2003
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From: Houston/Hope BC
Provided your automatic choke is working, the engine starts off with pulling a rich mixture, wich works fine with all that extra air from the leaks. Once the choke opens, the mix goes so lean it won't fire reliably.
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Thats true but the automatic choke is not working, all thats really hooked up on the carb is the fuel line, the throttle link, and a vacuum line to the distributer. We took the carb off and put a 351m one on that we had in the shop and with this carb the motor runs great at high rpm but wont idle at all, once it falls below about 1800 it completely quit.
Elder User
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From: Houston/Hope BC
Have you looked down the carb throat during those first few minutes when it runs OK to see if maybe it is dumping a lot of raw gas? This might fit your symptoms.
(Current theory: Engine starts, then loads up with a lot of excess gas, dies.... gas evaporates....engine starts again... runs fine at high RPM because air-flow is keeping gas from pooling.)
Sorry, this may not help, I mixed up a little on which symptoms go with what carb.
(Current theory: Engine starts, then loads up with a lot of excess gas, dies.... gas evaporates....engine starts again... runs fine at high RPM because air-flow is keeping gas from pooling.)
Sorry, this may not help, I mixed up a little on which symptoms go with what carb.
Last edited by Ecuri; Jul 16, 2005 at 07:34 PM.
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From: Houston/Hope BC
Do I have this right, you have tried two different carbs? And on the current one, the engine starts but will not run without a lot of throttle opening?
What type(s) are they?
What type of ignition system is on the engine? (may or may not matter.)
What type(s) are they?
What type of ignition system is on the engine? (may or may not matter.)
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dont know much about the carbs but the original 390 carb barely runs, the other one (the current one) is from a 78 351m and it runs like u said when the throttle is open quite a bit. It's got pionts and we've set the timing and the motor still wont run with throttle closed no matter where the timings at.
Elder User
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From: Houston/Hope BC
351M only had 2V option so i assume that is what you have. If it only runs at high speed you are still looking for some serious vacuum leak, IMO. If the jets where clogged it wouldn't run at any opening position. What about the port on the back of the carb for the PCV system, is it plugged or hooked up?
