Fuel vapor return line
February 19th, 2015, 09:55 AM
#1
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Fuel vapor return line
I have an aftermarket mechanical fuel pump installed on my '69 400G. The engine is relatively lightly modified (aluminum heads/intake, Demon 750cfm carb) and runs fine. When I installed the fuel pump, the manufacturer recommended I not hook up the vapor return line. His theory was that this could possibly starve the engine of fuel under hard accel conditions. I followed his advice and the engine has been running fine under light acceleration conditions.
About 2 weeks ago I got on it from a start (~75% throttle) and the engine started running a little rough and finally stalled once I slowed down.
My question is what symptoms might I expect to see by not having the vapor return line connected. Any comments are appreciated.
About 2 weeks ago I got on it from a start (~75% throttle) and the engine started running a little rough and finally stalled once I slowed down.
My question is what symptoms might I expect to see by not having the vapor return line connected. Any comments are appreciated.
February 19th, 2015, 11:58 AM
#2
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Join Date: Sep 2013
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I have an aftermarket mechanical fuel pump installed on my '69 400G. The engine is relatively lightly modified (aluminum heads/intake, Demon 750cfm carb) and runs fine. When I installed the fuel pump, the manufacturer recommended I not hook up the vapor return line. His theory was that this could possibly starve the engine of fuel under hard accel conditions. I followed his advice and the engine has been running fine under light acceleration conditions.
About 2 weeks ago I got on it from a start (~75% throttle) and the engine started running a little rough and finally stalled once I slowed down.
My question is what symptoms might I expect to see by not having the vapor return line connected. Any comments are appreciated.
About 2 weeks ago I got on it from a start (~75% throttle) and the engine started running a little rough and finally stalled once I slowed down.
My question is what symptoms might I expect to see by not having the vapor return line connected. Any comments are appreciated.
February 19th, 2015, 03:34 PM
#3
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Location: Dallas-Fort Worth
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I have the same problem, however I'm running a Holley 125 electric pump and same thing, under a hard run the motor starves-bogs until you get out of the throttle. What are you using for a vent system? 67-455 retro-mod 442, the robski
February 20th, 2015, 09:22 AM
#5
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Not sure of the specs on the pump but I could get them. It's a mechanical pump in the stock location running a 3/8" feed line. I installed an Earl's in-line filter about 4" upstream of the pump (low pressure side). My engine builder tried to convince me of leaving off a filter to avoid potential starvation problems but I don't like that idea. I could easily remove the filter to see if that makes any difference but I don't really want to run it without a filter. I'm not sure if installing the filter on the high pressure side would make any difference.
As far as a vent is concerned, I have the factory vent at the tank but nothing else. That's what got me wondering about the need for a return line. It would be easy enough to install one and try it but I figured I'd ask to see if anyone else tried running without one and if it caused any problems.
As far as a vent is concerned, I have the factory vent at the tank but nothing else. That's what got me wondering about the need for a return line. It would be easy enough to install one and try it but I figured I'd ask to see if anyone else tried running without one and if it caused any problems.
February 20th, 2015, 09:37 AM
#6
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Join Date: Sep 2013
Posts: 3,574
Not sure of the specs on the pump but I could get them. It's a mechanical pump in the stock location running a 3/8" feed line. I installed an Earl's in-line filter about 4" upstream of the pump (low pressure side). My engine builder tried to convince me of leaving off a filter to avoid potential starvation problems but I don't like that idea. I could easily remove the filter to see if that makes any difference but I don't really want to run it without a filter. I'm not sure if installing the filter on the high pressure side would make any difference.
As far as a vent is concerned, I have the factory vent at the tank but nothing else. That's what got me wondering about the need for a return line. It would be easy enough to install one and try it but I figured I'd ask to see if anyone else tried running without one and if it caused any problems.
As far as a vent is concerned, I have the factory vent at the tank but nothing else. That's what got me wondering about the need for a return line. It would be easy enough to install one and try it but I figured I'd ask to see if anyone else tried running without one and if it caused any problems.
February 20th, 2015, 09:42 AM
#7
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Location: Plano, TX
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A vapor line is to deal with vapor lock, which is normally when trying to hot-start the engine, not while running.
A filter on the suction side of the pump is a bad idea and can make the pump starve. Move the filter to the pressure side.
Bear in mind the carb bowls can run the car even at WOT for a few seconds. So if it bogs immediately then it's definitely a carb issue. In any case, I still expect it's a carb issue.
A filter on the suction side of the pump is a bad idea and can make the pump starve. Move the filter to the pressure side.
Bear in mind the carb bowls can run the car even at WOT for a few seconds. So if it bogs immediately then it's definitely a carb issue. In any case, I still expect it's a carb issue.
March 5th, 2015, 09:56 AM
#8
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Location: Western North Carolina
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Just a quick note to wrap this up. I tracked the source of the problem to a weak connection for the 12V HEI power source so it wasn't fuel related at all. However, I did take the advice given above and have moved the fuel filter from the low pressure side of the pump to the high pressure side.
Running good so far.
Running good so far.
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