Fuel Pressure drop in traffic ???

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twotone68

twotone68

lifetime mopar man
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My 68 dart with a mild 318 has a stock tank with a 1/2 in. line sending unit ,3/8 & number 6 fuel line tank to carb, running a carter, race mechanical pump, quick fuel 680 vac. sec. with guage on dual feed line. The Problem is ,when I fisrt start it in the morning it is unregulated at 9-9 1/2 psi. after running for 20 mins in traffic it is down to 5 to 5.5 psi ????? Pump gas premium, insulation around any hot exhaust with lines. Bowls are always half to 3/4 full. no vapor lock . Any ideas ????
 
Yes it Is. My 680 Quick Fuel Carb is Supposed To Stay At 6 1/2 psi. But I Know volume is the big key as long a pressure stays above 5 psi. Thinking of going all 1/2 in. and #8 lines front to back.
 
-8/half inch lines is way overkill for a mild 318.
Your changing psi numbers could be on the supply side. Such as a failed tank vent. Or a collapsing rubber hose. A volume output test would be my next test.
 
Sound like either the pump or the gage to me.
Dropping pressure in traffic doesn't sound like a volume issue, but a pressure issue.

Just thinkin :)
 
twotone68 said:
My 68 dart with a mild 318 has a stock tank with a 1/2 in. line sending unit ,3/8 & number 6 fuel line tank to carb, running a carter, race mechanical pump, quick fuel 680 vac. sec. with guage on dual feed line. The Problem is ,when I fisrt start it in the morning it is unregulated at 9-9 1/2 psi. after running for 20 mins in traffic it is down to 5 to 5.5 psi ????? Pump gas premium, insulation around any hot exhaust with lines. Bowls are always half to 3/4 full. no vapor lock . Any ideas ????
Click to expand...

Witch mechanical fuel pump? I run a homemade 1/2" tank pickup, Rubber A/N number 8 fuel line .Standard mechanical Carter street/strip pump, gets me 6.5 -7.0 at the carb feed line. You need to find the restrictions , in the fuel flow. Brass hardware fittings, usually kill liquid flow, think about it. Pictures, would help immensely.
 
TB
The volume test was just to prove the suction side was not restricted or collapsing. You will see it as the flow goes from full-flow at the test start, to almost zero-flow as the collapsed/collapsing hose shows up, or as a gradually reducing flow as the vent problem shows itself.Or some combination of suction-side issues.
But if it pumps a pint in 15 to 30 seconds, youve instantly proved its not a suction-side issue.But if it barely pumps anything out, then you have to figure out is it the pump or the line. And to the OP, no you dont rush out and buy a new pump. Its much easier to prove the line first.
 
AJ/FormS said:
TB
The volume test was just to prove the suction side was not restricted or collapsing. You will see it as the flow goes from full-flow at the test start, to almost zero-flow as the collapsed/collapsing hose shows up, or as a gradually reducing flow as the vent problem shows itself.Or some combination of suction-side issues.
But if it pumps a pint in 15 to 30 seconds, youve instantly proved its not a suction-side issue.But if it barely pumps anything out, then you have to figure out is it the pump or the line. And to the OP, no you dont rush out and buy a new pump. Its much easier to prove the line first.
Click to expand...

Having it happen at low speeds in town only already showed that., as driving anywhere else at higher speed would require more volume and the problem would be worse in those conditions.

That's why I said pump or gauge. :D
 
Thank You All . I have 2 month old carter mechanical, 110 gph. pump,All New 3/8 aluminum hard lines, except at connections, new 3/8 fuel injection rated rubber hose. Do Have push connect 3/8 fittings at pump . The tank Only has one line ,the feed line, the vent on the filler neck would be my only possibility ???? Plenty of volume in 10 seconds, more than a pint. I Have a Russells Big Filter at the tank n a polished Fuel Injection filter just before the Dual feed line at the carb. Both rated at 90 to 110 gph.
 
twotone68 said:
Thank You All . I have 2 month old carter mechanical, 110 gph. pump,All New 3/8 aluminum hard lines, except at connections, new 3/8 fuel injection rated rubber hose. Do Have push connect 3/8 fittings at pump . The tank Only has one line ,the feed line, the vent on the filler neck would be my only possibility ???? Plenty of volume in 10 seconds, more than a pint. I Have a Russells Big Filter at the tank n a polished Fuel Injection filter just before the Dual feed line at the carb. Both rated at 90 to 110 gph.
Click to expand...

There was a post here recently where someone had the same problem and it ended up being the gauge and not the fuel system dropping off, so maybe...
 
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