poor fuel and 350 eng with carb

allan Johnson

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have a 77 c15,350 with carb,the dew point goes over 60 when temp is 85-90 degress,195 stat,trys to vapor lock and doesnt want to idle,dont wanna change stat to 160 just to change it back soon,sounds like the alcohol gas is evaporating,will adding some leaded race gas help?thanks!
 

SirRobyn0

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I'll give my opinion on this. I'm a mechanic with lots of classic experience, but I don't live where 90F is typical.

Most of these fuel systems have a return line. If the fuel pump is weak very little if any fuel is returned and vapor lock is the result. Keep in mind vapor lock most commonly occurs in the pump or just before the pump, not after. A lot of people think it happens after the pump, but that section of fuel line is pressurized, and any liquid that is pressurized has a higher boiling point. If it's trying to vapor lock at those temps I would start by replacing the fuel pump and making sure the return line is open and not kinked or otherwise damaged.

We are having a heat storm here and I just drove my truck for an 1 1/2 hours starting out with city traffic and heading eventually to my home in the country. I ran the A/C made a couple stops and never had it vapor lock or even feel the slightest like it was going to and it was 110F outside. Truck handled it better than I did and I had the A/C blowing on me! I'm running a 195 stat as well, normal 10% ethanol pump gas 87, no additives. My point is what you are experiencing is not normal hot whether behavior, you have a fuel system problem, most likely a weak pump.

You should also check to make sure heat riser isn't stuck shut. If it's stuck closed that'll put a lot of heat into the intake manifold but those symptoms will be more of percolation rather than vapor lock.

Are you running dual exhaust? And if yes to you have H pipe installed?

Does it have an EGR valve? And if yes does it function?

Are you running the factory air cleaner housing? If yes does the thermac function correctly? Does it have air intake hose to the grill by the radiator?

Do you have duel tanks? If so will it do it on both tanks?

Let me know if any of this doesn't make sense.
 

Ken B

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wow what a complete assessment !
 

Grit dog

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My vote is fuel pump. And if it’s old, it’s cheap insurance even if that’s not the whole problem.
 

Matt69olds

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As mentioned before, make sure the fuel pump is correct for the application. Cars/trucks with A/C (or any application where high under hood heat soak is a common occurrence) will have a fuel system designed with a return line.

The return line serves 2 basic purposes: the first is to circulate a tiny amount of fuel back to the fuel tank during normal operation. The built in fuel leak helps keep the fuel pump cool. The 2nd purpose is one most people are unaware of: the calibrated fuel leak back to the tank also bleeds off fuel pressure in the fuel line.

Imagine this situation: your driving on the highway, A/C going full blast, it’s 95* outside, you pull over for some dinner. The fuel in the fuel pump/lines continues to absorb heat, increasing the pressure in lines. Eventually, the pressure rises enough to force the needle in the carb off the seat, resulting in flooding or hard hot restart.

The tiny orifice in the return line fitting of the pump provides a place for the pump to vent the excess pressure.

The return line is another one of those “ah, cars don’t need that crap!” that people don’t understand, and actually serve a purpose.
 

Raider L

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@allan Johnson,

My Holley performance fuel pump does bleed back to the tank when I shut the engine off, through a valve in the pump. I don't need a additional line for it. Also, I wrapped aircraft insulation we used to use around lines that ran near the engine or exhaust in the aircraft, around the whole length of gas line where it passed anywhere near the header tubes when we were rebuilding the engine and it was out of the truck. I saw how close it was going to be and to avoid vapor lock I figured it would be a good idea to cover the line. I guess it worked.

You might get some of that header tube wrap, I think it comes in rolls, Summit would have it, and motorcycle parts places would have it to, and you could put some of that around the fuel line where it runs near your exhaust and that would at least protect it from any heat source.
 

Crusader fixer

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I vote with the experts, above. I'm in the deep South, 90/90 heat and humidity are an every morning event and ethanol has had no effect on vapor lock. Lack of recirculation will do it though. In the real old days it was not uncommon to see a farm truck that operated at low speed and heavy load in the field have a return line locally rigged to fix the problem.
 

SirRobyn0

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I vote with the experts, above. I'm in the deep South, 90/90 heat and humidity are an every morning event and ethanol has had no effect on vapor lock. Lack of recirculation will do it though. In the real old days it was not uncommon to see a farm truck that operated at low speed and heavy load in the field have a return line locally rigged to fix the problem.
I haven't done an install in a few years now but larger box trucks and RVs based on the Dodge B-van cab and chassis, could have problems with vapor locking. Dodge did not put return lines on any of those rigs from the factory. The solution was go after the fuel pump, cut the metal line and add an aftermarket fuel filter with a return line built into the filter. Wix #33054 has 5/16" inlet and outlet with 1/4" return, which is what the Dodges had for fuel line so it work well on that. And then we'd plumb the 1/4" return back to the fuel tank. This cured even the absolute worst vapor lock issues. Back to squares, if you have a square with a vapor lock issue, and no return line, if you install a pump with a return, you can plumb the return back to the tank just using bulk rubber hose and securing it to the frame.
 

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