Vapor Lock? 78 K20
 

Here is yet again another post about a vapor lock haha sorry guys. Im trying to diagnose my issue which im pretty sure is a vapor lock... I have put a phenolic spacer under neath the carb ( which was recommended by my buddy who was having the same issue), Replace all of my rubber fuel lines blew out the metal lines, replaced the fuel filters. I did all of that and it was fine but noticed that i was going through gas rather quickly so I took another look under the hood and found that the mechanical pump was bad so it was spraying my fuel all over the road ( Trucks been sitting for 11 years and vapor locked before now). So during the time the mechanical pump was bad it didn't vapor lock but once I replaced it the truck began to vapor lock. When I would lock up on the side of the road id check under the hood and felt my lines to see if they were getting hot. The lines were warm but not to bad... I than check the mechanical pump and it was screaming hot so I already had a electric pump down by the gas tank to help work through the vapor lock so i just bypassed the mechanical and put a pressure regulator on the lines so i don't flood the carb. Still no luck.... So now hear I am asking for help :lol:.....The truck locks up at night and day makes no difference I have also replaced the sending unit, I Have also checked the timing its spot in.


I have a 1978 K20 Chevy
350 crate engine
Stock Quadrajet cab
Headers instead of manifolds

Re: Vapor Lock? 78 K20
 

Crack open the gas cap when it acts up, is it sucking air? May be a clogged vent.

Spacer should have worked, you've taken the fuel pump out of the equation.
You say vapor lock, could it be flat out starving for fuel?
could be vent issue, clogged filter(s), collapsed sending unit sock, hole in the sending unit sucking air...

Re: Vapor Lock? 78 K20
 

I had a Fram gas filter blow it's back flow valve out into the line.
Blocked gas flow, had to keep my foot into it or it would die, and no start.
Told on here it happens , the fix is to remove the spring and valve.
I think the 78 was the last year , without a return line.
Some will get a 3 line fuel pump and even add the extra line to fix it that way.

Re: Vapor Lock? 78 K20
 

One uninformed opinion - I suspect that your headers are boiling your gas. Extreme experiment, but do you have the stock manifolds/exhaust that you can re-install to test the theory?;) You can try wrapping the fuel lines with insulation (I got some from Summit for my 454 fuel lines, but it did not help me. My problem was starter heat soak -ultimately fixed with Hitachi starter.) Of course, header wrap or ceramic headers are an option, but I like the cheap options first.

You can check fuel line pressure at the carb, but I think you might still have pressure with no actual liquid fuel if you have vapor lock.

I dont belive it's sucking air when I open the cap...unfortunately I dont have the manifolds anymore so no luck there....this truck still does have a return line on it just not being use because of the electric pump. I did try insulating the fuel lines alittle bit and it seemed to help but locked up a little later while driving.....how would I Know if its fuel deprived?
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Took it out for a drive today first starting the idle is a little rough and idling low then once u give it gas the idle climbs...Then on my way back I noticed if i mash the gas and then let off it to crusing along from what it sounds like is that its backfiring in the carb or somplace... after it backfired the vapor lock symptoms appeared I pulled over and felt my fuel lines and they are weren't even warm?
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Re: Vapor Lock? 78 K20
 

How is your timing? Vacuum leaks? Any smoke when this happens? (black indicating to much fuel?)
I personally think there is an issue, but doubt vapor lock is it.

No black smoke, Timming is at 10 btdc cuz I'm in Colorado, just replaced all the the vacuum lines a few days ago so I don't think that is it. I think it might be being fuel depriived I've I bought a new pump today and going to put it on tomorrow. This pump pushes 5.5-9 psi which is what the mechanical pumps push so we'll see if this fixes it I'll keep you guys posted.
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Re: Vapor Lock? 78 K20
 

You may have to break down and buy a fuel pressure gauge to verify actual fuel pressure. I bought a couple fuel pumps before finding out my fuel pump lobe is half dead. The truck still screams with 1 1/2 lbs of fuel pressure though.

Re: Vapor Lock? 78 K20
 

Popping in the carb led me to question the timing. In fact, I still question it. Your sig says crate 350, how old is the engine? Would it be possible the timing chain is stretched and throwing cam timing off? (spitballing here, have had it happen before, but usually with high mileage engines.)
Questioned the smoke thinking you could be getting too much fuel. Over the years I've had choke plates missing linkage and slam shut pulling up to a stoplight and about a year ago a piece of carb gasket get wedged in a metering rod and jamb it open. Truck would actually flood out at idle in gear.
I actually like cadillac al's idea of the pressure gauge.

The crate has about 23,000 miles on it and then sat for 11 years...so idk...and there is no smoke visible when running so I think we are good there. I tested the old fuel pump today and found out the engine was starving for fuel and that's way it would act like it was out of gas... I throw the new pump on today and it ran a heck of a lot better was not holding back at all like it was and was supplying the carb with the proper fuel now. When driving the popping in the carb or the back fire went away didn't hear this time.
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Re: Vapor Lock? 78 K20
 

Cool,running better is always good news

Re: Vapor Lock? 78 K20
 

good job.