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Maybe vapor lock.

5K views 39 replies 16 participants last post by  Robinls5 
#1 ·
I feel I am chasing my tail.
I start up the chevelle and it shows 7 pounds fuel pressure on the regulator, go up the road and get it hot with a high speed pass, and pull back in the shop and it shows 3 pounds and will slowly drop to zero and pull all of the fuel out of the front bowl, so you think vapor lock right? Well that is what it acts like for sure, but its never did it before on hotter days than it is now and I did not change anything. All of a sudden after 3 years it acts up.
540 cubic inch.
clay smith mechanical big block fuel pump. 1000 cfm AED 4150 carb.
1/2 inch steel braided fuel line from the tank to the pump and then 1/2 inch line to the fuel regulator and then 1/4 inch lines to each bowl, they are all wrapped with reflective heat resistant tape, I used the original 3/8 fuel line for the return to the tank. I checked the inline fuel filter and it is good. I have have raced this car in vegas at 100 degrees without a any issue, hot lapping it.
Before I start throwing parts at it I would appreciate some insight from anybody who has seen something similar.
Maybe the fuel pump is giving up but I don't think so because the car will light right up 2 hours later when the fuel bowl fills with 7 pounds on the regulator. Thanks ahead of time for your insight.
 
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#3 ·
Have you been buying fuel from a different source?

The formulation may have changed, and you are now having the fuel boil. Wrapping the lines and installing the holley 108-70 heat shield eliminated intermittent issues I was having when driving on the street, but almost every run at the track.
 
#6 ·
Chevron premium fuel, from the same source.
I was also thinking maybe the bowls were boiling from heat.
I pulled it in the shop today and watched the fuel pressure drop from 3 pounds to 0 in about 2 minutes after a high speed run, and also watched the sight glass have fuel in the very bottom and then after 2 minutes the car sucked it dry, and right when the car died I took the fuel line off of the primary bowl on the front and there was no fuel in the line and had my daughter turn it over and and there was no fuel coming out of the line why she is turning it over. So that makes me think fuel pump giving up, but go out there 2 hours later and turn it over and the fuel bowl fills up in about 10 seconds. sheeeesh.
 
#5 ·
It can depend on when you bought fuel. Winter blend is a disaster. In June the fuel is much different.

My car struggles in warm temps with Winter fuel plus I’m at 6400’.
 
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#8 ·
Fuel Pump/Regulator are cooler when Engine/Car has sat for a while

After doing your Hot Runs they become Hotter thus they do not work as well

In the 80s I had put a 1/2" Copper Fuel Line on my 65 GTO with just a Stock Fuel Pump
and at the 1,000' Mark on the Track she would fall flat / no Acceleration

Added a Inline Electric Fuel Pump back near the Gas Tank and problem solved
 
#11 ·
i have exactly the same fuel pump and 1/2” fuel line but inly a 1/4” return line with a .70 jet. no vapor issues so I doubt thats your issue. I have had issues with regulator diaphram going bad bleeding off pressure thru vent when real hot
 
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#24 ·
I was also experiencing a vapor lock issue on super hot days so I bought a fuel tank sender assembly with a return line and ran a line back to the tank from the rear of my fuel log. Since I wasn't sure what size "jet" was needed in the return line I found an adjustable flow control valve to use inline. I started from a full closed position and slowly opened the valve until my idle fuel pressure dropped about 1/4 PSI. Then checked the volume of fuel flow by placing the return line is a bucket and letting the car idle for a few minutes. I was surprised how much fuel was returning... well over a quart in a couple minutes.

If I decide to go to the drag strip I just close the needle valve and I have enough fuel volume to make a full pass. With it set as detailed above it'll run out of fuel just past the 1/8 mile mark.... ask me how I know.
 
#13 ·
I feel I am chasing my tail.
I start up the chevelle and it shows 7 pounds fuel pressure on the regulator, go up the road and get it hot with a high speed pass, and pull back in the shop and it shows 3 pounds and will slowly drop to zero and pull all of the fuel out of the front bowl, so you think vapor lock right? Well that is what it acts like for sure, but its never did it before on hotter days than it is now and I did not change anything. All of a sudden after 3 years it acts up.
540 cubic inch. clay smith mechanical big block fuel pump. 1000 cfm AED 4150 carb.
1/2 inch steel braided fuel line from the tank to the pump and then 1/2 inch line to the fuel regulator and then 1/4 inch lines to each bowl, they are all wrapped with reflective heat resistant tape, I used the original 3/8 fuel line for the return to the tank. I checked the inline fuel filter and it is good. I have have raced this car in vegas at 100 degrees without a any issue, hot lapping it. Before I start throwing parts at it I would appreciate some insight from anybody who has seen something similar. Maybe the fuel pump is giving up but I don't think so because the car will light right up 2 hours later when the fuel bowl fills with 7 pounds on the regulator. Thanks ahead of time for your insight.
Hi Mike, I've had similar issues before with my old engine and combo. Be glad yours is a 70, my 69 is the one
with the extra tube in the gas tank so when it was 1/2 to 3/4 full, fuel would be siphoned out of the tank.

I also now have the Clay Smith BBC fuel mechanical fuel pump. When I replace my gas tank with the Tanks, Inc larger tank, I'll use the 3/8 line as a return line. I have a heavy duty continental 1/2 inch I.D. fuel line at the ready.
It's not stainless steel, but a 300 psi heavy duty industrial fuel rated hose.

I bet, if you drive her around and use of the winter fuel, you'll be okay with the summer blend.
But summer blends might not be out as usual, because I bet, with the lack of driving by Americans
during corvid-19 they've got excess stock of winter fuel and don't have room for summer blends
until they sell all the winter fuel.

Oh I don't know if this would help or not, but I'm using one of the clay smith in-line regulators
that are typically used with Enderly Fuel Injection, I have mine at the back end of the carb so fuel
is constantly flowing. Let me link that here: LINK
You can see the brass body underneith the fuel log with the arrow
pointing in the direction of flow. Ideally they say, 1/2inch should be used each way
for best results, but I think it should be okay sending back through 3/8 inch steel line.

Question, aren't the bowls in these carbs suppose to close when the float level is raised?
How are your bowls draining of fuel? Is it sucking it back through? I guess you don't know.

Good luck.

BTW What does your Chevelle run at the strip?
 

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#15 ·
I watch it run out as the car is sitting there idling meaning the car is burning it and nothing is coming to the fuel bowl. it basically runs out of fuel with some still in the tank,and an hour later crank it over and the bowl fills quick, its been good for 3 years and all of a sudden it starts this, somthing has changed but I dont know what.
In decent air it goes bottom 10ns at 130mph with a 1.40 60 foot off of a foot brake, fun stuff.
 
#16 ·
I dont know that but I will ask tomorrow.
like i mentioned earlier its not hot here maybe 85 to 90 in the middle of the day, i have raced it in over 100 degrees with no issues. this crap just started last week, something changed. im thinking movable parts like fuel pump or fuel pump rod but i am needing a little insight before I start throwing parts at it, thanks.
 
#22 ·
Mark

I live up in Castle Rock. Have you come up with any additive solutions?
 
#20 · (Edited)
Mike,

I’m running the same Clay Smith fuel pump as you with the same regulator as Spike.

Do you have this problem while driving down the road, or just when idling?

Theoretically a return line should solve any vapor lock issues. Ask me how I know.

Couple of thoughts in addition to the other members suggestions...
check for loose hose fittings that could be sucking in air.

If your only vent is the gas cap try running with cap off. If you have another vent check to see if it is blocked. I had that problem too.

If it’s only happening @ idle it could be a volume/pressure or fuel pump starting to fail issue.
Check regulator and fuel filter for problems.

Also look at needle and seats in the bowls. Sometimes they get gunk up from ethanol and stick.

If your not running PTFE hoses check to see if the rubber is disintegrating inside from the crap fuel.

If all these check out good then I would look at the fuel pump rod and then the fuel pump itself.
 
#27 ·
Gerry any time I am running on the primary bowl and pull to stop it dies.
But when you plant it on the floor it acts like a ten second car that it is.
The secondary bowl still has fuel, go figure.
Believe me that in no fun having it planted in a 35mph zone doing about 80 and then getting off of it and having it die, trying to get it back to the shop.
And there is no rubber in the filter, that was one of the things i checked.
 
#21 ·
Another test you could do, if you need to...Get everything you need to quickly hook up a temporary feed can...can, short length of hose to connect to pump inlet with the correct fitting, and wrenches needed.

Bring it back to the garage and watch it run out of fuel....as soon as it does, disconnect inlet line on the car and connect the temporary can to the inlet of pump and crank the engine.....if it fills the bowl, something is up with the line or tank on the car or a vapor lock issue. If it won't fill the bowl from the temporary can, something is wrong with the pump.
 
#36 ·
Does your fuel contain ethanol? I battled vapor lock for years, threw parts at it, nothing helped. Changed fuel, problem gone! It can be hard to find gas with no ethanol these days, there's only two stations around here where I can get it.
 
#38 ·
Good to know Tks Im not sure I would have checked pump internally

I have same pump CS will send you parts to rebuild or you send pump in I have replaced my pump arm
I have 1/2 line no sock but an Aeromotive cartridge filter at tank
wow thats a ton of trash that got in there hard to understand where that came from
 
#40 ·
On both of my 70s I have NO fuel sock--BUT-- at the tank,,, The line to the engine I have a METAL ( in line) fuel filter. I change them both every spring. WHY?
My 454 & 468 have Q-Jets, SO WHAT! In my lifetime I have learned, Screwing with the filter inside a Q-Jet is not a good thing. So I keep my paws off of the carbs. and use a filter at the tank.
The Q-Jet on my 468 has over 45.000 miles, No issues so far. Also on both engines, I have an Electric pump at the tank. When I want to pretend I am racing John Force, I turn on the Electric pump. I was doing this in the 70s when I was drag racing. ( I ran two Bendix Electrics at the tank) A stock type fuel pump is fine for street driving. I have found out that high RPMs, Its like flushing a toilet.... The Q-Jet (or any Carb.) needs LOTSA FUEL.
Thats my thinking.
Bob
 
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