Gas Boiling out of Carb???

[Ryan536]

Hello, I just joined this site and it already has been very helpful in answering a few questions I have had. I bought my 1973 Vette basically the way it looks but over the years it has been owned by racer wannabes and the cobble fairies.
One problem I encounter is that the car starts up fine when cold but after it runs for awhile and I shut it off it won't start again. It turns over and over but it won't "catch". Eventually it will start, say after it turns over 20 times and some pumping the gas pedal or holding it to the floor.
I have searched this site and everyone seems to say its a starter problem. I have noticed on my car that the wire from the block to the frame looks "green" and a little corroded so I am going to replace it.
I was told by some mechanics that the gas is "boiling" out of the carb. I've never heard of this. The car has a RPM Performer intake and a Edelbrock 750cfm carb and a mild cam. It also has headers and the usual go fast goodies. It's a 350cid with a 4 speed manual. I know the gas pump is the original.
Any help would be much appreciated!
ThanksView attachment 14437

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[MikeMc71]

We had the same typr of problem with my wife's '71. About the same set-up as yours, except its a 383 Stroker. I would suggest going to the Edelbrock web-site, they have some very good write-ups on tuning and troubleshooting their carbs. I have done things like backing off the choke, I believe they are too closed when stock. I have also replaced the metering rod springs ( because of the cam) to lighter than stock. There was jsut not enough vacuum (because of the cam) to hold the rods closed at idle, and the engine would "load up" at idle.

We still have a problem when its real hot (100 deg plus) out, it seems the gas does tend to evaporate out of the carb, the electric fuel pump helps that a lot!

 

[SIXFOOTER]

Well, you need to know for sure that the fuel IS boiling out of the carb, or disapearing from the carb for whatever reason. Next time out, before restarting when it usually does not start, pull the air cleaner off and look at the carb, see if the choke is all the way open. If it is then look into the throat and operate the throttle, gas should shoot into the throat, If it does not, then the bowl is indeed empty. This can be for a couple reasons and you will need to go thru them. You can put on an isolator gasket, its a thick phenolic or wood base gasket that thermally isolated the carb from the intake. Prevents the carb from getting as hot. You can put on a heat shield, it is a sheetmetal shield that prevents thermal radiation from the intake to the carb. An aluminum intake also helps. Last piece I know of is the fuel pump, it might be bypassing and allowing the fuel to be siphoned back to the tank.
On your year, I don't know if it has a return hose or not, if it does, use it.

 

[Ryan536]

Thanks for everyone's help, not even one day later! Awesome. Next time it happens I'll try what SIXFOOTER suggested.

 

[Vettehead Mikey]

I can guarantee you that it's not your starter motor. If it turns over (cranks) at a reasonable speed, it's got nothing to do with your issue.

Regarding gas 'boiling' and emptying your carb to explain hot start problems, I have a real doubts. It's true that today's gas is more susceptible than gas from the 70s, but it just does not evaporate that quickly.

You mentioned that you have to hold the pedal to floor to get it to start. More than likely your engine is flooding while it's stopped. Take off the air cleaner (as suggested above) while it's stopped and look down inside. If there's gas dripping or everything is wet, there's your problem.

Too bad the previous owner got rid of the stock intake and carb, I would bet you would notice no decrease in performance with them and certainly wouldn't be putting up with the present issue.

 

[SIXFOOTER]

Agreed, thats why I said to see if the choke is full open. If its not it will cause the flooding issue and the way to start a flooded motor is hold the throttle full open

 

[6880 Mike]

I'm in camp with Vettehead and Sixfooter, sounds like a choke issue to me.

 

[Hib Halverson]

Others have suggested making sure gasoline is actually "boiling" out of the carb after you shut the engine off. You definitely need to do that.

If that is the problem, it's called "percolation" and about the only way to stop it is put a heat insullator between the carb and manifold then make sure no fuel lines are touching the intake manifold the heads or the block.

Now, you didn't say which Edelbrock four-barrel you have. They make two 750s. One is a clone of the old Carter AVS and the other is a Quadrajet clone. If it's a Quadrajet clone, you want to make sure the well plugs at the bottom of the float bowl are not leaking.

Lastly, you make note that the car only starts after you either pump the throttle or hold the pedal down. Interestingly, if you have to pump the gas to get the motor fired, that usually means the engine is not getting enough fuel during cranking. If you have to hold the gas on the floor (ie: clear flood) that means the engine is getting too much gas. It would hard for the engine to lack fuel in cranking and be flooded at the same time, so you might want to more carefully identify the symptoms because the procedures to solve those two problems are vastly different.

 

[Ryan536]

I guess I have to add a little more information here. The car came with a broken Holley carb and a Torker "drag racing only" High Rise Single Plane Intake. Nothing but problems so I bought Brand New a #1407 Performer Series 750 carb and the Edelbrock Performer RPM dual plane Intake beacause on Edelbrock's website they say its the perfect match for what I want. The cam is a Crane Cam #100072. The car goes like Hell, no problems there.
Today I started it up and and let it run for 20 minutes, up to operating temp. Of course now it fires up everytime. I let it sit for 5 minutes and I just had to wave the keys in front of it and it started. The carb is dry as a bone inside and when I work the throttle and it shoots two nice streams of gas. The choke is fully open. I'll try it again tomorrow!
I also have to clear up the pedal pumping quote. When the problem occurs it's always at the worst time. Like when I am trying to leave a super busy gas station or I'll stall it at an intersection. (It has a racing clutch which isn't too forgiving or I'll do a third gear take off by mistake, it also has the Hurst short throw shifter). So I start cranking it and it turns over and over. Then I get mad and start pumping, sometimes it works sometimes it doesn't, then I mat it and eventually it will go.
I also have to add the old points distributor has been replaced with an HEI Distributor. The distributor was rebuilt and a all new cap, rotor, Module, and Mag pickup, The spark plugs and wires were also changed this year. The battery is brand new. The only thing that isn't fresh or new is the coil. A friend suggested that it could be breaking down. I also noticed some cobble wiring from the starter to the motor. The big wire from the frame to the motor looks a little corroded as well.
Like I said before it has been owned by racer wannbes and cobblers. I'm working out all the bugs slowly and surely. I love the car and I know it'll be a blast once a few gremlins are gone.
I hope this new information helps.
Thanks for all your help!!!!

 

[Ryan536]

After re-reading all your great advice and tips I remember that the carb came with one of those isolater gaskets. I have tons of hood clearance so I might just as well put it on. Can't hurt, right?

 

[Hib Halverson]

I'd run one of them.
In fact, if you have hood clearance, there are several aftermarket companies which make a sort of "superduper" heat isolater which is a two carb gaskets with an aluminum head shield sandwiched between them.

Now that you give a more detailed description of what happens when it stalls, I tend to think you don't have any problem with gas "boiling" or anything like that. Without seeing the car, I think the combination of stalling and hard starting might be related to calibration of the carb itself.

How's the engine run at idle?
How's it run just off-idle?

when you very slowly open the primaries a little ways, does the engine run smoothly or, just off idle does it start to run rough?

 

[Ryan536]

I might have to buy the superduper isolater because the one that came with the carb is only 1/8" thick.
When the motor is cold it likes to snap and pop a little but it goes away after a few minutes. It runs smooth but lumpy because of the cam at idle. When you give it a little gas there is no hesitation it ramps up and down perfectly. Give it a quick shot and it goes. Exactly what you would expect.
Now I might have the final piece of the puzzle here. Before I changed the distributor from points to HEI it has backfired through the carb lots of times. I contacted Edelbrock Tech Support to see if that would hurt the carb but they never replied back. On their website they claim, quote,"They are unaffected by engine backfires, which means that there are no power valves to blow out,..." And so on.
Thanks again!!!

 

[GerryLP]

Ryan,

I am not too familiar with your Edelbrock's carb model (specially the Carter-like one), but if there is percolation on your carb, then it could be related to a relatively-lean mixture. An engine running a relatively-lean fuel mixture runs hotter. The backfires could be evidence of this. Also, if the engine radiates more heat than usual, then this could also be explained by a relatively-lean mixture as well. The stalls in the middle of the intersection or pulling from the gas station could be caused by a float incorrectly set.

So it may all be caused by a lack of adjustment, like Hib stated, on your carb.

It would be worth your while to ensure that the carb is set correctly.

That quote from the web site is perhaps true (in that the Edelbrock is impervious by backfires), for the Holley used to be infamous for failing power valves after backfires. A failed power valve basically delivers power setting added fuel all the time when is blown.

 

[SIXFOOTER]

I'll stick with my original diagnosis. If after 20 minutes of idling the choke is full open thats good, but you still have not achieved the heat soaked condition you will after a 20 minute drive and a 10 or 15 minute sit. Your on the right track tho

 

[Red69]

I guess I have to add a little more information here. The car came with a broken Holley carb and a Torker "drag racing only" High Rise Single Plane Intake. Nothing but problems so I bought Brand New a #1407 Performer Series 750 carb and the Edelbrock Performer RPM dual plane Intake beacause on Edelbrock's website they say its the perfect match for what I want. The cam is a Crane Cam #100072. The car goes like Hell, no problems there.
Today I started it up and and let it run for 20 minutes, up to operating temp. Of course now it fires up everytime. I let it sit for 5 minutes and I just had to wave the keys in front of it and it started. The carb is dry as a bone inside and when I work the throttle and it shoots two nice streams of gas. The choke is fully open. I'll try it again tomorrow!
I also have to clear up the pedal pumping quote. When the problem occurs it's always at the worst time. Like when I am trying to leave a super busy gas station or I'll stall it at an intersection. (It has a racing clutch which isn't too forgiving or I'll do a third gear take off by mistake, it also has the Hurst short throw shifter). So I start cranking it and it turns over and over. Then I get mad and start pumping, sometimes it works sometimes it doesn't, then I mat it and eventually it will go.
I also have to add the old points distributor has been replaced with an HEI Distributor. The distributor was rebuilt and a all new cap, rotor, Module, and Mag pickup, The spark plugs and wires were also changed this year. The battery is brand new. The only thing that isn't fresh or new is the coil. A friend suggested that it could be breaking down. I also noticed some cobble wiring from the starter to the motor. The big wire from the frame to the motor looks a little corroded as well.
Like I said before it has been owned by racer wannbes and cobblers. I'm working out all the bugs slowly and surely. I love the car and I know it'll be a blast once a few gremlins are gone.
I hope this new information helps.
Thanks for all your help!!!!

You have plenty of good suggestion to help trouble shoot your problem. I can't help but add my 2 cents. I once had a problem with starting and having my 69 run properly after replacing a starter. It turned out that I forgot to tighten the frame ground end of the engine ground cable. Sometimes it's the simple things that get you. You mention your engine ground wire being corroded, so thought I'd mention it. A couple weeks ago I started my 79 to leave for the track, and the GEN light comes on. I pull the boot off the ALT (red wire) and uncovered corrosion. I quickly cleaned the wire, installed a new terminal and was on my way with the problem solved. You might like to check your ground cable if the carb insulators don't help.

 

[Ryan536]

I have ordered a new engine/frame cable and a new negative battery cable (I didn't like the look of it either) from Ecklers. Should be here in a couple of weeks. I also ordered the Edelbrock Isolator plate. It should be in next week.
Thanks for the help!!!

 

[TMPJoe]

X2 on getting the carb adjusted properly... the popping on the initial startup... also stalling when letting the clutch out and then hard starting.. could be the low idle adjustment..

where are you idling that cam?