Hot Rods Vapor lock/percolation. Its too damn hot

Morning guys
Ive got a 50 pontiac with a 1968 pontiac 350 under the hood. Ive put about 1500 miles on this setup and in this heat I've had a problem show up. Car doesnt over heat and runs 190 most all the time. But I have an afr gauge on it and when I sit in traffic at idle its starts to run more rich the longer I sit there. Im sure it would vapor lock if I gave it enough time to do so. Ive insulated the fuel line and that helped some. I need to add a phenolic spacer. The carb is a 500cfm afb carter thats been rebuilt.
Any advice on dealing with this issue? Should I run an electric pump? Should I run a return line?
I dont want to go with that 3 letter word thats not allowed here. I just dont want to worry about sitting in traffic either. Thanks

 

I just ran into the same problem with my '62 Mercury on Monday afternoon (ambient temp was about 97, and humidity was "gross" or typical Missouri humidity). Stop and Go traffic led to a couple instances of misfiring while accelerating away from the lights. Once I got a few miles down the road and some air flowing through the engine compartment, no more issues. This was the first time, I have ever had this issue come up, and also probably the first time I have driven the car in these temps.

Long story short. The most effective way to cure this problem is to lower underhood temps. More air flow, louvers, etc. Phenolic spacers definitely help, but can lead to clearance issues in some applications.

 

It’s interesting yours is running rich. Weirdly enough I think I can explain that as mine was doing something similar at the weekend. I thought it was vapour lock, as in out of fuel in the float bowl, as it was tricky to start and a few primes on the throttle didn’t help. I could hear the fuel boiling in the carb but when I looked down the carb the boiling fuel was forcing its way out of the venturi and it was flooding the engine rather than no fuel. Haven’t fixed it but maybe that will help.

 

On my 442 I removed the fuel line from the carb (double pumper) and installed it backward, facing the firewall. Re-routed the hard line up the firewall rather than to the front of the engine, put an electric pump in the tank and never had another issue. That was 10 years or more ago. Still running like a champ. That fuel line up the front of the engine on a Pontiac should at least be insulated with a sleeve like this

 

Billy. I have the insuation like that from the pump to the carb. But as you know on a pontiac that fuel line is right over the water pump so Im sure that doesnt help. Ive also got the heat crossover blocked on the manifold. Its an aluminum edelbrock manifold. I have thought of adding an electric pump in place of the mechanical and running the fuel line like you suggest. May try that soon. Thanks

 

I guess I’ll show how dumb I am.

I think term vapor lock is overused.
I think there is a lot more going on than the very mysterious “vapor lock”.
Hey, got clothes pins?

There can be many factors in hot weather but lets look at......
Electrical
Yep.....
How does the saying go? Batteries really die in July but they are buried in January.
I believe hot weather is much harder on a car than cold weather.
If there is a weakness in the electrical system, it will show up in hot weather.
Things to look at......
Connections!
Make sure all connections are clean and well done. Pay special attention to the main cables and grounds.

6 bolt cars to me seem to be cars but only more so, meaning they seem more sensitive.
In hot weather the starter is more apt to drag wow......wow...............wow.
It’s more apt to be “rich” and prone to flood.
Ok....
How can you tell when you have a bad ground or poor connection? It gets hot.
When does a starter or any electrical motor start having problems? When it’s hot.

Do you see where I’m going with this?
One reason for being rich may be poor spark. Being hot the electrical system is at a disadvantage.
It’s something to consider.
Check connections. Make sure the wire is a big enough gauge. Make sure there is enough ground.
I’m n hot weather the electrical system needs all the help it can get.
It’s something to consider.

Emergencies..........
Years ago my ‘50 would not start after after a 40 minute trip in hot weather. The solenoid was as hot as a fire cracker.
I poured some cold water on it and it immediately started and I drove home.
I was a kid then.
If you find yourself in a parking lot and the car will not start......
Some wet rags as heat sinks on the solenoid, distributor, battery, coil and starter (if you can get to it) may be a good idea. Just don’t forget bout them.

 

Heat is a killer as well as cold. One thing we didn’t worry about was our gasoline blend in the 50-70’s but today we need to factor that in. I would definitely add a phenolic plate under the carb. They do help. I also believe in good air movement thru the grille and into the engine. I have a 6 blade fan and a pulley change to make sure there is coolant flow thru the radiator when idling at a light…

 

The afb carb is prone to boiling fuel in the fuel bowls, after running the car up to temperature shut it down and open the hood there’s a good chance that you will here the fuel boiling in your carb. An electric fuel pump can help with startups but the boiling is a symptom of today’s fuel.

 

Go with a return line to the tank. Pontiac had a filter with a return line port on it in the 60’s. Cad had a return line on the fuel pump on air conditioned cars in the 70’s. Then fuel doesn’t sit and get hot, it’s always moving cool fuel from the tank.

 

33041 is a part number for a 3/8" inline fuel filter with a 1/4" return hose barb on it.

The cause is the fuel today, formulated to work with with EFI systems at 50 psi, but it boils in carbs at ambient pressure when the underhood temperature gets too high.

Lots of good suggestions here, good luck with it, I feel for you

 

If the engine isn't a show piece, you could try the carb gasket that is centered in a big metal heat shield that reflects the intake heat away. The phenolic spacer is great idea, I think the return line to the tank will be your most potent medicine, though since you've already done the spacer. Maybe block off the egr? Oh, double check your timing and timing chain slack just to rule out retarded timing throwing a ton of heat into the ports.

-rick

 

Thanks for all the replies. This is a engine I refreshed. Probably 1500 miles ago. New rings/bearings/cam/timing chain.
I think the main problem is the fuel getting too hot. I think I'm gonna go with a return line and electric pump. I'm gonna order a phenolic spacer too. No egr on this one but the exhaust crossover is blocked. Thanks again

 

"today's fuels" Had to add a phenolic spacer to the 331 Hemi I built in '10, would crap out at stoplights in hot weather. Didn't need one on the one I built in the mid-late 90s.

 

I've read lots of complaints about defective mechanical and electric fuel pumps on a lot of forums; just about all of the complaints are about fuel pumps made in China. unfortunately, just about all of the new pumps are. If you have an original American-made pump, hold onto it. You probably will be better off rebuilding it instead of buying a new Chinese pump. If you don't have/can't get an American-made pump, look for one made in Mexico or South Korea.

You might also consider installing an electric pump and installing an on/off switch. You can use that pump to prime the carburetor bowls. If your car sits for long periods, the gas (particularly/w ethanol) might be evaporating.

 

Your plan should fix the issue. I also run non ethanol fuel. I've found it has a higher boiling point.

 

Without the ability to do a hands on diagnosis, I can only offer some things that can help with this. Your cause may vary.
If there is a heat riser between an exhaust manifold and the exhaust pipe, make sure it isn't stuck and operating properly. Better yet, just get rid of it. Float level too high. Big cause. There should be a little valve somewhere in/on the carb that opens the fuel bowl to the atmosphere when the throttle is closed. That lets the vapors vent to keep from pushing gas out the main jets and flooding the engine. Proper fuel level and the little anti percolation valve are key to keeping from having this problem, or it was when gas was gas. If there is a heat passage in the intake manifold under the carb, try finding a way to block it off. A cold air intake setup helps, too. Use a 160 degree thermostat in the summer. Any way you can find to get under hood airflow will be a big help. The problem you have has been around forever but it is fixable. Ethanol in the gas? Ethanol boils at 170 degrees.
good luck

 

As said above a return line to the tank works real well. I use an electric pump with a bypass regulator so the fuel is in constant circulation. One other thing that I don't see mentioned often is blocking off the exhaust crossover that heats the carb area. I block mine as it heats up well enough with the rest of the engine and it never seemed to be a great idea to fry the carburetor on a well built engine.

 

If you still have a spreadbore intake manifold on it, Find a GM style linkage Carter Thermoquad along with the thick insulator gasket. I've been using these Thermoquads for damn near 50 years and I never have vapor lock. There are always a couple on Ebay, find a nice looking one that says GM linkage, put a kit in it and enjoy! The big spreadbore moan will put a smile on your face every time you dip into the secondaries. PM me if you like.

 

This problem has been going on for years, so I'm not so quick to blame "today's gasoline" as the biggest culprit, though I'm sure it doesn't help. I was chasing the same problems on my '55 back in the '90s, so it's probably more an issue of increased underhood temps compounded by lack of air movement, than the formulation of today's gas.

First, I would check your lines and be sure they're not touching ANYTHING that might be heat-soaking them while also checking to see if they can't be moved further away from any heat source (engine block, intake, water pump, heater hoses, etc).

Second, I like the idea of a Phenolic spacer as anything you can do to isolate the carb from the heat source (intake) will help.

Third, adding a return loop in the fuel line would keep the fuel moving at idle, bypassing the carb and preventing it from sitting still in the engine compartment and heating up. The return doesn't need to go all the way back to the fuel tank (though that would probably be ideal), you could "T" it off the carb and then "T" it back into the feed before the fuel pump.

Fourth, do something to either increase the airflow under the hood or increase the scavenging of idle air under the hood to reduce underhood temps. Examples of this would be a better fan (mechanical or electric) or adding louvers/removing inner fender material.

One thing to consider, when they sold these cars new, they didn't suffer from these problems. I don't believe the new pump gas is 100% to blame. It's because WE change things without the thousands of hours worth of engineering that the OEs used. Your '50 Pontiac as it rolled off the dealer's lot likely idled till it ran out of gas without vapor locking. But then something changed and it's the addition of the 350.

Just had a second thought, if it's getting richer the longer it sits and idles, it sounds like it's loading up and that's a carb adjustment issue, not a fuel starvation/vapor lock situation. On my '55, when it could start to vapor lock, it would start to stumble like it was running out of fuel (which it was) and I would have to drop it in Neutral and increase the revs (to get the fuel moving) to keep it from dying. That's your typical vapor lock/starvation scenario. I think your issue is more related to "loading up" due to an incorrectly adjusted/tuned carb.

 

Yup! I have one on my 351W and it works like a charm in all conditions. The moan when you open it up sounds like it's trying to suck up pets and small children in the area. I run ethanol free and changed out the x rings in the primary wells with ones that I made out of some bulk Viton B material so no more replacing the x rings what seems like every two years.

 

Has anyone running a mechanical fuel pump on the block compared the fuel temperature some distance before and after the pump? The pump sitting on the hot block (even with some insulation between) seems like an excellent place for the fuel to be heated. Especially at idle when the flow is very small.

An electric pump would let you bypass that heat source, halving the number of places where the fuel system has to be in contact with the hot engine (the remaining one being the carb). Maybe making a big difference, maybe not, but it's a relatively simple change.
A return line (all the way to the tank preferably) increases the flow through the system especially at idle, that reduces the time the fuel can absorb heat in the fuel line and the heated fuel in the line is constantly replaced by cooler fuel from the tank.

All that reduces the temperature of the fuel reaching the carb. Then you also have the options reducing the heat transferred to the carb, such as phenolic spacers, heat shields, and perhaps an air duct to feed the carb cool outside air rather than the hot air around the carb - the air drawn through the carb cools it internally, the cooler it is, the more good it'll do, feeding it hot air heats it up and makes the problem worse.

 

Pontiac used different pulleys for A/C cars and Non-A/C cars, you want the smaller A/C pulleys. Not only will they spin the water pump faster, the fan also spins faster, moving more air. Non-A/C pulleys are about the same size crank and water pump, A/C has much smaller water pump pulley and little bigger crank pulley. You also need to run as big of a fan as you can with lots of blades to keep the air moving. I have some older Pontiac parts book if you need pulley part numbers or need yours looked up.
If you go with Ram Air tubes, shorter the better, non-metallic, and pull air from the side of radiator. The tubes will heat up making them useless if too long or in the direct heat of the radiator.

 

LOL! I like " that three letter word" . I have it on my 1950 Buick. Never the problem you mention. In the mountains or in the desert. But I understand. To each his own.

Ben

 

All good suggestions but also running the highest octane you can get helps. We can get 93 with ethanol down here. If you can get non-ethanol fuel that helps as well.

 

According to info Summit published cyl wall wear is 3X higher with a 160 vs a 180-195 range one, plus a greater chance of sludging.

 

recently acquired a '65 T-Bird with 390. it has the boiling gas in the carb problem that makes restarting motor again when shut off after driving a while. Ford was aware of problem and tried to address it by using a spacer that has motor coolant tubes running through it. it also has a 5 degree wedge shape to address the angle that the motors were set at. owners of Birds have tried not running the factory spacer and using a phenolic with limited results. be sure that spacer is not made of plastic. one guy made one out of treated wood. changing carburetors has helped some a little but, still boiling gas at times. Mr Gasket has 1/4" and 1/2" thick rubber composite gaskets. just need to have tall enough bolt down studs to use if stock manifold. aftermarket manifold studs can be changed. I have just switched from the large canister air cleaner housing to a chrome 14" with recessed base and 2" filter. hopefully this will allow more air around and through the carb. will be adding Thermo - tec tubing heat shield covering on fuel line, etc. still open to ideas.

 

some of the Corvette guys run the fresh air tube that goes to the heater, to the carb instead. So it blows outside air on the carb. with a computer fan and a switch to make it work when you're stopped.

I tried this with limited success.

 

You want to think about insulating or putting a heat shield between the fuel line and the exhaust where it passes close such as going past the manifold and lead pipe. I had an exhaust leak so it was extreme but the gas was boiling in the line before it got to the fuel pump. That cost me a tow bill for truck and the 25 ft boat I was towing with it.
I went through a cooler of ice water with vapor lock with the 48 pulling a trailer up a long grade in Utah in 1977 on the only other time I ever had one vapor lock in 62+ years of driving. Once we got over that one grade we didn't have any other problems the rest of the trip.
I'd be under the car looking for any place that the fuel line was a bit too close to the exhaust or other heat sources and figuring out how to shield it from the heat.

It's spendy but non ethanal premium might help a bunch in hot weather too. Depending on how much you drive the car that might be the less expensive fix or at least worth trying for a tank or two.

 

Guys thanks for all the suggestions. Just to put it in 1 post. I've got an aluminum intake with the heat crossover blocked already. There is no heat riser in the exhaust manifold. I've insulated the fuel line and I have it ran the way pontiac did from the factory.
It's definitely boiling the gas in the carb causing a rich condition as seen on the afr guage.
The carb isn't loading up because of a needle seat issue or too much fuel pressure cause this only shows up after idling in this extreme heat were having. No problems starting or restarting.
I'm gonna install a electric fuel pump back at the tank and run a new fuel line to the engine.
I have a 18" 6 blade derale fan on there with a shroud. The car doesn't overheat.

 

I worked in a Pontiac dealership in Texas in the mid 70's in the hay day of those 350's and never heard of over heating problems with any that had good clean cooling systems.

They don't add to the looks of things but there are a number of carb heat shields available from the speed trinket vendors similar to this one.