Heat soak/vapor lock problem

[CudaJon]

Hello to the group,
Have a 340 73 Cuda, every year a deal with heat soak in the carb due to vapor lock. My solution is I play with the timing and for the most part it does start after sitting for 10 minutes or longer in the heat of the summer. But every spring after the long sleep of winter, it comes back - maybe I have another problem??  This year I wanted to try a fuel filter with a third nipple for the vapor escape.  (I'm hook on Uncle Tony's videos) My question is this. I have a vapor separator as shown in the photo. My cannister is gone and was wondering if I could use one of the four holes in the photo to run back my connection from the vapor nipple of the fuel filter. That is cut one of the four hoses and plump in my vapor return line from the filter. Hope that makes sense. I'm not a mechanic so I'm hoping I'm not messing anything up with ventilation. Have not traced the line from the separator with the four holes to the front part of the engine, not sure where it terminates.  Any thoughts on that logic? Thanks for the help.

[Barracuda1]

Something that has worked for me is a Cool Carb spacer.  coolcarb.com
I never had a vapor lock issue again.

[SilverFrost73]

Hi Cudajon,

Is this the same problem you were dealing with last year? I thought the timing adjustment and ported vacuum fixed this issue for you?

[CudaJon]

Hi SilverFrost73

-Great memory!! Yes it is and like I said the problem all summer was gone. Honesty to god gone.  Until, I want to say, about late September that same year, I'm in New England and the temperature on average at this time is mid to high 50°F and the problem reappeared - exactly the same issue. I stowed it for the winter and now I'm dealing with it again. I wrote down the procedure exactly on the timing which I plan on during on a warmer weekend but was curiosity about using a fuel filter with a separate vapor line. That's my story.  Thanks!!! 

[Rich G.]

Unless you have something crazy under the hood I can't see you having a vapor lock issue on a 73/340. I don't think adding a different filter setup would do anything. How did you determine it was a vapor lock issue?

[CudaJon]

Fair question and no I don't have anything then a standard issue 340. So the car always starts up no problem when cold, meaning sitting over night. However if I was to go for a ride and bring the engine to normal operating temp and then shut down the engine, I could start it up within 5 minutes but if I waited longer I would have to put the accelerator to the floor and it would take a very long time if at all to start. Now from there if I open the hood and let stand for say an hour, I can get in and start it with no issues what so ever.  And as I say previously and with your advice by tweaking the timing I believe I can rid the problem (I'm hoping for the summer).  If it's not heat soak/vapor lock it sure seems to act like it.  Thanks!

[Rich G.]

A couple things you can check next time your in a hot situation. When you shut the car off , take the air cleaner off and look into the carb and see if gas is dripping down inside the carb. , that would be a problem. If that's good when you try to start the car and it doesn't, look inside the carb and see if gas shoots in when you pump the throttle. . I'd also check the fuel pump and see what kind of flow and pressure you get. This is all assuming you have a fuel issue and not an ignition problem. Also if you had a vapor lock issue if you shot a spray of carb cleaner in the carb it should start right up. Just a few things you might want to check.

[Filthy Filbert]

I had a similar issue years back.   The coil would heat soak and break down. Then it wouldn't run until it cooled back down.   

However one difference is that I would start backfiring and running rough when hot; then it wouldn't restart hot.   

[torredcuda]

Having to floor it to start could mean a flooding issue, possibly carb leaking fuel into engine when sitting.

[CudaJon]

Great feedback - thank you all. Will report back with my findings hopefully I'll get to play car this weekend. Thanks again!!

[chargerdon]

Todays standard 87 octane gas and yes also the 91-94 octane premium contains up to 10% Ethanol.   Ethanol raises the octane rating, but, also evaporates quicker than pure gasoline.   

That doesnt matter at all in modern cars with sealed systems and fuel injection, but, in our older vented cars with carburetors it causes the exact situation that you mention.   I.e. Car runs completely normal and then when you shut it off and it sits the residual high heat off the engine causes the fuel in the bowl to "boil off".    So, after 1 or 2 mins the car will start right back up, but, about the 5 min mark, enough of the fuel has boiled off causing an extra rich mixture in the intake manifold that will "flood" the plugs.   Wait 15-20 mins or longer and the plugs are no longer flooded, but now the fuel is gone.    THIS IS HOW IT IS FOR MANY OF OUR OLDER HOT RODDED VEHICLES.   

Solution, well if starting after about 5 mins, simply hold the throttle wide open to allow air to clean out the plugs and recognize you might have to crank it for 5-10 seconds before it fires.    If you wait more than 15 mins, pump the throttle at least twice before cranking.  I.E..live with it.    I have the exact same problem on my 66 Charger and my 74 Challenger.     

Likewise if the car sits for more than 3 days without use, i will have to crank it for about 3-4 seconds, then pump it couple of times and then crank another 2-4 seconds and give it another couple of pumps, then crank again and it will start up.   Why? Because all of the fuel in the carbs bowls have evaporated, and you have to crank it to get fuel into the bowls and the pump accelerator to get it into the engine.   

[SilverFrost73]

Hi SilverFrost73

-Great memory!! Yes it is and like I said the problem all summer was gone. Honesty to god gone.  Until, I want to say, about late September that same year, I'm in New England and the temperature on average at this time is mid to high 50°F and the problem reappeared - exactly the same issue. I stowed it for the winter and now I'm dealing with it again. I wrote down the procedure exactly on the timing which I plan on during on a warmer weekend but was curiosity about using a fuel filter with a separate vapor line. That's my story.  Thanks!!!

I hope you get this figured out. I've also been worried about this happening to my car. I agree with what both Torredcuda and chargerdon said. The first thing I would check is the carb boiling over and flooding. I also agree with the evaporation of ethanol fuel. I would only run non-ethanol and see if that helps your issue. If that doesn't solve the problem then I'd starting checking ignition components. Hope this helps

[torredcuda]

I run 93 octane and we only have 10% ethanol available around here but I do not have a hot start problem since I rebuilt my carb. It does take a bunch of pumps to start after it`s been sitting for a few days but then starts easily cold, warm or hot.

"about the 5 min mark, enough of the fuel has boiled off causing an extra rich mixture in the intake manifold that will "flood" the plugs."
Maybe I`m not understanding this correctly but how does fuel boiling off, i.e. evaporating, cause a rich mixture?

[Filthy Filbert]

I run 93 octane and we only have 10% ethanol available around here but I do not have a hot start problem since I rebuilt my carb. It does take a bunch of pumps to start after it`s been sitting for a few days but then starts easily cold, warm or hot.

"about the 5 min mark, enough of the fuel has boiled off causing an extra rich mixture in the intake manifold that will "flood" the plugs."
Maybe I`m not understanding this correctly but how does fuel boiling off, i.e. evaporating, cause a rich mixture?

All that fuel vapor ends up in the plenum, air filter, etc.  when you crank, that fuel vapor heavy, low oxygen air sucks into the engine. 

[Xghobo]

A friend went through this and tried all the things mentioned here. It ended up being the fuel pump rod
Go figure