vapor lock

[anthony a zappone #2624]

On very hot humid days (90 plus) I am having a hard time with fuel on both my 41 and 47.  (on my 36 Pierce as well)  Lousy idle, bucking on hard acceleration, etc.  I have electric fuel pumps on everything, however they dont seem to push the fuel through as it did before.  I notice that the crossover pipe on the exhause manifold is merely a couple inches away from the intake line on the back of the carb, and the carb itself.  Has anyone put some insulating element between the two to help the problem?   Do most people think that this is where the problem is occurring?  Tony Zappone #2624

[Bill Ingler CLC 7799]

Hi Tony- You did not say what type of electric pump you have on your cars. The small A/C pulse pumps are famous for failing. They sound like they are working but in fact the additives in todays fuel ruin the thin rubber diaphram so they sit and click away doing nothing. The majority of the vapor lock problem probably occurs between the tank and the engine in the fuel line but most likely in the intake side of the engine driven pump. The pump draws fuel using vacuum, vacuum means lowering pressure to draw the fuel, put in high temps and you have the makings for vapor lock. The more restriction in the line to the tank the harder that engine pump has to work to pull through an electric pump or even a inline filter. The harder that pump has to work to get fuel the more chances for a vapor lock. What clears a vapor lock or prevents a vapor lock is fuel pressure and the only place in the original fuel system of the flat head CADS that is pressureized is the line from the engine fuel pump to the carb. If that engine fuel pump is getting adequate fuel from the tank then the pressured fuel it is sending to the carb should prevent a vapor lock in the pump to carb line.Some have found insulating that line helps. I have never found in my cars that insulating any lines helped prevent vapor lock but what I have found that if you use the proper electric pump with a one way check valve around that pump so that the engine pump draw is not being restricted, my vapor lock problem have gone away. I live in Phoenix and 80-95 with and occastional trip to the high country or across the desert has tested the car for vapor lock. One place that can also help to cause an inadequate fuel supply to your engine pump is the flexible rubber hose from the fuel line to the pump. In order to try to retain the originality of the car, repop flexible rubber hoses have been made.Theses are great for judging purposes but questionable for touring.The tank to pump line is 5/16 but if you cut in half one of these new repop flezible lines you will see that the fuel passage has been reduced through to the hose to about 1/8. For touring a 5/16th rubber fuel hose works just fine. If you really want something to do and read about what has been written about vapor lock go to the top of this web site page and click on search and then type in Vapor Lock. That should keep you busy. Havn`t see you since we were on the Finger Lakes CCCA CARavan.  Have fun

[Russ Hobden #1235]

Since all three of your cars are acting up in the same way, Could it be the fuel?  Ill bet you are buying gas for all your cars from the same place.  Try other brands and grades and see if that helps.  As the previous reply mentioned, gasoline companies put in all sorts of additives in todays fuels and it always isnt adequate for our old engines.  Russ Hobden

[John Washburn]

Anthony,

Remember, the lower the octane, the lower the boiling point. So in the hot summers use the lowest octane, since it has the highest boiling point (which is what vapor lock is, boiling), and these old cars were running on much lower octane when new.

There is also a great website, by, I think, Chevron, on gas. I think I picked it up on this web site. So as mentioned earlier, a search under vapor lock should get you to it.

Oh, the proper gas line hoses are called fuel injection hose with the designation 30R9 only, nothing else. Dont use what the folks tell you, because they might give you 30R7 hose which will degrade with todays gas....

Good Luck

John Washburn
CLC #1067

[Bryson Talamini 21505]

Tony, Ive experimented a lot on this over the last 2 yrs. Further to what Bill says modern fuels vapourise in the engine pump more readily but you can overcome this by reducing the suction load on the engine pump. I increased the line from the tank to 3/8 ID, giving 50percent more area. Full throttle up hills at 100F no worries. But extended idling at 90+ causes boiling in the pump, see the bubbles in the glass. So I fitted under the LH chassis rail an Airtex 6V rotary pump [12v available] model E8011 , retained 3/8 on its suction but reverted to 5/16 on the discharge. However you end up with too much pressure at the carb inlet so I removed the 2 disc valves in the engine pump and now get 4 psi at carb, just right. Problem all gone. But I still havent solved the problem of hard to start after switching off for 5+ mins when hot. You can watch the fuel dribbling from the main jets & down the carb throat making the mixture too rich. Its because the heat from the adjacent exh manifold boils the fuel in the line & the increased pressure forces the float valve open. [The level is correctly set]. Ive insulated the line from the engine pump [as was done in the factory] which helps a bit & now I might add a heat shield on the crossover exh manifold, but it then wont look original. My engine never exceeds 180F.
Bryson in Oz