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1921 Touring Vapor Lock

Guest DB21tour

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Guest DBtouring

I need some help in diagnosing my vapor lock problem. Here is what is happening; I take her out for a drive and for the first 10 miles, she does fine. Then it seems like it starts missing a little and within a mile I start having vapor lock. The gas is actually boiling in the bowl on the carb. I can hear it. I take the top off the bowl and what gas its left in there, you can't touch it is so hot. I let that cool off and drive home, same thing happens when I get almost home. I have done this twice now and same thing happens. Temperature outside is 80 degrees at the most, and the road I drive is an open road, not a lot of stopping and starting.

I have checked, and double checked the timing, setting it as descibed in the Blue Mechanics Manual, checked the compression, around 45 on each cylinder, using the hand crank. Checked the plugs and points. Checked for fire to each cylinder. Engine, at least by the moto-meter is not getting hot or boiling over.

I'm getting frustrated and don't know what to check next. Has anyone else ever had this problem? Any advice would be greatly appreciated. If you have any questions for me, feel free to ask those also.

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Just curious, when you say what gas is left in the bowl is hot; just how much is left? In other words I am wondering if you have a fuel feed problem rather than a vapor lock.

Yes the fuel in the bowl will be hot after you stop the engine due to heat soak, not necessarily that hot whislt fuel is flowing through it with the engine running; of course in the time you take to remove the carb top some of the bowl contents will evaporate.

As to the engine temperature, can you hold your hand on the bottom radiator hose with the engine running after you have done 10 miles? if not then your engine is definitely running too hot despite what the moto-meter is telling you. If you can, then temps are OK and unless you have an exhaust leak near the carb then vapor lock is probably not an issue whilst you are moving.

Chris H

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Guest DBtouring


When the car starts to miss and sputter, I know the vapor lock is starting and I pull to the side of the road, open the hood and can hear the fuel boiling in the carb bowl. It doesn't take more than a minute to pull the top off, after I stop, and feel just how hot the fuel is. Seems like when I pull the top off, it is about half full.

If it doesn't rain, I will try to drive it tomorrow evening and see how hot the bottom radiator hose gets after about 10 miles. I really haven't checked that, but the radiator is full or almost full and it has never boiled over - out the overflow.

If it does rain, I'll start taking the vacuum tank apart and checking it.

One other time, a couple years ago, I had a problem with vapor lock and found that the insides of my muffler had come apart and was blocking the exhaust to a large degree, causing the engine to build up extra heat. Just to be sure, I have pulled the muffler and tail pipe and they are clear this time.

Also on Saturday, I drained the fuel tank and filled it with new fuel to make sure I wasn't running old fuel.

Also, yes I am using the vacuum tank etc. and haven't had any problems with it thus far.

Thanks for the advice on things to check, this is what I am looking for.

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James, this is what JFranklin posted in the Model A forum:

"I just had my first tank of E-10 and My Model A started to run poorly so I opened the hood and heard the gas boiling in the carburator. Has anyone else had this happen? My car does not overheat or boil the radiator which is what I first thought I was hearing."

Maybe this is what is getting you too. I talked to Vern last night and discussed it with him as well. I did a little research and found ethanol boils at like 87 degrees.

Try going to a marina and get 5 gallons of boat gas which most likely is all gasoline still (check first) and see if this makes a difference.

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DB - while I have been as loud a critic as any concerning deathanol; your problem sounds more like ignition, specifically coil.

A bad coil can function perfectly when cold; then short out and cease to function when it gets hot. Possibly you can insulate around the coil, or direct some coiling air at the coil for testing purposes.


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