Misses and backfires through exhaust

[barkleydave]

OK I have had this symptom happen on warmer days after stopping warm engine and fueling. Does not always happen.

* Warm engine stop for fuel for 10 min or so. Truck running normally.

* Start up and start running... no power misses and backfires through exhaust. Within a quarter mile of nursing engine returns to normal and off we go.

Truck runs very smoothly, easy startup, GAV run at 1/4 + turn open

Spark advanced 1/2 to 3/4

Timing is correct, Clean plugs, FSI ignition module.

It feels like vapor lock to me and it is starving for fuel. Once cool fuel flows all returns to normal.

Suggestions?

Thanks

[wmws]

Sounds like vapor lock to me to. Try opening the right side of the hood while fueling and see if that helps.

[BILL WILLIAMSON]

Old Datsuns have a BIG glass window in the float chamber. After working on them for YEARS, I KNOW how quickly the gas BOILS, after you shut off a warm engine!
On hot restarts, Old Flathead V-8s, with stock air filters, are always "flooded", from an air cleaner FULL of fuel fumes! You just hold it to the floor, 'til it goes "RRRRR"---
Bill Flooded

[Patrick L.]

Something that I think helps is to make sure the fuel line is pitched down from the the strainer to carburetor with no flat/horizontal or upward slopes at all.

Also, some folks have found or thought that the car may be grounding thru the fuel line. So checking the battery ground may help. I'm one that happen to use 2 battery ground locations.

[Benson]

1. In the winter time most refineries change to a blend that vaporizes sooner (easier) for use in lower temperatures so engines start and run better when cold ...

2. Summer gas boils at a higher temperature to prevent vapor lock / boiling ...

NOW is about the time of year that they change to summer gas but many tanks still have winter gas.

If you got a load of the winter gas then it will boil and vapor lock easier when temperatures get above 80 or so or when it is hot under the hood, which happens when you stop engine to fuel.

It is important to remember that: Many low volume (slow sales) stations will still have the winter gas for months after the change over. Some in the hottest part of summer still have winter gas!

[JOHN CT]

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Misses and backfires through exhaust
Something that I think helps is to make sure the fuel line is pitched down from the the strainer to carburetor with no flat/horizontal or upward slopes at all.

Check the gas cap. Fuel can be getting warm and expanding in your gas tank. Witch can cause a vacuum in your tank. It happened to me once. I thought gas filters were clogging up. It drove me crazy. By loosing the cap all of my gas problems were solved

[Mitch//pa]

Congrats ,,you made the first vapor lock post of the season...
The prize is on its way
Lets get out and buy clothespins before they run out of stock

[Y-Blockhead]

Originally Posted by barkleydave

OK I have had this symptom happen on warmer days after stopping warm engine and fueling. Does not always happen.

* Warm engine stop for fuel for 10 min or so. Truck running normally.

* Start up and start running... no power misses and backfires through exhaust. Within a quarter mile of nursing engine returns to normal and off we go.

It feels like vapor lock to me and it is starving for fuel. Once cool fuel flows all returns to normal.

Suggestions?

Thanks

Sounds exactly like what my Town Sedan would do. I think the gas was percolating in the carb as it sat there. I installed a phenolic spacer between the carb and the manifold to help isolate the carb from the heat. I have not had this happen since.

Before I installed the phenolic spacer I found that if I stop the car it helped to shut off the gas and run the engine until it died to empty the carb. This helped but not every time.

[denniskliesen]

Originally Posted by Mitch//pa

Lets get out and buy clothespins before they run out of stock

I had put some on my old truck last year and I had guys in their seventies never seen it before and thought I was nuts.

[Y-Blockhead]

Originally Posted by JOHN CT

Check the gas cap. Fuel can be getting warm and expanding in your gas tank. Which can cause a vacuum in your tank. It happened to me once. I thought gas filters were clogging up. It drove me crazy. By loosing the cap all of my gas problems were solved

Ummm... Let's think about this for a minute. If the gas "expands", the pressure will build in the tank, not cause a "vacuum in your tank"...

[katy]

Ummm... Let's think about this for a minute. If the gas "expands", the pressure will build in the tank, not cause a "vacuum in your tank"...

Perxactly

[Tom Wesenberg]

Sounds completely normal for this 10% ethanol crap gas we have to use these days. My Model A did the same thing, but since I only buy the gas without ethanol now it runs OK. Even with "good" gas, it still boils easier than the good gas of the 60's and 70's, so I had to add a heat shield between the manifold and carb on my 1950 Studebaker. Any heat shield you can add to the fuel line and carb will help.

Also some guys report success by adding a quart of diesel to 10 gallons of gas. I bought a couple gallons of diesel and will give it a try this summer.

[JOHN CT]

Originally Posted by Y-Blockhead

Ummm... Let's think about this for a minute. If the gas "expands", the pressure will build in the tank, not cause a "vacuum in your tank"...

Yep your right! My brain wasn't working right

[Jacksonlll]

When it happens, just pour some water on the carb bowl. Happens all he time.