car fire this afternoon - thoughts on why?

[sunlion]

I'm safe and the car has some minor damage but pretty scary few moments.

Lost most power in my 912e going up a slight incline from a stop, while pulling over noticed flames out the deck lid. Fire was in left carburetor, soon spread to interior. Had a can of extinguisher which was soon empty. Luckily, passerby had a real extinguisher and got the flames extinguished within a few minutes. Still smoking heavily when the fire dept arrived and they put out the smoldering. Had the car towed to my shop.

Carbs were balanced a few weeks ago, was running better, but with frequent popping from left carb and no back firing from the muffler. Only today, had scheduled an appt to have it looked at. All the fuel lines were intact - I checked once I was sure everything was out and had cooled down. I guess, with a popping carb, the intake valve is open slighlty when the spark is delivered. When I accelerated to go up the hill, it probably got a good jet of fuel which was ignited in the carburetor. Any other thoughts on why this happened?

[ivangene]

saw the car - looks like the carb retrofit did not account for fuel return line

just a quick guess

[ivangene]

PS nice coupler - survived the fire

[sunlion]

Yes, the coupler was unscathed.

A fuel return line would not have allowed a large fuel build up in the carburetor? It goes from where to where?

Valve timing must be off also to allow spark or ignited fluid into the carburetor.

[HawgRyder]

Fuel lines are the most suspect.
The return line must actually go back to the tank.
I have seen some where the return line was blocked off....letting the fuel pressure to build in the float bowls...and overflowing.
Fuel pressure for carbs is usually around 3 to 5 PSI....hopefully with adequate flow to supply the needs of the engine at that pressure.
If the car has a fuel injection pump it will be giving a great deal more than 5 PSI...!!
You state it's a 912...but has the pump been replaced at the time of carb rebuild?
In any case...the lines will all be 40 or more years old if not exchanged.
Bob

[ivangene]

^^^ what he said

spark source could be bad plug wire or any number of other things, once fumes build up in there it doesnt take much -

look at the IN fuel line on the drivers side and then right behind that is another ??? fuel line??? was that capped off????
(looks like the return line but goes no where) - I am no 912 expert so this is guess work here...plus I only spent 2 minutes looking at it

[sunlion]

There is no fuel return weber IDF 40 duals. But apparently, they are very sensitive to fuel presure, needs to be 3-3.5psi. The port which was capped off is not a fuel return but a port which is uesd if the carbs are run in series for some reason.

[70SC]

I had a fuel fire ( years ago ) . The 44mm Webers had a lead passageway plug that fell out causing the fire. Not alot of damage thank god !!!Good luck with the reconstruction .

[vracer]

What was the size and type of your "Too small" extinguisher?

[sc_rufctr]

Unfortunately there's no real way to know 100% what caused the fire.
Fuel lines, carburettor fault, return line. It could have been any combination of these.

Also brass fuel fittings can come loose from the alloy body of the carbs especially from older Webbers.

I hope it works out OK.