Weber carb fire

gumba · 06-28-2006, 04:50 PM

I had a strange (to me) carb fire at the track. I had a fire on the inside of my weber 46mm carb, left side. Must have been buring for a while as the #3 auxilliary venturi was pretty much completely melted, #1&2 melted about 50%. The majority of the venturi ran down the sides and collected at the top of the venturi (choke tube) welding them in place. The K&N filter contained the fire within the carb.
Any guesses what would cause the fuel to come back up, and continue to burn?
The car had been running fine prior to this.
Harold
3.4 twin plug 46mm Webers, Electromotive HPV-1

Phoenix-MN · 06-30-2006, 08:20 AM

I had a fire like this years ago. Melted down the auxiliary venturi's and plastic velocity stacks on one bank. The carbs did not have the anti-percolation holes drilled in them and this happened on a hot day after startup on a hot engine. I suspect there was a lot of heat-soak from the engine to the carbs and boiled the fuel in carbs. Had the carbs drilled, replaced the parts, and has been fine ever since.

Paul

BReyes · BReyes's Garage

Sorry about that. Such a scare.

Who has a pic of this mod(anti-percolation holes drilled)?

Thank You.

gumba · 07-02-2006, 12:09 PM

The fire was contained by the water shield. The carbs that were on the car were suppose to have this mode done. I have another pair of 46mm webers I will check. I'll post some shots when I can confirm this.
Also curious if this could still happen even though the carbs were drilled.
Someone suggested putting in a fuel shut off switch and turing the fuel pump off and letting the car run out of gas after a run session.

911pcars · 07-02-2006, 01:10 PM

Possible cause:
Excessively high fuel level in float bowl. Excess fuel spills into the throttle bore and into engine. Could ignite if there's a hot spot in the combustion chamber or from spark plug or a backfire through the intake.

External fire can begin if raw fuel spills out into the throttle bore with the throttle plate closed. Fuel overflows; spark from ignition wire ignites the fire. Watershields or equivalent can contain the flames for awhile.

If float needle valve doesn't close, fuel continues to fill up and over the float bowl discharging out of the main nozzle.
If float pivot pin falls out, float no longer can close the needle against the seat (ask me how I know).

Sherwood

gumba · 07-02-2006, 01:37 PM

The car started and ran fine down to grid (wife driving) As she went on track she said the car didn't have much power and later was smelling something burning. As she pulled into the garage I could hear the motor was off, Idle was about 2,000 rpm and a faint smell. Wasn't until I opened the engine lid I saw the fire coming out from under the water shield.
Looks like some chunk of carb went down #3 cylinder as the motor only rotates about 120 D.

911pcars · 07-03-2006, 09:47 AM

Gumba,
Sorry to hear of the outcome.

The heat could have melted the primary and/or secondary venturis, even a portion of the carb housing. The slug of molten al./potmetal could have run into the combustion chamber. Got any pics for morbid viewers?

Sherwood

gumba · 07-03-2006, 10:34 AM

The secondary venturis melded. The carb housing is fine. I figured some stuff had to get into the motor.

911pcars · 07-03-2006, 11:33 AM

Amazing if the idle and transition ports are still open. I hope everything is fixable.

Sherwood

BReyes · BReyes's Garage

any pics on "anti-percolation holes drilled"

Thanks,

Regards,

gumba · 07-15-2006, 07:26 AM

99% of the sludge came off in the ultrasonic tank. The other few chips pryed off. The idle ports on top were redrilled. Other than that the carb is good to go. Some carb debris got into #3 and bent the valves.

No "anti-percolation" were drilled. It was implied that the down side to the anti-purcolation holes, in my case would have been that the gas would have been pumped out through the holes, spilling gas on the outside of the carb which could have made the fire worse. Luckily the carb fire was contained by the watershield.

911pcars · 07-15-2006, 08:34 AM

Harold,
Upon reviewing this thread, it isn't obvious what actually cause the fire. Any idea? I'm glad injuries to the carbs were minimal and the engine wasn't too badly damaged.

Drilling anti-percolation holes is an often-used fix to vent the float bowl. That modification, along with insulation blocks between manifold and carb seem to help in many cases. Perhaps an engine condition, under specific running and ambient conditions contributed to your situation?

Sherwood

Henry Schmidt · Henry Schmidt's Garage

Hi Harold
Sorry to hear about this problem

I would look to a float (cracked ?) or a needle and seat for the bowl that supplies fuel to that throat.

Do you think the engine swallowed any debris?

gumba · 07-15-2006, 09:37 AM

We have insulator blocks, they are between the heads & manifolds, not between the manifolds & carbs.
I'm hearing opinions on what caused it, but who knows. Paul's description is pretty accurate to the conditions we were running in. Temp was 104, the car been sitting during the lunch break. One of my friends thought he heard a backfire wihen the car was started at our pit. She drove it down to pit lane (Buttonwillow), sat for a few minutes then waved on. She said it wasn't running right when she started to go out on the track.
Someone suggested installing a fuel kill switch. On a hot day, before turning the key off, turn the fuel pump off and let the car run out of gas emptying the float bowls.
We have a kill switch and a halon fire system, as well as a halon bottle mounted under the driver's seat.
H.

gumba · 07-15-2006, 09:42 AM

Hi Henry,
I'll check the float. The needle & seat were working. I'll replace all that when I rebuild it. Debris went into #3 bending at least the intake valve. I have the left bank off. I sent the head out, the cylinder & piston look fine.
H.

Steve@Rennsport · 07-15-2006, 10:49 AM

Gumba:

Those insulator kits are FAR more effective when they get mounted between the heads & manifolds.

Wayne 962 · Wayne 962's Garage

I had this happen to me 12 hrs after I bought my RS clone. Big fire, lots of melted secondary venturi = big mess. Car didn't die - kept running, albeit poorly.

The timing was way off due to a defective distributor and a host of other problems. In addition, the car had PLASTIC VELOCITY STACKS, which I feel are an engine fire waiting to happen. The plastic stacks apparently caught fire, and then helped to melt the secondary venturis. The metal stacks apparently don't have this problem - I recommend tossing the plastic ones ASAP, and replacing them with metal if you have them...

-Wayne

Henry Schmidt · Henry Schmidt's Garage

Hi Harold
In your email you suggested broken rings. As far as I know only two things break rings in a new engine. Detonation caused by ignition timing or bad gas is the first. The second one is insufficient ring gap. No ring gap will generaly damage the cylinder.
When you couple your backfire at starting with broken rings my thought is ignition problem.
I would check both during the reassembly.
All of this damage is sad to hear.
Cheers

gumba · 07-17-2006, 06:57 AM

Wayne,
I have metal stacks. The car was running (not very well) when she drove it in.
Henry,
We found broken rings when we pulled the #3 cylinder. Since #4 & #5 leak down numbers are same and worse, I'm guessing (I'll know tonight) that those rings are also broken. We'll check the ring gap on the other pistons.
Don't know what to say about the cause. This is the same ignition set up I ran on the 2.8 twin plug for the past 2 years, Electromotive HPV-1. I confirmed the timing was set to 28 degress. My friend said he heard a backfire but who knows if it was my car. He was parked a few garages down from us. The car has never backfired on start up before.
Thanks for the input.

gumba · 11-16-2006, 05:25 PM

In regards to the fire, the consencus seems to be that the valve guides were not sized properly. The mechanic that disassembled the motor noted carbon build up on the back of the 1-3 intake valves caused by the valves not closing all the way. After the fire I took the intake manifolds off and noticed the #3 intake valve was stuck open. I thought the valve was bent from carb debris. The mechanic took the old valve out and went to put a new one in and it too got stuck. We sent all the heads out to get disassemble and checked. Some valves worked smoothly and some bound up. The heads were completely redone and new guides were installed. I sent the Webers to Bieker Engineering to see if the bodies were damaged. They seem to be o.k. and are being rebuilt. Also having new secondary venturies made as they nla.

We found broken rings in #3,4 & 5. The shop measured the ring gaps of the unbroken rings, top .025", second .025", bottom .020".