Hi all,
I would like to get your thoughts on an incident that happened to me about a month ago. The airplane is a 1979 Warrior II (160 hp O-320 w/ about 950 hours SMOH).
I was at a fuel stop. We made a quick turn of it: bathroom break, fuel up, let the fuel settle, sump, and go. Thirty minutes on the ground. Because the engine was still warm, I did not prime, just turned on the fuel pump, mixture rich, cracked the throttle as usual (1/4"), and engaged the starter. The airplane had zero interest in starting. The starter was certainly spinning the prop enthusiastically, so a weak battery or starter was not the issue. We discovered later that the coil on the left mag was weak and would completely fail when hot. So that's well-explained and the airplane starts briskly now, warm or cold. The part I'm concerned about is what happened next.
Thinking that I maybe needed to prime after all, I added a couple of shots (with the primer, NOT the throttle - I know better). The engine made a couple of feeble attempts to start (owing to the weak coil, I think) and then we heard a muffled popping sound which I assumed to be a backfire. I did what I've always been trained to do and kept cranking. The engine never started and I eventually stopped cranking to avoid burning up the starter.
Long story short, I DID have a backfire of some kind. The carburetor was completely fine, nothing singed, melted, or misshapen (saved by all that cranking?). But, a rubber grommet on the back side of the carburetor heat airbox did catch fire. We saw a tendril of smoke escape the front of the cowling, shut off the fuel, shut off the electrical system, and scrambled out of the airplane.
The fire was localized to that rubber component and burned itself out quickly. Obviously, I needed a carb heat airbox repair afterward but, beyond that, we could find no other damage to the engine. *whew*
So here's my question: why did the engine backfire? I know that induction system fires can occur if the accelerator pump is used to dump fuel directly into the carb while starting. I didn't do that. I used the primer and put fuel directly in the cylinder. How would a fire back up into the induction system from that?
The only theory I've heard so far is that, with my engine approaching mid-time, perhaps an intake valve did not seal well at the time the engine fired such that the combustion mixture was able to back up into the induction system.
Any thoughts? The next annual is in April and, at a minimum, I am thinking that a borescope examination of the valves is a good idea.
I would like to get your thoughts on an incident that happened to me about a month ago. The airplane is a 1979 Warrior II (160 hp O-320 w/ about 950 hours SMOH).
I was at a fuel stop. We made a quick turn of it: bathroom break, fuel up, let the fuel settle, sump, and go. Thirty minutes on the ground. Because the engine was still warm, I did not prime, just turned on the fuel pump, mixture rich, cracked the throttle as usual (1/4"), and engaged the starter. The airplane had zero interest in starting. The starter was certainly spinning the prop enthusiastically, so a weak battery or starter was not the issue. We discovered later that the coil on the left mag was weak and would completely fail when hot. So that's well-explained and the airplane starts briskly now, warm or cold. The part I'm concerned about is what happened next.
Thinking that I maybe needed to prime after all, I added a couple of shots (with the primer, NOT the throttle - I know better). The engine made a couple of feeble attempts to start (owing to the weak coil, I think) and then we heard a muffled popping sound which I assumed to be a backfire. I did what I've always been trained to do and kept cranking. The engine never started and I eventually stopped cranking to avoid burning up the starter.
Long story short, I DID have a backfire of some kind. The carburetor was completely fine, nothing singed, melted, or misshapen (saved by all that cranking?). But, a rubber grommet on the back side of the carburetor heat airbox did catch fire. We saw a tendril of smoke escape the front of the cowling, shut off the fuel, shut off the electrical system, and scrambled out of the airplane.
The fire was localized to that rubber component and burned itself out quickly. Obviously, I needed a carb heat airbox repair afterward but, beyond that, we could find no other damage to the engine. *whew*So here's my question: why did the engine backfire? I know that induction system fires can occur if the accelerator pump is used to dump fuel directly into the carb while starting. I didn't do that. I used the primer and put fuel directly in the cylinder. How would a fire back up into the induction system from that?
The only theory I've heard so far is that, with my engine approaching mid-time, perhaps an intake valve did not seal well at the time the engine fired such that the combustion mixture was able to back up into the induction system.
Any thoughts? The next annual is in April and, at a minimum, I am thinking that a borescope examination of the valves is a good idea.
