Carb fire season

[PilotDAR]

Yeah... it's carb fire season. I was starting the 150 to take my grandson flying this morning. I'd left it plugged in all night with the engine blanket on, so it was nice and warm in the hangar. The air wasn't though. I had it running after start, though not as smoothly as I wished, I think I'd frosted the plugs in a cylinder, and that cylinder was not running yet, but, the engine was idling, with assistance from some primer strokes. It then stumbled and stopped, so I restarted it. I saw a waft of smoke come up through the windshield defog vent, and then around the cowl. Hmmm.... carb fire! So I kept cranking until the battery would not turn it over any more, while selecting mixture lean, and fuel off. Then, master and mags off, hopped out, and it was still smoking. And my cell phone was ringing. It was my wife, telling me that the plane was on fire, she could see it from the kitchen.

Upon opening the battery/oil door, I could see fire in the low forward area of the engine compartment, so I emptied a fire extinguisher into it - no more fire. I removed the cowls, to be sure all the fire was out, which is was, and pushed the plane back into the hangar. Yup, there'd been a fire in there! The insulation around the oil breather was what had been burning (about the only flammable thing in there). Lots of black on the bottom cowl and nose strut. That corresponds to my wife's later report of "lots of fire under the engine". I'll have the carb and airbox off for inspection/overhaul as required, and inspect everything else. Interestingly, my primer is four nozzles, one into each cylinder, rather than just one into the induction pipe, so I'm surprised that enough gas could drain that far back, but obviously, it can, it was not being sucked through the non firing cylinder, while the others were running.

I've only been involved in one carb fire before, while right seat in a buddy's 182 while I was a student. Luckily, on that occasion, even though he did the wrong thing, the fire self extinguished. He said: "We're on fire, get out", which, while a great briefing to a passenger, resulted in him not cranking through, to pull the fire through the engine. I'm here to tell you - it worked this morning! Keep cranking!

[CpnCrunch]

I thought the standard primer injected into the manifold. That is what the POH shows anyway (although it might depend on model). How would fuel get from the cylinder back to the carb?

Were you using the primer after starting? Did you pump the throttle at all?

Just curious, so I can avoid the same thing happening, although I don't really fly when it's below 0. My plane has an engine heater, but I don't have anywhere to plug it in. (But there's only a few days a year here when I would need it anyway).

[PilotDAR]

This O-200 is configured with a primer nozzle on each cylinder injection port, rather than one nozzle above the carb, so I too was surprised that fuel could find its way back to the carb and airbox, but, they are down hill! No, I don't pump the throttle, the accelerator pump is not designed to work that way - it will, but it's not designed to. I prime with the primer. The added (while I would normally have said safety) factor is that you can only pump as fast as the primer will refill, so, in theory, you can only put so much fuel in with the primer, where throttle pumping could put much more in, as the accelerator pump refills immediately. Obviously, that's not an absolute safety element though!

I electric heat my planes every time before cold weather flying. Normally, I get great starts that way. I guess the air was just too cold today, and I frosted plugs. Normally, that resolves itself after 15 seconds or so of idling, but not this time. Obviously, I over primed it to keep it running the first time. I only used two pumps of the primer (as I always do) to start it, I just have another pump ready to go on cold days. For my O-200, three prime pumps is too much.

I didn't do anything during this start, which I would not normally have done, other than the one cylinder did not pick up quickly enough to burn the excess fuel. I did try to get the engine scanner on, to see which one it was, but the scanner is on the avionics master, so it took too long to come on, to be helpful.

The key message is, as worked today, and made the difference, keep cranking. I cranked it until the battery went flat. A starter is less cost than a whole C 150. That said, I don't think the whole event lasted even a minute, and the starter should survive that on a cold day. As I continued to crank, I thought about exceeding the starter time limit, but figured that it would be dumb to not do everything I could to suck the fire through the engine first time around. Of course my wife was alarmed, as all she saw was me continuing to "start" an airplane which was obviously on fire! My hangar fire extinguisher was not terribly helpful (perhaps too cold), so my workshop fire extinguisher did the job, as it was warm - know where your fire extinguishers are, and keep them maintained!

I also removed the battery right away, and put it on charge inside, as it probably would have frozen in a short time, flat and outside on a cold day. If got the airbox off, and there seems to be no real damage, just smoke stains. But, it's a good opportunity for a carb overhaul anyway. Mine has the two piece ventrui, and they can be deformed by a carb fire - it's an AD inspction. I haven't looked up in there yet...

[JasonE]

Interesting you post this today. The last time I flew the J3, it was fairly cold and didn't get a good enough preheat. I was thinking about carb fires as I used the primer to keep it running... Being in straight skis I had to keep it moving to warm it up. A few laps of the runway before it was smooth running.

[digits_]

Is using the primer while the engine is running a common practice? This is the first time I've heard about it.
Is it normal that an engine would require the primer to keep running? Or would that be an indication of a wrongly set full mixture setting or someting similar? Would turning on the carbu heat have helped, to enrichen the mixture and get some warmer air in?

Thanks for sharing!

[ahramin]

Is using the primer while the engine is running a common practice? This is the first time I've heard about it.

It's common practice for anyone who understands intimately that an engine needs fuel/air/spark to work and wants to keep it running when for various reasons it needs more fuel. There are downsides though, which the title post makes abundantly obvious.

[ahramin]

I did try to get the engine scanner on, to see which one it was, but the scanner is on the avionics master, so it took too long to come on, to be helpful.

One of the aircraft I fly is like this, I start it with the avionics master on.

[TWSC]

Is using the primer while the engine is running a common practice? This is the first time I've heard about it.
Is it normal that an engine would require the primer to keep running? Or would that be an indication of a wrongly set full mixture setting or someting similar? Would turning on the carbu heat have helped, to enrichen the mixture and get some warmer air in?

Thanks for sharing!

Pretty common starting a beaver when cold. A couple of primer shots will catch a dying engine and get it going. Don’t want to try and catch it with throttle for the same reasons pumping throttle is a bad idea.

[digits_]

Is using the primer while the engine is running a common practice? This is the first time I've heard about it.

It's common practice for anyone who understands intimately that an engine needs fuel/air/spark to work and wants to keep it running when for various reasons it needs more fuel. There are downsides though, which the title post makes abundantly obvious.

Owkay...

Let me rephrase: the OP didn't want to use the "pump the throttle" technique because the accelerator pump isn't designed to prime the engine. This makes me wonder if the primer is designed to prime the engine while it is already running. I was under the impression that priming should only be used to get an engine running, not to smoothen out an already running engine.

[photofly]

Data point: The manufacturers instruction for one of my planes, with a very boring Lycoming O320, say to use the throttle pump three times to augment the electric primer (which is just a solenoid valve on the fuel line to the carb, pressure provided pre-start by the electric boost pump) before starting.

[CpnCrunch]

This O-200 is configured with a primer nozzle on each cylinder injection port, rather than one nozzle above the carb, so I too was surprised that fuel could find its way back to the carb and airbox, but, they are down hill!

Ah, ok. All the C150 POHs I can find say the primer injects fuel into the intake manifold. C172 POHs say it injects into the "intake ports" for the cylinders, which I guess is how your O-200 is set up.

It would be interesting to take a look at that primer to see exactly where it is introducing the fuel and whether some of it could be dripping back to the carb.

How did you manage to get frosted plugs with a preheated engine? Is that not unlikely? Could you have had a stuck valve?

[CpnCrunch]

Owkay...

Let me rephrase: the OP didn't want to use the "pump the throttle" technique because the accelerator pump isn't designed to prime the engine. This makes me wonder if the primer is designed to prime the engine while it is already running. I was under the impression that priming should only be used to get an engine running, not to smoothen out an already running engine.

Yeah, I'm somewhat skeptical about the need for priming a running piston engine. I've flown quite a bit in -15C and never had any problems. As long as the engine is primed and preheated it should start and keep running. The only problem I've had is with too much preheating it can cause vapor lock (in a carburetted O-360).

[Heliian]

I wonder if the activation of the primer while running caused the fuel to be sucked through the system freely until it flooded instead of the "measured" pumps.

Excellent move on the continuous cranking. Some might not have reacted as properly.

[Bede]

I had my plane warm and run up in -27 and it still had a hard time starting again once warm. I have a similar thread going here. http://www.avcanada.ca/forums2/viewtopi ... 7&t=143964

No fire though.

Is using the primer while the engine is running a common practice? This is the first time I've heard about it.

POH says to leave primer out and prime if required on cold start.

In extremely cold temperatures, it may be necessary to continue priming while cranking.

No, I don't pump the throttle, the accelerator pump is not designed to work that way - it will, but it's not designed to.

Why can't you pump throttle? What are risks, damage?

[PilotDAR]

Why can't you pump throttle? What are risks, damage?

Well, to begin with rapidly changing the throttle setting on an engine you're trying to get to idle nicely is counterproductive.

That said, carbs with an accelerator pump (most) will squirt a stream about like a water pistol, when the throttle is opened. This stream shoots about ten feet if the carb is not on the engine when you do this. It's a single, steady stream. The primer directs fuel through a primer nozzle, which is more like a pressure washer, it atomizes the fuel into a mist, which is why there's so much resistance in the primer plunger, you're creating hydraulic pressure in the fuel.

The atomized mist of fuel is much more likely to burn fast and completely at idle, than a water pistol stream, and so is better at slow engine speeds for complete combustion. It would be great if the accelerator pump also misted the fuel, but then who would want primer plunger pressure on the throttle!

The systems work fairly well, when used as intended - well, nearly always!

By the way, upon disassembly, the inner venturi of my carb was very obviously heat damaged, and distorted, and no longer airworthy. I will have it replaced. This is a required AD inspection for good reason!

[AirFrame]

Data point: The manufacturers instruction for one of my planes, with a very boring Lycoming O320, say to use the throttle pump three times to augment the electric primer (which is just a solenoid valve on the fuel line to the carb, pressure provided pre-start by the electric boost pump) before starting.

To clarify, it says to pump the throttle *while turning the starter*, correct?

I've seen people do it the other way around... Pump first, then starter, and end up with airbox fires. Again, a lack of understanding of the engine system.

[photofly]

Nope. It says "When the engine is cold move the throttle two or three times up to full throttle before setting it to roughly a quarter of its full travel. Press the primer button for approx 5 seconds....
When the engine is warm move the throttle one time up to full throttle before setting it to roughly a quarter of its travel."

[CpnCrunch]

POH says to leave primer out and prime if required on cold start.

Here is the exact quote (from 1968 model POH):

Engage starter and continue to prime engine until it is running smoothly, or alternely, pump throttle rapidly over first 1/4 of total travel.
Pull carburetor air heat knob full on after engine has started, Leave on until engine running smoothly.

Note that the above is in the "without preheat" section. With preheating there are no more instructions after "engage starter".

So, in the case where you have preheated the engine but still not running smoothly, perhaps a good idea to leave carb heat on

[digits_]

The systems work fairly well, when used as intended - well, nearly always!

I'm not claiming, or even implying, this was your fault or that you did something wrong, however I am curious if you know if the primer was originally designed to be also used on a (rough) running engine as opposed to only during start? Is there some kind of certification or tests done in those circumstances?

[PilotDAR]

I am curious if you know if the primer was originally designed to be also used on a (rough) running engine as opposed to only during start?

The POH says: "Continue to prime the engine until it is running smoothly, or alternatively, pump the throttle over the first 1/4 of total travel."

I'm not keen on the pumping the throttle technique, for reasons I've already stated, though I guess the Cessna people are okay with it.

My quick check of the POH for the 172 and 182 of that era shows similar wording to the 150. I have never used more than three strokes of prime, for fear of over priming it, and if the first start attempt on three strokes didn't get it running the second same attempt always would, and did the other day too. It was the priming during rough idling which seems to have introduced the problem, though I don't know what I'd do differently next time!