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Post-backfire inspection?

AAflyer

Well Known Member
On my last flight, my engine backfired and ran backwards a few revs.
What should I look for, as far as possible damage?

Aeropower/Superior IO-360 8.5:1
Cold-air sump
Dual PMAGs
Catto 3-blade
Veterans 4 into 2 exhaust

This happened after a quick fuel stop, so was using my hot-start procedure:

Pmags-OFF
Throttle - cracked
Mixture - ICO
Push the starter...
after a couple of blades, turn PMAGs on,
Mixture - enrich after firing

Thanks,
Bruce W
 
For starters... the airbox and filter.
And then... timing. On cranking ignition is supposed to be retarded and so running backwards should never happen.
 
I’m not Bruce, but having the pmags off for the first couple blades can give the starter a little more time to get the prop swinging at full speed and reduce the potential for kickback. This delay is built in to the pmag software so shouldn’t be necessary on initial startup attempt, but i do use this technique on the rare occasion that my first start attempt fails.

I had to add in a 1.5-2 tooth retard in my timing to get rid of the occasional kickback during startup. As an added benefit, my CHTs dropped a bit and are now in the mid 300s range in cruise.
 
This delay is built in to the pmag software so shouldn’t be necessary on initial startup attempt, but i do use this technique on the rare occasion that my first start attempt fails.
.

As you stated, correctly operating pmags (or other electronic ignitions, or impulse coupled mags, etc., ) should have their timing retarded enough to prevent any kickbacks. Why would you treat second or third start attempts differently?
Same subject: OP said his engine ran backwards for ‘several revolutions’. I don’t understand how that is possible.
 
There is a potential for Kickbacks on startup when equipped with pmags, a light weight prop, and high compression engines, and it’s exacerbated by a weak battery. A onetime retard on startup is therefore built in to the software to help with this. If the engine doesn’t start first attempt, that auto retard won’t happen on subsequent attempts unless the ignitions are cycled off and back on. As an alternative, you can have the ignitions turned off, hit start, let the prop start spinning and gain some speed and momentum, and then turn on the ignitions once that has happened.

Pmags are known to have an aggressive advance curve. One way to compensate for this is to build in a little retard into the timing. For me, a couple of teeth on the ring gear past top dead center lowered my CHTs and also fixed the occasional kickback problem while having no noticeable effect on speed. This was for an IO-360B1B with 9:1 compression, a whirlwind 200RV prop, and a PC-680 battery.
 
As others have said, I leave the PMAGS on for hot and cold starts. I have no information about exactly how the pmags work, but since it is an electronic device, my guess is that it might take a millisecond or two to "boot". During this time, are you sure that the timing is perfect? I've never seen any advice to power them on while the prop is spinning.

Please let us know what you find after the inspection.

Also, for anyone with a pmag, I strongly recommend investing the time to understand how the timing works. It's not complicated, but extremely important. I also recommend an engine monitor like the eicommander (https://eicommander.com/) or enginebridge (https://www.enginebridge.com/).
 
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It is a bad idea to turn ignition off during cranking. You say backfire, but suspect it was a kickback. The former sends combusting gas back into the intake and the latter just reverses the rotation when the spark lights the charge too early (advance must be retarded for slower RPMs). Did you observe flames or smoke coming from the air filter area? No damage from spinning backwards, though kick backs can break teeth on the ring gear if the starter was engaged, so I would check it.
 
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SNIP

Pmags are known to have an aggressive advance curve. One way to compensate for this is to build in a little retard into the timing. For me, a couple of teeth on the ring gear past top dead center lowered my CHTs and also fixed the occasional kickback problem while having no noticeable effect on speed. This was for an IO-360B1B with 9:1 compression, a whirlwind 200RV prop, and a PC-680 battery.

The pMag advance at lower RPMs is less than that set at timing. In other words less than 25 degrees BTDC for parallel valve engines. If you have a standard 25 degree mag and a pMag (with jumper in) set at 25 degrees, the pMag timing will be retarded as compare with a standard mag at lower RPMs.

Adding more timing retard may be what is needed for higher compression engines - just like you would do with a mag. As with any ignition system, retarding the timing will lower CHTs. I would not however reach for this as the first tool out of the toolbox for high CHT issues unless there are other considerations.

Side note - for our flying always run the pMags with jumper in (four cylinder pMags). For six cylinder pMags in RV-10s I recommend install with the timing setpoints as 25 degrees BTDC base and no more than 34 degrees for the most advanced setpoint.

I agree on the points made about starting with a weak battery.....

Carl
 
Ignition timing

As far as I know, most electronic mags have a built in retarding function below a certain rpm (400).
Some manufacturers of electronic mags is stating that timing should be set at TDC. This may work if the battery is in god condition and the outside temp is >32F. To be safe from kick back during starting, the ignition should be set to fire a few degrees after TDC. This will prevent a kick back that can be expensive.
Depending on the electronic mag that is installed, the advance can be set
to what is desired for normal operation 20-30 deg.
If you have timed to 5 deg After TDC and set advance to 30 deg, you will have
an effective timing of 25 deg under normal operation.

Good luck
 
Howdy Everyone. Sorry in advance for the long reply. The short of it is, in my opinion, kickback is a simple timing problem. Pmag has suggestions for the issue. The Pmags have a jumper wire at the Pmag connector that can be installed or not to adjust the advance curve. Having the jumper installed renders a less aggressive curve, no jumper installed is more aggressive. Users should also know what firmware version their mags have installed.

Board version and FW version are imbedded in the “VL” code on the ignition data label.

“VL XXYYZZ:

XX is the board number

YY is the FW version

ZZ is internal tracking"

I suggest reading these documents:

Go to the Pmag website, at the top, hover over:

MANUALS/GUIDES/TOOLS

Click: DOWNLOADS

Click: 114 MANUAL

See page 13, Variable Timing.

Also check out the TROUBLESHOOTING GUIDE.

Copied from their TIPS and TRICKS page:

"Low Mass Props: Kick-Back Precaution

(NOTE: This issue has been addressed in firmware V40, and after, by adding a 4 degree startup firing lag.)

Lightweight low mass props, by their nature, have less flywheel effect on the engine. This allows the engine to decelerate much quicker as the starter motor pulls against each compression stroke, and as the engine approaches TDC – where the plugs fire for startup. Under the right conditions, the combination of low mass prop, weak or undersized battery, corroded terminals, cold engine, long cable runs, tired starter (or any combination) can cause the engine to come to a (virtual) stop by the time it reaches TDC. At this point there is a 50/50 chance the engine will move forward or backward (a so-called kick-back) when the plugs fire. While the ignition may be performing as designed, the results are the same. Fortunately, there is an easy defense.

Simply re-time the ignition with the engine parked two or three degrees “AFTER” TDC (instead of “AT” TDC as you normally would). AFTER TDC means (from the TDC position) rotate the prop in the normal direction of travel to your revised target timing mark. Note: This will also slightly retard the operating advance range, but this small amount is not likely to be noticeable for most flyers. If desired, the shift can be nulled with the EICAD interface."

Your Aerosport Power engine plate will have the timing recommendation stamped on it.
 
As far as I know, most electronic mags have a built in retarding function below a certain rpm (400).
Some manufacturers of electronic mags is stating that timing should be set at TDC. This may work if the battery is in god condition and the outside temp is >32F. To be safe from kick back during starting, the ignition should be set to fire a few degrees after TDC. This will prevent a kick back that can be expensive.
Depending on the electronic mag that is installed, the advance can be set
to what is desired for normal operation 20-30 deg.
If you have timed to 5 deg After TDC and set advance to 30 deg, you will have
an effective timing of 25 deg under normal operation.

Good luck

It takes time for combustion to take place. This is why we advance timing more and more as the RPMs increase. If you spark at TDC, it is almost impossible to get a kickback, as the combustion will not be producing pressure for at least several more degrees at cranking RPM levels and the pressure cannot reverse direction, as you are on the other side of TDC. The real issue is whether or not your EI is retarding enough and how accurate it's TDC reference is. If these cannot be set and maintained with some level of precision, then yes more retard can add safety.

The previous post mentions weak staters allowing the prop to stall at TDC and would agree that this could cause issues in some cases. However, most mags are set with a 25* lag on the impulse and drift to increasing advances as the points wear over time. Therefore, most of the fleet is starting with advances of 0-3 degrees BTDC and history has shown few problems beyond starting with weak batteries. I have seen my engine reverse rotation a couple of times with a weak battery in very cold weather. I don't see that as a problem with my ignition, but a problem with the pilot trying to push a bad situation. Generally when the starter is too weak to prevent this it is also too weak to resist the force and therefore doesn't break teeth most all of the time.
 
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