Violent start when starting warm/hot motor.

[WingnutWick]

Hello all - re attacking this issue on here and hopefully trying to figure it out:

IO-540 with dual PMAGS. I have experienced, since having the aircraft, what I can only describe best as a violent kick when starting a hot/warm motor. This happens if I have just ran it for a few minutes (say starting up and taxing to get fuel and it will happen on the subsequent start) or after running for time and attempting restart while engine is till hot. When I do this, most of the time, I will get an extremely violent/hard kick that feels very not good for the engine.. The best way to describe it is what I would imagine starting with improper timing (ie without an impulse coupling) or some sort of detonation issue where there is one massive explosion that violently forces the cylinder away vice a smooth burn and gradual acceleration of the piston on the ignition stroke. This may be totally wrong but that’s what it feels like - and it feels terrible for the motor. The PMAGS are timed properly and they retard their timing for start. It also never occurs when starting from a cold motor (which I would imagine it would if dealing with timing problem). My only possible thought with the PMAGS is that I believe there is “wasted sparks” that occur on strokes and that this possibly is igniting some residual gas vapor filling a cylinder due to a sticking valve or something? But my understanding of how it all works makes that an uneducated speculative guess at best. Reaching out for help in this matter and to hopefully figure this thing out!

My typical hot start procedure is mixture idle cutoff, no priming, throttle cracked. Usually starts pretty easily albeit with this thud.

Thank you for your help!

CW

 

[joeboisselle]

Are your injectors bleeding out vapor during the power stroke, then your PMAG hits it at the top of the exhaust stroke?

 

[WingnutWick]

Are your injectors bleeding out vapor during the power stroke, then your PMAG hits it at the top of the exhaust stroke?

Good question, is it normal for vapor to bleed out of injectors on a hot motor? You would think that this would be a common issue with PMAGs if so, but I haven't heard of these hard kicks with other PMAG users.

 

[pitts12driver]

I went to start my plane recently (RV-7A, IO-360-M1B, dual P-Mag) with a still hot engine, no priming. It kicked back and broke something in the starter.

I poked around on the web and found this on VAF:
Time your emag 1-2 degrees retarded. If you have a composite prop kickbacks are common unless you do this and practice shutting down the emag in the event of a no start. The reason for this (assuming you have the latest firmware installed) it will delay sparks for a few revolutions during the start sequence. If you stop after cranking that logic goes away. To reset the logic you must remove power from the emag.

Remember the emag is a wasted spark system and fires spark on both the desired compression stroke and the exhaust stroke of the other 2 cylinders. This is what can be either a kick back or a “prop stop” event. Maybe this will help. Brad at Emag explained this to me when I had problems and I finally got them figured out."

So you have full data on our event, during this morning's second start sequence: the engine did start briefly on the first attempt but did not stay running, then I tried again but without shutting down the E-Mags. WE NEED TO SHUT THE E-MAGS DOWN AFTER ANY FAILED START ATTEMPTS BECAUSE THEY NO LONGER RETARD THE IGNITION UNLESS POWER CYCLED! This btw is not in the installation/user manual!

 

[lr172]

Good question, is it normal for vapor to bleed out of injectors on a hot motor? You would think that this would be a common issue with PMAGs if so, but I haven't heard of these hard kicks with other PMAG users.

This is not from wasted spark. That happens at top of exhaust stroke, so at least one of the valves or both are open. This means there is no compression and therefore no combustion. Wasted spark systems are used on 10's of 1000's of engines with no consequence.

if you injector leaked it sill wouldn't matter, as it would leak into the intake chamber, not the cylinder. The intake valve is JUST beginning to open when the spark happens.

 

[lr172]

What you describe sounds like too much advance during cranking. You say the pmags are retarding for startup; How do you know that? Have you put a timing light on the engine and confirmed it? My first suspect here would be an issue with the pmags advancing too much in the startup RPM range or not obeying the retard command for various reasons. Have no suggestions on the specific issue or error mode. Pretty easy to confirm or reject that possibility via testing with timing light. Let me know if you want test instructions.

Pmags have had issues in the past with timing resetting itself uncommanded. Unsure how prevalent it still is.

 

[WingnutWick]

Well, there you have it. In my mind, this points DIRECTLY to problems with the pmag! Don't forget that it is using a processor and sotware to make decisions on when to do things. Possibly poor sw coding. Most systems use RPM level to decide when to retard. Possible Pmag jyust assmes first RPM movement is start and all others are not. I certainly would hope that is a problem and NOT how it is designed, as that would be poor engineering.

Correct me if I'm misreading this but this is regarding subsequent starts following a failed start. My problem occurs on initial start after turning off power but with a warm motor. So it is the first attempt following a power down.

 

[joeboisselle]

Well, there you have it. In my mind, this points DIRECTLY to problems with the pmag! Don't forget that it is using a processor and sotware to make decisions on when to do things. Possibly poor sw coding. Most systems use RPM level to decide when to retard. Possible Pmag jyust assmes first RPM movement is start and all others are not. I certainly would hope that is a problem and NOT how it is designed, as that would be poor engineering.

But the OP didn't say there were multiple attempts at starting.

The installation and operating guide includes this note on wasted spark and valve overlap.

At startup, E-MAGs will fire plugs slightly after TDC, at

which time the “wasted” side will be in a period of valve overlap. [Valve overlap is

when the intake and exhaust valves are open at the same time.] When starting an

engine where vapor has accumulated in the “wasted” cylinder, this vapor can be

ignited by the wasted side spark. Vapor accumulation might be due to a) improper

shut down, b) over priming, c) a hot start, or d) other reasons. In the test cell

17instances where this was observed, the event itself was rather unremarkable. It was

heard as a hard “puff” prior to engine start.

"Rather unremarkable" doesn't fully line up with the OP's description but (edit)-> looks like it's related to A, B, C, or D. And "D" can't be solved while sitting at a desk chair.

 

[lr172]

Correct me if I'm misreading this but this is regarding subsequent starts following a failed start. My problem occurs on initial start after turning off power but with a warm motor. So it is the first attempt following a power down.

Correct. Sorry, I was qouting a post that I assumed was from a buddy clarifying details about your event. Apparently that was not accurate, so please disregard my comments on that. I still do suspect that your issue is related to too much advance while cranking.

 

[lr172]

But the OP didn't say there were multiple attempts at starting.

The installation and operating guide includes this note on wasted spark and valve overlap.

At startup, E-MAGs will fire plugs slightly after TDC, at

which time the “wasted” side will be in a period of valve overlap. [Valve overlap is

when the intake and exhaust valves are open at the same time.] When starting an

engine where vapor has accumulated in the “wasted” cylinder, this vapor can be

ignited by the wasted side spark. Vapor accumulation might be due to a) improper

shut down, b) over priming, c) a hot start, or d) other reasons. In the test cell

17instances where this was observed, the event itself was rather unremarkable. It was

heard as a hard “puff” prior to engine start.

"Rather unremarkable" doesn't fully line up with the OP's description but (edit)-> looks like it's related to A, B, C, or D. And "D" can't be solved while sitting at a desk chair.

Sorry, I read the following from pitts12 and mistoke it for a clarification of the OPs issue

So you have full data on our event, during this morning's second start sequence: the engine did start briefly on the first attempt but did not stay running, then I tried again but without shutting down the E-Mags. WE NEED TO SHUT THE E-MAGS DOWN AFTER ANY FAILED START ATTEMPTS BECAUSE THEY NO LONGER RETARD THE IGNITION UNLESS POWER CYCLED! This btw is not in the installation/user manual!

 

[Rdog420]

But the OP didn't say there were multiple attempts at starting.

The installation and operating guide includes this note on wasted spark and valve overlap.

At startup, E-MAGs will fire plugs slightly after TDC, at

which time the “wasted” side will be in a period of valve overlap. [Valve overlap is

when the intake and exhaust valves are open at the same time.] When starting an

engine where vapor has accumulated in the “wasted” cylinder, this vapor can be

ignited by the wasted side spark. Vapor accumulation might be due to a) improper

shut down, b) over priming, c) a hot start, or d) other reasons. In the test cell

17instances where this was observed, the event itself was rather unremarkable. It was

heard as a hard “puff” prior to engine start.

"Rather unremarkable" doesn't fully line up with the OP's description but (edit)-> looks like it's related to A, B, C, or D. And "D" can't be solved while sitting at a desk chair.

this depends on the software of the pmags. Older models may not fire 4 degrees after TDC like the manual. OP can do the pull through test to confirm where his plugs are firing at startup RPMs.

 

[sansoneservices]

Hot start on my surefly requires throttle to be at the idle stop or I get kickback. Just saying

 

[Walt]

Hot start on my surefly requires throttle to be at the idle stop or I get kickback. Just saying

That doesn’t sound normal.

 

[lr172]

That doesn’t sound normal.

+1

I would be checking things out on the surefly. Maybe the timing is off a bit or there are settings for the retard on cranking, including the rpm threshold. In my experience, throttle position should not affect the propensity to kick back. One exception would be too much advance on cranking and the closed throttle blade creates a significant enough air flow reduction to counter that propensity. Either way, it would seem on the surface that you may have too much advance during cranking.

Worth looking into

 

[Bcone1381]

Hot start on my surefly requires throttle to be at the idle stop or I get kickback. Just saying

I had this too. It will damage things if its ignored...ring gear and starter gear.

I retimed the Surefly with the prop at 5 degrees ATDC and That totally fixed the kickback. But that required a second adjustment to the Surefly microswitches so normal timing still fires at 25 BTDC. I loose that 5 degrees of timing advance at the lower power settings but I'm happy with it.

 

[sansoneservices]

Timing is set according to the engine data plate, and the surefly start position is at TDC as instructed. I understand it's possible to retard the Start timing position and adjust the Run timing switches. Since it hot starts fine with the throttle at idle I'm satisfied for now. Thanks for the helpful responses my friends

 

[rv8ch]

this is regarding subsequent starts following a failed start. My problem occurs on initial start after turning off power but with a warm motor. So it is the first attempt following a power down.

I agree that it sounds like one or both pmags are firing earlier than you want. Some things to check:

1. have the pmags been updated with the latest software? Older software didn't retard timing at start.
2. are you 100% sure that they are timed right? They are not timed the same as a magneto, as I'm sure you know.
3. are you sure that when you say that they are powered down, they are actually powered down? We've seen a few pmags that are wired "strangely". Is your switch and key manipulation identical at the fuel station as it is when you first pull the aircraft out of the hangar?

Best of luck, and please keep us posted.

 

[WingnutWick]

I agree that it sounds like one or both pmags are firing earlier than you want. Some things to check:

1. have the pmags been updated with the latest software? Older software didn't retard timing at start.
2. are you 100% sure that they are timed right? They are not timed the same as a magneto, as I'm sure you know.
3. are you sure that when you say that they are powered down, they are actually powered down? We've seen a few pmags that are wired "strangely". Is your switch and key manipulation identical at the fuel station as it is when you first pull the aircraft out of the hangar?

Best of luck, and please keep us posted.

I will double check the software but these were basically replacements for me seeing as the two I purchased from them had some serious problems (to which resulted in a recall of a lot of their mags due to some improper board coating). I received them about tow years ago.

I am pretty dang sure they are, I have an 8 with dual PMAGs and never an issue. Timed at a TDC verified with crankshaft mark and #1 at TDC of compression stroke. Not too long ago I retarded the timing a few degrees to see if it would help, but to no avail. I will conduct a pull through test next.

I wired them up to toggle switches with power coming of the master bus, so when I kill the master they are powered off. But I will test by pulling the breakers after a shut down next to verify I didn’t somehow mess that up when I wired it years ago.

 

[Rdog420]

I will double check the software but these were basically replacements for me seeing as the two I purchased from them had some serious problems (to which resulted in a recall of a lot of their mags due to some improper board coating). I received them about tow years ago.

I am pretty dang sure they are, I have an 8 with dual PMAGs and never an issue. Timed at a TDC verified with crankshaft mark and #1 at TDC of compression stroke. Not too long ago I retarded the timing a few degrees to see if it would help, but to no avail. I will conduct a pull through test next.

I wired them up to toggle switches with power coming of the master bus, so when I kill the master they are powered off. But I will test by pulling the breakers after a shut down next to verify I didn’t somehow mess that up when I wired it years ago.

Are you running a key switch for your P leads? Or push button starter with toggle switches for mag p leads?

 

[WingnutWick]

Are you running a key switch for your P leads? Or push button starter with toggle switches for mag p leads?

Push button with toggle switches.