VW engine timing Question - Help needed

[spriteah]

Hi all,

I have been trying to help a fellow flyer with a VW engine. I am having trouble getting the baby to start. Looking for ideas or advice.

It's a VW aero conversions.
Magneto ignition that has been re-timed. It's a Bendix dual mag D4RN 2021. Has been timed to the K mark and E gap at R which I think is 8 deg BTDC. It has an impulse system to delay 20 Deg to assist with starting.

When I check timing on the engine I can adjust it to open points at 8 Degrees BTDC. However the motor will not start. I get the occasional fire but just wont turn over.

Any ideas? Does the timing sound correct? Any advice from some that have experience with the VW's or Magneto would be greatly appreciated.

Thanks in advance
Jim

[blue up]

Running Fred Video by foggythomas | Photobucket (http://s82.photobucket.com/user/foggythomas/media/2005_1011Image0022.mp4.html)

This one also has a built-in impulse, albeit electronic. Fires at just after TDC (IIRC) for starting and runs at about 24 BTDC.

The timing marks on the crank flange may be i a different place to where you'd expect them. Mine didn't have TDC on it but a mark for 28 ATDC. I seem to recall that the woodruff key is at 12 o'clock at TDC on number one. It may be best to ensure you have the marks in the right place to begin with.

Are you 100% sure you have the impulse running correctly? Also, sure it isn't trying to RUN at 8BTDC?

I'm not much help, I'm afraid. Best of luck.

[cockney steve]

If your static timing is 8 before, a 20* delay would see you trying to start at 12 AFTER!!!!..doesn't sound at all right to me....As previous poster said...check your marks and if necessary, go back to basics, remove a plug poke a bit of wire in (maybe a thin screwdriver) and "feel "for TDC.
8* fixed does not sound nearly enough , to me, but i'm NOT an aero-engineer.

Have you tried asking the LAA?...they, no doubt, have the knowledge.

[SawMan]

Make sure #1 is beginning the compression stroke. With the plug removed, take a piece of threaded rod or a long bolt and insert it into the #1 plug hole till it touches the piston. Rotate engine manually while carefully letting the piston push the bolt out. Hold your finger against the head during this process and use it to 'mark' the bolt as TDC passes. Now thread a nut onto the bolt past where you marked it so that the nut will rise from the head before TDC. Tape or lock-nut the first nut in place.

Cycle the engine back to the beginning of #1 compression and insert bolt till the nuut seats against the head. Very slowly turn the enging and stop as soon as the piston touches the bolt. Make a mark on the flywheel at that point. Continue to turn engine with the bolt and nut rising, then beginning to fall back into the head, stopping at exactly where the nut touches again. Mark that spot on the flywheel. TDC will be exactly in the middle of those two marks you made. If you have the nut set somewhere around a few degrees BTDC/ATDC you can use a rule held straight across the two marks at the edge of the flywheel and it will be very close toi the edge of the flywheel in the middle, making the center mark quite accurate if you did the rest right.

I use this technique to check when I suspect the timing marks to be off on an engine. Tuners use a "degree wheel" which can be had in plastic rather cheaply, and a threaded adapter to replace the bolt which positively stops the piston before TDC, then they rotate backwards till the piston stops rotation again. That gets it perfect, but the way I related here will get you within less than 1/2 degree if you're careful, and that will get the engine started and running well. Look at automotive cam manufacturer and seller sites for a degree wheel and how to use it, the plastic ones are all you need and can be had for less than the price of one lunch. Hope this helps!

[bcgallacher]

Not trying to be clever - we have all done it - are the leads connected to the correct cylinders ? Is it a single or twin mag. installation?

[spriteah]

Thanks all that have contributed so far. I have made a plastic wheel to determine TDC as mentioned by sawman so I'm happy I have that correct.


Leads, will check today as I did not remove them (someone else did and they have been marked with Pen.


Its a Single Mag (dual ignition). With this in mind if the mag is internally timed correctly it can only be installed correctly or out 180 degrees. I have altered it 180 degree out of frustration and could not get it to fire. Manually spinning the mag I can get spark on all plugs so that side is working.


Jim

[blue up]

It is often said that 25% of ignition faults are inside the Carb. A squirt of NEW fuel into the intake? I have the same problem with my old cars along with the nasty effects on the carbs themselves that the ethanol brings.
Also, could you have an earth fault to the mag switch or leads? Beginning to run out of suggestions.





Spriteah, A H Sprite???

[stevef]

I haven't worked on a dual mag for many years so I'm a bit rusty but assuming the timing is correct, I'd try swapping the P leads over just in case they've been crossed. Maybe the switch wiring is arranged so that the engine normally starts with left magneto selection only and now it's trying to start on the right with the impulse spark effectively grounded.
Just a guess.

[cockney steve]

^^^^^^^ Single mag, dual ignition, 20*impulse retard...says all that in previous posts :p

OP- I still think this is a timing issue....you said it fired but wouldn't run.....was that one cylinder?--or more?.... I'd agree that it could be possible that some cylinders have the wrong leads attached....but i'd expect some of these to "knock back" when you crank....we also make the sweeping assumption that nothing other than ignition -timing has been interfered with since the engine last ran.

check the fuel supply is actually filling the carb. float bowl! I'd be tempted to increase the timing advance to ~15-20 degrees before TDC...that would give you a spark on impulse, , around 5 before, to tdc itself.....at 5 before, the flame-front still has to propagate and the mix start to expand......by which time the crank has gone over tdc and the gases are chasing the piston.

this is one reason why fixed-timing aero-engines seem to stop the prop momentarily, when cranking....they need a lot of advance to run efficiently at normal cruise revs, so the timing, static is grossly over-advanced....enter the impulse to A-delay the spark until the piston is much further up it's stroke.(It also means the charge is more compressed and hotter , so easier to ignite.
B- give the magneto a sharp flick across the magnetic "break" thus inducing a juicier spark.

further to point "A"... the VW is not noted for high compression (or revs!)....I would expect the start-timing to be more critical than a lycosaurus!.


Advance static to 20* BEFORE tdc....if it's wrong, the engine, IF it fires, will merely kick-back...A running engine will usually tolerate the ignition being swung ~40* either side of TDC before stopping....but it won't produce much power at extreme over-advance or retard. also, not recommended if under load.

[Bye]

i would replace the condenser/s if you can as a precaution as the failure mode can manifest itself in some strange ways and is often very hard to detect.

assuming of course your system has them still.

GB

[stevef]

Single mag, dual ignition, 20*impulse retard...says all that in previous posts.

Not quite - it's two magnetos in one unit driven by a single accessory case gear shaft. I've never understood the logic of having a 'fail-safe' ignition system with a common mechanical drive but problems do seem to be rare.
Spriteah - can you give us the full history of the recent engine problem. Did the non-starting begin after timing adjustment or was there a previous failure of some sort?
Re points adjustment, magnetos usually have them set on the bench (along with the E gap) and final timing is carried out by rotating the magneto on its flange on installation after setting No 1 piston on its compression stroke and using marks on the starter ring gear (or a timing disc) and a timing synchronisation light. My memory is a bit hazy re dual mags though.

[dubbleyew eight]

I had a lot of trouble on my continental O-200 with new magnetos until I realised that the earthing of the magneto must have been dodgy.
I put earth wires on each magneto case (slicks) to the engine block and have had no further problems.

clarification. I'm not referring to P lead grounding in the above.
you would think that metal to metal contact where the maggies bolt to the accessory case would be enough but gaskets and oil make it problematic.
a lead from the magneto case to the engine block sorted it out.

[spriteah]

Gents, Thanks all for the responses. The story is that the aircraft experience ignitions problems and another person before me removed and serviced the Mag. That person replaced the points re-installed but then couldn't start the plane.


I check it and found it was about 45 degrees out. Checked in manuals and believe the mag should be internally set to K mark not C mark. I did this and set the E gap to "r" at 0.4mm as per spec. I can then test in situ and using a mag timing device get the points to open around the 8 Deg BTDC. The mount allows me to alter the timing about 20 Degrees either way.


When rotating the prop (hand start) I get an occasional fire (little movement of prop more a back fire) I have tried altering the mag locations in all directions hoping it would roar to life but not so.


The spark plugs are correct and mag is correct (not 180 out). I have tried with aero start still no roar. The P leads are removed and connections isolated with heat shrink being it not grounding.


I wonder if the Mag needs to be timed different to normal. I have sent an email to Revmaster hoping to get some advice. I will try to get it retarded about 28 degrees but I know the mag will be on an angle and not fit into the planes firewall. So if this is required then I feel the problems will be internal of the mag. As stated I didn't pull it down so I don't know what it looked like before. I got it.


Regards and thanks all. Keep ideas coming.


Jim

[spriteah]

Spriteah = AH Sprite.. Sold it now. Had a Mark 4.

[MurphyWasRight]

Dont think this has been mentioned yet:

Verify that you are actually getting a good spark at each cyclincder.

Use a spare spark plug clamped to a grounded part of the engine to verify each cable, looking for nice fat spark. Do not try to hand hold the plug:)

This verifies almost everything to do with ignition except for timing.

BTW: I am -not- an aircraft mechanic but ages ago owned/worked on a VW bug engine. Nothing like setting the valve lifter clearances every 3000 miles.

The good thing about VW engines is that they were easy to swap out, the bad thing is that they needed to be.

[cockney steve]

get the points to open around the 8 Deg BTDC. The mount allows me to alter the timing about 20 Degrees either way.


When rotating the prop (hand start) I get an occasional fire (little movement of prop more a back fire) I have tried altering the mag locations in all directions hoping it would roar to life but not so.

read my previous posts again!:) IF you are timing properly "true" static and NOT "impulse!" at cranking speed, you'll be ~12 AFTER TDC

THE FACT IT IS KICKING BACK means EITHER timing is out (advanced too much)....you are switching the impulse side "on" for starting???
OR you have plug-leads crossed..... IE...they are not all connected to the correct cylinder.....check your firing-order and check you have not got the cylinder-numbering mixed-up.

[spriteah]

Steve and others

Spark is good. I have gotten some feedback from Revmaster. It's definitely the timing internal of the Mag. I am now hoping they inform me what the setting should be before I start to experiment.

From Revmaster the firing should occur about 28 Degrees BTDC. Impulse delay (they say 25, manual says 20).

So if anyone knows the Mag internal settings to suit a VW please impart such knowledge this way.

Thanks again for the advice so far.

Jim T.

[cockney steve]

Sprite, Thanks for the update!...It's always nice to learn the outcome of mysteries like this.
Spark is good. I have gotten some feedback from Revmaster. It's definitely the timing internal of the Mag. I am now hoping they inform me what the setting should be before I start to experiment.

That appears to be a contradiction in terms.

IF you can disable the impulse mech, both mags should spark simultaneously. any correction would be a matter of altering the points-gap (and therefore the dwell-angle and the spark-timing)

The internal timing, IIRC, refers to the dual function of a magneto...IE, it is both a generator (dynamo) and HT transformer...OK, it's also a distributor inthe case of a multi-cylinder setup. however we are currently concerned with the generating side.... as the armature rotates through the pole-shoes, there isa "break" as the flux across the pole-shoes is broken...at this point, the primary winding has a current induced in it......ths break has to be synchronised with the points ....the primary needs maximum energy in it at the instant the points open , to collapse the field in the primary, thus inducing a high voltage in the secondary... the capacitor ensures the current does not arc over the points, thus extending their life and aiding the rapid collapse of the primary field.


If you are getting a healthy spark, it would suggese the magneto's internal timing is correct (well, near-enough) Therefore, assuming it's gear-driven from the engine, you need to check if the gearwheel is affixed to the magneto drive shaft in the correct orientation , that the timing-marks line up when the correct cylinder is firing.

Sorry, I don't know the particular magneto or engine details...but you confirm what i suspected....you still need advance at cranking speed , because the ignited gases will use those few degrees of crankshaft rotation, to expand...by which time the crank has gone over TDC and the gases start pushing the piston down at the earliest opportunity.

to summarise....goodspark means mag is probably OK...next thing is to time the spark so it occurs at the correct point in CRANKSHAFT rotation.

DO NOT BE MISLED by spurious flywheel markings or the wrong datum-point

Find the timing-cylinder (usually No.1...but not necessarily the "front" cylinder of a straight, Vee or flat configuration)

If using flywheel or crankshaft-pulley marks, establish where the TDC datum lines up with the "timing-cylinder" at TDC on the compression stroke

the datum can be a cast-in pointer, a screwed-on one, a crankcase-joint, etc

check , also if there are BOTH timing-marks AND TDC marks...there could also be camshaft -timing marks!....
you KNOW what the timing *should* be...so, once you've established the engine at TDC, rotate it BACKWARDS (yes, I've seen people think "advance " means "forward" in rotational direction)...It's NOT intuitive as a term, the old "early-Late" was a better description! by the timing-amount of degrees

THAT is the point at which your fixed-timing mag should "break" and spark to the timing -cylinder.

Sorry if I appear to be "talking-down" It's easy to assume that someone else "knows"....so going through the whole thing will hopefully help others who don't

Try googling Magneto principles, Magneto setting-up .magneto theory, magneto timing....I'm sure there will be a tutorial somewhat better than my attempt!