TR6 Ignition Timing Setup

[mikeyb]

Help!  I'm suffering from brain fade   :-/

I had to remove metering unit to replace dodgy fuel pipe to No.2 injector and dizzy pedastal to cure an oil leak.

Having previously had a good running car with timing set to 4 ATDC as per the pulley marks, I set the engine to TDC, marked the dizzy/pedastal up and pulled it apart.  Having reassembled, setting up drive gear, dizzy & MU up as per the book (yes the correct book, not the erroneous haynes etc.!), I now find the car runs sweetly but not at the 4 ADTC timing mark. More like 20-30 deg ATDC.  

I have tried moving the dizzy drivegear around a tooth, but seems not to make a difference.

What have I done to cause this?    :(

[sparky_spit]

Hi Mike - moving the dizzy drive gear round one tooth does not change the timing, it only changes the dizzy's rotational position relative to the engine.  The timing will only change if you change the relative positions of the cam lobe and the dizzy baseplate. As you are moving the whole dizzy, these have stayed the same .

When you say ATDC, do you mean BTDC?  This would normally be the timing marks for most cars firing point.  Or do they refer to something else?

If it's running fine at these settings, then I'd leave it as is, and adjust for power Vs pinking on the road. The best running timing point on my Spitfire is actually 19deg BTDC as per the pulley marks, but you can bet it's really nothing like that in practice.

[mikeyb]

That's the strange thing - the cam lobe position should not have changed relative to the TDC marking on the crank pulley. Although I removed the dizzy pedastal and separated the dizzy from it, the baseplate is bolted directly to the pedastal, the pedastal itself direct to the block, both in fixed alignment.

The car does run fine, I intend to re-check the timing using the vacuum method to be sure. What worries me is bearing in mind the car was adjusted and ran fine to its prescribed timing mark before, it doesn't at the same position now - which is off scale.

Early TR6's if I'm not mistaken, time up at 4 deg after top dead centre (dynamic at idle)

Edit - I've just thought about this and by baseplate, you are referring to the moving plate in the dizzy, yes?  Above I was referring to the clamp plate.  The dizzy has not been apart, but i wonder if the baseplate had previously been stuck in one position and has now freed off?  Would still not expect to be off scale with respect to timing marks though...

[sparky_spit]

What you say makes sense, and it is odd that the correct ignition point is now some way off where it was before.  It can only mean that something has moved.

I had similar problems with the Green Slug when I first got that and had problems getting the rotor arm pointing at a cap segment when the points opened.  It turned out the be the vacuum advance spring/hook had become detached from the peg on the points baseplate, and the baseplate had turned fully anticlockwise as far as it would go.  Before I found that I fannyed around for ages putting the dizzy drive in next tooth round.  It made no difference.  It was then that I realised that the drive pin on the dizzy shaft, the cam lobe, and the rotor must always be in the same orientation, as they are all fixed in place on the same shaft. It makes no difference, all other things being equal, where you have teh drive gear; it just means that the dizzy might be in an awkward place to connect vacuum pipe, coil lead, etc.

If your points baseplate is now say 8deg rotated from where it was, that equates to 16deg of crankshaft rotation (remember the camshaft turns at half engine speed). It's worth checking the the baseplate - see if there are any witness marks to show where it might have moved from?

[Nick Jones]

Is the mechanical advance in the distributor working properly?  IE rotor arm can be twisted against spring pressure and returns cleaning to it's start point when released?  I wasted alot of time on the PI due to this and got some very strange timing numbers, seemingly at random.  Difference is the car did not run fine though.

Nick

[Nick Jones]

Is the mechanical advance in the distributor working properly?  IE rotor arm can be twisted against spring pressure and returns cleaning to it's start point when released?  I wasted alot of time on the PI due to this and got some very strange timing numbers, seemingly at random.  Difference is the car did not run fine though.

Nick

[mikeyb]

Well, here's an update.  I took the dizzy out again yesterday, pulled it apart to make sure everything inside was moving as it should - it was, although I did find a tiny unused rivet stuck in the bottom - looked similar to those holding the centrifugal weights together. I also changed the vac advance unit (albeit not used on the TR6) as the vernier adjustment has seized on the old and the wheel had been mullered!  

The result - no difference whatsoever!  By ear the car timed up at an indicated 20 deg BTDC, on the road drove nicely.  Timing then rechecked using the vacuum method resulted in an indicated setting of 24 deg BTDC, but with some pinking on the road test. There was also a perceptible increase in fume from the exhaust.  So I have ended up back at 20 deg BTDC, the car is a pig to start from cold, tickover is lumpy and hunts but on the road she absolutely flies - way better than ever before.

Still concerned, cannot work out the root cause of this change  :-(