TR3 Ignition timing, can this be real?

[Mick Forey]

A few years ago I fitted a Piper "yellow" cam a mild, high torque, cam for touring in my beloved TR3 as the old one had worn out, badly. I also had the carbs re-built by SU Burlen. As the cam was non-standard I took the car to Pete Burgess in Alfreton for him to set up the timing and mixture. I had done it according to the book but he squeezed some more performance out of the engine by tweaking both but not by much. I was a happy bunny and the car drove even better, returning good torque and 35-40 mpg on a tour. The only issue was that I had no idea what the ignition timing was set up to at what speed so I could reproduce this later. Therefore I purchased a Gunson timing gun with advance feature model 77008. Got a good steady reading at idle but it jumped around a bit as the engine speed increased.

I decided to plot the results on the ignition advance curve in my Service Instruction Manual for the TR2 (with addendum for TR3). The first point at 500rpm was measured at 8deg BTDC, right on the top tramline, the next at 1000rpm was 12deg BTDC, again a perfect spot on the line. Then it all went seriously haywire as I measured 20deg at 1500prm and even 28deg at 2000 rpm. I then ran it at about 4000 rpm for a few sec and measured 42deg BTDC. I did not understand the result, see the photo attached.

I then realised I had not disconnected the vacuum advance, I did this and repeated the above and got the same result on the four measuring points, what is going on? Clearly the vacuum advance does nothing under these conditions, I thought it should. So I checked the movement of the backplate, yes it does move against the spring and I heard the sound of air being moved by the diagram. I checked the centrifugal advance weights and springs were still in place, yes all looked OK. The rotor arm moved OK against the springs. Can the timing light be faulty? Am I picking up a false signal? I tried the pick up on No. 4 HT lead and got exactly the same result. Have I got the correct dizzy for the car, is the chart in the book correct?

I am now at a loss of what to do next. The car drives brilliantly and if I had not bought the timing gun I would still be happy or blissfully ignorant. Anybody got any ideas what is going on here?

Mick

[iain]

I suspect the Timing light, at 42 degrees advance I would expect horrible and potentially very expensive noises under load

Iain

[Andy303]

Distributor degrees are half the number of crankshaft degrees, which is what you are measuring. Divide your readings by two and replot. You should be close to the factory numbers.

[iain]

I had taken it as read that we were reading Crankshaft and dividing by 2, perhaps not. The readings do then make sense.

Iain

[Lebro]

This is plotting the above measurements, taking the 2:1 reduction into account. They do seem odd above 2000 RPM crank  (1000 RPM dizzy)

 

Bob.

Edited March 16, 2019 by Lebro

[Mick Forey]

Thank you for the input. Yes, I did not spot that the chart in the book was distributor degrees not crankshaft degrees. I have also borrowed another timing gun and repeated the exercise. I have learned the following:

1) both timing guns produce the same results on a steady reading. 

2) both guns introduce a level of dither when the advance knob is turned away from zero.

3) there is also a lot of scatter (non-repeatability) in the readings between 1000 and 2000 rpm up to 12 deg from one extreme to another.

4) I eliminated any gun sensitivity to voltage ripple on the battery voltage by using an independent battery, no change detected.

4) A plot of the lowest of any data generated produces a sensible result. Therefore, an additional level of advance is being introduced into the system, mainly between 1000 and 2000 rpm.

5) my back aches after so much bending over staring at the timing marks

6) I stripped the distributor and found that the pin holding the vacuum advance return spring was very worn. The backplate can move independently, does it when the engine is running?

7) the weights and springs look OK but given the above pin wear, I think they need checking properly.

New "lowest" data points re-plotted correctly. Notice the small circle at 1250rpm, 22 deg advance, that was the worst extreme of measurement scatter, only got it momentarily once but plenty in between with the image dancing about, sometimes settling at one end or the other then would move about even at a steady rpm.

Thanks for the help, much appreciated. It might be a call to the Distributor Doctor. Anybody else seen these phenomena?

[foster461]

Mick, is there anyone near you that you can swap a distributor with for a test ?. A dizzy with a worn shaft and weak/incorrect weights will also show timing results all over the place. Dropping in a pertronix in place of the points and rotor would also be another easy test.

Stan

[Drewmotty]

The last plot looks great to me. A couple of degrees extra on the adjuster would bring it right into the tramlines and more or less the same figures as my cars run happiest at (32 to 33 deg max)

if the car runs well as it is then maybe leave it alone but that extra couple degrees may give you some extra edge.

[Mick Forey]

Another day bending over under the bonnet. I had a call this morning from a good friend offering both a spare Pertronix and a spare dizzy. The Pertronix turned out to be a dud, he was given it so no loss. The dizzy was fitted with points so after a good clean and setting it up, all worked well. Exactly the same symptoms as mine, interesting. However, the centrifugal advance mechanism was seized but the vacuum advance worked much better than my own dizzy. The image created by the timing gun still leaped about above 1000rpm as soon as I turned the advance knob on the gun. Very steady at idle. This was true of both dizzies and both guns. The old gun was better than the new one. 

I wonder if the voltage regulator oscilation is causing interference? Next step is to disconnect the dynamo and try again. 

My dizzy definitely needs help from the doctor. 

Mick 

[DesNoble]

Hi Mick,

Did you resolve your advance issues? I have a 59 TR3A with a DM2 P4 (40480A) distributor. When I rebuilt my engine I used  87mm pistons (2188cc), a TR4a inlet manifold, a TT 4 into 2 into 1 exhaust manifold, std cam. My advance looked like yours. I'm presently running on about 15 deg. BTDC and my advance goes up to about 50 deg all engine crank readings. I'm thinking of limiting the max advance to about 34 deg by changing the springs but really the hardest thing to get around is what people are actually talking about when they convert distributor to crank rpm. They seem to take what suits the advance curve fit regardless of doubling both the rpm and the degrees. Anyway what have you come up with.

Regards

Des.

[Mick Forey]

Hi Des,

Yes. I sent it to the Distributor Doctor with a detailed spec of my engine. It turned out that his 4 pot is almost identical to mine so he set my dizzy up just like his. Runs very well now but it took a few iterations to get the static timing right. I could not believe how much I could advance it without any pinking. I was very gentle with this until I had done over 500 miles on the engine rebuild.

I can recommend the Doc.

Mick

[DesNoble]

Hi Mick, Thanks for the info. I had the Distributor Dr refurb my vacuum on the disco. Like yours I seem to be able to advance the ignition without pinking, the 4A inlet manifold I think has that affect, however I have currently only done 200 miles so I don't want to stress the engine until I have run it for about 1000.

Thanks.

Des.

[Mick Forey]

I partially stripped my dizzy before sending it off, everything was quite badly worn, so had the whole thing done. Now it is like new and the strobe light image does not leap about.

Mick