View Full Version : TR2/3/3A Ignition Timing - Again

06-28-2018, 10:44 AM
I recently installed a Pertronix distributor and was able to set the static timing at 4 degrees BTDC. The car starts and runs well without any apparent detonation. The plugs however are black with a very small bit of white showing at the center electrode and the underside of the ground electrode. The car when hot seems to load up and the idle decreases and will eventually stall. I'm assuming that the problem is with fuel mixture and have turned in the jet adjusting nuts a quarter of a turn. At the same time I would like to check the timing with a strobe. My strobe is the older variety without an advance mechanism. The literature I have states that at 4 degrees from TDC the hole in the pulley is 1/8" or 3/16" counterclockwise before the pointer depending on which page you're looking at. It also states that at 400-600 RPM, timing is also at 4 degrees before TDC. Since my car will not idle at that speed and is more comfortable at 700-750 RPM what is the advance at that speed? I can't seem to find information of full advance for a TR3 and am not sure that the Pertronix distributor has the same advance as the original mechanical distributor. I've read that 15 degrees is used for timing with a modern strobe but I don't know what RPM is used at that reading. Timing with a strobe is difficult anyway since there is much in the way in order to point the light accurately and guessing how far the mark on the pulley is advanced from the pointer is not very accurate. I'm guessing with an older strobe the mark should be somewhere between 3/8"- 1/2" from the pointer at 15 degrees BTDC (but at what RPM?). I know investing in a more modern strobe would be helpful and I plan on it. They certainly are cheap enough. Any help would be appreciated.

06-28-2018, 11:08 AM
Have a look here https://www.britishcarforum.com/bcf/showthread.php?90580-TR4A-with-TR3-distributor-and-Vacuum-Advance

06-28-2018, 11:41 AM
According to the advance curves, at 750 RPM, the distributor should be set somewhere around 9 degrees BTDC which would put the hole in the pulley about 3/8" before the marker.

06-28-2018, 11:57 AM
Each degree is just short of 1/8 on the pulley hub. Advanced will be with the pulley short...or left...of the stationary marker. So, 4 degrees will be about 3/8 on the pulley hub. 15 degrees would be about 1-5/8 on the hub. The math is .104 per degree. 9 degrees is closer to 1 on the hub.

Remember, this is just to get the motor started. By the book, timing should be set forward as much as possible until you get preignition...then back off just enough to prevent preignition.

I think your idle mixture is too rich, and not timing related...

06-28-2018, 12:33 PM
The book I have says that 4 degrees is 1/8" before the stationary marker. It also says that at 400-600 RPM the pulley marker should be 3/16" before the stationary marker and claims that is also 4 degrees. Your claim of 3/8" is twice as much. Confusing.

06-28-2018, 12:54 PM
Much of the confusion comes from some numbers being in crankshaft degrees, while others are in distributor shaft degrees. The difference of course is 2:1, but the various books tend to be sloppy about specifying which units they are using.

3/8 at the pulley is closer to 8 crankshaft degrees.

The 15 degree number is probably full centrifugal advance, in distributor degrees. According to the book, it should happen at 2700 rpm; but again I believe that is distributor rpm.

So, by the book, total advance should be 34 degrees (4 initial plus 30 centrifugal, ignoring vacuum) at 5400 rpm (all at the crank, which is the usual way to measure such things).

I don't have the exact measurement handy but the pulley is pretty close to 6" diameter. That makes the circumference (distance all around the edge) 6 * pi, or 18.8". Divide that by 360 to get distance per degree, which is .0522" per degree, slightly less than 1/16".

06-28-2018, 01:53 PM
So that makes sense. 3/16" (per book) before the marker would be approximately 4 degrees BTDC. So according to the advance curve submitted on another post, if my car is idling at 700 RPM the advance should be around 9 degrees before TDC and the hole in the pulley should precede the marker by about 3/8". Better to get a new timing light.

06-28-2018, 05:25 PM
Randall’s right...2(pie)R. = 6” times 3.142/360 , so .052” per degree.

Cut my numbers above in about 1/2!

But I still think you have a mixture rather than a timing issue.

06-28-2018, 06:48 PM
Timing the ignition at idle can best be set by advancing timing to highest smooth idle at 750 or so and retarding it to lose about 100 rpm. (Neither of my TR3s would really idle below 700 or so, regardless of other adjustments.) Unless your distributor is newly rebuilt, higher rpm advance is dictated by the worn-ness of the dist. and condition of the springs. You can also set timing at speed in 4th gear by advancing it until you get a couple slight valve pings when you open the throttle fully at about 3500 rpm. Again, a worn distributor renders this method unconnected to what the advance will be at idle. I prefer a smooth idle, so the best idle minus 100 is what I use.

06-28-2018, 10:09 PM
Is it normal to get a smooth idle only at 1200 rpm with a fast road cam....mild street cam?

06-28-2018, 10:17 PM
In my TR3 service instruction manual, in the electrical equipment section, page 33 can be found the "automatic advance curve" chart in distributor degrees/RPM. When I first installed my points, and again when I installed a Crane Ignition I made a small "pointer" on a length of tape around a tooth pick. I pre-marked the tape with a few points of interest including 1,000RPMs (engine speed). Then I put the toothpick into the crank pulley hole and transferred the marks with different colored dots onto the pulley. Then I could run the engine and see the exact mechanical advance as it happened with my non-advancing strobe light. Of course, then I had to find an accurate method of measuring the engine speed, because you can bet your tach is not as accurate as you'd wish. -And later on I needed to know how to test the vacuum advance, and how much could be expected.

Without going into detail how I finally measured engine RPM, I can say that after it's all said and done, I still have no idea where the "right" spot is to set the timing at any speed, or how much it should be advanced. Without a dyno, we are just guessing. Once you know how it should run, you can loosen the distributor at idle and rotate the distributor and make it sound right without any instruments. If it starts running hot, or pings or has no power, maybe it needs adjusted a little.

You should also be aware of the significance of the steep advance curve at very low RPMs. -So as you slightly change the timing, the RPM changes, which changes the advance... or if the engine speed is slightly changed with the idle screws, the advance changes as well. If you really want to be curious, you can look at how many different advance curves are available simply by changing the springs on the rotating weights, or you could get a programmable curve and not be limited by a pair of "linear" lines. I tend to over complicate things, so I enjoy my method and my TR3 runs nicely -but it doesn't start with the quickest bump of the starter button like my friends who times his by ear.

06-28-2018, 11:09 PM
Is it normal to get a smooth idle only at 1200 rpm with a fast road cam....mild street cam?

I would say "no" with either of those choices. That said, I used to run a "3/4 race" cam that was pretty lumpy at 1200. Engine sure ran like stink above about 3200, though.

With the stock cam, etc and what they sell for premium fuel here in California, I find that I can use a bit more than 4 BTDC. I like to get the engine thoroughly warm, find what advance makes it just barely ping if I floor the throttle at around 1500 rpm (30 mph in 4th direct), then back off a degree or two. The result seems to be closer to 8 BTDC static.

Depends on a whole lot of things though, including what fuel you use and what altitude you are at; even outside air temperature (heat makes them more likely to ping). So your mileage may definitely vary.

Also, if you think you might ever want to actually hand-crank the engine; it might be best to stick to 4 before. It's my opinion that the hand crank is why they chose such a slow spark at idle, and a fast advance to bring it up at higher rpm. The more common 10-15 static advance makes it more likely that the engine will "kick back" during cranking and potentially injure the operator.