TR2/3/3A "Vintage" engine analyzer tach unstable?

[Snowkilts]

I bought a "vintage" Auto-Tune engine analyzer with an inductive pickup to help diagnose a rough idle issue. Engine is running rough and shaking pretty badly below 1500 rpm.

I'm finding that the rpm on the Auto-Tune does not agree with the mechanical tach and becomes very unstable below about 1500 rpm, which correlates well with where the engine is also running poorly. The analyzer shows a higher rpm than the engine is actually running, sometimes as much as double. The behavior varies depending on which cylinder I clip the pickup on. Some are worse than others.

Anybody have any insight as to what is going on here? I have never use one of these engine analyzers before.

For reference, here's what I have done to work on this issue. The shaking made me think it was timing related, so I started there. Normally I don't like to just throw parts at a problem, but looking at my records, many of them were very old and probably due.

- verify correct operation of both mechanical and vacuum advance (distributor was rebuilt by Jeff at Advanced about 5 years ago)
- reset static timing to zero, then dial to 4 deg advance. Driving test resulted in going to 12 deg advance which is where I am now.
- replace points (they were pitted), cap, rotor (broken and loose on the shaft), cap, plugs, wires
- new Pertronix coil (old one did test OK though)

I'm planning to tune and balance the carbs next. They have relatively few miles on a rebuild and should be in good shape.

Thanks!

 

[Sarastro]

I don't know what kind of analyzer you are using, but some of them sense the spark-plug current and count the current pulses to get the RPM. If that current is erratic, it can count multiple pulses and give the kind of readings you describe.

What is the coil's primary resistance? It should be about 3.5 ohms. If you have one of the low-resistance Pertronix coils, it may have considerably lower resistance. Then you'll get a lot of arcing at the points and can overheat the coil.

It's unlikely that carb tuning is your problem, and if you try to retune the carbs you'll almost certainly mess up the settings and get nowhere. You can't tune the carbs successfully until the engine is running normally.

 

[DavidApp]

Have you checked the condition of the mechanical advance weights and springs?
Worked on a TR3B that shook badly and found the springs were not connected and the weights were lose.

David

 

[Snowkilts]

Have you checked the condition of the mechanical advance weights and springs?

Yes, they are in good shape. This was one of the items that was addressed when I had the distributor rebuilt a few years ago.

 

[Snowkilts]

I don't know what kind of analyzer you are using, but some of them sense the spark-plug current and count the current pulses to get the RPM. If that current is erratic, it can count multiple pulses and give the kind of readings you describe.

That's what I am assuming is happening. I'm wondering what could cause this? I believe I have replaced just about everything that is involved in producing spark.

What is the coil's primary resistance? It should be about 3.5 ohms. If you have one of the low-resistance Pertronix coils, it may have considerably lower resistance. Then you'll get a lot of arcing at the points and can overheat the coil.

It's a 3 ohm coil. I have no ballast resistor. It measured at about 3.5 ohms.

It's unlikely that carb tuning is your problem, and if you try to retune the carbs you'll almost certainly mess up the settings and get nowhere. You can't tune the carbs successfully until the engine is running normally.

But an engine running rich or lean could certainly idle erratically, could it not?

 

[CJD]

If the pickup is receiving an erratic signal, I think you found the problem. Burned points, bad condenser, or coil issue.

 

[Snowkilts]

If the pickup is receiving an erratic signal, I think you found the problem. Burned points, bad condenser, or coil issue.

New points, new condenser, new coil. The new points helped a little. No noticeable change with the new coil.

 

[Sarastro]

Hmmmm...it does sound like you did just about everything possible with the ignition. In my experience, the carbs can be quite far out of adjustment and the car will still run reasonably smoothly, at least at idle. If it didn't, you wouldn't be able to adjust them in the first place.

Be sure that the high-voltage wires fit well into the terminals on the cap and especially the coil. I have seen cases where the wire didn't go all the way into the connection on the coil and the car didn't run well as a result.

What about a vacuum leak? check the tube for the vacuum advance and make sure that the diaphragm at the distributor isn't leaking. If you had Strombergs, I'd suggest checking the diaphragms, but with a 60 TR3 you are probably using SUs.

Also, maybe a valve is way out of adjustment, so it's not opening enough, or too much, but I suspect that would cause more problems at high speed than low.

 

[Snowkilts]

What about a vacuum leak? check the tube for the vacuum advance and make sure that the diaphragm at the distributor isn't leaking.

I did put a vacuum pump directly on the advance unit and verified that the unit is working and holds vacuum. I've had to replace that multiple times.

I didn't check the actual line or the fitting into the carb yet. That's worth a look.

 

[TFB]

Did it ever run correct?
Do you have good compression?
It can be easy to not get the intake manifold seated and have intake leaks which are more noticeable at idle.
If you look at carb jets is the fuel level just below jet top and not flooding?
Does the tach have a switch for4,6 or 8 cylinders?
Good Luck,
TOm

 

[CJD]

Remember that new does not mean good in our hobby. In fact, for many parts (like condensers) it increases the chance of a part problem.

If you need to verify the old diagnostic set, hook it up to a good running car and see what it reads there. If it is erratic with a good car, then set it aside while you work on the TR. If it reads smoothly on good car, it still means you have an ignition problem.

Also, mechanical tach and speedos use a magnet, which tends to lose strength over time and heat cycles. If yours have not been calibrated in a decade or so, reading low is the way they go, since magnets never get stronger over time.

 

[Snowkilts]

If you need to verify the old diagnostic set, hook it up to a good running car and see what it reads there. If it is erratic with a good car, then set it aside while you work on the TR. If it reads smoothly on good car, it still means you have an ignition problem.

So the learnings continue.

I put the tach on my neighbors older pickup that has separate coils but still had a wire to the actual spark plug. Readings were erratic, but the truck runs fine.

Then I noticed that the inductive pickup has the words "Spark Plug" molded into it, with an arrow. Apparently it matters which way you clamp the pickup onto the plug wire. Once I put it on correctly, it was rock solid. I have no idea if I was doing it right or wrong on the TR.

I'm now reasonably confident that the tool is good, but unfortunately I can't actually try it on the TR, since yesterday I gently bumped a another part against the starter wire and the copper connecting bolt on the starter broke off and fell on the floor. So that will be today's problem.

 

[Snowkilts]

Did it ever run correct?

Yes. It's been good for the last 5 years since I had the distributor rebuilt.

Do you have good compression?

Meh.

It can be easy to not get the intake manifold seated and have intake leaks which are more noticeable at idle.

Never had the intake manifold off in 27 years.

If you look at carb jets is the fuel level just below jet top and not flooding?

I'll check. I didn't know this would be visible.

Does the tach have a switch for4,6 or 8 cylinders?

No. I just tried it on a V8 and it seemed to be reading correctly.

Good Luck,
TOm

 

[sp53]

After all the work you have done, I would look at spark plugs and see if any fouled or how different they looked. If the engine runs at 1500 best, I would look at oil in the dampers in the carbs OR something simple and ugly like bad compression on the rings or valves.
steve

 

[TFB]

Fuel level is visible if you take off the air cleaners and lift pistons to view the jets.Crank the engine with coil off and level should hold slight below jet top,and both carbs should be same.
Tom