AH 3000 BJ8 tachometer

[davidb]

Evening gents

Here's a tachy question for the technophiles (we know you NASA types are out there, with your flux capacitors secretly hidden away behind the radio.....)

By the sound of things, my tachometer reads higher than it should, probably 3-4 thousand rpm, as a best guess, and while I'm saving up to get my speedo and tach re-built, I'd like to get a better reading on how much it's off.

Is there a relatively simple way to determine this, because I've gone thru previous threads and several Victoria's Secret catalogues, and haven't come up anything other than a few naughty ideas for Christmas for my wife...

Thanks

Dave

 

[Roger]

Got a GPS? Drive at a steady speed, say 50mph. Note your speedo and tach readings.
Calculate what your RPM should be at that speed. (There are a number of free programs on the internet to help)

Presto!

 

[Bob_Spidell]

Get something like this (some timing/advance meters have dwell and rpm readouts, too):

https://www.amazon.com/Actron-CP7605-Tach...0560&sr=1-1

-- or --

https://tinyurl.com/248csh7


The tach can be calibrated. There's a potentiometer (variable resistor, aka "pot" or "trim pot") on a circuit board inside the tach. You'll need to drill a small hole so you can get a jeweler's screwdriver in there to turn the pot. Tachs tend to drift over the years--I calibrated mine and now it's spot on.

See: https://www.hazelden.ca/austinhealey/RebuildingBJ8Tach/

 

[davidb]

My thanks to both Roger and Bob for comments/suggestions on the tachometer problem. Bob - your website referrals were particularly helpful. When can you come over to recalibrate my tach? I can pick you up at the airport....

Cheers
Dave

 

[Legal Bill]

Bob, what is Dwell? I never heard a good explanation of that. Also, can that meter be used to time the engine? If so, how?

 

[Randy Forbes]

From Wikipedia: Dwell angle, a term used in automotive context, indicating the degrees of rotation of the distributor cam during which the ignition contact breaker points in the distributor are closed

Key point here, is the amount of distributor <span style="font-weight: bold">degrees of rotation</span> that the points are closed.

The longer the points stay closed, the more time the coil has to build up a charge before the next time the points open (collapsing the magnetic field) and discharge energy to the sparkplug.

 

[Bob_Spidell]

Dwell is how long the points stay closed, allowing primary current to flow through the coil. When the points open, the magnetic field generated by the primary windings collapses, inducing voltage and current flow in the secondary windings (the primary and secondary windings are concentric loops or 'coils' of wire--separated by an insulator, usually oil--in the coil, hence the name). The voltage generated is determined by a simple ratio: number of secondary windings divided by number of primary windings. For example, if you have 10 primary windings and 1,000 secondary windings you will amplify the primary voltage by 100. Coils are essentially voltage 'step up' transformers.

Back to dwell. Since the amount of time the points stay closed varies with engine speed, dwell is measured in degrees of rotation of the distributor shaft. The distributor shaft is driven by the camshaft, which turns at half the rate of the crankshaft, so if the dwell is, say, 36deg then the points are closed one-tenth of one rotation of the cam, or one twentieth of the rotation of the crank. Setting the dwell with a dwell meter is generally more accurate than setting the points gap with a feeler gauge. As the rubbing block on the points wears down, the dwell will gradually increase. Too little dwell and the coil won't become saturated enough to generate a good spark, too much and the coil will get hotter than necessary (eventually breaking down the insulation). Points-driven ignition systems have trouble generating a good spark at very high RPM--the dwell angle stays constant but the time the points are closed decreases as engine speed increases.

Timing determines when the spark is fired in the cylinder, measured in degrees of crankshaft rotation. When the points open a high voltage is generated in the secondary ignition for the appropriate cylinder as determined by the rotor and the terminals in the distributor cap. You set timing by changing the position of the distributor relative to the camshaft.

The meter I posted a link to is a Dwell/RPM/Voltage meter. You cannot set timing with it--you need a timing light or advance meter (which allows you to set a particular advance then time to a Healey's single mark on the crank pulley). There are timing lights/advance meters with the dwell/RPM/Voltage functions built in--they have an LED or LCD readout and you can select which function you want. They are, of course, more expensive than a simple timing light.

Here's a good article: https://en.wikipedia.org/wiki/Ignition_timing

 

[wheelguy]

There is a ajustable pot. on the inside of your tach. behind the cover. It will allow you to adjust it to read right with a external gauge hooked up to go by.
Allen

 

[Legal Bill]

Bob and Randy, thank you very much. Excellent information.

Bob, your post gets at eactly what I am trying to figure out: how to best set the engine timing, given the paucity of timing marks on the engine. The timing light with an advance feature sounds perfect. Do they work on positive ground cars?

 

[Bob_Spidell]

... The timing light with an advance feature sounds perfect. Do they work on positive ground cars? ...

Yes. There was a long and involved forum topic on this a while ago (I'm too lazy to look it up--maybe someone remembers). Some people get confused by the term 'ground;' it's just one half of an electrical circuit--nothing magical--although it's the one that's exposed that things generally connect to for the return path to the battery. I also documented how I put a GPS power adapter--cigarette lighter type--in my positive ground car.

The issue with using 'conventional' devices in positive ground cars is isolation--any time a negative terminal and positive terminal come in contact current flows, sparks fly and, well, you have a MIG welder. If the polarity of the device and the car are different and the case of the device is metal and grounded you have a potential problem.

Here's an example of an advance meter:

https://www.harborfreight.com/timing-light-with-advance-40963.html

The red clamp is positive, the black is negative. For a PG car connect the black clamp to a current source--the hot lead on the starter solenoid is convenient--and the red lead to bare metal on the chassis (opposite to how you'd connect a negative-ground car). Clamp the inductive lead on the #1 plug wire, dial the desired advance--usually, 15deg BTDC at 600RPM with vacuum advance disconnected and sealed for a 6-cyl Healey--on the meter and turn the distributor until the mark/line on the crank pulley lines up with the pointer on the timing cover (engine running, of course). If you want, set 35deg and rev the engine to see if you get full advance around 3,000RPM, or set for total advance and let idle advance fall where it may (don't forget to tighten the clamp on the distributor). Some people set advance for max vacuum.

I haven't used this particular advance light, but it looks like it has a plastic case so it's PROBABLY inherently isolated. I had a nice Sears one with a plastic case but it got stolen years ago--I'm still ticked-off about it.

 

[Legal Bill]

Many thanks, Bob. that is excellent information. Everything you say makes perfect sense. I have a great inductive timing light now, but it does not have the advance knob. fortunately, our friends at Harbor Freight are able to provide us with the finest products from China at rates that would embarrass a prison work shop manager. Thanks for the link.