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MG MGB Technical - Fast reading tachometer

I have recently purchased a 1967 mgb roadster. The tachometer reads fast by about 16%.

At 1000 rpm in fourth gear the speed is 15 mph and it should be 18 mph.

Is there any way to adjust the tach to correct this difference?

Thanks Andy
Andy Preston

<<<snip>>>
At 1000 rpm in fourth gear the speed is 15 mph and it should be 18 mph.
<<<snip>>>
How do you know it's supposed to be 18 mph? Why are you comparing the two at such a low speed? You should be doing the comparison more at 3000 RPM.

It's more likely the speedometer is off. Do a speed check using mile markers and a stop watch.

Speedometer errors happen because of the use of incorrect tire size, or someone's added OD to a non OD car without correcting the speedo, or changed the diff ratio, all without recalibrating the speedo.
Blake

Hi Andy.

Better and easier to compare the tach with another instrument, maybe it is the speedo that is off. But if it is the tach rumour has it there is an adjustable potentiometer in some, if not all.

BTW, do you know anything of the whereabouts of Paul Kile from Sacremento? I'm trying to return something to him but have not heard from him for many months.
Paul Hunt

Andy
Check the tachometer by connecting a second tach to the car. I used a good (New) tach/dwell meter to set my tach which was running very fast. To adjust the tachometer you will need to remove the tach from the car. Disconnect the ground and the power supply. Remove the ignition light and the light for the tach. The impulse lead to the coil can be removed by unscrewing the knob and pulling the lead from the tach. Remove the tach from it's case. First remove the chrome ring and glass by rotating the chrome ring to line up with the tabs. Don’t bend the tabs. I removed the glass by also removing the metal ring that it is set in. You will know what I mean when after you remove the chrome ring. Next you need to undo the two screws that hold the tach to the case. There are four screws on the back of the case. Two hold the tach together and will pass trough the case. The other two hold the tach into the case. Remove the two screws that have holes the same size as the screws, not the same size as the screw heads. I did all of this on the kitchen table as it gave me a clean work space. Do not touch the needle and be real careful with the face. On the back of the tach there is a white knob about 1/2 to 3/4 inch in diameter. This is the adjustment pot for the tach. You need to drill a hole through the case that will line up with this knob. You will notice that the knob has a slot in the middle that will accept a screw drive. Now is a good time to also paint the inside of the case a bright white. It will help with the lights at night. The hole that I drilled was 3/8 inch. I also when to my local hardwood store and found a plastic plug that would fit the hole. Re-assemble the tach and put it back in the car. The hole that you drilled in the case will allow you to reach the adjustment pot for the tach. It only takes a slight turn of the pot to effect the tach. I used my tach/dwell meter to set the cars tach at about 3,000 rpm's. When done with the adjustment, and after you have run the car to check it out ( ran the wires for the tach/dwell meter into the car so I could compare both when running the car) place the plug in the hole that you drilled in the case. If you need to adjust later just remove the plug and try again.

Jim
67 MGB
Jim Lema

Whilst you're at it, you might want to check that the difference is consistent. I have a '67 tacho that reads high but variably. There is a lot in the archives about this and the consensus seems to be a capacitor the develops temperature sensitivity with age. It would be worth changing it whilst it's in bits. Have a look at http://www.classictiger.com/techtips/motach.html for the detail.
Steve Postins

Andy
The Smith impulse lead type of tachometer is not lineal in it's calibration. That’s why I selected 3,000 rpm’s as the point to set the tach. That is near the middle of the range and close to the rpm’s at 60 mph. It is still a little off at 1,000 and near redline, however the difference is not enough to worry about. The article that Steve points to talks about a calibration unit driven by a computer. I don’t have that unit, but it would make setting the tach easy and you could check several different rpm ranges.
Jim Lema

Just for information:

Some years ago, I installed an Allison electronic ignition, whereupon my tach began to read about 200 rpm high. When the Allison unit began to cause problems, it was replaced with a pertronix, and the high tach readings went away.

I have checked my speedometer on a rolling road and thus have a good idea of the performance of my instruments.
Dan

Thanks for the input guys. I have an electronic digital volt meter that also has a tach on it for automaotive use. I think I'll follow Jim suggestion and dismantle the unit and adjust it at 3000rpm with my digital tach.

Paul, I'm going to a car show in Dixon this weekend, which is near Sacto, I'll keep a look out for Paul Kile. If not I'll see if I have his number in one of my MG magazines. Good t ohear from you again.

Andy
Andy Preston

This thread was discussed between 07/05/2003 and 13/05/2003

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