MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Unstable Rev Counter

Hi Everyone
My 1969 MGB has begun suffering from unstable readings from the rev counter. From cold everything is fine throughout the journey, no matter how long. However when the engine is hot and then restarted I get tickover readings between 3000 to 6000 rpm. when its clearly about 850rpm. Thus most of the time the rev counter remains hard against the stop at 7000 rpm during normal driving. Sometimes when I first switch the ignition on the needle spins straight to 7000 rpm for a couple of seconds before falling back to zero.
The car is fitted with Petronix ignition that I fitted about 5 years ago, it starts and runs perfectly otherwise.
I have checked the ignition wiring (via a white cable) and it appears intact with good conections, but for some reason the heat now upsets it.
Any ideas please?
B Rowbotham

Hello,
Some internal electronic components suffer from heat and get less and less reliable on time. A capacitor is weel known to become leaking inside the tacho. You may have a look at the following URL, chapter Servicing tacho. I beleive C2 is involved in the fluctuations. But check also a compatibility issue when used with a pertronix system together.
"http://www.mgexperience.net/library.php"
Hope that help.
R.G
Renou

An unstable tach often indicates problems in the ignition LT circuit, definitely so if it is accompanied by any hint of missfiring or poor running. A 69 would normally have a current-pulse triggered tach and these are known to suffer problems with after-market electronic ignition systems, although usually it is a steady double reading. Erratic and full scale readings as here are more likely to be problems inside the tach, possibly dry joints. From 73 onwards it was a voltage-pulse triggered tach that didn't have the double-reading problems. What sometimes works with the earlier type is to modify the pickup so that the white wire to the coil only passes through the pickup once, and not twice (i.e. one full turn). Originally these tachs had the pickup on the outside and the harness wire passed through the pickup, making it relatively easy to change. But at some time the pickup was moved inside the tach, making it a much bigger job to modify. In that case it may be easier to fit the later voltage-pulse tach.
Paul Hunt 2

This thread was discussed between 30/09/2007 and 03/10/2007

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.