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MG MGB Technical - lazy rev counter?

Hi Folks,
this is something that if easily fixed i will, if not then i will leave alone, as it bothers me more from a 'why is that' point of view as opposed to a 'must fix that' standpoint. My rev counter functions fine, and reads pretty good when compared to an external measurement. However, when i start the car up, the rev counter doesnt do anything unless i rev to about 2500 - 3000 rpm. This 'wakes it up' and thereafter its fine, showing a good idle of around 600rpm. Anyone know why it needs to be be rudely awoken to get a reading? The car is a 73 bgt, everything seems to be in order, new alternator last year (but rev counter was doing this before the alt change).
put me out of my ignorance please!
all the best
mick
m rae

No idea...but my 77 MGB does exactly the same thing...but it's well down the list of things to look at and fix....anyone?
P J KELLY

I have to tap mine to "wake it up", then it runs just perfectly. Sometimes, if the car has been off just alittle while, it will work on its own, but usually I have to tap it.

Erick
Erick Vesterback

I have to tap mine on my '73 'B, or I sometime don't even notice it's sitting at zero as I drive off and the needle movement catches my eye as it springs to life. Doesn't always do it and always after sitting at least overnight.I love having to tap it. Makes me feel like Biggles in his Sopwith Camel!

This has come up before. I think the general concencus was that there's a bit of dirt causing the stiction.

TTFN

Derek Nicholson

My '73 B does exactly like Derek's but tapping is basically just to keep me busy - it doesn't help the tach much. If I reach up into the wiring rats' nest behind the dash and jiggle the "hot" wire leading into the thing it springs to life. Usually.

I'm gonna replace mine one of these days, but since the speedo works well at any speed over 20 mph, I can get a sense by feel, speed and sound about where I'm running it.

R.
Rick Stevens

Sticky pointer, either against its end-stop or possibly some gunge in the narrow air-gap between the magnets and the moving coil. You *might* stand a chance of cleaning it with solvent, but OTOH you stand a chance of buggering it up all together. You could always rig up a solenoid to give the case a clout a few seconds after starting ...
Paul Hunt 2

Paul, i think your reply has convinced me to leave well alone! Seems its a common thing to happen, and to be honest, i can live with it (have a vision of soemone asking me why the rev counter doesnt function at all, and me having to reply 'because i fixed it'). Thanks to all for your replies.
mick
m rae

OK, now the opposite problem. A friend of mine has a '68 B where the tach goes stark raving bonkers. The needle bounces around the red zone while the engine is at idle. BTW, the engine runs like crap and starts backfiring around what I guess to be about 3500 rpm in 2nd gear.

This is an engine that had sat without running for 15 years. A mechanic of some kind got it running, but ever so badly. I tried to help out a bit, starting with head retorque, valve adjustment, new points, condenser, rotor, cap, wires and plugs. I also set the timing to 15 degrees BTDC at an estimated 1500 rpm with the hose disconnected. There seemed almost no lateral play in the distributor spindle, the ground wire for the breaker plate was in good order, and the centrifugal advance was free - even before I oiled it. Then I reset the jets to nominal (six flats below the bridge. We replaced the float bowl needles, gaskets, etc and reset the floats. I balanced the carbs as well as possible with the engine shaking badly from the rough idle. The plugs still fouled very quickly despite revving the engine frequently to an estimated 2500 for a minute. We kept cleaning them.

Unfortunately it is not my car so I've had little opportunity to work on it. I'm not sure what the tach is counting, but the erratic behavior was unaffected by the new points and condenser. The primary connections at coil and distributer are clean and tight. If it were my car, I'd completely rebuild the carbs next, replacing jets and needles as well. Who knows what trash lurks therein. But I hesitate to tell my friend just to keep throwing money at it until we find the problem.

To the extent that this engine runs, it seems VERY fast. I strongly suspect that it has some major "tweaks" inside, but don't know what they are and no PO information is available. That too makes it difficult to tune.

Any suggestions?
Thanks,
Allen
Allen Bachelder

On a 68 (and up to 72) the tach counts current pulses through the coil and points. If the crazy tach is *always* associated with misfiring at the same time then the problem is almost certainly to do with the points, condenser (new ones noted), coil or interconnections, including a very flexible ground wire in the distributor running between the points plate (which moves under changing vacuum) and the distributor body (your check also noted).

However if sometimes the crazy tach still happens when the engine is running smoothly, or sometimes the tach will be steady with misfiring, then you may well have more than one problem. For the tach check the security of the 12v and ground connections to the back, if they are OK it could well be internal. For the engine the problem is probably HT or fuel related.

How were the plugs fouled? Wet with fuel (rich mixture)? Black with oil (rings/guides)? Have you got the plug order correct i.e. 1 3 4 2 *anti*-clockwise? Are you sure No.1 is firing when its piston is at the top of its *compression* stroke?

Having said that a highly tuned engine can run like crap under certain conditions where it wouldn't for a standard engine.
Paul Hunt 2

Paul,

Thanks! Your advice is always valued. Regarding the plugs, they were just black with lots of carbon, not wet with fuel. Come to think of it, the fuel smell was not extremely strong.

There was no correlation between the tach activity and the backfiring. The backfiring was only under load at an estimated 3500 rpm and rather irregular at that. The engine was running roughly, but not seemingly missing - at least not regularly. I checked to make sure I got the plug wires in the right order. Again, guesswork, but at what seems like 1500-2500 it settles in smoothly enough that I'm pretty sure the ignition is OK. I was particularly careful in checking that flexible wire from breaker plate to ground as I've seen similar symptoms when that wire breaks from the plate.

It may be a couple of weeks before I see the car again. I have a funny feeling this is all related to sitting for 15 years. Perhaps a ring was siezed and broke when the car was first started? Or else a sticky valve? My friend reported to me by phone the other day that the car seems to be backfiring less for no apparent reason.

Thanks again!
Allen
Allen Bachelder

About Allen Bachelder's question:
I've seen two cases simlar to that, where the tacho needle bounces around.
The first was where I found the fuel pump wired to the + side of the coil. The pump pulses caused the tacho needle to jump whenever the pump pumped.
The second was where the tacho supply was taken from the voltage stabiliser instead of +12v direct. The stabiliser switches the gauge supply on/off quickly, with the average voltage being around 8 to 10 volts. The rapid switching caused the tacho needle to jump all over.
Cheers! Fraser
Fraser Cooper

+ve side of the coil i.e. 12v side? Or -ve i.e. points side? Circuit-wise the fuel pump and coil +ve are connected together anyway. If the pump were connected to the *points* side of the coil then I could see it affecting the tach reading, but only on 72 and earlier cars.
Paul Hunt 2

Ditto on the tapping to wake up the tach on my '72B. Doesn't always happen, and only when it has sat for a while. I have learned to live with it, and clean the glass on the tach of "smudge" marks from my finger tapping at least once a month.
Joe

To Paul Hunt 2--
The fuel pump WAS connected to + side of coil (supply side). But the tacho's current loop was in the coil 'hot' lead, so that both coil and pump currents ran through the loop.
If the current loop, in that case, had been in the breaker points side, there wouldn't have been a problem.
Fraser
Fraser Cooper

Gotcha.
Paul Hunt 2

This thread was discussed between 12/12/2006 and 23/12/2006

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