MGB 70 MGB runs rough - sputters & backfires esp. cold

[RickB]

The ongoing saga continues...

It was horrible driving home today, I'd rather drive the Midget with one broken motor mount than try driving this thing again. :frown:

Yes, I am about to admit defeat.

At least until I can get some parts together and replace a bunch of stuff.
I'll start with the coil, then the condenser, then the plugs, etc...

 

[jlaird]

On that vehicle if you disconnect the tac. does the engine not stop. ie it must be hooked up for the engine to run.

I still say coil or wire from coil to tac or from tac to dizzy????

Or am I just all wet.

 

[dklawson]

No Jack, you're not wet at all.

Which tach is this (sorry I don't remember the age of your car). The early RVI electric tachs are wired in series with power to the coil. If you've got a problem with the sense wiring for an RVI tach, you will loose spark and have all kinds of problems. The later electric tachs were the RVC type and use a single sense wire to coil (-). If that wire shorts to ground your ignition system stops as it's a perfect kill switch.

The fact that it's dropping to zero periodically could be a very, very good indicator that your problem IS in the ignition system. The tach will say somewhere on its face whether it's RVI or RVC. Let us know which type you have.

 

[jlaird]

Sure glad I am not all wet, cold outside tonite even way down here.

 

[RickB]

I will look tomorrow at the type tach I have.

BUT:
It ran perfectly for a long time before I started sussing out the tach thing and found that if I pull on the wire is works.

 

[dklawson]

Rick, do let us know which tach you have. If you have an RVI tach and pulling on the tach wires fixes the problem... it could be as simple as replacing the white wires to/from the tach and cleaning all the connections. If it's an RVC tach, a simple test would be to unplug it and tape up the loose wire. If that fixes the problem it could be time for a new/used tach. When you look at your tach, also look on the back (under the dash) if possible. There are two types of RVI tachs, early and late. The early type use an external loop of white wire on the back of the gauge. The later type has bullet connectors (male and female) for the white wires. On the later type the looped wire is moved inside the gauge case.

As a side to this topic, my father-in-law's Formula-Vee had an ignition problem where the engine simply refused to run. We eventually traced it down to his Veglia-Borletti tachometer. Somehow it developed a short to ground which effectively worked as a kill switch. He's still using the tach... he just isolates it from the car's chassis. I only bring this up because your tach may not jut be indicating an ignition problem, it could be the cause of it.

 

[RickB]

That's interesting Doug, the strange thing though is that it drops to zero and bounces back up randomly even when the engine is running smoothly.

I know it just needs a connection there, regardless of whether or not the tach is "working".
Also I know that if I pull the bullet connection loose when the car is running it immediately dies. Does that mean it's an RVI?

 

[dklawson]

If disconnecting the tachometer wires while running causes the engine to die, you probably do have a Smiths RVI.

I guess a little more detail is in order. The RVI tachs pass the coil current through an induction loop on the back of the gauge. Each time the coil charges and discharges the coil current flowing through the loop causes a "pulse" and this pulse is used to fire transistors inside the tach circuit. (Not entirely accurate... but that's the Reader's Digest explanation). With the RVI tachs there is no "physical" connection to the coil or its power... just the induction loop.

OEM installs of the RVI tachs involve a white wire from the ignition switch going to the tach, through the loop, then on to the high-side of the coil. On aftermarket installations the common connection scheme is for a white wire to be routed from the coil's low-side (the side going to the distributor) to the tachometer induction loop, then from the loop on to the distributor (points). The result is the same, coil power flows through the induction loop on the back of the tach.

Regardless of where the wires connect to your car's wiring, if you disconnect those white wires, you are interrupting coil power. The engine will die... and with no current flowing the tach won't operate.

For quick contrast, the RVC tachs are a more modern (voltage pulse sensing) design. RVC tachs look for the swing from 12V to 0V as the points open and close. If you pull on the RVC tach wires and accidentally disconnect them... nothing should happen apart from the tachometer not working anymore. However, once the wires are completely disconnected, allowing the sense wire to touch ground will stop the engine as this is equivalent to the points never opening.

 

[RickB]

Thanks Doug, I appreciate the explanation.

Yes it definitely is an RVI tach.

 

[RickB]

Maybe it's a bad ignition switch, that would cause the trouble I'm having.
I'll check that out.

 

[Jagboy69]

Rick, I know this might sound absurd, but my BGT would actually run and drive with the spark plug order screwed up. It was the strangest thing I had ever seen. The entire time I was search for something that was causing it to run goofy and I would have bet the farm I had it wired right. Uh, I didn't. Just food for thought.

If you are suspecting ignition switch issues or coil issues, bypass them. Bypass the tach too while your at it. Screw the tach for now, disconnect it. With it OUT of the equation, It won't matter if it's good or not if you can't determine it's bad in the first place. Run straight wires from the battery for the coil. Try the car. If it still sucks, put it back the way it was. Then hot wire the ignition switch to constant "on" and just bump the starter wire to start the car. Take it for a spin. Try running straight FULL TIME power to the fuel pump from the battery. Drive again. It has GOT to be one of these things. Your not anywhere near west texas are you? I'd say drop by with the car.

 

[RickB]

Thanks Jason,
No - I've not been near west Texas in a very long time.
Last time I spent any time there was my 21st birthday, I was taken out to an Air Force base where I got to see one of the trials of the space shuttle, it was attached to the back of a 747. That was really cool to watch.

That's what I'll do, I'll just start bypassing stuff that 'might' be the problem until it goes away.

 

[dklawson]

Rick, any updates or is it too soon to ask?

 

[RickB]

Raining here, I have a policy about laying under a car in the rain.

 

[dklawson]

Understood. One of the worst experiences I had was replacing wheel cylinders on an old car of mine... while lying on my back in the gutter full of melting snow. I hope you get some good weather soon.

 

[RickB]

<div class="ubbcode-block"><div class="ubbcode-header">Quote:]while lying on my back in the gutter full of melting snow[/QUOTE]

Yup - btdt - thus I now have a policy.
Mainly because I do not like pneumonia, bronchitis, and that coughy, sneezy, achey, bad feeling you get from doing things like that.

Also, my other policy is to have at least two MG's so I don't have to be stressed about one of them not running.

Two of them not running, now that's a stressor! :wink:

 

[Jagboy69]

If it makes you feel better Rick, I just got done dropping a small fortune into my Jags radiator only to watch the heater core leak all over my driveway.

 

[RickB]

Well I put in new plugs and cleaned up that big brown wire connection at the starter today.

No joy.

Still runs very rough, sputters & backfires.

I know there's a whole list of possible things to switch out for new in the process of elimination, but I'd rather not spend that much money!

I can get a new cap & rotor, that's the next thing I would normally do anyway.

 

[dklawson]

Rick, I'm sure I asked this before but I don't remember the answer. Is the backfiring out the exhaust or the carbs?

 

[RickB]

Rick, I'm sure I asked this before but I don't remember the answer. Is the backfiring out the exhaust or the carbs?

Exhaust