MGB MGB Cuts out, Tach goes dead.....

[borjis]

I just came back from a brief drive and 4 or 5 different times I hear the engine miss and the tach drops to zero,then back to normal.

I checked my connections all seem fine, the dipstick was touching one of my #3 spark plug wire but I don't think that was it.

Anyone know what might cause that?

Is my coil going out? (its a gold lucas sport coil about 2 years old)

 

[Steve_S]

Do you have rubber boots on the spark plug wires? If not, it could be arcing to ground through the dipstick. Other than that, I would bet money it's as simple as a loose connection somewhere. Another possibility is moisture in the distributor cap.

 

[Bob Claffie]

Look for a bare spot on the pigtail inside the distributor. Happened to my sons MGB. Not the easiest thing to find. Bob

 

[DrEntropy]

It's iggy related. Points, ground pigtail to the advance plate... it's gonna be something "silly" for sure,

 

[100DashSix]

I had something sounding like this problem and it turned out to be a combination of the low tension leads connecting to the coil, and one of the wires underneath the dash; the under dash wire was fraying and shorting.

 

[borjis]

Look for a bare spot on the pigtail inside the distributor. Happened to my sons MGB. Not the easiest thing to find. Bob

bare spot on the pigtail?

can you be more specific?

If a coil goes bad does it slowly fail or just quit?

 

[dklawson]

As others have said, the tach dropping to zero indicates a problem with the low tension side of the ignition system. Also as said above, it could be a problem with the points, condenser, coil, the interconnect wiring, or the advance plate ground wire. Start by looking at the cheap stuff.

The pigtail thing is the wire from the points that goes out to the black/white wire heading to the coil. It's supposed to be insulated with a fiber jacket. It's not supposed to touch anything except the points. Likewise, there should be a similar wire between the breaker plate and the dizzy housing. It too has the fiber insulation and shouldn't be touching anything. Also be sure to check the position of the lug on the end of the condenser. I've seen those installed such that the crimp lug could accidentally touch ground creating a "kill switch".

 

[Nunyas]

well, I haven't seen it explicitly mentioned, so here goes...

I had a similar problem with my '76 B. For a while, I had no idea what was causing it. Then one day, for giggles I decided to check my ignition switch wiring. I found that the molex connector was slightly corroded. Plugging and unplugging the sub-harness a few times allowed it to work for a little while longer. Ultimately though, I had to use some electrical cleaner spray (readily available at auto-parts chains and hardware stores) to obtain a reliable connection.

Of course, your miles may vary.

 

[borjis]

Thank you all for the replies.

I will look at all suggested.

I do indeed have spark plug boots on my ignition wires.

I actually have not had the distributor cap off since
the engine was rebuilt some years ago, so its high time
I inspected that area anyways.

Thanks again, MUCH MUCH appreciated.

I have but one nagging question, does an ignition coil
go out all at once when it dies, or does it slowly become un-reliable?

 

[dklawson]

Coils are difficult to diagnose. They're also expensive which is why I said to check everything cheap first.

Most of the people I know who have had an honest-to-God coil failure (and haven't replaced one as part of a shotgun approach to troubleshooting) have experienced misfires under one of two conditions: 1) The car starts and runs fine through most speeds but misfires at higher RPM, or 2) The car starts and runs fine until the engine gets up to temperature... then it starts to misfire and potentially, dies.

 

[1965_MGB]

I had an issue with a coil breaking up when hot. It haunted me for a month or so, until the car quit and when I rested my hand on the coil, it about burnt my hand.

I put on a new coil, and all was well again.

Coils can go out all at once, but more often they start breaking up gradually.

 

[Steve_S]

If the ignition components are that old, check the dizzy cap and rotor for cracks. In fact, change them because you can't always see the cracks, especially in the rotor.

 

[borjis]

If the ignition components are that old, check the dizzy cap and rotor for cracks. In fact, change them because you can't always see the cracks, especially in the rotor.

The car has just over 1,200 miles on it since the rebuild 3 years ago.

I thought points, cap, rotor lasted longer than that?

 

[Steve_S]

Sometimes they last for 100,000 miles. Sometimes they are bad out of the box. Reproduction ignition parts are nowhere near the quality of the original factory issue components. Even if they were, it's a cheap way to rule out those parts. And you should have a complete tune up kit in the trunk at all times anyway so that if something does go wrong, you can fix it in five minutes and be on your way. It's better than calling a tow truck!

 

[borjis]

Sometimes they last for 100,000 miles. Sometimes they are bad out of the box. Reproduction ignition parts are nowhere near the quality of the original factory issue components. Even if they were, it's a cheap way to rule out those parts. And you should have a complete tune up kit in the trunk at all times anyway so that if something does go wrong, you can fix it in five minutes and be on your way. It's better than calling a tow truck!

Steve your totally right, thanks for the advice, the reproduction parts are definitely an issue. Its 50/50 that something will fit let alone last these days.

I had a brand new su fuel pump (points version) crap out on my after less than 500 miles and left me stranded, but lucky me only 2 miles from home. (hitting it didn't fix it like it did on my '69 B I drove back in '91)

Great website by the way! /bcforum/images/%%GRAEMLIN_URL%%/smile.gif

 

[DrEntropy]

NAPA "StanPart" Standard ignition parts are still pretty good in a pinch. And there's a NAPA store in most places I've ever been.

 

[borjis]

I think I found the problem.

I just got back from a 1 hour drive and none of the symptoms (cutting out and dead tach) happened.

I took the distributor cap off and noticed that the 2 wires (one comes from the coil, the other is an internal ground) had the cloth insulation coming off (almost all the way gone on the ground one) and they were both touching each other the entire length, so I separated them and I think thats what it must have been.

These look like the original wires since they had that cloth for insulation like the old days.

I would wager the new ones from moss have plastic insulation on them.

 

[jlaird]

As I remember the new ones have like a plastic tube around em like shrink tubeing.

 

[dklawson]

I'm glad you got it sorted out and that it was something as simple as the pigtail mentioned earlier.