MGB & GT Forum

Well, today my car started to act very strange. It was running fine and then just started missing and backfiring. I got it home and pulled the plugs and they're not fouled (shouldn't be, they're only a week old), checked the cap and rotor, both were fine, so the next thing I thought of was my Pertronix unit. My question is, those of you who had one die on them, did it die completely, or did it work on and off? I am thinking that it is going out and not signaling for spark correctly. The engine seems to run fine at higher RPM's (above 4,000), but anything under that and it just misfires.

Fuel flow should be fine as the car was running great this morning in similar weather. I did check to see if I was getting fuel at the carb and I am.

I didn't have much time to look into it, but will have more time tomorrow. I figure I will check the timing and see if maybe the dizzy came loose and moved. I might also check and see if I am getting enough fuel pressure. Anything else that I should check out?

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Andrew Bird

Andrew
Check your primary resistance on your coil. It should be 3 to 3.5 ohms and the secondary resistance should be about 8.5 K. If that's OK make sure the ignitor is tight and all the connections are tight. If you ever leave the ignition switch on for a length of time the coil can overheat and also damage the ignitor. I don't know what they act like when they start going out but rule out everything else first.

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Dan Wood
70BGT driver, OD, Pertronix, HS4's, Peco, .060 over, Elgin cam, Superlite wheels, poly bushings, panhard rod, rear tube shocks, 1" lowered front end, HD shock valves, etc, etc.
68 MGB rebuilt engine D9 cam balanced, still a work in process but runs real nice.
88 Saab SPG Turbo
86 Vanagon Westy (South African conversion engine 2.0 OHC 135HP) This car burnt to the ground. Bought another, same year and color but with a 2.4 Waterboxer.
2004 Dodge Ram 6sp with HO Cummins diesel
Lucas= Loose
Unsoldered
Connections
And
Splices

How do I test the primary and secondary resistance? Obviously with a multimeter, but where do I test it from? The coil wire and thne the + and - posts on the coil?

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Andrew Bird

I second Don's suggestion. I had Sapphire totally quit two weeks ago. Last Sat. morning when I got further into my testing, I tested the coil using the same specs Don listed. I had good secondary resistance, but the primary was only 1 1/2 ohms. You'd think the diff from 1 1/2 to 3 wouldn't make that much difference, but a new coil fixed the problem post haste.

If you have doubts, pull a plug again and check for spark. Mine was so faint I could barely detect it.

If you have nothing at the plug, check for spark from the HT wire from the coil to the distributor.

If no spark, check the little wire from the coil to the distributor using a continuity tester. It should create a flashing result when the engine is cranked, telling you the Petronix unit is opening and closing signal, which should draw spark through that HT wire to the cap. If that test is fine, and the problem is ignition, it's likely the coil or that HT wire to the center of the distributor.

Just finished Ignition 401 last weekend!

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Frank
MGBJockey

I should have said the primary resistance is measured across the two terminals with a multimeter, preferably a digital one. The secondary resistance is measured between the coil tower and one of the primary posts.
Frank
Glad you got her running good again.

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Dan Wood
70BGT driver, OD, Pertronix, HS4's, Peco, .060 over, Elgin cam, Superlite wheels, poly bushings, panhard rod, rear tube shocks, 1" lowered front end, HD shock valves, etc, etc.
68 MGB rebuilt engine D9 cam balanced, still a work in process but runs real nice.
88 Saab SPG Turbo
86 Vanagon Westy (South African conversion engine 2.0 OHC 135HP) This car burnt to the ground. Bought another, same year and color but with a 2.4 Waterboxer.
2004 Dodge Ram 6sp with HO Cummins diesel
Lucas= Loose
Unsoldered
Connections
And
Splices

Andrew,
The Primary test is by putting one multimeter lead on the + terminal of the coil, and the other lead on the - terminal. The terminals are the small wires. Set your meter on 10 ohms (0 ~ 10). You should get 3 to 3.5 ohms with that test. Mine only read 1 to 1 1/2 ohms and that difference, even at that low level of ohms, was enough to be faulty.

The secondary test is from the + terminal to the center output of the coil. This spec is in mega ohms, so you have to use one of the higher range settings on the meter.

Make sure you reconnect the small leads from the harness to the right terminals when you are done.

The test of the small lead from the coil to the distributor should be done with a continuity tester or test light, and you touch one lead to either the - wire on the coil, or the spade on the terminal block on the side of the distributor (those are the respective ends of the same wire), and the other lead is hooked to a known ground, like an unpainted bolt. Have someone crank the engine over. You should see the light flash rapidly. If successful, that indicated the distributor is signaling the coil to release spark.

For spark test, remove a plug, connect it back to it's plug wire, and hold the threaded base against an exposed ground with a pair of rubber insulated pliers while someone cranks the engine. You should see a detectible spark in the gap to the plug's electrode. You do the same test with the distributor end of the HT coil wire (the plug-type wire running to the center of the distributor cap).

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Frank
MGBJockey




Edited 1 time(s). Last edit at 2006-02-03 12:27 AM by mgbjockey.

With the spark test make sure it is a nice blue / white spark. I had problems and mine showed up red / orange, a sign of a very weak spark. If it ends up being the Pertronix module I would reccomend a unit that is mounted on the inner fender. An ignition unit inside the dizzy is subject to heat and vibration, not a good environment for important electronics.

Get that extra set of pionts that every Pertronix owner should have in his glove box and put them in. Should take 10 min. if the problem goes away you have your culprit. At that point you only have to decide to reorder a Pertronix or stay with points.

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We can find the one cow, out of millions of cows, with Mad Cow Disease, but we can't find tens of thousands of illegal aliens. Lets put the USDA in charge if immigration!

Andrew,
Okay, we're waiting patiently for an update here........ How far have you gotten?

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Frank
MGBJockey

Just got done and it's still running bad. I checked the coil and the primary measured 2ohms and the secondary measured ~11ohms. This is not a Lucas coil, it is a "NASCAR Heavy Duty" coil from a local auto parts store if it matters. I regapped and cleaned the plugs, checked the timing and tried it again still with no avail. I cleaned the cap and rotor and checked the wires coming out of the Pertronix unit. All that is fine.

The car runs a whole lot better with the choke out running richer so I am led to believe maybe an intake leak. I just replaced the intake gasket, so I wouldn't think it would be bad, but who knows.

So, either the coil is bad, the Pertronix unit is dieing or there is an intake leak. What do you guys think I should do next?

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Andrew Bird

Anyone else here near him? Coil at 2 ohms is low. That confused me last week. If someone is near him they could pull a coil and let him try it. Where do you live?

Andrew, most parts store will not allow you to return an electrical item like a coil.

ALWAYS think ignition first, and fuel after that. There are good threads here about that.

Also, did you do a visual test for spark? i.e. pull a plug, snap wire back on it, ground it (threads) to something that is bare metal, and watch for spark as someone cranks the engine over. A message above tells you what to look for. I was only getting very weak spark.

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Frank
MGBJockey




Edited 1 time(s). Last edit at 2006-02-03 02:36 PM by mgbjockey.

Since you just replaced the intake gasket, I'll suspect that first. Spray some carb cleaner around it with the engine running and see if it changes it's tune.

Also, in the past people using non-Petronix coils with the Petronix unit have had failures. Many haven't, but many have. Tom B. won't use anything but a Petronix coil with them for that reason.

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Joe Reed, Cordova TN

'02 Subaru.....'95 Miata.....'78 MGB

Ok, well I did a visual test on all the plugs. All check out good. I also did a compression test just to be sure and it was even across the board. I sprayed some gasoline around the intake manifold while the engine was running and didn't notice a difference, but I couldn't really get fuel around ever edge of the manifold (DCOE). This has to b my problem though. The engine is sputtering and backfiring and runs better with the choke (enrichener) out. Ignition is fine, so I guess I will order an intake gasket and swap it out. Thanks for all the help.

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Andrew Bird

Check the gasket or soft mount O-rings (whichever you have) between the carb and the manifold too - could be an intake leak there. Don't know that I'd spray gasoline though!! I'd prefer to use a spray can of carb cleaner, starting fluid, etc. using the little red tube that comes with them. That lets you pinpoint the spray where you want it better.

If you don't have a manifold gasket leak, replacing the gasket is going to be a waste of time....

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Joe Reed, Cordova TN

'02 Subaru.....'95 Miata.....'78 MGB

This may not be it, but when I installed the Ignitors on both of my LBCs I had to advance the ignition past the factory specs to get them to run right. Both of them would miss and even backfire untill I turned the distributer to add advance. I even sent one of them back to the place that I had bought it from and when I replaced it again it did the same thing. I could reinstall the points and it would run well. Might not be your problem, but wont cost anything to try.

Paul

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70 TR6
78 MGB
80 Spitfire
66 Mustang
89 CRX Si
96 Integra GSR