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MG Midget and Sprite Technical - Misfire when engine is warm

Hi, my 1500 midget starts and runs fine from cold, but starts to misfire badly when warmed up.
I've changed the distributor cap, rotor arm, plugs and plug leads without any joy.
The car is fitted with a Luminition Performance Ignition and coil, it's been suggested that the coil may be faulty,(coil does seem to run hot).
Question is, can I fit an ordinary 12 volt coil to check if the "Performance" coil is faulty.
A replacement Luminition coil is about £40 which is a lot of money to spend if the coil turns out ok.
Can anybody advise?
Steve

Just a differant thought....do you still have a heat sheild between the carbs and the exhauste system......or do you have anything running hot along the fuel lines or tank?


prop
Prop

Steve, I don't see why you can't use a different coil. I think the differences is the voltage output to the plugs.

Is the other coil designed to operate with a ballasted system?
Clive Reddin 76 Midget

Those are classic symptoms of coil failure. You should be able to use an alternative coil so long as it has the same voltage and ballast configuration as your current one. In particular, you need to check whether your current coil is ballasted or not.

Originally your car would have had a ballasted coil but it may have been changed when the Lumenition kit was fitted.

While it is possible that the wrong coil was fitted with the kit (ie non-ballasted rather than ballasted) that would have shown up in being more difficult to start. If you have not had cold starting problems then you should be able to assume safely that the current ballast configuration is correct.

To check what your system is wired for, measure the voltage at the coil's positive terminal with the engine running and compare that with the voltage at the battery's positive. If it is the same, fit a non-ballasted coil. If it is a few volts less (typically around the 8-9V mark) then you need a ballasted coil.
Paul Walbran

Can i ask, as I find the engine idles badly (948cc Sprite) when engine warmed up. Is that likely to be a faulty coil as well? Is there a way to check it?
Thanks
G Vickers

Somewhere in the archives is a relevant discussion about coils. I also had the lumenition coil which failed on the way to Le mans. I did another 600 miles on a random 2nd hand Bosch coil from a French garage so in short "yes". any un ballasted type coil will work fine to test.

I asked about replacements when I got back and there was lots of good advice about measuring the resistance etc.

I now have the gold lucas one and am very happy.
Dean Smith ('73 RWA)

Coil thread here...
http://www2.mgcars.org.uk/cgi-bin/gen5?runprog=mgbbs&access=&mode=archiveth&subject=97&subjectar=97&thread=2007062616364429426
Dean Smith ('73 RWA)

Bear in mind that Spridgets used internally-ballasted coils (approximately 3 ohm resistance across the primary terminals) until the 78-79 models which are the only ones with an external ballast resistor. Simply saying your car is a 1500 is not sufficient; if your mfg date is after 08/78, your coil should be externally-ballasted (1.5 ohm) otherwise it should be internally-ballasted (3 ohm). Likewise, if your starter solenoid has a wire which is white with light green tracer connected to it, you should be externally-ballasted.
David "loads of ballast" Lieb
David Lieb

Ooops, I meant to indicate a manufacture date of 08/77, not 08/78. My Haynes manual implies that the solenoid changed at the end of 1977 production, so I am assuming that it would coincide with the serial number 200000 changeover, although Horler appears to be remarkably reticent about this change, seeming more concerned with changes in the spring of the reverse lever. So, until one of you tells me otherwise, I will assume that this was among the many changes implemented in August of 1977.
David "procrastinating again" Lieb
David Lieb

Hi, thanks for your responses.
The car is a 1979 model.
The luminition coil has 12 volts printed on it, and unless it's remotely mounted, there's no sign of a ballast resistor.
Should I just obtain a standard 1979 1500 coil and fit it?
Steve

Steve, this is how mine started, also missing under load. I thought it was electrical right up to the time the head gasket blew completely!! See my post.
Also I have replaced evrything before now assuming that because the parts I was putting on were new that it was fixed!! Big mistake !! With current QC on parts you just never know for sure. In my case it was a set of Lucas points (nos) that turned out to be faulty. It fooled me for hours, nay, days !!
Chris
Chris Sargeant

Steve,
What you need to do first is to take a digital VOM (Volt-Ohm Meter) and measure the resistance from the + side of the coil to the - side with at least one side of the coil completely disconnected from all wires.

Next, re-connect the wiring to the coil, turn on the ignition, and measure from earth (NOT the - side of the coil) to the + side of the coil. If all is well, you should see significantly less than the 13.2VDC you would see at the battery; about 9 to 10 volts). Since the wiring diagram does not assign a letter to the resistance shown in the WLG circuit, I assume they used a high-resistance wire rather than a discrete ballast resistor. Readily verified by doing an ohm reading from the WLG wire at the solenoid to the + side of the coil. If yoou see about an ohm and a half, you have a high-resistance wire.

Borrow a known good 12V coil (NON-internally-ballasted) as a test. OTOH, pertronix Flamethrower coils are available in both flavors, 40011 for your externally-ballasted application and 45011 for internally-ballasted applications.

David "yup, I have a Pertronix product or two" Lieb
David Lieb

Steve. MG used a resistor wire, a long length of wire with a built in resistance, rather than a separate ballast resistor. This reduces the input voltage to the coil, when the ignition switch is in the "run" position to less than standard system voltage of 13.5-14.5 volts. I seem to remember about 9 or 10 volts, with the engine running, back when I was doing some tests.

I would disagree with David on the output voltage of the coil. He is correct for a 12V input, but a little high for a coil using an external resistance system. As I remember my tests, about 6-8 volts was the output.

One of the problems with the various electronic points replacement systems is that they are difficult to trouble shoot due to the electronic components. Unless one is an electronics technician, and has a test bench set up, the only trouble shooting one can do is to swap our parts. The Luminition in my daughter's car failed, on a random basis, when running and I replaced the distributor with a points type. No problems since then.

Both the coil and the "power board" for the Luminition can fail when hot. The coil is easy to test by installing a factory coil, which would be an easy test. If the problem remains, installing a points type distributor should allow you to determine if it is a distributor related problem.

It would be good to check out the ignition system throughly to see if that is a part of the problem. But, it is not the only source of a mis-fire condition. It should, however, be throughly checked so that, if you do not find the problem, you can begin to check for vacuum leaks/air leaks at the carb and induction system and water leaks onto the ignition system.

Les
Les Bengtson

Having just changed the coil to fix the snag as outlined at the start of this thread I am now somewhat dismayed to think I have fitted the wrong one.A local supplier issued the coil as fitted to Midget and minis etc. I am now going to check if the wire is ballasted etc.
By the way the coil gets very hot on use but the car flies along.

Another point - Blackie goes into the local garage for his repairs post crash so he should be up and running for spridget 50

Dave'how can a small car be so complicated'Price
Dave Price

Hi, I took the Midget to a local rolling road to check out the ignition system.
The coil turned out ok, but they traced the fault to water in the fuel? (no idea how that got there!!).
The car made 72 BHP @ the wheels at 4900 rpm.
As the engine won't rev beyond 4900, and seens to run rough for a Piper 270, they recommended checking the camshaft timing as it may be incorectly timed in.
Steve

I agree with Les' disagreement with me ;-) It SHOULD be 6 to 8VDC, but I have always seen it measure high, likely because there is seldom an actual load on it at the time of measurement. Perhaps if the points happen to be closed at the time...

I am also going to disagree with David cuz the silly a$$ didn't think the WLG circuit through properly. You should NOT see any significant resistance on the WLG line from the solenoid to the coil cuz that is the BYPASS. You should see that ohm and a half or so on the other lead to the coil, but my manual is not here, so I am not sure where the next point in that circuit would be.
David "where's that grain of salt?!?" Lieb
David Lieb

This thread was discussed between 10/05/2008 and 12/05/2008

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