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MG Midget and Sprite Technical - Engine misfire when hot

Have a problem with 1969 MK-III 1275 (stock) with engine misfire after just ten miles of driving. From cold the engine starts, runs, and drives very well. My first thought was the ignition coil, but it tested good (Les Bengtson's method). I have replaced the distributor, cap, rotor and wires. Spark plugs all look to be in good condition.

I have pertronix ignition installed, could their be a problem internally with this system to fail when hot?

Regards,

Larry C.
Larry C '69 Midget

What is the condition of the wire/connection for the + side of the coil. If this wire is tattered, it could be loosing connection as it gets hot.

but... there are plenty of other possible and more likely causes. Has it ever totally died? Are you sure it is a spark issue and not a fueling issue?
Trevor-Jessie

could be a blocked tank breather - when the symptom appears, stop and open up the filler cap, if it goes with a sucky-whoosh then it's a partial vacuum causing fuel starvation.
David Smith

Trevor-Jessie,

'Has the car totally died'?

When the condition occurs it sounds (feels) like it is running on two, or three cylinders and will stall at slow speed. I will re-check all of the wiring this evening. Interesting thing is after about five minutes or so, it will re-start although not run cleanly?

David,

In the US we had vented caps until 1970, and this car has an aftermarket 'Aston' cap from MOSS. Your point well taken. On that same line, last evening I did change the fuel pump as I had a new one on hand because I did not know the true age of the one installed by previous owner but safe to say it was more than twenty years. It was of German make, same size as the original S.U, and seemed to work well on the bench. I hope to re-test the cap/pump this evening weather permitting. Do not like pushing any car in the pouring rain.

Good points all, thank you for your input.

Regards,

Larry C.

Larry C.

did the misfire start after you installed the new pump....it could be a bit to strong in the prussure ...should be 2-4 psi

is the choke working okay not sticking....thats the problem Ive got....it just hangs a bit ... a simple lite touch with a finger releases it....still cant find the hang up....but it runs like traash in hot weather if it catches

prop
Prop

Prop,

I installed the new S.U pump just yesterday, and not had a chance to re-test. I also had a problem with the choke a few years ago, and this is entirely different.

Regards,

Larry C.
Larry C.

Plugs..it's happened to me several times..you have a plug breaking down, I'd put money on it!
Robin Cohen

Robin,

Good thought, while the set of NGK's I am using now are fairly new this could be a possibility. You're thinking an internal break in the copper core then?

After my testing the new fuel pump, I will try another set of spark plugs. This would be a first for me, I've been using NGK's for fourty years without a single failure.

Regards,

Larry C.
Larry C.

Not to hi jack your thread...(who me??? lol) But, has anyone tried those platnuim NGK's that have the 4 grounds to the 1 elctrodes...HHHmmmm, maybe those are boesch.

Personally I have never seen an NGK go bad...but they sure loose there gap setting in a hurry...go wide


prop
Prop

I had the same thing. New plugs sorted that out.
Clive Reddin 76 Midget

The car started and ran well in the garage this evening with new coil, fuel pump, and old spark plugs. Fuel tank cap was checked and is well vented. It has been raining so no driving test this evening.

Another thought, I now have concern about a new fuel filter (clear plastic type) that I installed a few weeks ago. It's mounted vertically near the radiator expansion tank (where it has always been) and noticed it is filling no more than a quarter of the height of the clear globe. I would think three quarters full would be a minimum for good fuel pick-up. Any thoughts about this?

Robin & Clive,

I do not have a new set of plugs on-hand, and will try a new set later this week along with a new fuel filter when I see my parts vendor.

Regards,

Larry C.
Larry C '69 Midget

We talked about fuel filters resently,,,and reallised that this is one of those odd rareitys of nature....cause how does the carbs get fuel when the filter only fills about 1/4 of the way.....strange indeed

so yes its working correctly....its not just you.


prop
Prop

Had something very similar a few weeks ago, turned out to be the coil. It tested OK with an ohmmeter when cold but after it got hot something went down and the car would cough and splutter and eventually die.
First I thought I had cleared it with new points and condenser but then it died when I was out after 15 - 20 mins, bought a new coil from a local spares shop and the problem went away!
Graham P

Larry. The fuel filter commonly is less than fully filled. It fills on an "as necessary" basis and will often be seen less than completely full.

Coils can break down internally, especially if they have been clamped too firmly by the ring clamp. Just snug enough that the coil will not move under moderate hand pressure is the way to go.

What does the tach do when the engine dies? If it drops to zero, there is a problem in the low tension ignition circuit. If it winds down evenly, a fuel problem is more common.

With the engine hot, at idle, spray some carb cleaner around the junction surfaces of the carbs and intake manifold, especially the ends on the SU intake manifolds, to see if an air leak is developing as the engine gets warm. There is a plug at each end of the balance tube on the SU intake and these plugs have been known to loosen over time, causing an air leak and poor engine running.

Les
Les Bengtson

I have had a number of plug's fail recently..the quality of them seems pretty poor at the moment. One was a cheapy autojumble one, one was an NGK and I've had another one go recently..missfire's intermitently when hot etc. Modern plugs just seem cr&p!
Robin Cohen

Does the tachometer behave erratically when it misfires (low tension ignition fault) or just drop with engine speed (fuel or HT fault)

Could also be be a sticking valve. What are your valve clearances set to?

Guy
Guy Weller

Guy,

The tachometer drops with engine speed when she misfires. Valve clearance was set this past spring to standard spec for 1275, as I have done for the past thirty years. Performance is sparkling before the misfire.

Les,

I am aware of the manifold test you suggest, and will get into that later this week. I read something in my latest Moss catalog about the carbon content in the newer plastic ignition rotors being too high, and they now offer a new series rotor to correct this. I am, and have been running Kingsborn sets. Any thoughts about this?

Regards,

Larry C. '69 midget (still)
Larry C '69 Midget

I had the exact same symptom... my coil/resistor was not properly wired, it would get really hot in about 15 minutes of driving. 1/2 hour to cool down or a couple cups of water poured over the can would solve the missfire for a few minutes and then die again as it reheated. Turns out the start/run ignition wiring to the ballast resistor was backward so the engine was starting on 8 VDC and running (till over heated) at full 12 volt volts. Swaping two wires fixed it. M
Marc Judson (2- 1978's)

I tested the voltage at my ignition coil (Lucas Sport) between the positive terminal with ignition (PerTronix) on, and good earth. I am reading 10.41 Volts which is way over the (4-6 volts). My Lucas fault manual suggests a faulty coil, or amplifier. I will substitute my standard (reserve) coil tomorrow when I have a little more time to see if I get a different reading.

My questions then are these:

Could this be an internal fault of the PerTronix module, or a fault with the new coil that I replaced after the first occurence of the problem?

Regards,

Larry C. '69 Midget (still)

Larry C '69 Midget

After obtaining a new PerTronix Ignitor kit yesterday I discovered a crack at the base of the magnet sleeve that fits over the distributor shaft point cam. When lifting it out the base came loose, and one of the magnets came free. Cannot be sure if the magnet was free inside the sleeve, or when it separated?

New kit installed, and timing re-set all seems to be right with the system now. More testing to follow later today.

Thanks to all who assisted with this problem. Although I'm sure this was a rare occurrance I will add this check to my annual maintenance.

Regards,

Larry C. '69 Midget (still)
Larry C '69 Midget

Larry. Not sure what you were trying to get at with your next to last posting. The information about the coil makes no sense without more information. Your 69 Midget should have a full time, nominal 12 volt ignition system.

Were the Midgets negative ground by 1969? I know the MGBs were in 68. Input voltage to the coil (positive terminal on the negative ground cars) should be 12.0 to 13.0 volts with the engine cranking and 13.5 to 14.5 volts with the engine running. The output side of the coil (negative terminal on negative ground cars) should have a voltage reading of about 8-10 volts. The internal resistance of the coil will drop the output voltage to the points to lower than system voltage.

The factory supplied coil had an internal resistance of about 3.5 ohms. The Lucas Sports Coil has a lower internal resistance of about 2.7 ohms. Thus, it does not lower the output voltage as much as the original coil would.

This has some minor significance in that the lower voltage equates to longer points life--not a factor for people who are running an electronic points replacement system.

It sounds, from your last post, that you found the cause of the problem--your electronic points replacement system was bad. Not at all uncommon, like any device they will go bad after some period of operation. Just far more difficult to trouble shoot problems with such units than it is with points systems.

I hope you did take the opportunity to clean and lubricate your distributor while it was apart. So many people believe that you really can install the various points replacement systems, then forget about the distributor for years. The Lucas distributor needs annual lubrication, regardless of the coil triggering device used with it, if it is to perform well.

Les
Les Bengtson

Les,

The car is negative ground, as you suggest. The coil is good, this was my misunderstanding of the Lucas test proceedure. I was in the wrong section (Opus) instead of the (standard) coil ignition system in my service manual.

We have a local club member who (DNF) within a mile of the finish of a rallye when his magnetic sleeve came apart. As you say, it happens. I will be carrying a point and condenser plate ready to install should this happen again. I serviced my distributor just this past winter when I installed the PerTronix ignitor.

Side note; I installed a PerTonix Flame Thrower Distributor (Moss) in our 1974 B/GT in April, and I must say while the car ran well prior, it has not run as sooth, and cleanly since it was new! Disclaimer, I have no affiliation with either company in any way, it just made that much difference in this car.

Regards,

Larry C. '69 Midget
Larry C '69 Midget

Try to elimate things. First, use a timing light to check if all sparks are sparking, or one or aren't (and which ones). Check the HT coil lead as well. If one or two cyl are failing this MIGHT be the lead or the spark or the cap.

Also use a hairdryer to heat up the Pertronix unit to see if this makes things worse. Keep us posted
anoldr

I have never had a problem with a faulty sparkplug.
Tom

Two yrs ago, I had a similar problem. I could drive for awhile (20 minutes) then the car (1970 1275 stock) would run or seem to run on two cylinders just no power. weeks of trouble shooting. Anyway to make a long story short, everything mentioned in all posts above were fine, the one thing that was not was the regulator, I had a good working spare, swapped them out, and two years of carefree driving have followed....
Greg

Larry is your coil a 12 volt coil or a ballasted coil?
With that amount of volts applied to it you should have fitted a 12 volt coil and not the one you are imagining which would be a ballasted coil. By the way your meter is either on its way out or your battery was going flat!

I too have never had a spark plug fault since the early 70s when I stopped using crappy champion and bosch plugs and began using the best NGK. Look at any race winning clubman race car and you will only see NGK plugs. I now change the plugs in all my cars about every 5 years just to do something!
Bob (robert) Midget Turbo

Tom

"I've never had a problem with a faulty sparkpug"
You are a lucky (&&er then! I have a couple of dud plugs you can have!
Robin Cohen

Bob (Robert),

I am running a 12V Lucas Sport Coil which has tested fine since replacement of the PerTronix with a spare lent to me for testing. My problem was traced to a faulty "magnet sleeve" (fits over the distributor cam shaft) that cracked, and partially came apart. I have contacted PerTronix, and they claim to have a new replacement in the post to me now. Locally this has happened to at least one other club member.

All,

Many years ago I did have a few spark plug problems, but never with NGK. Champion, KLG, and Bosch mainly. NGK seem to be most forgiving with temperature range to burn off oil, etc. in most engines including two stroke. While I do carry spare plugs, I've never used them in my cars.

Regards,

Larry C. '69 Midget (back on the road)
Larry C.

This thread was discussed between 05/08/2008 and 12/08/2008

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