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MG MGB Technical - SPARATIC MISFIRE

My 79 MGB has developed a sparatic misfire. It is not always consistent. The car has a new Weber DGEV carburetor, a Lumitronix ignition in the 45D distributor. New plugs, cap, rotor, and wires, including coil wire.


Idling at 950-1000 smooth idle with occasional misfire. The tachometer did not bounce or waver. Check the plugs with 3000 miles on them and found light brown deposits on all 4. I also "dremmeled" the connectors to the -ve and +ve terminals, resulting in some improvement.

Here in Florida the temps are in the mid 60's F, mid teens C.

AM: Engine starts one pump to set the choke, turn the key and we're up and running in a snap. No miss. Drive one mile to the first traffic light (engine warms up)still good idle and a slight sproatic miss.

Green light, through the gears and we start to get a slight sputter, and once or twice, a carb backfire. There is a lag in acceleration but we are running well at 25-3000 RPM

PM: Homeward bound the temps are in the mid 80's F and Mid 20's C. (Engine warms up well) Now we have sputtering even at 25-3000 RPM, uneven idle and lag in acceleration.

As the tach does not falter, my thought process turns to the (blue) rotor.

Home now, replaced the blue rotor with the "old" black rotor I kept in my tool chest. Seems to be better, but haven't road tested it yet.

Am I on the right track? Any other thoughts??

Cheers

Gary
79 MGB
gary hansen

Sounds like you're onto something in the right direction
The only thing I would add ---at this stage, is that the Weber carbs are fairly touchy on idle mixture and if set too lean they will give an intermittent little spit back
I'm thinking while your electric choke is operating ,cold everything is ok but as it warms and the choke goes off, then the mixture could be too lean
Has the carb been jetted up for you or is it straight out of the box---Usually they come with fairly lean jets in them
willy
William Revit

Agree. If you had a condenser, I would recommend changing it, but since you have electronic ignition, I would check ground wires.

Do one other test. Hook you timing light to each plug wire and see if there is any noticeable miss in any cylinder. Also check for vacuum leaks.

Electrical is what I would think is the cause
Bruce Cunha

Willy, the Weber was pretty well set right out of the box. Did some mixture adjustment to get a smooth idle.

Bruce, you really learned about dodgy condensers. The ignition in mine is electronic and I did clean up the coil contacts to the distributor. That did offer some improvement. After i got home from work, I changed out the (blue)rotor for a (black) rotor which worked well from the last tune up.

I did check for vacuum leaks and all seems well.

Tomorrow's trip into work should tell the tale if I get better results in performance with the rotor change.

Will post results.

Thanks

Gary
79 MGB
gary hansen

Gary,
I had similar symptoms on my '73BGT with HIF carbs and 25D distributor.
Turned out to be a clogged line from the vapor canister to the tank. Clogged almost solid. Had to cut out a small section and replace. Also replaced the anti run-on valve during my diagnostic process, but the clogged line seemed to be the culprit.

Pat
Patrick Callan

If the tach is steady, and with electronic ignition, then it is HT or fuel. If it's HT then I'd expect to get an exhaust backfire, at least some of the time, even if some of the time it was from the carb. I don't know what can cause purely a carb backfire, other than perhaps sticking valves.

Check fuel by disconnecting the fuel delivery pipe (it may spurt if the ignition has been on recently), direct it into a container, turn on the ignition, and it should deliver at least one Imperial pint per minute, and in practice in less than half that time, in a steady series of pulses with minimal bubbling.

Another thing is if it starts spluttering and stumbling, as opposed to a single sporadic miss, is - when safe to do so - to turn off the ignition when it is happening, bring to car to a halt, then turn on the ignition and listen for the pump. Chattering from the pump means the carbs are starved of fuel. One click or none (assuming SU), and it fires up and runs cleanly, is not fuel. Supply at any rate, can still be problems inside the carb.

Also check the float chamber to see if there is any debris present.
paulh4

Gary
I'd try just opening the idle mixture out say 1/8 turn to richen it a tiddle and see if that gets rid of your little idle speed spit----but also
New in the box these carbs usually come with 140 primary main jet and 145 secondary main
They work a whole lot better on a MGB with 155 primary main and 150 secondry main

willy
William Revit

Thanks to all. I replaced the new (blue) rotor with the old (black) rotor and got MAJOR improvement, both starting cold from home and return from work when the temps were hot. There is still a v-e-r-y slight miss at idle (900-1000 RPM)

WILLY, did some richer mixture setting on the carb and got a not so good tone (somewhat "blubbery") from the exhaust. So I set it back to the leaner "smoother" setting.

PAUL, the carb backfire was sort of a once,short, quick "spit" and had no exhaust backfire at all. The lag in acceleration is now minimal. I got a good response when I cleaned the spade tabs connectors on the coil. I will now replace the female part of the connectors to the +ve and -ve terminals.

My knowledge of the electrical pales in comparison to yours, but I feel the HT part of this issue is the guilty party here. I am also going to look more closely inside the distributor cap for "tracking" and burned contacts. With the engine running, I ran a screwdriver over the (new) spark plug and coil leads to check for "leaks"... none.

PATRICK, for the most part, my car is pretty well "de-polluted" It still has the vapor canister set up with a hose line to the base on carburetor air filter.

GENTLEMEN,as it was a 10 hour day at work, and it is now 1:00 AM EST, with a second glass of wine, I am pretty well knackered. Will delve into the car tomorrow.

Will post with progress

Thanks again.

Gary
79 MGB

gary hansen

Gary,
Good idea to look for possible "tracking" in the dark! If your coil has spade terminals screwed on, get rid of the spades and replace the ends of the coil wires with eyes. Better contact, corrode less and won't work loose!
I had an Edelbrock/Weber on my V8 Roadster. I seem to remember you adjusted the idle to get the best idle speed then "leaned" it till it dropped 20 rpm.
Have you got any type of insulation between the exhaust manifold and the Weber?
Allan Reeling

You may see the effets of tracking, but having had problems with caps on two cars I couldn't. Also more likely to be on the inside I suspect, possible to rig up something to see (in the dark as suggested) with the cap removed, but not easy.
paulh4

ALLAN, good thought process on the coil terminal connectors. There is also a white with black striped wire on the +ve and -ve posts that go to what I perceive as a ballast (finned aluminium bar approx 3" long).
The carburetor adjustment I will redo using your method. There is no heat shield protection from the exhaust header to the intake manifold. I bought a 50' roll of exhaust wrap to reduce the heat build up.

PAUL, I do see some sort of "burn" on the 4 rotor contacts. I will compare the condition of the old cap to the new and swap it out if feasible.

I will post results later

Thanks again

Cheers

Gary
79 MGB
gary hansen

OK. Changed the new distributor cap for the old one after finding "burn" marks on the 4 contacts of the new cap. Also, replaced terminal connector ends on the coil +ve and -ve posts. Used he Dremel tool to clean up posts and spade ends. The ends of the ballast were very dirty and corroded.

So it seems this issue has been laid to rest. Thank you for all your input.

Cheers

Gary
79 MGB
gary hansen

" I perceive as a ballast ' A '79 would be fitted with a ballast resistor wire built into the loom from the fuse box. It's white and green at the coil, pink in the middle and white and brown at the fuse box. If the box is a ballast and the ballast wire hasn't been by-passed, you could be running on limited voltage to the coil, with a big reduction in spark energy. Are you sure the "finned" object isn't the amplifier for the electronic ignition? There should also be another white and green at the coil +, which comes from the starter solenoid and feeds 12v when cranking If you have a meter check the resistance across the coil terminals. 3 ohms is a 12 v coil, 1.5 a 6v, used with a ballast circuit, less than that an electronic coil, OR a faulty 12v.
Allan Reeling

ALLAN, There is a white wire with a green stripe that comes out of a wire harness to one side of this "ballast" (or amplifier as you mentioned) which resides next to the coil, then a white wire also coming from the same harness to the other side. I have no idea where in this wiring harness these
two wires originate.

Cheers
Gary
79 MGB
gary hansen

ALLAN, There is a white wire with a green stripe that comes out of a wire harness to one side of this "ballast" (or amplifier as you mentioned) which resides next to the coil, then a white wire also coming from the same harness to the other side. I have no idea where in this wiring harness these
two wires originate.

Cheers
Gary
79 MGB
gary hansen

Gary, On the 79 there is something called "distributor resister fed by a white and a white and green. The white and green goes to the distributor multi-plug. Thats on the original electronic Lucas dizzy, presumably to reduce the voltage to the electronic unit. The white probably to a bullet connector together with a white to the dizzy plug, the pink resistor wire and the white and brown from the fuse box. Whether your new electronic unit needs the resistor or not, you'll have to check the unit instructions. But still check what voltage coil you have and go back to basics based on what the installation instructions to your new unit tell you.
Allan Reeling

ALLAN, Hmmm, there is a multi-plug harness that went to an old distributor. I'm guessing it was an OPUS unit. The OPUS was changed probably long before I bought the car. That plug appears to come out of a harness and is hanging in limbo attached to nothing. When the engine was rebuilt some 33k miles ago the distributor was physically broken I guess during the re-installation of the engine. It was replaced with a 45D. So perhaps this "amplifier/ballast" does nothing today.

Cheers

Gary
79 MGB
gary hansen

Gary, I would guess it is the dizzy resistor is probably not needed, especially if the plug is just hanging there. I hate things just being there! That plug will have a W/Bllue,( to the resistor) a W (which goes to the 12v end of the ballast wire and a W/black (to the coil neg). Has someone bridged any of the plug contacts?
Are you sure the ignition unit is Lumitronix? I can't find any reference on google.
Allan Reeling

Gary, look here;
http://www.advanceautowire.com/mgb.pdf
Get your meter out and check what coil you have and whether the resistor wire is still in circuit.
Allan Reeling

ALLAN, looking at the wiring schematic,this item is listed as a Distributor Resistor. From what I can see on the schematic and what I see in my car, this resistor no longer has a function.

In the car, a WLG wire comes from a harness to the resistor as does a W wire. Neither of these two wires go to the coil. As I do not own a meter, there is no way I can trace its origin or terminus. I do not know how many previous owners this car had,but the OPUS unit is long gone. So if this resistor is part of the OPUS circuit, it has been retired long ago along with the multi-plug and its wires.

I don't like to see unused wiring harnesses hanging in the air, but I'm reluctant to remove it for fear of cutting off a functional wire that may be feeding a necessary circuit.

Cheers

Gary
79 MGB
gary hansen

Sorry Gary, got my colours mixed the resistor, if that's what it is, has w/Blue and white wires. I never trust the codge ups PO's may have done. I've corrected lots of such problematic wiring, the only way is to start from scratch. What make of ignition is it? By the way a meter is a sound investment and they're not expensive.
Allan Reeling

"If you have a meter check the resistance across the coil terminals"

Remove all wiring from the coil terminals before measuring them as otherwise you will get confusing results.

You also need to determine whether you have a ballasted feed to the coil or not, which is a little trickier to determine if a factory electronic system has been supplanted by an after-market one. If you have a ballasted feed to a 12v coil, you will be getting weak sparking. If you have a direct feed to a 6v coil, it will be 'overdriven', possibly overheating, and quite likely over-stressing the electronic module. To determine, you need to find out which wire is the ignition supply wire, which should show 12v when disconnected from the coil with the ignition on. That should be connected to the coil +ve, an earth connected to the coil negative, and all other wires removed from the coil. Measure the voltage on the coil +ve:

If you see 12v it is a direct feed, and you need a 12v coil i.e. one measuring 2.5 ohms (typical sport) to 3 ohms (standard).

If you see 6v, you probably have a ballasted feed, and probably a 6v coil.

If you see 9v you probably have a ballasted feed and a 12v coil, which is incorrect.

It's further complicated on North American spec cars by original 45DE4 coils being lower resistance than points coils to compensate for the reduced current of electronic systems. 45DM4 coils were even lower - about 0.8 ohms, which gave a higher intensity spark without overheating as that system is variable dwell which results in very short (compared to points) coil pulses at lower revs.

All cars after 1972 had two white/blacks on the coil -ve, regardless of whether they had points, 45DE4 (Opus) of 45DM4 distributors. One is the trigger wire from the distributor (points and 45DE4) or external ignition module (45DM4) to fire the coil, the other extends that trigger to the tach.

All rubber bumper cars had two white/light-greens on the coil +ve, again regardless of distributor. One was the ballasted ignition feed, the other is the 'start boost' wire from the solenoid.

As said, the 45DE4 system had a reduced-voltage 12v supply on a white/blue from an external resistor, as well as a full 12v supply, and all the electronics were internal. These were rather unreliable, many were replaced under warranty. I would have expected it to have been factory replaced by the 45DM4 by 1979.

The 45DM4 had a black and a yellow/red between the distributor trigger and the electronic module.
paulh4

OK got some time to review the connectors.

Connected the +ve post of the coil is:

Condenser,(radio static suppression I guess),Red to distributor, 2 W/GRN.

Connected to -ve post of the coil is:

Black to distributor,2 W/BLK

Cheers

Gary
79 MGB


gary hansen

That's as it should be assuming the black and red are right for your ignition module. You just need to check which coil you have.
http://www.ebay.co.uk/itm/Small-digital-multi-meter-multimeter-detector-electrical-test-automotive-car-DIY-/401173492947?hash=item5d67cdb4d3:g:jy8AAOSwSv1Xl1qv
Allan Reeling

ALLAN, So, I do have it wired correctly. Yes, the Red and Black are the for the ignition module.

Now you mention which coil I have. The coil is blue in color and believe it be a LUCAS coil. Having no idea how old this coil is, should I be considering a replacement and if so, which unit would be the best choice with regards to my car's set up?

Cheers

Gary
79 MGB
gary hansen

Gary. Blue coil is probably a Bosch. Age of the coil has little to do with how it is working (I am running the original coil on my 50 TD). Some good video's on testing the resistance, but I think one of the other members can walk you through it.
Bruce Cunha

Gary,
If the coil has a part number on it, google it. You might be lucky and it is marked with a voltage, or "electronic". But you still need to know whether the supply is ballasted. Get a meter!!!
Allan Reeling

Connections sound correct - up to a point see below - for an under-cap ignition module such as a pertronix.

It does seem to have ballasted wiring, but you do need to check the coil resistance and whether the ballast is genuinely in circuit as described earlier.

If it is, then I'd advise moving the red wire to another ignition switched source such as the white or white/brown at the fusebox. As it is, the voltage to the ignition module is continually varying between 12v and 6v, instead of being at a steady 12v as it would be on an unballasted system.
paulh4

Attended a G.O.F the past w/e. It was 170 mile R/T and the car ran good, and much better on the return trip. It did experience some missing heading home, but ran well after a 60 MPH 50 mile highway leg.

A fellow who owns a British car repair shop informed me I have a Bosch 12 volt coil and the distributor resistor is doing nothing and should remove it along with the former OPUS wiring harness and plug which is hanging in space.

I also noticed a lack of a "12 Volt hot line" from the starter to the +ve post on the coil. So my system is not ballasted. I was also informed the ballasting in the 79 MGB's had a long silver wire that runs inside a wiring harness and terminates somewhere to a coiled resistance in the front of the car forward of the radiator(?). Got me there.

Ordered a 7mm insulating spacer for the Weber carb. I did pitch the old one as it was pretty well tattered. I cannot help but feel there is a fuel vapor issue here. Will also buy a digital meter. This ballast/non-ballast issue has me curious.

I will address to all this after our holiday. Thursday morning we will arrive at Gatwick and will be with relatives in Newport Pagnell, Bucks, for about 5 weeks.

Till then

Cheers

Gary
79 MGB
gary hansen

This is getting very confusing.

"I also noticed a lack of a "12 Volt hot line" from the starter to the +ve post on the coil. So my system is not ballasted."

If you don't have a direct 12v feed to the coil +ve, on;y the two orginal white/light-greens, then the system probably IS still ballasted.

"the ballasting in the 79 MGB's had a long silver wire that runs inside a wiring harness and terminates somewhere to a coiled resistance in the front of the car forward of the radiator(?)"

The resistive wire is pink and that is inside the harness wrapping, with white/brown wires at the supply end and white/light-green at the coil end. There is no coiled resistance in front of the radiator.

The Opus/45DE4 did have a second resistor, an identifyiable component this time, with white on the supply side and white/blue on the distributor side.

There was also a white wire going into the distributor i.e. 12v., and a white/black from the coil -ve.

Far from 'doing nothing', the ballast in series with a 12v coil is significantly weakening your HT spark.

But before doing anything, and regardless of what the coil or anyone else says, measure the resistance between the primary spades - all wires removed. A 6v coil - which is needed for a ballasted system - measures about 1.5 ohms. A 12v coil measures about 3 ohms.
paulh4

PAUL, again thank you. Yes, this has me quite confused. I will address this after we return home in June. I will start a fresh post. I will also buy a meter and write down the readings so I can ascertain what I have.

Previous owners have done their handy work and I am not that electrically savvy. So I will learn with the meter and research what to do.

Thanks

Cheers

Gary
79 MGB
gary hansen

I just got back from the Wallingford parade. My car suffered from a misfire when hot. I thought it was fuel vapourisation, but comparing underbonnet temperatures with another B there showed no difference. Also the issue didn't clear once we got some clear road, and the new Revotech fan kept the water temperature just where it should be.
The car started off fine on the way home but when it warmed up the misfire returned. I had a quick look around but couldn't see any problems. I know mixture and timing are good, its just off a rolling road (Novatech in Slough, highly recommended).
A plug check showed no 3 darker than the others.I took the dissi cap off and everything looked fine but I noticed the screw on the no. 3 post seemed a lot further out than the others. Its a 25D with side entry HT leads metal cored contacted via a screw that pierces them. Well it was very loose and went in a long way with very little force, some of the others were a bit loose as well. I have already fitted a red rotor arm.
This seems to have totally cleared the problem, much to my surprise. If that was it, why did it run fine cold? I allowed the car to idle for some time after putting it back together and a drive around locally including some time idling and no sign of a misfire.
i suppose the screws can work loose so should be checked sometimes.
Stan Best

Stan. I happen to be in Newport Pagnell , Bucks at the moment. I won't be back to the states until June.. So the issue with my car is still unresolved

One of the things I will do upon returning to the states is to change the plugs. Let's see what happens after that

Cheers

Gary
79 MGB
gary hansen

Things move around when they heat up, loose connections are often intermittent because of that.
paulh4

Hi Gary, I was having trouble with the car misfiring after cruising on a light throttle opening. While I was waiting for the rolling road I put a set of these in
BPR6EIX spec as MGB upgrade on the NGK website.
They are rhodium tipped, much finer electrode. They masked the problem until Andy set the mixture to a good fit across all operating conditions.
The plugs look white when they are removed after idling, but the lambda is good. So those old guides to mixture strength dont work now.

Paul yes, thermal expansion has to be a candidate for what was going on. I wonder of the spark was jumping the gap, until the insulation closed up and stopped it.
Stan Best

This thread was discussed between 13/04/2017 and 12/05/2017

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