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MG MGB Technical - Very Bad Engine Miss

Hi All, I own a 1980 MGB Limited. All Emmissions have been removed. I have a New pertronix Dizzy and Coil installed, Weber Downdraft Carb, New performance Wires and switched to a Double Roller Timing Chain, complete Peco Exhaust System. These have been on my car for about a year. During that time the MG always ran great. Last Friday I took my car out for a ride. It idled like a new one. I took off and ran through the first 3 gears to 4,000 RPM and slipped into 4th. This is where my problem started. All of a sudden with no warning the car went from power to a complete breakdown in the engine. it sounded like it was running on just 2 cylinders. I could barely get it back home, about 3/4 mile. could anyone tell me what might have went wrong? your help would be very appreciated. Thanks to all. Barry Crum USA rbcrum@atlanticbb.net
B.R. Crum

*Is* it just firing on two cylinders or not? You need to do some diagnosis, starting with a timing light on each plug lead and the coil lead watching the flashes for any leads flashing erratically or not at all. While doing that check the timing as well, then work from there, either back into the ignition system or forwards into the fuel system.
Paul Hunt

Additionally, and specifically, check that the vacuum hose for the power brakes hasn't broken or become unattached (or otherwise leaking somehow). And that there is no moisture in the distributor cap.

Wayne
Wayne Pearson

BR: I had a similar problem last year. Turns out the lead wire from the coil to the dizzy was working its way out of the coil. The spark had to "jump" from the coil to the lead wire. The car ran fine until 2800 RPM. After that, it started backfiring through the carb, tach readings "bounced".

So, check all lead connections for solid contact

check inside the dizzy cap for carbon arcing

Let us know how it turns out

cheers

Gary
79 MGB
gnhansen

Hi everyone, Paul, I took the plugs out when I got it home. #1 was normal, #3&4 were sooty black and #2 was wet with gas. Now I could only go about 20mph as top speed because any more throttle would cause it to sputter and backfire through the carb so I don't know if that fouled the plugs or not. I do know that before this happened the plugs were all normal looking, that is tan. Wayne I did the check and all was fine in that department, Thanks. Gary, I checked what you said and they were fine also. Thankyou. Paul I started the car to check the spark with the timing lite but the motor could barley stay running. Then as I revved it slightly I heard a very noisy rapping sound. It didn'nt sound like a tappet or anything else from the valve train. I'm now thinking the worst!!?? If you have an idea or anything as to what may have caused this noise please contact me. Again Thank all of you for your input and for the comeback. Sincerely Barry
B.R. Crum

did you jump a key on th e timing chain?
Peter Murray

Barry: If you can get a "doctor's stethoscope" start the engine and "listen" all around the engine to pinpoint the noise. I have found that to be a handy device. You can also try a length of metal rod held to your ear and the opposite end to the engine block and crankcase.

Also, hook up the timing light and see if the timing has changed. If so, that would probably indicate a timing chain "slip" or "jump"

Has the oil pressure dropped at idle and or at "RPM" ?

A rapping at worst could be rod bearings. A thumping would be mains. This is where the stethoscope comes in

Cheers
Gary
79 MGB
gnhansen

Timing chain slip would affect all cylinders equally, and is virtually impossible. Wet with gas implies a cold cylinder and not firing. Black and sooty could be either fouled plugs causing non-firing, or non-firing causing fouled plugs, so you need to clean the plugs and then see what happens. Unless you are going to dismantle the engine without doing any more diagnosis you are going to be running it, so I'd still strongly recommend the first test you do is with the timing light as I said. Correct any ignition and/or fuel problems (could be a stuck float valve causing flooding for example) and then see what mechanical noises you have.
Paul Hunt

Barry,
Have you checked the compression on all the cylinders?
It would tell you whether the engine itself is mechanically healthy, and only takes about 10-15 min.
I would do that before anything else.
Good Luck,

Ralph
Ralph

Okay guys, I'm going to check all you suggested if only I can get the motor to cooperate. Thanks a million, I'll check back to let you know if anything changes. Later Barry
B.R. Crum

I can't help but to think that the high volt (40,000 v) Pertronix coil
you are running is causing a short to develope between the rotor
and dizzy shaft.

It kept happening to me until I swapped to a moderately lower voltage (but higher than stock) Bosch Blue coil.
Daniel Wong

You could try removing one plug wire at a time to see if the tapping goes away, narrowing it down to which cylinder (probably #2). With a lot of soot and a wet plug, maybe there's a lot of carbon buildup in the combustion chambers.

The wet plug must not be firing, or wasn't just before you shut down last, as Paul says.

Is the whole distributor new/rebuilt, or a new Pertronics module stuck in one that still needs attention? Wiggling shaft, stuck advance, etc. I have read someone having a problem with the rotor not fitting all the way down.

A lot of rotors are questionable, it seems, especially with higher voltages.
Tom

Doesn't matter what coil you use, the peak voltage is governed by the plug gap. Once the voltage rises high enough to jump that it doesn't rise any higher, no matter what coil you have. You can see this by testing a coil by pulling the HT lead out of the centre and seeing just how far the spark will jump, and it will jump 1/2" or more. But your HT leads only have about 1/8" of insulation, so why don't they break down? Because the plug gap shouldn't be more than 32 thou so the HT will rather jump that than through the insulation.

One problem can be that if you open up the plug gaps beyond 32 thou in a mistaken attempt to try and make use of the increased voltage of an HV coil, then all sorts of things can break down.

There are also a lot of bad rotors out there, the ones with the domed rivets inside the base circle. Those with domed rivets outside the base circle, or the original flat rivets, are fine. See http://www.mgb-stuff.org.uk/rotors.htm
Paul Hunt

Hi again! I found out that when I took the dizzy cap off, that little carbon shaft was broken and the spring inside the cap was broken up in little pieces. This may be the problem. I put a new cap and rotor on (the one with the rivet outside the circle). Now for the dumb part!! I failed to mark the plug wires on the engine and on the dizzy. I know the firing order of 1-3-4-2, but how do you match up the wires from the engine to the cap. I did put them on as I tried from memory and it backfires through the carb and the tailpipe. Please help as I am going crazy here. Thanks guys!! Barry
B.R. Crum

If you havent moved the dizzy (i.e. timing), this is how I would do it....

Disconned the coil, take the cap off and get someone to turn the engine over, look what direction the rotor on the dizzy spins (I cant remember if its clock wise or anti clockwise)., then rotate the engine by hand till you are at TDC on cylinder one, take the rocker cover off and check that both the inlet and exhaust valves for cylinder one are closed (thus proving you are at TDC on cylinder 1 on its FIRING stroke).

Then loo where the rotor arm is pointing, it wants to be pointing in the direction of what will be the number one plug place on the cap!

So if the rotor is pointing at 2 O clock (I am sure that is what it is on mine but i could be wrong, its a year or so since I did this), put the cap on and connect the number one plug lead to the socket on the cap that the rotor is pointing at.

Now, assuming the rotor rotated clockwise (reverse if it was the other way around!), out the lead for clyinder three in the next socket going clockwise, then put the lead for cylinder 4 in the next socket on the cap and 2 on the next one.

That should do it! Sorry I cant remember if the dizzy goes clockwise or anti clockwise! I marked all mine up so I wouldnt have to remember!
Chris B

Nice write up, Chris.

The distriubor rotates counter (anti) clockwise. Tweak the rotor by hand clockwise to get it to the un-advanced position.

Both valves are closed when the rockers are 'loose'.

From number one, go around counter (anti) clockwise 1-3-4-2.

warmly,
dave

Dave Braun

Thanks Dave and Chris, I now have it right! Well I started the car and it will run however, very roughly. I put my timing light on each plug wire to see if they were firing, as Paul had instructed. Here's the kicker, each plug is firing, the light is very bright on all cylinders. I reset the valves and checked the valve train and all seems in order. The engine does not backfire through the carb or the tailpipe now, but it still sounds like it is missing. It doesn't have any power at all. Does anyone think I should pull the head? The carb is getting fuel and I would think the Dizzy is OK due to the plugs getting spark. Right now I'm trying to think what else to do to the motor, but I am stumped!! If anybody has an idea what to do next please let me know. Let me remind all that the car ran beautiful one minute and without warning it started a terrible missing that the B could not get out of it's own way!! Thanks for listening, all you guys are great. Later Barry P.S Maybe I'll try new plugs, that's alot cheaper than some of the other alternatives however I'll do what I have to to get her back on the road. Thanks
B.R. Crum

Before I pulled the head, I would double and triple check my spark plugs.

1) if you went around clockwise with the wires instead of counter (anti) clockwise, your firing order will be 1-2-4-3. I've tried that before and it is terrible! So swap wires two and three and see if it fixes it.

2) if you found top dead center and yet you were really on TDC for number four (as one and four come up together, and two and three come up together) and you went around clockwise instead of counter (anti) clockwise two and three will be in the right places, but one and four will need to be swapped. If instead you found TDC on four and went counter (anti) clockwise, I can't see the engine running at all.

3) finally, if you just pick a wire and start at one and go counter (anti) clockwise from there, you should have your firing order in one of four tries.

4) check the resistance on all four spark plug wires and the coil wire. It is not unheard of for a carbon wire to fail. The resistence will vary as to type, but for wires of similar construction and length should be roughly the same. If not, replace them.

warmly,
dave
Dave Braun

Hi Dave, I did go counter clockwise on the dizzy and have the order correct, thanks for that. Now here is something I need to know. My dizzy may be installed backwards, that is the vacuum advance module is pointing to the left of the engine. The problem with that is the module hits the starter not allowing me to get the right timing numbers. I saw a picture of an MG engine and it has the vacuum module pointing to the right towards the heater valve at say around the two o'clock position. Could I have the dizzy in wrong? I think I may have seen it installed both ways I don't know. Thanks Dave Barry
B.R. Crum

Bary,

The Distributor on the MGB I care has the vacuum advance pointing rearward. I believe that the only way to turn it around would be to reverse the dog on the bottom of the distributor shaft, as the dog is a one-way installation relative to the driving shaft inside the engine.

Something not mentioned is spark plugs.

Here is O'Connor Classic Autos take:
http://www.oconnorclassics.com/techtalk_plug.php

They describe symptoms exactly like yours.

Incidently, here is a picture of where number one plug ends up on Maggie.

good luck,
dave


Dave Braun

and another picture


Dave Braun

Barry,

I certainly would not pull the head unless you find a reason to do so. As suggested earlier, a compression test may show if a problem exists that would lead to pulling the head.

I suspect the reason it ran well one minute and not the next is because of the failed carbon contact you found in the dist cap. And now I suspect you haven't done everything to get over that problem. New plugs are a good idea since you may have fouled the originals when the problem started.

Also, what about pieces of carbon causing a problem in the distributor? Maybe blow it out with an air gun? Do you have the old points set up that you could put back in to check it?

Given that your dist is installed in a non standard orientation, you need to take extra care to see that you have the wires correctly attached. Since you have the timing light, go ahead and check the timing.

Charley
C R Huff

You don't alter the drive dog to get the vacuum capsule pointing in the right direction. Simply remove the leads from the cap and turn the distributor body so the vacuum capsule is pointing where you want it to. The rotor will still be pointing in the same direction as it was before, so now put the leads back in the cap to get the correct firing order.

Where you *do* alter the drive dog is when the *rotor* is pointing in the wriong direction for No.1 cylinder at its firing point, and you want to get that right. However that is purely cosmetic and less of an issue than the vacuum capsule pointing in the wrong direction which causes problems for its pipe to carb or manifold and possibly getting the correct timing.

The rotor should be pointing to about 2 o'clock, the vacuum capsule points to about 11 o'clock so the vacuum pipe can go round the back of the rocker cover.

The broken carbon contact would definitely cause sudden bad misfiring, with cap and rotor changed one would hope that part is now OK. Direction and order of the leads is of course very important, but the starting point i.e. where No.1 lead goes is crucial. You have to look at where the rotor is pointing when No.1 cylinder is at it's firing point. Each cylininder has two TDCs in its 4-stroke cycle so you can't simply go by piston position i.e. position it at the top of the cylinder nor by the timing marks. Without taking the rocker cap off the easiest way is to remove all plugs and turn the engine in its normal direction with your thumb over No.1 plug hole. When you can feel the compression lifting your thumb off the hole that is the compression stroke, so keep turning until No.1 piston is at the top of the cylinder, then see where the rotor is pointing and connect that cap contact to No.1 plug, and continue 342 from there anti-clockwise round the cap replacing leads.

As I say the *rotor should* be pointing to 2 o'clock, it could be 180 degrees out if the distributor has been dismantled and reassembled incorrectly, or if an incorrectly assembled distributor replaces a correctly assembled distributor (or vice-versa if the drive gear was altered to suit the incorrectly assembled distributor). The rotor could also be in any one of 6 (or 8, depending on how many teeth there are on the drive gear) positions if the drive gear has not been inserted correctly.
Paul Hunt

Thanks Dave for the article and the pictures. I installed new plugs and "feeling dumb" on my part the engine sprung to life. I could hear a metallic pinging in the motor as it ran, but then a miss started again. I started to take the plugs out to check them and when I got to #2 plug it had the hook peened against the electrode. Something in that cylinder did that so I put the piston to TDC looked in the hole and there was a piece of metal setting on top of the piston! I put a magnet in and took the piece out. It is the size of a half of a dime and 1/16" thick. I don't know where it came from, but obviously it came from the head or a valve. It does'nt look familiar. Anyway I regapped the plug, installed it and it started up. I set the timing and the idle and it seems to idle and rev fine! No metallic sound! I'll test drive it today and let you all know how it went. I can't thank everyone involved with my problem enough. Thanks again, Barry
B.R. Crum

Can you post a picture of the foreign object?
Art Pearse

Barry,

Since the # 2 plug got hit, I would replace it. There is a pretty good chance that the hit cracked the porcelain.

As for what you found in the cylinder, I can’t think of anything that small that would come out of the head unless it was a piece of valve. If it were a piece of valve, it would not be running well at all.

The other small thing I can think of that would have come from the cylinder would be a piece of the top piston ring if it punched through the top of the piston. If it is that, it should be easy to identify by appearance.

I think it is more likely to be something that went in through the intake. If it were mine, I would go ahead and run a compression test on the engine, and I would inspect the carb/air cleaner / intake for possible loose parts.

Charley
C R Huff

Charley and everyone, I put a new plug in #2 and took it for "The Ride" I was impressed with it's behaviour! It ran like a new one. No noises, hesitations or missing, just a joy to drive again. The car is responsive with no smoke from the tailpipe so maybe that foreign object did no damage to the engine or it's components. Couldn't have done it without all of the input and help you guys provided. Thankyou all so very much!! Cheers, Barry
B.R. Crum

Barry: Great!!! The car is up and running. So the rapping noise you posted turned out to be this F.O.D. in cylinder #2 and not a bearing. I would definatly take Charley Huff's advice and look real close at anything related to the intake system of the car and search for any missing bolts, washers etc. Just in case there is a "relative" waiting to follow in its footsteps.

Were you able to ID the piece of metal you retreived from the cylinder?

Cheers

Gary
79 mgb
gnhansen

Gary I'm not sure at least not yet but I think it might have come from the weber carb!! I noticed that a piece is missing off of the choke linkage. I'd send a picture but i'm not having any luck! Must be the program or something. I'll figure it out and see what happens. Thanks Gary Cheers Barry
B.R. Crum

Barry: I have a Weber DGV-E on my 79. The choke assy has a teardrop shaped lever which peened atached to the butterfly shaft. That end has a rectangular hole on the bottom of the teardrop. The upper end of the teardrop has a hole for the actuator rod which goes to the bi-metallic spring. This is what operates the butterfly. If that teardrop lever is missing, I would be willing to wager, this is the item you retreived from your #2 cylinder.

You can google weber carburetors and look at an exploded view of the mechanism. I think you will find your culprit.

cheers

Gary

79 mgb
gnhansen

This thread was discussed between 10/08/2009 and 24/08/2009

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