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MG MGB Technical - Misfiring on No 1 Cylinder
|Hi all, im a bit new to this classic car ownership so forgive me if the answer is obvious, Ive recently bought a 79 MGB Roadster which i collected and drove home the 80miles with no issues, however its cut out on me and wont start. On inspection the wiring to the coil had fractured in several palces so i made a new section of loom...still no joy...the car starts on 3 cyl then within 15sec goes onto 2 then back onto 3 then immediatley cuts out as if ive turned her off. Ive changed leads, plugs, cap, rotor its got fuel and set the timing all to no avail, when plug No1 is removed its wet with fuel and makes no difference with the HT lead on or off, ive checked compression and tappets all seem ok....HELP!!!!|
|Welcome to the mad world of MG's and I hope you get it fixed quickly and don't become disappointed with your new fun machine.|
It certainly sounds like a HT fault but there again it could be fuel or LT! You'll need to be pretty methodical in explaining what and when things changed. When it cuts out how long does it take before it starts running again?
For starters check you still have a voltage at the coil when it stops. If you do, remove the main HT lead from the dizzy and hold it close to the block (10mm or so) to see if you have any spark (probably not but best make sure). If not work your way back through the wiring to the dizzy and check for bad connections, try manually opening the points. do they spark a little? if not work back again to the ballast resistor and check that.
We could go on for a whole lot more but lets take a few steps at a time. Come back with more info and we'll chip in or see if you can find a MG friend close by that has a bit more experience. Find the local club, someone's bound to be able to help.
|Thanks Mike, its good to know im not alone...the car originally drove fine when collecting it, i drove to my work fine but on return the car wouldnt start at all. On inspection i found the wires to the coil were wrapped in tape, i removed this to find the wires brittle/snapped in places and corroded, i made a new short loom section and refitted it thinking id solved the matter. I did find a spare green wire taped to the original wiring but no connector on the end, this has remained as found. The original wiring was 2 white/green wires to positive on coil and 2 white/black to negative on coil and to distributor. I took the opportunity to fit new leads..this is where i get confused...firing order is 1-3-4-2..rotor turns anti clockwise but car will only run all be it with a misfire if the leads are set up in a clockwise direction, any other combination it wont run at all. It starts on what seems 4 cylinders then immediatly develops a misfire then will allow me to hold the revs at say 2000 rpm for 15 seconds then shuts off as if ive turned the ignition off...then turns over and over before begrudgingly starting on 2 cylinders then onto 3 then cuts out again??...any ideas on where to go from here..i checked coil wiring again today and all seems ok. Ive points and condenser on order for tomorrow even though i have a spark..ok its a pretty odd spark from the plug not your usual lightening bolt arcing but more a weak flash which jumps all over the electrodes if this helps.|
|Oh you are definitely not alone. We've all been there at one time or another and there will be lots of advice coming your way when MG50 finishes and the Boys and Girls get back online. You might even get some of our international brethren adding to the mix from time to time.|
I find it amazing that a PO can rig a car to run just long enough for the new owner to get it home and no more! I certainly couldn't manage it ;0)
Back to the problem at hand...
1-3-4-2 anti clockwise is right but you need to ensure that 1 is where you think it is. I'm guessing you changed the leads one for one? Even so, you need to set a base line and check pot 1 is on its compression stroke and that the rotor arm is pointing to the correct lead.Personally I would do this by lifting the rocker cover and checking both valves are closed. Others like to use a finger over the plug hole to test for pressure. Once that's confirmed you can confidently set the leads in the correct order.
Two white and light green wires on the +ve of the coil and white and black on the -ve is correct for the year but the spare green has me a little suspicious! Measure the voltage on it and report back. One of the white and blacks goes to the dizzy the other to the tacho. One of the white and green wires should run to the starter solenoid and the other runs to the ignition relay via the ballast resistive wire in the loom (I hate these and this is my guess as the offending item, but let's not assume ;0). My easy way of eliminating a lot of the potential problems is to run a new clean 12v supply to the +ve by connecting a new wire from the brown wire connector on the fuse block to the +ve of the coil but only for a short time (couple of mins max). If it starts and runs we can use the same wire at different points of the circuit to find the fault. Try it after checking the leads as above and see if it will run for a while.
That should be enough for now. Let me know how you get on.
|Stick with what Mike is saying, and proceed methodically through the various things said. However one thing you haven't said that you have replaced is the actual Coil itself. I've had a similar thing happen to our Road Midget and the coil had broken down. |
It worked fine for a short while then as it got hot it started to play up and gave the same symptoms you describe. It would cool down enough to run for a very short time only to repeat it again.
Worth a swap if you have a spare one available.
|Andrew, sound advice and I was going there next... The same happened on my 1300 with a "new" 12 month old unit. I decided to go back to an old one of unknown heritage but probably of 70's vintage I had knocking around the garage and it's been fine ever since (touching wood)..|
The symptoms also apply to a duff condenser or a duff ballast wire, both of which I have seen in a 74 Midget over the last few years.
|Well new points and condenser fitted but no difference, ive double checked firing order, timing and plug leads on correct and all seems ok however both your theories about a duff coil maybe the issue. Im still running the original one and tested the feed from the ignition to the coil, when testing the live i had a really nice bright test lamp, connect the live to the coil and the test again and the lamp barely illuminates so im thinking new coil req...or is this normal when connected?? Also i noticed today just moments before the car cuts out my ignition light comes on, but it no longer illuminates when ignition is turned on anymore...i keep telling myself it will all be worth it in the long run hehe.|
|the lamp will dim significantly (or go out) if the points are closed when you connect the wires to the coil. So that could be a red herring. Check it when the points are open when there should be no change.|
|check the resistance across the + and - on the coil. If it's a 6V coil, i.e., for use with a ballast resister it should read around 1.5 ohms, a 12 v coil double that. The voltage at the coil with ignition on will be approx 6V if it is wired through the resister wire 12 V + if the resistor has been by-passed. Often you find this wire by-passed but the 6 V coil is left in place, it then gets fried and the primary windings break down!!!!|
|Well ive no idea how this thing is running but ive sorted it..i think. I changed the coil today and she fired up and ran without cutting out, ignition light works again but i still had the misfire. The way the vehicle ran i presumed it was only No1 cyl not igniting even though i had a good spark cause thats the first lead i pulled off with no difference, but it appears No4 isnt igniting either...swopped the leads around so 1 is now 4 and she runs the best she ever has. This is where im really confused, the timing set to 10deg BTDC the rotor points to the front of the engine straight at what i thought was No1...with the leads as they are the firing order is 1-3-4-2 making No1 against the bulkhead not radiator end??...Any ideas....Steve|
|I don't know if it's possible on a B series engine, but I've been told to watch out for the dizzy being a 180 degrees out on the A series engines. Is that a possibility?|
|If the vacuum outlet is still pointing upwards, the dizzy drive could have been been put back in the block at a different angle or the dizzy shaft has been assembled 180deg out. but check that the rockers on No 1 are "rocking" when you think you have TDC. It makes no odds except for the orientation of the plug leads, as long as at TDC the rotor arm is set to a contact which is then N0 1 and then the rest are in order from that. Did you install a 6V coil or 12Volt?|
|I installed a 12v non ballast type coil and as for the timing TDC is set with the rotor firing on the plug nearest the radiator which has now become No4 plug making No1 against the bulkhead.|
|Well done on getting it running. Now you really do need to understand how you've done it to be sure it's not going to go off again or worst still, if it does, how to get it back.|
If you've fitted a 12v coil, did you run a clean 12v supply or are you still using the two original white and green wires (or the stray green you found)? Running a 12v coil on a ballast system will result in a weak spark which may/will give problems when running under load.
From what you've written it would appear the dizzy drive is 180 from standard which is not a problem so long as you know it. As I mentioned in an earlier post, check this with the rocker cover off, rotating the engine forward past TDC, if one of the valves opens on the first (nearest the radiator) cyl the dizzy is 180 out. Both should be shut for the compression stroke (half a revolution of the crank) and the exhaust should open as the piston comes back up to TDC. Setting the timing on either 1 or 4 makes no difference so long as you get the leads in the right place relative to the rotor arm and the leads in the correct sequence. Reading between the lines a bit, I would say the stray green wire is 12v which has become detached from the coil causing the initial fault. Then you've fixed the loom and in changing the HT leads you managed to get them crossed over and lost the 180 set-up but as luck would have it, managed to get 2-3 in the right place which is not a surprise if you tried the leads in the clockwise direction.
You will need to work out if you should stay with the 12v coil or go back to the ballast coil assuming that's what you have. It is possible the PO connected 12v to a 6v coil and this has now burnt out on your run home. To do this you really need to know the voltages at the coil. Go buy a cheap multimeter to check and it will also help find out what the stray green wire is.
Well done again and remember the next problem is just around the corner ;0)
This thread was discussed between 19/09/2012 and 24/09/2012
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