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MG MGB Technical - misfire

i'm experiencing a msifire on the over run or at constant revs with my 67 roadster. it sounds like one cylinder isn't quite firing properly. would i be correct to change condensor and plugs?

could it be anything else? the combustion chamber pressures are all equal @ 160
d buck

Well you might be and also if you have a known good cap and leads put that on as well. That at least should eliminate the ignition because I don't think that's where the problem lies. I'm pretty sure that the problem is a weak misture and this is a classic case. The weakness could be caused by an induction leak in the inlet manifold gaskets or even the carb spindle and bushings being worn. When did this start and was it as a result in some change that you made i.e tuning the carbs etc as this would be a sure pointer. I'd advise that you check the carb tuning carefully, make sure that you have the correct needles fitted and preferably adjust the carbs to the rich side of ideal for best performance rather that to the weak side. If you have a Colortune you can have a look at the mixture in number 1 and then number 4 cylinders and adjust them both accordingly.

I'm not sure that you will ever eliminate the popping back during over run conditions but you shouldn't get misfiring during constant light throttle.
Iain MacKintosh

Could be lots of things! But often best to start with the electrical stuff. Condensor is cheap so good start; plugs next-especially if the're a bit old. They may give a clue as to which cylinder is affected. Also check/change plug leads and the dizzy cap and rotor arm. HTH M
Michael Beswick

i know the vehicle has had recent new leads, and a carb tune up not long ago. however i have taken the carbs off and refitted them, although careful so as not to disturb the linkages.

it has appaeared after some fuel pump issues, i rebuilt the pump at the weekend with new contacts, works perfectly now. however i have noticed this misfire on the day i could start the vehicle at work, when i also changed the plugs. it misfired all the wya home no friday and gave up outsidemy house. the pump was not ticking over properly and the point were worn. however i left the old plugs in and i think this is the reason for the misfire.

i will change the condensor over lunch and re-fit the plugs.


also on friday i forgot to close the bonnet and whilst driving home it flipped back and broke, so this weekend i have also rebuilt the bonnet using a steel item from SC parts.

needless to say i'm gutted the car had a new fibreglass roof from the b hive [which wasn't great] and a respray about a year ago.

i must say i'm not feeling on top of the world at the moment.

d buck

It would help if you mention all the things you've fiddled with when you pose the original question!!!! M
Michael Beswick

The fuel pump could very easily be causing the mixture problem that Ian described. One of the problems with repairing a fuel pump at home is that it may run perfectly when hooked up to a battery, but most people don't have the means of testing the pump under actual operating conditions until it is on the car. So while the pump may have checked out fine on a battery with nothing pumping through it, it could well be failing under load. I would also suggest removing the fuel filler cap for a bit and see if the problem goes away. If it does, the vent in the cap is clogged and you need a new cap. If you will e-mail me, I'll send you a fuel delivery troubleshooting guide that may be of help to you. Good luck - Dave
David DuBois

I've most always found some minor problem seems to make things look like a major mess.

Regarding misfiring - I have the Crane electronic ignition system, so points and condensors are not concerns for me. However, I've found keeping the rotor brass arm clean is very helpful for good sparks. I've got the side entry cap (1964 B, 18GA) and use the yellow bumble bee wire -- if you have the "plug-in" spark plug wires, clean those connections in the distributor cap and make sure the connections are good and tight.

best of luck glg
glg

cheers guys, i bought a new condensor as i didnt have one spare, fitted that and cleaned the rotor arm, all is well, no more misfiring.

regarding the pump, i forgot to mention i fitted a new filler cap about 4 weeks ago. i found when testing the pump to hold my finger over the inlet, is this a good way to test?
d buck

Well, just goes to show what we know, not a lot. This seemed a classic case of shortage of fuel but we are of course trying to diagnose from a distance and that is never easy. Well done and thanks for letting us know.
Iain MacKintosh

I have to say that I always recommend checking ignition first as it is the easiest, for non-starters as well as misfires. Clip a timing light with inductive pickup onto the coil lead and each plug lead. Any erratic firing on the plug leads that isn't also on the coil lead indicates the rotor and/or cap are breaking down, and this will probably get worse when you blip the throttle wide open. Also ease the HT lead out of the coil and see how far you can get before the engine dies, you should be able to hear it arcing over a distance of at least 1/4" and up to 1/2" or more. If the engine cuts as soon as the physical contact is broken it is probably a failed condenser. This also usually causes retarded timing from normal, and if you remove the distributor cap and flick the points open manually they will also be sparking much more than normal.
Paul Hunt 2

Re starting on this post, what would be your advice in such a situation happening only after a one hour troublefree travel ? Cooling permits to restore proper
running conditions. And of course it is repeatable .
Thanks in advance.
Renou

This one might be into parts substitution. If you have a known good coil then change that first.
Iain MacKintosh

I have been told that original coil was replaced by a 40KV LUCAS sport and the issue is still on .
Renou

D. Buck - Holding ones finger over the inlet port of the pump will introduce a load on the pump and if you could get a full seal from your finger and from the check valves inthe pump, it would stop the pump altogether. While this might tell you something about the pump's ability to pull a vacuum (if you do this, always make sure that you have one hand on the pump and when it starts getting warm, shut the power down or damage will occure to the pump), but unless you want to hold your finger there for a long period of time, it will not pick up any long term problems with the pump. I always run the pumps on the test stand for 24 hours after I restore them. It has been a real eye opener to me to see how often a pump will fail after 23 hours if it is not adjust correctly. Doing the 24 hour endurance test of the pumps has almost completely eliminated pumps coming back after restoration. Cheers - Dave
David DuBois

cheers dave, i had the pump in such a way that i could release the wire quickly.

the pump now seems to provide proper thuds when switching on. before it was almost silent.
d buck

Renou - the same diagnosis method as with a non-starter at any other time. Check for HT with a timing-light, if none check back through the LT with a voltmeter. If you have a spark at the plugs check the fuel. Can't sniff hot plugs though, so in that case I'd check the pump was pumping into a container from a pipe removed form a carb. Then if HSs it is easy to lift the lid on the float chambers and see if they have fuel, on HIFs you are reduced to blowing gently in the float chamber pipe and seeing fuel bubble up from the jet, for which the air cleaners to be removed. Works for HSs as well, of course. The problem comes if it repeatedly fixes itself part way through diagnosis, and in the past I have wired a voltmeter to various circuits and positioned it in the cabin where I can see.
Paul Hunt 2

Paul thanks for your reply. I told the french guy in trouble to have a glance at it. He did modify static advance which was too high and the probleme is still alive. I remember to have had a hard HT problem to fix that I happened to diagnose in the garage at night and the light off: one of the main leads was arcing.
Many thanks.
Renou

Renou - I actually forgot to mention the first bit of diagnosis that you should do immediately it happens, and before you turn off the ignition or disengage the clutch or gears. With the forward momentum of the car still spinning the engine look at the tach (only works with electronic tachs, not the mecahnical rev counter) and the ignition warning light. If the tach is showing zero the ignition LT has failed i.e the circuit through the coil and points, if the warning light is on the ignition feed has failed i.e. from the ignition switch (which will also cause the tach to stop registering). If the tach is still registering the problem is either HT or fuel.
Paul Hunt 2

Hi Paul, very intersting tip. His car is a US 72. I explained the check to carry on with the speedo.
Best regards.
Renou

LAst but not least, the B was a 72 US market model. It had been equipped with a fuel vapour system and that system might be the reason for which the car fails after one hour trip. Did anyone ever experienced such a situation?
Renou

Not necessarily US cars but I have just remembered something else and that is a blocked or restricted tank vent system. On non-emissions equipped cars this is a vented filler cap, on emissions cars a non-vented cap and a seperator and hoses back to the charcoal canister and from there to the atmosphere. If this gets blocked the car will run for a while, but as fuel is used a partial vacuum starts developing in the tank, and eventually defeats the pump. Leaving it parked can allow air to seep back in so it starts again, but removing the filler cap will result in a big 'whoosh' of air and it will start right up. Be aware that a vented cap (non-emissions system) can contain a spring-loaded valve to prevent leakage in a roll-over and so a small 'whoosh' can often be heard even on a good cap. This effect shouldn't occur at all on an emissions system.

Blocked emissions hoses anywhere will cause problems, either fuel starvation or fuel overflowing (float chamber hoses).
Paul Hunt 2

This thread was discussed between 20/02/2006 and 01/03/2006

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