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MG MGB Technical - missfire at 3,500rps

My MGB GT started yesterday to missfire/backfire at about 3,500 in 4th and under load. Started when it was raining so I thought it was due to damp, but the same is happening today - after being dried in the garage overnight and today is bright and warm.
Have checked the timing and fuel lines (for air) and all seems to be OK. HT leads seem OK - they are each giving a spark. Points appear OK to.
I'm at a loss as to why this is happening. Any ideas anyone?
Thanks
Mike
M Kemp

How is your condenser?
.

Could be the condensor or the coil is breaking down at higher RPM, also check that you have oil in the dampers on the carbs.
John H

Gooday .

Had the same problem last two weeks. pulling up hill in 4th it would break down. My problem, which i have had once before, was the plug leads,One to be precise. if they are silicone leads with the carbon fibre centre, they can burn away at either end and will fire the plug with no load but under load can break down. Check for a blackened lead end where the metal end fits on. if there are marks ie burning, then remove the end and check the core. alternatively, change leads.
hope this helps

cheers.
Cactus

M.Kemp
I'm running a carbon resistor wire from the coil to the distributor cap, but because of the graphite breaking down and causing problems like you are experiencing, I'm running copper leads to the plugs. The one resistor wire from the coil seems to keep it quiet enough for the radio to work fine.
Barry
Barry Parkinson

Thanks Guys,
I was thinking it might be a lead, I removed the one from cylinder 2 and barely noticed a change to the running of the car, very marginal if at all. I swapped the leads round and found it better (perhaps a stronger plug), but still not good.
I will change the leads and the points (to clear the capaciter question) and see if that helps.
Otherwise, I think it must be poor combustion on a cylinder, but I'm not getting much in the way of smoke and my oil pressure is fine.
Mike
M Kemp


I had this problem and after changing plugs and leads still had it over 3k. It turned out to be a clogged Fuel Filter. After that no problems!

Mark

Hi there,
Well two weeks later - points, capaciter, leads, plugs, coil all changed, and still the problem persists. I have however, found a temporary solution - pull out the choke all the way and the backfire goes away, but the car runs flat (very little power).
OK I thought, check the mixture - plugs are a good colour - light grey deposits (a bit of oil but not to bad - it is an MG after all)definately not whitein the centre! - so they indicate the mixture is OK??/..... So.....
Any thoughts anyone...
Could it be air getting into the carbs?, induction system?.....something else?
HELP!!!
Mike
M Kemp

Mike, see my thread ''total misfire and stalling''

Maybe your floatlevel in the carbs is also very low?

Jacob.
Jacob Halma

MIke, if your car runs better with the choke pulled out, then its not getting enough fuel. I would recommend going through the entire fuel delivery system from the fuel pump to the carbs. It could be a bad pump, restricted filter, or even junk in the float bowls.
Jeff
Jeff Schlemmer

If you follow what Jeff says you will need to know the fuel delivery rate. To test this remover the fuel pipe from the carbs an put it into a suitable container. The pump should deliver at least 600ml or 1 pint in a minute.

The back fire at high rpm temorarily cured by pulling the choke out does sound as if the mixture is leaning out due to a fuel shortage as Jeff says.

What happens is that the mixture fails to burn fully in the combustion chamber because it is too lean for the flame front to travel across the chamber efficiently. As a result unburnt fuel enters the exhaust port where it finishes burning. The fact that is under less pressure once it leaves the cylinder speeds this up. Bang.
David Witham

Am I the only one thinking the timing may be off?
I recall going through the same problem a long time ago. Not knowing what I was doing (still don't pretty often) I simply pulled out the choke. The resulting enrichment was able to briefly mask the problem. After taking apart my carbs twice, I found out that the real issue was my dizzy.
Philip

Thanks all for the comments. I checked the timimg, backing it off slightly from 13-14* (dynamic timing) to allow for unleaded, and then checked the fuel mixture. I closed the inlet completely and then opened it 2 complete turns (12 flats) as per the manual. I then ran the car and gradually opened the value untill I didn't get any backfire/missfire (took another 6 flats). Curious now whether this is about the normal setting, or if the value has been "overclosed" and ridged the needle?. Anyone have any comparrison?
I still need to balance the Carbs, but at least the car now accelerates smoothly through the rev range (well up to 5500 rpm which is high enough for me!).
Thanks again for all you comments and help.
Mike
M Kemp

I had the same symptoms on my 73 roadster plus the car wouldn't idle with the choke out. So I was running with the choke out and pushing it in every time I came to a stop sign!

To try and fix this I first worked on the timing and solved some distributor problems. However, even though the car ran better it till needed the choke out to run well under any kind of load.

So, being unfamilliar with the carbs I took it to my local British car guy. He did a minor rebuild on the carbs and the problem was solved. I'm afraid I can't be too specific on the parts he replaced but it was not a full rebuild.

BH Davis
B.H. Davis

This thread was discussed between 02/06/2006 and 22/06/2006

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