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MG MGB Technical - Sporadic Misfire

My 1980 UK spec. roadster suffers an occasional loss off power accompanied by a light backfire in the exhaust. It feels as though it's only firing on two cylinders. It can run like a dream for 30 miles then the rough running occurs out of nowhere.
There seem to be two main types of driving when it will occur. One is under hard acceleration in third gear at about 3000rpm, the other is when cruising on a light throttle when making constant light corrections to the throttle to maintain distance behind a vehicle in front.You can feel a slight hesitancy build into the full symptoms in this case. It can also happen when trying to accelerate on part choke when cold.Backing off the throttle, "blipping" it a few times with the clutch depressed and maybe changing down a gear,and after a few seconds it clears.The tacho doesn't drop when it happens.
I fitted an unleaded head during the cars rebuild.It has done around 400 miles since then. The symptoms were initially worse after getting back on the road with the car losing power completely with a horrendous backfire that caused everyone around to duck! Leave the car to cool down and it was once again driveable.
The fuel tank, pump (Moprod contactless), fuel lines and filter were renewed in the rebuild.I cannot detect any leaks in the manifolds.Replacing the coil, HT leads, plugs, new distributor, points, condenser and a set of recon HIF carbs have only made the symptoms less severe. Dwell is good, timing set at 9 deg.btdc plgs a healthy colour, good compression and running on 99 octane unleaded.
An mg specialist has checked everything including carb balance & mixture and found nothing wrong.
I don't know where to look next.....any suggestions please!!!
ph barton

Phillip - See my article, Fuel Delivery Troubleshooting Guide at: http://www.custompistols.com/cars/dave/ddFuelDeliveryTroubleshooting.htm
for information on troubleshooting from that rospective. Be aware, that even though it is an all electronic pump, the Moprod can still fail intermittently or permanently and when it does, unlike the points style SU pump, tapping (or even beating) on it will not bring it back to life. Having experienced a problem with a clogged fuel tank vent, my first check would be to remove the tank filler cap and then going for a drive. Cheers - Dave
David DuBois

PH----- I believe that your 1980 MGB has dual carburettors. I had this problem occur on an MG-TF. It was one ofe the piston dampers hanging up on the wall of the chamber. It too was intermittent.
Sandy
Sandy Sanders

PH,

if there is a Lucas Opus distributer on the engine, it might be an electronic problem. Sometimes the small boards tend to crack and give problems when getting hot.
Other things to check are the cap of the distributer that also can become faulty and the fuel pump, filter float valves, coil.

Ralph
Ralph

Dave, your article mentions a charcoal filter as a vent for the fuel tank on late cars. Is this a US spec thing?...I can find no reference to it on UK cars. What would be the equivalent vent on a UK car?
I'm interested in the the concept of on carb piston getting stuck. Would this cause a unburnt fuel getting into the exhaust to backfire?
I shall be working through the HT side of the electrics. Does anyone know where I can get a set of copper HT leads rather than silicone?
ph barton

Charcoal filter/cannister was an US/export thing only. UK cars must have a vented filler cap.

Backfiring can be caused by a weak mixture such as on the overrun. Also if it is in the exhaust it can be caused by intermittent HT, or a sticking exhaust valve. If in the intake then a sticking intake valve. A sticking carb piston won't cause unburn't fuel in the exhaust, but it will upset the mixture. However unless this happens just as you have moved the throttle, i.e. the pistons are going to move up or down, it is unlikely to be the pistons.

Copper leads are unlikely to solve the problem, and you will need alternative suppression. If the leads *are* the problem they are faulty and you would be better off with a new set of silicone cored. I've had a set on my V8 for 75k miles and 13 years and they are fine. I've only had one lead fail in dozens of years and hundreds of thousands of miles on several cars and that was something to do with a connector, not the lead itself. By comparison the original carbon string start going high and variable resistance in just a few k miles, as well as being much more expensive.
Paul Hunt 2

Try a new cap and rotor - some of the current products are known to have "invisible" faults.

Which "new" distributor did you install? Part of the problem may be an incompatible igniton curve and the wrong vacuum advance module for your application.
Jeff Schlemmer

This thread was discussed between 19/08/2007 and 22/08/2007

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