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MG MGB Technical - Intermitant Power loss.

Hi, On two occations in the last fortnight the car was running well and then there was a loss of power. Each time this happened i was doing approx 40 mph and when i accelerated the car struggled to go beyond 45 mph. The first time it appeared, i stopped and it idled normally and revved ok and then ran perfectly for the rest of the journey. The second time i was again doing 40 mph and the same thing happened but this timei did'nt stop and after about 30 seconds it ran normally. The weather on both occations was sunny (rare this year) and about 25c. The B is an early rubbernose with original engine (84,000 miles) K&N air filters and 123 ignition. Both times the oil pressure was 55psi and engine temp was reading normal. There was no misfiring or uneven running at all. Any advice would be welcome. Chris.
Chris82

Sounds like a fuel delivery problem to me.

If you have HS4 carbs, the next time it happens immediately shut off the ignition and coast to the side of the road. Remove both float lids and see how much fuel is present. If low / none check the fuel lines, filter and the fuel pump. Check your plugs too.

Tom
Tom Sotomayor

Tom, The carbs are HIF4s and were reconditioned units about 6 months ago. Thanks, Chris.
Chris82

Chris - See my Fuel Delivery Troubleshooting Guide at: http://www.mgexperience.net/article/fuel-tsg.html The first place I would look is at the tank vent. If the vent is clogged, the tank will draw a vacuum and the pump can't draw any more fuel through until the vacuum drops. My article covers this and other causes. Cheers - Dave
David DuBois

You can do much the same test as Tom says on HIFs by shutting off the ignition, then turning it on again. If you get lots of chattering (you shouldn't get more than one click) then the float chambers *were* low, for whatever reason. IMHO if it were a fuel tank cap vent problem it is unlikely to suddenly 'release' the vacuum to allow you to proceed normally, especially if the engine wasn't even switched off in the meantime. It sounds such a rare occurrence that diagnosis is going to be difficult. My roadster used to do something similar but even less frequently, the only common feature I could identify is that sometimes it happened shortly after I had swung out to overtake. I never found the cause, but after I had the carbs rebuilt it hasn't happened since. There used a be a little red dust at the bottom of the float chambers (HSs), that was with a new tank, but no filter on a 73. With HSs it is possible to get debris trapped inside the jet pipes, which can move about causing problems to come and go. But with HIFs the bottom of the jet is sitting in the middle of the float chamber. Unless something of significant size is floating round in there you shouldn't get the same problems. You will have to remove and invert the carbs to get a good look inside the float chambers on an HIF, which will mean replacing the large O-ring (they don't like being disturbed.

Ignition problems could cause similar symptoms, next time look at the tach to see if it is fluttering up and down. Even if not it could be HT.
Paul Hunt 2

This thread was discussed between 13/08/2007 and 14/08/2007

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