Suddenly not running right.

[whitestu]

The old B has been going well lately until today whilst during a bit of spirited driving and a hump back bridge it suddenly will only rev with the choke out (literally just after the bump from the bridge!). I limped home driving on very light throttle which was OK but without the choke it won't rev past about 1500 rpm. The car starts and idles prefectly.
First thing I though was a pipe had come off and I had a vacuum leak but all seems fine. Next I thought fuel starvation but delivery is good and pump easily delivers several pints per minute.
Leads, cap and rotor arm all seem fine - seems like it can't be electrical as the car goes fine with the choke on, but probably worth swapping rotor and cap tomorrow before exploring the carbs further.
Can anything in the carbs move / come loose after a jolt? Can the float valve close fine but not full open? All the linkages seem to be working fine.
There's oil in the dashpots.
I'll check the idle mixtures with the colour tune but as it idles fine I don't expect to find anything wildly out.

Standard 18v engine with HIF4s.

Suggestions welcome!

Thanks

Stu

[Vic Butler]

Worth checking if both dashpots in the carburettors are moving freely. It seems odd but the jolt must have upset something. I presume the throttle is opening correctly?

[Charles Farran]

As Vic suggests , (and also an easy one to do) it would be worth removing the dash pot covers & removing & cleaning both valve pistons & springs with fresh petrol or carb cleaner & then doing the "upside down test" to see if both fall back into the covers (without the springs) with a rapid drop & satisfactory clunk - taking care to keep the front & rear carb covers & valve pistons matched. Whilst they & the filters are removed i would also spray some carb cleaner into the carb bodies & also check the state of the connecting fuel pipe between the carbs as they do perish unnoticed & either leak or compress when the carbs demand more fuel when the accelerator is depressed & thus starve fuel to the second carb.
I take it there are no other air leaks around the carbs?
Having said all that 80% of fuel supply problems turn out to be electrical!!!
You don't say whether you have traditional points or electronic ignition......
Cheers
Charles

[Vic Butler]

Charles suggested electrical which could be the fuel pump. Make sure both wires to it are still intact and that nothing is loose.

[Paul Hollingworth]

On the Friday before the MG Social at Gaydon last week, I experienced very similar symptoms. The car would start OK and idle perfectly, but soon as it was on-load it would splutter. I slept on it and decided I would look at ignition first to rule it out of my enquiries before going to fuel. It turned out that one of the ends of the leads where it goes into the cap was very corroded (probably caused by the leaking heater valve). On examination I could see that there was evidence of tracking on the inside of the cap. I cleaned it all up and sprayed it with WD40 and the car ran OK so I concluded that was the problem. I fitted a new cap and leads to make sure. Car ran to Gaydon and back with no problems apart from being freezing inside, blasted heater valve was stuck in the 'off' position where it leaks worse. Usually the heater performance is more than adequate. I've bought a new valve now too but I wont put it on just yet until I've bought some new anti-freeze and its warmer in my garage. Forced the old valve into the 'on' position.

[Vic Butler]

On my previous MGB GT there was a pinholed core plug which did the same to the distributor.