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MG MGB Technical - 1982 Roadster poor acceleration when warm

Hi, I've recently purchased this car with 98k on the clock. generally all is well on my return to classic british motoring (previously a 75 BGT in late 80's)
If I go on a longish run, the car is hesitant when accelerating almost like petrol starvation or vapourisation. It picks up above 3k and is ok. When switched off it is also overrunning but not to a great extent (2-3 turns)It idles well and has no other outward signs of problems except the usual rattles. The heatshield isn't in great condition but as this problem happens while driving (at 0 deg C last week)I'm not sure vapourisation is the cause.
I'm thinking ignition/fuel but where to start? As I'm not sure of the history of the vehicle I'm contemplating a carb rebuild anyway, would this be a good place to start? any ideas would be appreciated.
P M Gregory

1982? 1980 were the last ones built.

Forget vapourisation, often mentioned, rarely happens, especially in a UK winter, these cars run in desert states without problems. It *can* be a problem with HS carbs with the exposed jet pipes where the heat-shield is faulty, but even then highly unlikely in a UK winter, and when it does happen it is more likely to be when stuck in traffic or on hot starts.

Acceleration hesitation is a sign of insufficient oil in the carb dampers.

If it needs lost of choke to get and keep going then it could well be weak mixture.

Running-on is very common, especially with 95 octane, although tends to affect mid-70s cars more. It can be a sign of weak mixture or over-advanced ignition causing hot-spots in the cylinders.

Fuel starvation is easy to check - remove a feed pipe from a carb and direct it into a container. Turn on the ignition and it should deliver at least one Imperial pint per minute, and in practice closer to two, in a steady series of pulses with minimal bubbles.

If the tach is steady when this hesitation occurs it isn't ignition LT i.e. coil primary, points or connections. If you can reproduce it at a standstill then clipping a timing light to coil and plug leads and watching the flashes will tell you if it HT if the flashes become erratic.

But as the car is new to you a full set-up from scratch is called for, i.e. plug condition and gaps, HT lead, cap and rotor condition, points condition and gap/dwell, timing, vacuum and centrifugal advance operation, and only when all those are good moving on to the carbs and doing a full jet centering (fixed needles), airflow and mixture balance. Only if you have problems doing the carbs would one normally consider a rebuild, you could waste money rebuilding perfectly good carbs only to find the problem was still there. Start with the easy and cheap stuff.

P Hunt

I think the clue is everything works over 3k.
check oil in dash pots.
I've just had similar problems and have cleaned out the dash pots,( pro re build!!) the crud in the piston damper hole, yuk!! At present I'm experimenting with oils. 20/50 OK. ATF runs a little nicer I think, I have 2 stroke motor bike oil (sae 30)as suggested by Peter Burgess, standing by.
r Tudor

I posted elsewhere on the forum, but found the problem was almost certainly with the carbs. I have now rebuilt (with help from the forum) and the problem has gone.
P M Gregory

This thread was discussed between 23/01/2010 and 13/05/2010

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