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MG MGB Technical - Rough starting 78B

My 78B with twin SU's has just started rough starting. When it starts, it coughs and stutters and runs rough as if only 1 carb is working. If I pulse the gas to over 3K it seems run normally. Once the car is warmed up, 3 to 4 minutes, I can reduce the manual choke and it will idle more normal.

When I am driving, if I run it about 2K rpm, I feel and hear a surging in the engine.. If I bring it up to about 2,200 rpm, the surging goes away..

Maint. this spring I had new wires, plugs and cap installed and the timeing adjusted to deal with a very rough start.. teh car ran great for about 6 weeks and then the above started..

Any thoughts??
Jeff Lanaus

Jeff-
I've read your message a few times and am drawing some conclusions (please note I am not one of the seasoned MG nutters here, but perhaps with a bit more info from you we can coax them into some assistance).

You wrote, "I can reduce the manual choke and it will idle more normal."
So, after three to four minutes (which is way too long for a properly tuned and adjusted B) you are merely reducing the choke, but still leaving it partially "on"...

This signals that the carbs are clearly running too lean. More on this in a bit.

You wrote, "I had new wires, plugs and cap installed and the timeing adjusted to deal with a very rough start"
So, you paid a shop to perform this task to correct the same problem you are now experiencing again...

My short fast answer is if you are not comfortable turning a wrench on your MG is to take it back to the shop. But that would take all the fun out of owning one of these cars, wouldn't it?

It appears to me that they did not adjust the carburettors...or that there is excessive wear on the internal bits.

If you did the work yourself and/or you are willing to do a bit of wrneching then check the following:

Pull the spark plugs and inspect after a drive (if possible at least 20 min after you are done faffing about with the choke). They should be a biscuit colour. If black and sooty then the mixture is excessively rich from you compensating with the choke, but I'd be prepared to find plugs that are glazed white or have the central insulation cracked-which signals an overly lean mixture.

Remove the air filter(s) and check for smooth operation of the pistons...when you reach in and lift one up does it come back down smoothly or does it stick? Does one come down faster than the other? Is there oil in the dampers?

Be sure to advise what you find.

Hope this helps
Matt
M Ruch

My first thought is that one of your replacement parts has suffered infant mortality, it isn't unknown. If you can get the poor running to happen with the car at a standstill and your head under the bonnet, I'd clip a timing light onto the coil lead and each plug lead and watch for any erratic or irregular flashing. If they seem to be regular, then when on plug leads 1 and 4 point the light at the crank pulley and see if the timing is about right and not jumping about all over the place. If *that's* OK, then it looks like fuel, although it might be worth replacing the condenser as a matter of course, as these can be difficult to diagnose.

For fuel I'd set up both carbs from scratch, and if one or both has to be moved too far from the basic starting position or each other then a problem is incicated, which could be flooding, starvation, vacuum leak etc. Also check the rise and fall times of the pistons with the air cleaners removed.
Paul Hunt 2

Oil in the dash pots?
Peter

Shouldn't make any difference, the oil only affects things when opening the throttle, lack of oil causing stumbling.
Paul Hunt 2

This thread was discussed between 26/06/2007 and 01/07/2007

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