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MG MGB Technical - Help Needed With 1980 MGB

Still having problems with a "well used" 1980 MGB after doing a whole bunch of stuff to try and correct a "rough running" problem that does NOT occur at the idle. It only happens when there is a higher rev load on the engine once you get up to 2800 - 3000 RPMs [when the revs are increased manually when the car is stationary or running in any gear]. After getting through 1st gear and into 2nd, and the revs drop down to about 1500 - 1800, it is smooth again, and when you bring up the revs to about 2800 - 3000 RPMs in 2nd gear, the problem appears again and so on through 3rd and 4th gear. This 1980 MGB is pretty typical of a daily driver. It is not in showroom condition but has travelled all about over the last couple of summers and has operated pretty well throughout. Single carb. No special stuff added. Only of late has this problem come about and nothing has been done to the car that would be cause for concern or alarm.

The basic issue is rough running, coughing, sputtering, bucking and loss of power when the engine gets to about 2800 - 3000 RPMs.

Here is what we have done so far:
new spark plugs - set to 0.035"
new plug and coil wires
new points - set to 0.015" by removing the distributor and setting the gap on the bench
new condenser
checking of dwell angle confirmed at 52 degrees
new rotor
inspection of distributor cap shows no signs of problems
checking for air leaks on the intake and exhaust areas (with propane tank) shows no problems
checking for loose bolts and nuts all about the engine area (headers, manifold, carb,etc etc) shows nothing out of whack
checking for problems with hoses for vacuum advance and emission stuff appears all OK
engine timing with strobe light to about 14 degrees BTDC (timing advances when engine revs are increased)
adjusting valve clearances to 0.015" [cold] (most were OK - a few were a little out of adjustment - and a running test gives every indication they are within tolerance)
compression test shows a bit low on all 4 cylinders ( 1=140 / 2=110 / 3=110 / 4=130)
new fuel filter

and when this did not appear to fix the problem, we went through the checking and adjustment again to be sure we had not made a mistake in one of the settings but the problem is still there. Idle is OK but when the engine speed is brought up to about 2800 - 3000 RPMs the rough running, coughing, sputtering and loss of power occurs.
Leonard & Deborah Fortin

Sounds to me like fuel starvation. Run it up till it starts acting up, then shut it off immediately. Check the fuel level in the float bowl, if it's empty (or nearly so) you've found the problem.
Ken Lessig

I agree wit Ken that it sounds like fuel starvation given that you have changed all of the electrics. May be also check your throttle spindles. I had a similar problem recently at lower revs. If the spindles have any wearthen replace them. I found there was a ridge on my spindles of about 1 to 2 thou around about half the shaft. At idle it was OK but as the throttle opened the spindle moved because of the wear and let air past causing fuel starvation. Richard
Richard Evans

Another way of checking for fuel starvation is that the pump delivers at least 1pt in 30 secs at the carbs. There is more in the electrics than you have changed in your list. If you can reproduce it while stationary clip a timing light onto each plug lead and the king lead and watch for any irregularities in the flashing. If so, HT is the problem, and regular at the king lead but irregular on the plug leads points to cap/rotor, I have found two breaking down this way even though a visual inspection showed nothing. Is the dwell and timing consistent when the problem is apparent? Does the tach flicker when the problem occurs? If so ignition LT is the problem, which should also show up on the timing light if not dwell.
Paul Hunt



I had a similar experience and discovered it was fuel floding the cab. I would always step on the gas pedal(a couple of times sometimes) and then turn the ignation to start the car. this resulted in poor performance, similar to leonard's experience.

Solution: stay off the gas pedal, just turn the ignation key and only step on the gas pedal after the engine start running.
also make sure screws on the top/tip of spark plugs are tight.
nate

I had a similar problem with the 1.8 previously fitted in my B. Turned out to be a partially blockwed pipe from the tank to the fuel pump. Looked good from the outside, perished like mad inside and restricting flow.
Worth a look.
Richard Day

This thread was discussed between 04/08/2003 and 07/08/2003

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