MGB: Ghosts are Back!

78 B with 70 head, new street cam and SU HS 6 's.
The Story:
Running great for the past two weeks. Yesterday, I heard a backfire and from then on, the idle went south. There was a constant miss and genrally poor running. At one point, the car stopped. I did not hink that it would refire, but it did.
It struggled to rev past 4000 and the tach seems to malfunction around the 4000 mark. It's like the car will continue slowly accelerating, but the tach holds steady at 4000.
The backfire tells me fuel or exhaust.
The tach issues and miss tell me ignition. The car will now idle about 500 rpm lowere than it did previously and many times want s to die.
The carbs seem ok. Both sides are lifting and have vacum. Would a fuel filter possibly be at fault?
Anyone have a similar situation? Does it sound like a distributor problem?
Your thoughts are greatly appreciated.
Matt

Check the static timing after checking the points gap. Something could have moved.

Is 78 a points type distributor?

Matt,

although the HS 6 carbs are way to big for the demand of your engine, is should run fine with them.
The head of a '70 B is also not that different but the dizzy of the '78 is!

I had the same symptoms on a '75 B when changing to HS 4's years ago. Although it has a differnt type of tach the
readings were close to what you observed.
As the missfiing could not be cured by checking valves and exhaust gas CO as well as setting the ignition advance, I changed the distributer for an old 25D one from a MG1300, that was lying around in my shop for more than 20 years. Set the dwell and flashed the timing and the problem was cured on this car.
May be this can work for your '78 too.

Ralph

Years ago had a car that acted like that, turned out the timing chain jumped or slipped. Don't know if that's possible with MG's or not

Same thing happened with my 77B. I replaced fuel lines, filter, fuel pump, plugs, wires.........
Coil was the problem.

This may be out there - but I just had it happen. I had managed to "bend" the tip on a spark plug - and it caused the misfires you described. I had run some compression tests and apparently the plug reinstalling it at some point...... rough idle, "missing" under load........... new plug and all is well.

Update
The distributor is a 25d4. When I had the car in the shop, they disconeected the vacum advance and plugged the line. For the heck of it, I reconnected the vacum advance and the idle went up by 500 rpm and smoothed out. The car ran like crap.
I just checked the timing and it is at TDC. I believe that it should be at 10 degrees before at 100 rpm, correct?
I

Is it possible for the float tab to get bent from a severe backfire? Even though the most probable cause is timing jump.

Not sure what you mean by float tab.

I tried to set the timing. It is moving from 5 to 10degrees. I might guess that there is something gone bad in the distributor.
It should stay right on 10 degrees at 1500, correct?
Drove it again after moving the timing. Really drives terrible now. Backfiring and missing.
Might be time for a trailer and on to the shop.

Matt,

when there is a 25D instaled, it should be connected to ported vac, not to the manifold vac that was used on later cars.

Ralph

Forgive me for missing the 78 part; it should have had pointless though yours is converted. I got lucky. You could have any number of things going on right now. I'm thinking the backfire is a symptom, not a cause.
Do a thorough tune; points, static timing, plug gap, valve setting, not in that order necessarilly. While you are doing the valve setting, look to see that you don't have one stuck open or a pushrod jumped out and jammed up.
If all of that checks out, then you can start looking for the wild cards like bad condensors and the like.

Thanks for the recommendations. I will start head to toe and try to figure it out.

I think that by playing around with the distributor, I may have made things worse. Does the fact that the timing would advance but would not hold one position at 1500 rpm mean anything in particular?

Just to add closure.
I took the car into the shop this aftenoon. All that mess was caused by a bad condensor. Chris Roop was on it.
While it was there, we convered the points to a Crane ignition.
All is well again.
For now.