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Smoking MGB

Posted by Little Nellie 
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Little Nellie Avatar
Bill B
Fresno, CA, USA   usa
1979 MG MGB "Little Nellie"
All I have just spent the weekend pulling out the motor from my 1979 MGB as it was smoking during de-celleration and acceleration.

Here is what was found and what I did.

1) The head was cleaned magnafluxed checked for cracks, machined flat (.005"removed) new valve stem seals fitted, it has hardend inserts, valve guides and valves good, all looked ok.
2) Pistons removed, rings checked and new piston rod bearings fitted as the existing showed wear.
3) Motor re-assembed with Payen head gasket, torqued to 45 lbs
4) Motor re-assembled and installed back in the car.
5) Compession check (cold showed)
#1 170
#2 170
#3 165
#4 165
5)started the motor everything ok until about the time the choke shut off and then motor idling rough. Checked the spark by pulling off each plug lead and # 3 not much difference???
6)Shut it off and Pulled the plugs 12&4 looked lean # 3 a little soot.
Compression test hot showed
#1 180
#2 182
#3 180
#4 175

7)Removed the rocker cover and adjusted the tappets and let it cool overnight
8)This morning checked fluids all ok, installed the plugs fired it up still a little rough adjusted the carb then took it for a run.
9) "The bugger still smokes" (oil) on de-celleration when changing gear......and the engine does not de-cellerate as it should when changing gear (races a little)
10) Did not rev the engine to see if it smokes at high rpm as I want to run in the rod bearings.
11) engine idles rough still missing on # 3

Any thoughts on # 3 cylinder and the smoke issue?????????
It is a california car with all the emmission stuff.


dickmoritz Avatar
Dick Moritz
Philly 'burbs, PA, USA   usa

Did you hone cylinders and replace piston rings? Did you check all the cam lobes for wear?

FYI, head gasket torque for all MGBs, I believe, is 50 ft-lbs., but that shouldn't affect what you're experiencing. And no need to particularly baby the new rod bearings, although there are specific break-in procedures for new cam or lifters, and new piston rings...

Are you using 20W-50 non-synthetic oil? Are you running OE electronic ignition and original ZS carb?


Errabundi Saepe, Semper Certi
(Often wrong, but always certain)

Fogliner Avatar
Mark Vanherd
Thornhill, British Columbia, Canada   can
1974 MG MGB
Did you do anything with the front sidecover when you had it apart? The mesh inside gets cooked from being so close to the Catalitic converter and breaks down.This allows the oil vapour that it is designed to trap,to go right up the pipe and get drawn into the intake (if it doesn't plug itself off).
I read an article on here somewhere on how to cut the back side open enough to get the mesh out and clean then replace the mesh with a stainless choregirl and close it up again.
I always thought it would be a good idea on a CC equipped car as the cat gets extremely hot.

Something to check anyway,might not solve the prob but it's easy to overlook the sidecover.


Little Nellie Avatar
Bill B
Fresno, CA, USA   usa
1979 MG MGB "Little Nellie"
Dick thanks for your comments

The cylinders were not honed as it looks to be that the motor was rebuilt recently and has 040" oversize pistons.

Cam lobes look great

Castrol 20/50 liquid engineering

Yes OE Ignition and rebuilt ZS with new jet and choke calibrated new diaphram etc

Little Nellie Avatar
Bill B
Fresno, CA, USA   usa
1979 MG MGB "Little Nellie"
Mark, the vent tube from the crank case to the carb is as clean as a whistle and the Cat although in place has no guts inside it.

dickmoritz Avatar
Dick Moritz
Philly 'burbs, PA, USA   usa

Perhaps pursue one issue at a time. Try swapping the number 3 spark plug with another to see if the miss moves. If it does, replace the plug. If no change, try swapping the number plug wire with another to see if the miss moves. If it does, replace the wire. You might also try connecting a timing light to each plug wire in turn to see if you see any difference in signal from any of the four wires.

Keep at it; you'll get there...


Errabundi Saepe, Semper Certi
(Often wrong, but always certain)

ErnieY Avatar
Ernie Y
Nr Cahors, France   fra
1976 MG MGB GT "My Mistress Of Nearly 30 Years"
1977 MG MGB GT "Runner But Parts Car"
Are you sure the smoke is not brake fluid from a punctured servo diaphragm, a pin hole would be enough to do it ?

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