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MG MGB Technical - Diagnosis of missing -- help!

69 B Roadster - Car is missing at low RPM's (below 4,000) intermittently. Improves when I get above 4,000 rpm. Starts O.K. but not as easily as I would like when engine is cold. Also, seems underpowered vs prior performance.

Possible problem: While adjusting timing I had clamp for power for timing light hooked to the top brown wire on front of fuse box and shorted it against the hood prop slot next to it. Fried the wire down to the starter. Pulled harness apart and replaced fried wires and repaired and re-wrapped.

So far have:
-Replaced points and re-set points gap - confirmed by checking dwell and it comes in at 60 degrees as specified
-Re-set timing (17 degrees .... I have seen published stuff from 14 to 20 recommended... had it at 20+ previously and may take it back toward that)
-Cleaned dizzy cap inside contacts...they were a little corroded up even though it is fairly new cap. Does that point the way to anything?
-Cleaned rotor contact edge with my points file - it was a little corroded too
-Replaced condenser
-Had previously set valve lash - should I re-visit that?
-Pulled dash pots and air piston off and cleaned out as there was some yellow creamy looking crud on my front damper and replaced with 20-50W oil. Didn't put washers back as they were hardened and so thick the dampers wouldn't stay in. Will get new ones. Think the P.O. used something non-stock as they seemed too thick and very hard.


Planning on:
-Re-checking compression just be be sure. All were within 145-155 cold last time
-Pulling plugs and checking to see if anything strange there. I am getting fire to all 4 as I did check that.
-Checking coil spark - Seems like that is a bit of a rough test. Coil is fairly new. Still have old one I could put back on to test.
-Advancing timing a couple more degrees
-Checking carb set-up.

Any suggestions? Driving me crazy not to be driving it more but don't feel comfortable doing that if it's not working right.
J.T. Bamford

Not so much corrosion on the cap contacts and rotor but burning, this is normal, the HT has to jump a gap inside the cap as well as at the plugs.

Any flickering of the tach with the missfire? Does a timing light clipped onto the king lead and each plug lead show any erratic flashing when the missfire occurs? Use your 12v with inductive pickup, I presume) timing light as these will show up both lack of HT and also plugs breaking down internally. The older series type won't show the latter, as won't pulling the leads away a bit and watching for a spark jumping the gap. Dwell consistent during the misfire? Timing ditto? Inlet manifold leaks? Spray carb cleaner round the gaskets and plugs, a leak will make the idle go up. When did the carbs last have a full, and proper, setup (and not just a twiddle here and there)? How old are the needles and jets?
Paul Hunt

By all means, pull your plugs and check for their condition. I assume when you say you have "fire to all 4" you are using your inductive timing light as an indicator. Keep in mind that the timing light only senses current flow and that a plug shorted closed will still flow current. I would check plugs and plug wires. Also, take a look at your engine in the dark. While running, any shorts from deteriorated plug wires or cracked dizzy cap will show up as sparks as pretty blue arcs.
Bill Boorse

Change your plugs and check resistance of individual spark plug wires. Without a scope, the only thing you can check on ignition is whether something is obviously broken or not. Usually intermitent misfires are ignition related but often caused by excessive fuel fouling the plugs. Make sure you also check the compression while the engine is running to rule out intake or rocker arm problems. You can have good compression cranking, but no running compression due to valve train fault, however, this would not be intermitent.
Joaquin

This thread was discussed between 06/08/2003 and 08/08/2003

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